The paper describes the structure of the ICF program and highlights its latest results. The pre-construction activities of the National Ignition Facility (NIF) project have increased their momentum as a result of the recent Congressional decision to appropriate the requested funds for Fiscal Year 1997 to continue the project. The facility is an essential part of United States nuclear defense program in the absence of underground testing. The near-term goal of the U.S. ICF program is to achieve ignition in the laboratory (i.e. NIF) and to achieve high yield in the future. This paper describes some current results in ignition physics and related areas, as well as the progress in some new concepts that may have application to high yield. International collaborations that have been developed in recent years have contributed to the increased rate of progress. The paper closes with a summary of current and planned international initiatives.

The National Ignition Facility (NIF) Project being constructed at Lawrence Livermore National Laboratory will use advanced Nd-glass laser technology to deliver 1.8 MJ of 0.35-micrometers laser light in a shaped pulse, several nanoseconds in duration, achieving a peak power of 500 TW. A national community of U.S. laboratories is participating in this project, now in its final design phase. France and the US have collaborated on numerous technology developments for NIF and the French Megajoule Laser Project (LMJ). Separate and more limited collaborations between the US and UK have taken place under agreements predating NIF and LMJ. This paper presents the status of the laser design and development of its principal components and optical elements.

The LMJ (Laser Megajoule) Project was approved by the French Atomic Energy Commission (CEA) in 1993. The LMJ is a major component of the French Stockpile Stewardship program (named `Simulation' by CEA) as the National Ignition Facility is part of the US Science Based Stockpile Stewardship Program. An evaluation of the feasibility (Avant Projet Sommaire) describing the technical, financial and schedule aspects of the project was completed in 1994. In 1995, the French Ministry of Defense authorized the Project and preparation of the advanced design report. The CEA laboratory, CESTA, near the City of Bordeaux, has been chosen for the project site.

In the USA, France, and Japan large-scale theoretical and experimental efforts are devoted to creation of high-power laser facilities for DT-ignition in laboratory conditions. The available experimental data and calculational-theoretical models allow to predict the laser energy level needed for realization of the DT-gas ignition under laser irradiation of targets. For direct-drive targets this level is 300 U and for indirect drive targets it is 2 MJ. Achievement of ignition conditions require high uniformity of target irradiation 2%. For indirect—drive targets this value has already been achieved, but the energy level in this case is 5-6 times larger than for direct-drive targets. Methods for achieving the high target irradiation uniformity are complex, but quite real and currently are intensively developed in a number of laboratories in the world. The greatest success has been achieved in Rochester on the facility OMEGA Upgrade.

The proposed 100TW laser system to be built at AWE is based upon LLNL technology currently under development for the National Ignition Facility (NIF). The optical configuration of the AWE system is somewhat different to that for the NIF and presents different problems. This paper illustrates some of the problems and proposed solutions.

Beamlet is a single beamline, nearly full scale physics prototype of the 192 beam Nd:Glass laser driver of the National Ignition Facility. It is used to demonstrate laser performance of the NIF multipass amplifier architecture. Initial system characterization tests have all been performed at pulse durations less than 10 ns. Pinhole closure and modulation at the end of long pulses are a significant concern for the operation of NIF. We recently demonstrated the generation, amplification and propagation of high energy pulses temporally shaped to mimic 20 ns long ignition pulse shapes at fluence levels exceeding the nominal NIF design requirements for Inertial Confinement Fusion by Indirect Drive. We also demonstrated the effectiveness of a new conical pinhole design used in the transport spatial filter to mitigate plasma closure effects and increase closure time to exceed the duration of the 20 ns long pulse.

The status of diode-pumped, transverse-gas-flow cooled, Yb- S-FAP slab lasers is reviewed. Recently acquired experimental performance data are combined with a cost/performance IFE driver design code to define a cost- effective development path for IFE DPSSL drivers. Specific design parameters are described for the Mercury 100J/10 Hz, 1 kW system (first in the development scenario).

An exhaustive set of Beamlet and Nova laser system simulations were performed over a wide range of power levels in order to gain understanding about the statistical trends in Nova and Beamlet's experimental data sets, and to provide critical validation of propagation tools and design `rules' applied to the 192-arm National Ignition Facility at Lawrence Livermore National Laboratory.

Starting from Frantz-Nordvik equations and using the recurrence relations proposed by Lowdermilk et al., an iterative computing method has been proposed to solve the inverse problem of pulsed laser amplifiers, i.e., to find the initial fluence and pulse profiles from the required output fluence and the given amplifier parameters. Numerical calculations have been performed for single- and multi-pass Nd:phosphate glass amplifiers to confirm our results.

The Miro software is being developed for simulating propagation and amplification of laser beams in laser Nd- amplified setups as Megajoules or NIF. The physical effects taken into account are essentially: saturated amplification, absorption, Kerr effect, birefringence and aberration. The models of propagation are either non linear geometrical optics of parallel beams, or Fresnel diffraction. Reverse calculation is also possible. A graphic user interface has been included to allow interactive management of optical chain and results. A Unix environment with X-Windows ans Motif is required to run Miro.

We have developed a fully 3D model for the performance of flashlamp pumped laser amplifiers. The model uses a reverse ray-trace technique to calculate the pumping of the laser glass by the flashlamp radiation. We have discovered several different methods by which we can speed up the calculation of the gain profile in a amplifier. The model predicts the energy-storage performance of the Beamlet amplifiers to better than 5%. This model will be used in the optimization of the National Ignition Facility amplifier design.

A replacement for the HELEN laser has been proposed that would involve a considerable increase in performance up to 100 TW from 32 beams. The design for the new laser is to be based on the technology being developed for the US National Ignition Facility. The pulse generation and preamplification stages employ novel technologies and represent a significant departure from previous designs. As part of the laser replacement development program a pulse generator and preamplifier have been built and installed on the HELEN laser at AWE, based on those employed on the Beamlet laser physics demonstration facility at Lawrence Livermore National Laboratory. As well as providing experience of the technologies involved, this system represents a significant enhancement of the performance of HELEN. Initial pulses are generated by a diode-pumped Q-switched Neodymium-doped Yttrium Lithium Fluoride ring oscillator. These are then launched into optical fiber and transported to a series of two integrated optics amplitude modulators for pulse shaping. The resulting pulses are then transported by optical fibers to a ring regenerative preamplifier for amplification to a level suitable for input to the HELEN laser. We describe this system, its performance and the enhanced capabilities of the HELEN laser resulting from its installation.

Microchip lasers are used as pulse generators in the LMJ front end. These lasers are made with Nd:YLF using collective fabrication processes. A linearly polarized, single-frequency, 1053 nm emission has been achieved with very good efficiency. The beam of the microchip lasers has been coupled into a polarization-maintaining single mode fiber using microlenses, with coupling efficiencies better than 70%.

We have modeled the output of a feedback stabilized regenerative amplifier (regen). We solve the rate equations including upper- and lower-laser-level lifetimes explicitly. The complete regen dynamics including the losses due to the feedback stabilizer are modeled. We provide a prescription for determining the injection-pulse shape required to produce a given output-pulse shape from this region. The model shows excellent agreement to measured regen output. This model of the regen along with our code RAINBOW, completely describes the temporal dynamics of the OMEGA laser system allowing OMEGA users to specify on-target pulse shapes in advance.

We have analyzed the availability and reliability of the flashlamp-pumped, Nd:glass amplifiers that, as a part of a laser now being designed for future experiments, in inertial confinement fusion (ICF), will be used in the National Ignition Facility (NIF). Clearly, in order for large ICF systems such as the NIF to operate effectively as a whole, all components must meet demanding availability and reliability requirements. Accordingly, the NIF amplifiers can achieve high reliability and availability by using reliable parts, and by using a cassette-based maintenance design that allows most key amplifier parts to be replaced within a few hours. In this way, parts that degrade slowly-- as the laser slabs, silver reflectors, and blastshields can be expected to do, based on previous experience--can be replaced either between shots or during scheduled maintenance periods, with no effect on availability or reliability. In contrast, parts that fail rapidly--such as the flashlamps--can and do cause unavailability or unreliability. Our analysis demonstrates that the amplifiers for the NIF will meet availability and reliability goals, respectively, of 99.8% and 99.4%, provided that the 7680 NIF flashlamps in NIF have failure rates of less than, or equal to, those experienced on Nova, a 5000-lamp laser at Lawrence Livermore National Laboratory.

Amplifier prototypes for the National Ignition Facility and the Laser Megajoule will be tested at Lawrence Livermore National Laboratory. The prototype amplifier, which is an ensemble of modules from LLNL and Centre d'Etudes de Limeil- Valenton, is cassette-based with bottom access for maintenance. A sealed maintenance transfer vehicle which moves optical cassettes between the amplifier and the assembly cleanroom, and a vacuum gripper which holds laser slabs during cassette assembly will also be tested. The prototype amplifier will be used to verify amplifier optical performance, thermal recovery time, and cleanliness of mechanical operations.

We developed a number of active devices for use in the regenerative amplifier, which is one of the specific sub- assemblies of the preamplifying section in the french L.M.J. design. The first one is a modular-square-diode pumped laser head, which provides 0.65 Joules pumping energy at 800 nm into a 4 mm side Nd³⁺:phosphate glass, in the form of a close coupling-uniform-configuration. Our original side pumping scheme makes use of symmetric diode stacks and optimized rod holders with a thermal conductivity. Some heat is waste in the volume of glass, as a result of the pumping process, and it is efficiently--since very closely--removed by the latter holders. A (pi) /2 rotation of one pumping section with respect to the next one allows uniform pumping and thermal conduction together with birefringence reduction. No water is required. A complete 3D thermal model is developed, in order to describe temperature and stress distributions, inside the laser head and glass rod. Peak stress values in glass at F equals 10 Hz, with flexible rod holders including Indium parts, equal 5.8 Mpa when the mean thermal power is 22 watts. Preliminary laser tests are experienced at 10 Hz with two similar laser heads around a square Nd³⁺:LHG750 glass rod. With 1.3 Joule pump energy at 400 microsecond(s) pumping duration, the assembly is placed inside a Qswitched stable, multimode plano-concave, resonator. The output energy is 40 mJ, within 80 ns fwhm pulses.

The concept to use a slab as active element, working in zig- zag geometry, and also as Fresnel rhomb, seems to be rather attractive. However, in this case different depolarization effects in active element are of crucial importance. We have carrier out the estimation of depolarization effects arising both due to mechanical loading of an active element at its fastening and due to thermooptical distortions. To check up these rigid requirements to depolarization (0.1% - 0.01%) careful measurements of depolarization effects and their sources are being carried out. Mechanical loading gives one of the main contributions in depolarization at fastening of active element. Using model experiments with glass Fresnel rhomb under mechanical loading we have measured depolarization effects. It is proposed to use additional glass plate to compensate beam depolarization in zig-zag slab. The received results allow to expect successful use of the slab amplifier as a Fresnel rhomb providing rather high quality of optical materials of active element.

We are currently developing large-aperture amplifiers for the National Ignition Facility (NIF) and Laser Megajoules (LMJ) lasers. These multi-segment amplifiers are of the flashlamp-pumped, Nd:Glass type and are designed to propagate a nominally 36 cm square beam. The apertures within a particular amplifier bundle are arranged in a four- high by two-wide configuration and utilize two side flashlamp arrays and a central flashlamp array for pumping. The configuration is very similar to that used in the Beamlet laser, a single-beam prototype for the NIF/LMJ lasers, which has four apertures arranged in a two-high by two-wide configuration. In designing these amplifiers, one of the more important criteria is the layout and composition of the pump cavity. The pump cavity consists of the laser slab, blast shields, flashlamp cassettes, and any high- reflectivity components that may be used. The current pump cavity design for the NIF/LMJ lasers is in many respects very similar to that of the Beamlet laser: Brewster-angle laser slab, central and side flashlamp cassettes, and top and bottom reflectors (in both the flashlamp cassettes and in the slab cassette). However, there are a number of important differences between the two amplifiers. The NIF/LMJ amplifiers will use 180 cm arc-length, 4.3 cm bore- diameter flashlamps as opposed to the 91 cm arclength, 2.5 cm bore diameter flashlamps used on Beamlet. In addition, in the NIF/LMJ lasers there will be eight lamps in the central lamp array and six lamps in the side lamp array, vs 16 lamps in the central array and 10 lamps in the side array on Beamlet. Finally, the NIF/LMJ flashlamp pulsewidth will be 360 microsecond(s) as compared to 550 microsecond(s) on Beamlet. To understand the effect of these differences, we performed a series of experiments in order to characterize the optical performance of the amplifier. These experiments were done on a Beamlet amplifier that was modified to accept the different-sized flashlamps. In addition, our pulsed-power bank was modified to produce the 360 microsecond(s) pulsewidths needed. We will present results regarding the full-aperture gain distribution and the effect of pre-pulse conditions on flashlamp pumping efficiency. We also investigated the possibility of steering the pump light to selected regions of the laser slab. We will show that this can ameliorate the effects of amplifier spontaneous emission on gain uniformity.

The laser architecture of the NIF beamlines requires small- area beam dumps to safely absorb back reflections from the output and leakage through the PEPC switch. The problems presented by these beam dumps are that fluences they must absorb are very large, beyond the damage threshold of any material, and ablation of beam dump materials potentially contaminates adjacent optical components. Full scale tests have demonstrated that a stainless steel beam dump will survive fluence levels and energies as high as 820 J/cm² and 2.5 kJ, respectively. Small scale tests with tungsten, tantalum, and stainless steel have demonstrated erosion rates less than about 0.5 micrometers /shot, with stainless steel having the smallest rate. They also suggest that increased angles of incidence (>= 60 degree(s)) will greatly reduce the material ablated directly back along the beam path.

A preliminary error budget for the third harmonic converter for the National Ignition Facility laser driver has been developed using a root-sum-square-accumulation of error sources. Such a budget sets an upper bound on the allowable magnitude of the various effects that reduce conversion efficiency. Development efforts on crystal mounting technology and crystal quality studies are discussed.

A large aperture optical switch based on plasma electrode Pockels cell (PEPC) technology is an integral part of the National Ignition Facility (NIF) laser design. This optical switch will trap the input optical pulse in the NIF amplifier cavity for four gain passes and then switch the high-energy output optical pulse out of the cavity. The switch will consist of arrays of plasma electrode Pockels cells working in conjunction with thin-film, Brewster's angle polarizes. The 192 beams in the NIF will be arranged in 4 X 2 bundles. To meet the required beam-to-beam spacing within each bundle, we have proposed a NIF PEPC design based on a 4 X 1 mechanical module (column) which is in turn comprised of two electrically independent 2 X 1 PEPC units. In this paper, we report on the design a single 2 X 1 prototype module and experimental tests of important design issues using our single, 32 cm aperture PEPC prototype. The purpose of the 2 X 1 prototype is to prove the viability of a 2 X 1 PEPC and to act as engineering test bed for the NIF PEPC design.

Experiments and calculations indicate that the threshold pressure in spatial filters for distortion of a transmitted pulse scales approximately as I^-0.2 and (F#)² over the intensity range from 10¹⁴ to 2 X 10¹⁵ W/cm². We also demonstrated an interferometric diagnostic that will be used to measure the scaling relationships governing pinhole closure in spatial filters.

Designing a computer control system for the National Ignition Facility (NIF) is a complex undertaking, because of both the system's large size and its distributed nature. The controls team is addressing that complexity by adopting the object-oriented programming paradigm, designing reusable software frameworks, and using the Common Object Request Broker Architecture (CORBA) for distribution. A prototype system for image-based automatic laser alignment has been developed to evaluate and gain experience with CORBA and OOP in a small distributed system. The prototype is also important in the evaluation of alignment concepts, image processing techniques, speed and accuracy of automatic alignment for the NIF, and control hardware for alignment devices. The prototype system has met its initial objectives, and provides a basis for continued development.

A new dynamic alignment procedure, with a high repetition rate, taking into account all the fluctuations of the components of each laser beam, including the driver, is proposed, using the driver itself. Diode pumped solid state drivers, operating at 1 - 10 Hz, allow the alignment in thermal stabilized thermal conditions and its control until the main shot. An alignment sensor designed for such a procedure is presented. It is based on a set of two CCD cameras with their optics, located behind the last spatial filter pinhole, a set of boresights inserted at the pinhole locations and an image processor. This equipment fulfills the requirements in terms of delay, precision and reduction of the number of components.

In order to correct wave front distortions, a technique has been developed based on adaptive optics used in a pre- compensation configuration. The system consists of a wave front sensor and an optically driven liquid crystal Spatial Light Modulator as a wave front corrector. The main advantage of the sensor is its ability to detect phase deformation of several tens of wavelengths with an accuracy around (lambda) /10. This approach appears to be a good candidate for phase shaping ultra-intense laser system exhibiting strong wave front distortions. The use of a optically addressed single large liquid crystal cell covering the entire beam aperture makes this device diffraction free. The choice of a pre-compensation geometry is discussed and preliminary results are presented.

This paper describes the major optical and mechanical design features of the Beamlet Focal Plane Diagnostic system as well as measurements of the system performance, and typical data obtained to date. We also discuss the NIF requirements on the focal spot that we are interested in measuring, and some of our plans for future work using this system.

The operational requirements of the National Ignition Facility place tight constraints upon its alignment system. In general, the alignment system must establish and maintain the correct relationships between beam position, beam angle, laser component clear apertures, and the target. At the target, this includes adjustment of beam focus to obtain the correct spot size. This must be accomplished for all beamlines in the time consistent with planned shot rates and yet, in the front end and main laser, beam control functions cannot be initiated until the amplifiers have sufficiently cooled so as to minimize dynamic thermal distortions during and after alignment and wavefront optimization. The scope of the task dictates an automated system that implements parallel processes. We describe reticle choices and other alignment references, insertion of alignment beams, principles of operation of the Chamber Center Reference System and Target Alignment Sensor, and the anticipated alignment sequence that will occur between shots.

This paper is concerned with the statistical comparison of 2D smoothing by spectral dispersion and smoothing by optical fiber, both techniques being proposed for uniform irradiation in plasma physics. In the asymptotic framework of a large number of elements of the random phase plate or excited optical modes of the fiber, closed-form expressions for the contrast and spatial spectrum of the integrated intensity of the speckle pattern are derived so as to put into evidence performance differences between these methods. These differences essentially originate from the much longer time delay induced by the multimode fiber with respect to the one induced by the gratings and from the interplay between the nature of the delay line vs. the nature of the spectral broadening.

Energy-efficient laser phase conversion, using fully continuous distributed phase plates, has been achieved for solid-state laser drivers in ICF. Optical lithography has been demonstrated to be an excellent means of generating deep, continuous surface-relief structures in photosensitive materials that subsequently are replicated with embossing or etching techniques. In addition, the method of simulated annealing has been shown to be a superior technique for designing continuous phase plates to control the focal-plane profile.

It is now widely recognized that spatial beam smoothing (homogenization) is essential in coupling the laser energy to the inertial confinement fusion (ICF) targets'. For the indirect drive approach' to ICF, it is desirable to distribute the laser energy into a uniformly speckled profile that has a flat-top super-Gaussian envelope (8th power or higher) and contains greater than 95% of the energy inside the super-Gaussian profile. Spatial smoothing is easily achieved by introducing a binary random phase plate (RPP) in the beam. This produces a homogenized far-field pattern which consists of an overall envelope function determined by the RPP element superimposed with a fine scale speckle pattern arising due to the interference among the various RPP elements. Although easy to fabricate and currently in routine use in many fusion laboratories2, the binary RPPs do not meet the ICF requirements stated above since the far-field intensity profile is restricted to essentially an Airy function containing only 84% (an upper limit) of the energy inside the central spot. Approaches using lenslet arrays (refractive or diffractive)3'4 have limited use since they operate in the quasi-far-field and have a short depth of focus.

The laser wavefront of the NIF Beamlet demonstration system is corrected for static aberrations with a wavefront control system. The system operates closed loop with a probe beam prior to a shot and has a loop bandwidth of about 3 Hz. However, until recently the wavefront control system was disabled several minutes prior to the shot to allow time to manually reconfigure its attenuators and probe beam insertion mechanism to shot mode. Thermally-induced dynamic variations in gas density in the Beamlet main beam line produce significant wavefront error.

This report represents the results of mathematical modeling of thermo-optical distortion in active elements with rectangular section of the laser systems. The thermo-optical distortions (TOD) in slabs are superposition of the two effects: the change of length of beam path; the change of refraction index due to temperature and stress distributions. The very important mechanisms of TOD had been investigated with numerical simulations. (1) The pumping of slabs which results in inhomogeneity of temperature space distribution along the short sides of slabs. (2) The regular inhomogeneity of pumping along the long the sides of slabs that results in S-type bending. (3) The local inhomogeneity of pumping as a result of pumping defect. (4) The slab support influence on the TOD increasing.

The performance of the Beamlet laser with 1D SSD implemented is investigated. Simulations indicate that the critical issue for laser performance is the amount of additional divergence owing to SSD in comparison to the size of the spatial filter pinholes. At the current +/- 200 (mu) rad pinholes used on Beamlet, simulations indicate that the levels of SSD divergence anticipated for the National Ignition Facility results in a very slight degradation to the near field beam quality. Experiments performed with the Beamlet front end show no degradation to the near field beam with up to 100 (mu) rad of SSD divergence. MEasurements of the smoothing of a far field speckle pattern generated by a phase plate show the expected improvement in contrast with increasing amounts of SSD divergence.

The Laser Megajoule is the project equivalent in France to the National Ignition Facility project in USA. The laser has been sized to achieve ignition of a small amount of DT and to produce fusion energy in laboratory with a significant gain, by imploding small capsules filled with a DT mixture. The target area has to be designed to contain the maximum threats imposed by large emissions of neutrons, x-rays and debris due to the explosion of the target. We describe the 10 m diameter target chamber. Prediction calculations of the x-ray threats on the debris shields and the first wall are also detailed. We show that the first wall has to be protected by refractive materials and that low porosity plasma-sprayed boron carbide is potentially a good candidate. We also show that high velocity shrapnels generated by neutron, x-ray and ion deposition in the materials close to the target could also be a threat for the target chamber first wall and optics.

The preliminary design of the target area for the National Ignition Facility has been completed. The target area is required to meet a challenging set of engineering system design requirements and user needs. The target area must provide the appropriate conditions before, during, and after each shot. The repeated introduction of large amounts of laser energy into the chamber and subsequent target emissions represent new design challenges for ICF facility design. Prior to each shot, the target area must provide the required target illumination, target chamber vacuum, diagnostics, and optically stable structures. During the shot, the impact of the target emissions on the target chamber, diagnostics, and optical elements is minimized and the workers and public are protected from excessive prompt radiation doses. After the shot, residual radioactivation is managed to allow the required accessibility. Diagnostic data is retrieved, operations and maintenance activities are conducted, and the facility is ready for the next shot. The target area subsystems include the target chamber, target positioner, structural systems, target diagnostics, environmental systems, and the final optics assembly. The engineering design of the major elements of the target area requires a unique combination of precision engineering, structural analysis, opto-mechanical design, random vibration suppression, thermal stability, materials engineering, robotics, and optical cleanliness. The facility has been designed to conduct both x-ray driven targets and to be converted at a later date for direct drive experiments. The NIF has been configured to provide a wide range of experimental environments for the anticipated user groups of the facility. The design status of the major elements of the target area is described.

The principal technical goal of the National Ignition Facility (NIF) is to achieve thermonuclear ignition in a laboratory environment via inertial confinement fusion. This will enable NIF to service the DOE stockpile stewardship management program, inertial fusion energy goals, and advance scientific frontiers. All of these applications will make use of the extreme conditions that the facility will create in the target chamber. In the case of a projected 20 MJ yield scenario, NIF will produce approximately 10¹⁹ neutrons with a characteristic DT fusion 14 MeV energy per neutron. There will also be a substantial amount of high energy x-rays as well as solid, liquid and gaseous target debris produced with directly or indirectly by the inertial confinement fusion process. A critical design issue is the protection of the final optical components as well as sophisticated target diagnostics in such a harsh environment.

Various target irradiation systems allowing to obtain a high irradiation uniformity degree of direct drive targets are analyzed. The required irradiation uniformity of the target <EQ 1% can be realized for laser beams number of N_c? >= 100, Waist diameter on the order of target diameter and gaussian or supergaussian transverse laser intensity distribution on the target.

The Beamlet laser is a nearly full-scale, single-aperture prototype of the driver design for the National Ignition Facility (NIF). As part of a test and validation plan for the NIF design, Beamlet was recently equipped with final focusing optics and diagnostics for the purpose of evaluating integrated component performance and equivalent target-plane irradiance conditions at the 0.351-micrometers output wavelength specified for NIF targets. A 37-cm aperture two- crystal converter scheme generates the third harmonic of the Nd:glass 1.053-micrometers wavelength with high efficiency. The efficiency of the converter has been characterized and is reported, along with detailed measurements of the near-field and far-field UV irradiance distributions at operating conditions up to and exceeding red-line levels for the NIF. Dependences of observed beam quality on critical laser parameters including output power, B-integral, and spatial filtering are discussed and compared with numerical simulations.

The characteristics of the 3(omega) focal spot are determined by the properties of the 1(omega) beam and the frequency tripling process. The size of the 3(omega) focal spot depends on the spectrum of spatial noise in the 1(omega) beam. A perturbation theory for ripple transfer in frequency tripling is used to predict the characteristics of the 3(omega) focal spot. The theory predicts that 1(omega) phase noise grows 9X in power in the tripling process. This can cause a significant reduction in 3(omega) energy delivered to the target. Results from Beamlet 3(omega) focal spot characterization experiments are also presented.

Experimental and theoretical studies at Los Alamos of nonlinear optical phenomena important to the design of the National Ignition Facility are summarized. These include measurements of nonlinear index coefficients, Raman scattering in atmospheric oxygen, and theoretical studies of harmonic conversion.

The main goal of the work was the analysis of approach, based on the use of pulse compression technique in laser systems for inertial confinement fusion. This approach promises improvement of parameters of existing `direct amplification' laser systems. The compression technique provides the additional instrument for control of spatial and temporal structure of the pulse, necessary for ISF realization.

KDP (KH₂PO₄) single crystals up to 47 cm in size have been grown by the rapid growth technique on the point seed in glass crystallizers of 1000 L in volume at growth rates of 10 to 25 mm/day in both the [001] and [100] directions. Measurement of the optical quality of 41 X 41 cm single crystal plates are presented.

WYKO Corporation is currently designing and manufacturing specialized phase shifting interferometers to aid in the qualification of large optics for the U.S. NIF program. The interferometers will be used to qualify homogeneity of raw material and provide in-process inspection information and final inspection qualification data. The 24' systems will be the largest commercially available Fizeau phase shifting interferometers ever manufactured. Systems will be produced using traditional CCD cameras as well as megapixel CCD camera for applications requiring higher lateral resolution. Mechanical and optical design considerations include vibration and distortion control of critical optical elements, polarization control of the laser source, imaging system design, and optical transfer function optimization. We also address effects in the test cavity arising from measuring transmitted and reflected wavefronts of optics mounted at Brewster's angle.

A new method for figuring the surface profile of optical plastics and optical glass have been proposed and demonstrated. An ArF excimer laser is used to ablate very thin layer of the surface of the substrates. The shape of the ablated surface is monitored by an interferometer in site condition. The ablation rate of PMMA is 0.08 micrometers per pulse at the energy density of 50 mJ/cm². The optical glass (BK-7) can be ablated 0.15 micrometers per pulse at the fluence of 1.5 J/cm².

The possibilities of very high damage threshold optics at the Nd:YAG third harmonic are discussed for Megajoule applications. Only two possibilities which are either electron beam oxides materials like Al₂O₃/SiO₂ or IBS fluorides like YF₃/LiF seem to be able to meet the specifications. Current state of the art of the two solutions is given as well as their possible improvements.

At the first ICF conference some of the present authors described advances towards low-index silica AR coatings and high-index zirconia HR coatings on fused silica for use at 1.06 micrometers , where silica and zirconia coatings showed good laser-induced damage thresholds. Here progress towards full 3(omega) mirrors at 351 nm using well defined sol-gels to produce the necessary SiO₂-ZrO₂ stacks is described in detail. Results suggest that these SiO₂-ZrO₂ stacks have higher laser damage resistance than previous ones and that MgF₂ and Sc₂O₃ alternatives may improve the mirrors still further. The nature of damage caused to these coatings by Nd-YAG laser at 355 nm operating with pulses of 11 ns duration and a beam 1 mm in diameter is also reported.

Sol-gel derived and base-heat treated antireflective (AR) coatings with good abrasive/scratch resistant performance have been investigated and developed. The comparison of abrasive/scratch resistant property of the AR coatings from this method to that from other methods demonstrated that the base-heat treated AR coatings have still more abrasive/scratch resistant ability. After more than 30 times of drag wipe and 1000 times flashlamp glow discharges of 300 microsecond(s) duration and 15 J/cm² for each discharge, the AR- coatings indicated no visible crazing or peeling. In the mean time, more than 6% transmission gain related to the spectrum of Nd⁺³ absorption can be obtained and maintained.

Most of the glass laser based inertial confinement fusion systems around the world today employ non-linear frequency conversion for converting the 1.053 pm light at the fundamental frequency (referred to as 1o light) to either its second harmonic (called 2o) at 527 nm or to its third harmonic (called 3w) at 351 nm. Shorter wavelengths are preferred for laser fusion because of the improved coupling of the laser light to the fusion targets due to reduced fast electron production at shorter wavelengths. The frequency conversion process, however, is only about 60-70% efficient and the residual 30-40% of the energy remains at la) and 2w frequencies.

Scalar and resonance domain diffractive optical elements are proposed for use within high power laser systems. Resonance domain elements are described for beam deflection and polarization selection. Scalar domain elements for harmonic separation filtering and beam shaping in the near- and far- fields are also described. Experimental results are presented for far-field beam shaping and harmonic separation filtering elements.

We are engaged in a comprehensive effort to understand and model the initiation and growth of laser damage initiated by surface contaminants. This includes, for example, the initial absorption by the contaminant, heating and plasma generation, pressure and thermal loading of the transparent substrate, and subsequent shockwave propagation, `splashing' of molten material and possible spallation, optical propagation and scattering, and treatment of material fracture. The integration use of large radiation hydrodynamics codes, optical propagation codes and material strength codes enables a comprehensive view of the damage process.

We recently demonstrated the production of over a petawatt of peak power in the Nova/Petawatt Laser Facility, generating > 600 J in approximately 440 fs. The Petawatt Laser Project was initiated to develop the capability to test the fast ignitor concept for inertial confinement fusion, and to provide a unique capability in high energy density physics. The laser was designed to produce near kJ pulses with a pulse duration adjustable between 0.5 and 20 ps. At the shortest pulse lengths, this laser is expected to surpass 10²¹ W/cm² when focused later this year. Currently, this system is limited to 600 J pulses in a 46.3- cm beam. Expansion of the beam to 58 cm, with the installation of 94-cm gratings, will enable 1 kJ operation. Target experiments with petawatt pulses will be possible either integrated with Nova in the 10 beam target chamber or as a stand alone system in an independent, dedicated chamber. Focusing the beam onto a target will be accomplished using an on axis parabolic mirror. The design of a novel targeting system enabling the production of ultrahigh contrast pulses and an easily variable effective focal length is also described.

From 1995, ILE has started development of a high power chirp pulse amplification Nd:glass laser for investigating the fast ignitor concept as a new approach toward high gain inertial confinement fusion. The output of the new glass laser line is estimated to be 100 TW, 100 J, 1 ps and is focused on a high density compressed plasma by the GEKKO-XII laser for the additional fast heating.

The process of Chirped Pulse Amplification (CPA) as presently implemented on the VULCAN glass laser is capable of delivering 30 J to target with pulselengths in the sub- picosecond regime. Results from various experiments have shown that intensities of 10¹⁸ - 10¹⁹ W cm^-2 have been achieved on target enabling users to carry out important new experiments in X-ray laser research, laser fusion, basic plasma physics and particle acceleration. An EPSRC (Engineering and Physical Sciences Research Council) facility upgrade grant has been awarded to increase the current operational level of 35 TW, to over 200 TW. This upgrade forms Phase I of a two phase upgrade to raise the performance of VULCAN to the Petawatt level. This paper details the design of the upgraded system and describes a new interaction chamber which takes full advantage of the availability of synchronous kJ multi-beam long pulse operation on VULCAN with the CPA capabilities.

The effect of a change in the system parameters upon the one micron laser's power, energy and beam quality will be discussed. The parameters varied in the study were the optical losses, the gain and gain profile of the amplifiers. Additionally, the effect upon power, energy and beam quality as a function of slab count and position will be presented.

Design optimization of the 1 .8 MJ, 500 TW National Ignition Facility (NIF) laser has proceeded with the use of a suite of new computational models. Cost-effectiveness of alternative fundamental architectures was considered using CHAINOP. A very fast, lumped-element energetics code, CHA1NOP includes an extensive cost database, a runtime choice of optimization algorithm, and a set of heuristic rules for diffraction and nonlinear effects and for operational constraints. Its ability to flexibly consider many alternative configurations at a few seconds per chain made it the ideal "first-cut" tool for narrowing the investigation to the switched, multi-pass cavity architecture that was chosen.

Test results from the Optical Science laser are presented that validate the ability of propagation codes to predict beam filamentation in UV optics. A discussion of IL scaling rules will be given, along with system design curves that relate the allowable rms phase front roughness.

The design of the National Ignition Facility (NIF) is the result of optimization studies that maximized laser performance and reliability within a restricted cost budget. We modeled the laser using a suite of tools that included a 1D propagation code, a frequency conversion code, a 2D ray trace code for calculating the gain profile, thermo- mechanical codes for calculating the pump-induced distortions in the slabs, a database giving estimates of optics bulk/finish quality, and costing models of the laser/building. By exploiting parallel processing, we were able to consider approximately 750 possible designs per hour using a cluster of 28 workstations. For our optimization studies, we used a temporally shaped (ICF indirect drive) pulse producing at least 2.2 MJ and 600 TW in a 600 micron diameter hole at the target entrance plane. We varied as many as 20 design variables (e.g., slab counts, slab thickness, Nd concentration, amplifier pulse length) and applied as many as 40 constants (e.g., flashlamp voltage and fluence damage/filamentation at various points in the chain). We did not vary the number of beamlets (fixed at 192 or the aperture (fixed at 40 cm). We used three different optimization approaches: a variable metric algorithm, an exhaustive grid search of more than 50,000 candidate designs, and a parabolic interpolation scheme. All three approaches gave similar results. Moreover, a graphical analysis of the parameter scan data (analogous to sorting and pruning designs using a spreadsheet) has allowed us to understand why the optimizers eliminated alternate designs. The most inexpensive main-switch-boot slab configuration meeting the mission requirements and satisfying all constraints was 9-5-3. The cost of this configuration is approximately $DOL10M less than the 9-5-5 conceptual design. However, the NIF Project has chosen a slightly more expensive 11-0-7 configuration for continued Title I engineering because of its similarity to the Beamlet 11-0-5 design and a lower B-integral.

The issue of thermal recovery of the NIF amplifiers has taken on increased emphasis as program goals move toward increasing the shot rate to once every four hours. This paper addresses the technical issues associated with achieving thermal recovery in the NIF amplifiers. We identify two temperature related thermal recovery quantities: (1) the difference between the average slab temperature and the temperature of other surfaces in the amplifier cavity, and (2) the temperature difference in the slab over the aperture. The first quantity relates to optical disturbances in the gas columns in the system, while the second quantity is associated with optical aberrations in the laser media itself. Calculations and experiments are used to quantify recovery criteria, and develop cooling approaches. The cooling approaches discussed are (1) active cooling of the flashlamps with ambient gas and chilled gas, and (2) active cooling of the slab edge cladding. Calculations indicate that the NIF baseline cooling approach of 20 cfm per lamp ambient temperature gas flow in both the central and side flashlamp cassettes is capable of meeting thermal recovery requirements for an 8 hour shot period, while to achieve a 4 hour shot period requires use of chilled gas and edge cladding cooling. In addition, the effect of changing the amplifier cavity and beamtube fill gas from nitrogen to helium is addressed, showing that a factor of 8 reduction in the sensitivity to thermal disturbances is possible.

Slabs in NIF/LMJ large-aperture amplifiers are set at Brewster angle between two lamp arrays. A 2D+ ray-trace code is used to predict pump and gain coefficients profiles in the aperture. Code predictions, in good agreement with experimental results, show the high decay rate due to amplification of spontaneous emission leads to a strong depletion of the stored energy on the edges of the slabs. To compensate for this roll-off, shaped reflectors were calculated at CEL-V, using a dedicated routine added to the ray-trace code. They were fabricated and tested on the modified beamlet amplifier at LLNL. Numerical and experimental results agree to show that it is possible to redirect light using shaped reflectors, and that directing the pump radiation towards the edges of the aperture leads to a more uniform gain.

The Mégajoules Laser (LMJ) is comprised of 240 separate beam lines that each produces about 20 kJ of 1 .053 microns light. Simply stated the architecture consists of 240 optical pulse generation system that each delivers a 10 nJ shaped and modulated pulse. Next, 240 preamplifier modules amplify the tailored pulses to 1 J, whereupon they are transported to the large amplifier chains where the laser energy is increased to the 20 kJ level. The preamplifier modules contain a regenerative amplifier ( regen), a spatial beam shaping module and a fourpass amplifier. The specifications for the regen include a total gain of i0, an output energy of 10 mJ, operating at a 1 Hz rate, a square-pulse distortion < 2, a signal-to noise-ratio < i0, and a pulse-to-pulse output energy stability ofbetter than 3%. In case the Mégajoules pulses are smoothed with a multimode fiber, we designed and tested a multimode regenerative amplifier that is described in this paper. The current design of this cavity is shown on Fig 1 . The 4.25 long cavity consists of 2 lenses and 2 spherical mirrors that make it equivalent to a confocal cavity. A square and a circular diaphragms are used in order to have the required Abbe's invariant and to shape the beam in the cavity; the square plane will correspond to the pupil, whereas the circle plane will correspond to the focus plane of the laser. Near a pupil plane is the 4 mm diameter LNA rod amplifier, that is side-pumped by laser diodes. The laser diodes deliver 2 J in a 400 ts pulse, and about 500 mJ are stored in the upper laser state. The total avalaible energy for the cavity modes is about 150 mJ. For the experiment, the input of the regenerative amplifier was a 1 0 pJ pulse of 20 ns duration, delivered by a LNA microchip laser coupled to a multimode fiber. The output beam was mode matched to the regen with 2 diaphragms (square and circular ) and 2 lenses. The s-polarized input was then introduced into the cavity via reflection on a GLAN polarizer. After one round trip the Pockels cell is turned on, trapping the pulse in the cavity untill saturation is reached. The pulse is switched out by energizing one more time the Pockels cell. We measured an output energy of 30 mJ; gain and transmission were determined using Findlay-Clay (relaxation oscillations ), and found to be respectively equal to 20 and 0.91 per round trip. The signal-to-noise ratio was determined by comparing the pulse buildup in the cavity when the input seed is blocked and unblocked. It lead us to a ratio equal to 300, so we can expect 3.1 5 with 10 nJ injected. The amount of square pulse distortion was found to be less than 3.

Megajoule is the laser facility being designed by the French Centre d'Etudes de Limeil-Valenton to obtain ignition and burn. The Laser Megajoule consists of a 240 beam high-energy glass laser system and target chamber, intended for inertial confinement fusion research. About 2 MJ at 351 nm are required. To effectively control the optical pulse on target for different types of experiments, the front end system must be versatile enough to easily control many pulse parameters over a large range. The Megajoule front end system is composed of two major subsystem: the pulse generation system and preamplifier. The following descriptions track the optical signal path through the pulse generation system and initial results are discussed.

We present the design of a diode-pumped Neodymium glass amplifier delivering one Joule output pulses with a 1 Hz to 10 Hz repetition rate, good stability due to diode pumping and good beam properties due to the uniformity of energy deposition. The amplifier architecture is simple and rugged. We show that, when an amplifier with an amplification section of about 1 cm³ is considered, 5 J useful stored energy yields an output pulse with a more than 1 J energy and with low spatial and temporal distortions.

We describe the prototype preamplifier for the NIF laser system and discuss the performance of the regenerative amplifier and 4-pass laser systems that comprise the preamplifier.

We have developed amplitude and phase modulation systems for glass lasers using integrated electro-optic modulators and solid state high-speed electronics. The present and future generation of lasers for Inertial Confinement Fusion require laser beams with complex temporal and phase shaping to compensate for laser gain saturation, mitigate parametric processes such as transverse stimulated Brillouin scattering in optics, and to provide specialized drive to the fusion targets. These functions can be performed using bulk optoelectronic modulators, however using high-speed electronics to drive low voltage integrated optical modulators has many practical advantages. In particular, we utilize microwave GaAs transistors to perform precision, 250 ps resolution temporal shaping. Optical bandwidth is generated using a microwave oscillator at 3 GHz amplified by a solid state amplifier. This drives an integrated electrooptic modulator to achieve laser bandwidths exceeding 30 GHz.

For the LMJ (Laser MegaJoule) project, a programmable low- voltage temporal pulse shaping system of the fiber laser pilot beam is required. A possible solution consists of a LiNbO₃ integrated optical amplitude modulator driven by a programmable electrical waveform generator. Thomson Microsonics has addressed this solution and this paper presents the results of the preliminary study supported partially by CEA fundings. We report the TMX original approach for the design of the non trivial fast waveform generator (based on low cost components) and the fully compliance with the requirements of a first subscale prototype. We present then the first very promising experimental results on a `proton exchange' processed amplitude modulator which exhibits insertion losses and on- off ratio in compliance with the requirements.

Acquisition of an optical signal intensity as a function of time can be performed with precise synchronization (jitter inferior to one picosecond) using an external picosecond pulsed laser as a timing clock driving a photoconductor to generate a streak camera sweep signal. This allows for averaging of successive optical signals, thus increasing the dynamic range of the measurement. With this method the input signal of the ICF laser amplification chain could potentially be directly analyzed in its entirety without need for dynamic range compression devices. A dynamic range of 10⁵ is shown to be feasible goal from test experiments using a 1 kHz Ti:Sapphire laser. The use of laser timing for precise diagnostics of other functions of the ICF operation is also of potential interest.

A variant of the flashlamp for NIF was developed on the base of the experience of manufacture and application of high- power flashlamps in Russia. Features of flashlamp design as well as first test results of the experimental samples are presented.

Experiments were performed on the 100-J class Optical Sciences Laser at LLNL to characterize the saturation fluence and small-signal gain of a solid-state Nd:glass amplifier utilizing LG-750 and LG-770, an amplifier glass developed for the National Ignition Facility (NIF). These high quality measurements of gain saturation at NIF level fluences, i.e., 10 - 15 J/cm², provide essential parameters for the amplifier performance codes used to design NIF and future high power laser systems. The small- signal gain, saturation fluence and square-pulse distortion were measured as a function of input fluence and pulse length in platinum-free LG-750 and LG-770. The input fluence, output fluence, small-signal gain and passive losses were measured to allow calculation of the saturation fluence. Least squares fits of the output vs. input fluence data using a Frantz-Nodvik model was used to obtain an average saturation fluence for each data set. Overall, gain saturation in LG-750 and LG-770 is comparable to long pulse lengths. For shorter pulse lengths, < 5 ns, LG-770 exhibits a stronger pulse length dependence than LG-750, possibly de to a longer terminal level lifetime. LG-770 also has a higher cross-section, which is reflected by its slightly higher extraction efficiency.

Weare in the midst of constructing an amplifier laboratory (Amplab) that will be the physics and engineering proving ground for full sized segmented glass amplifiers ofdesigns that will outfit the National Ignition Facility (NIF) and Laser Megajoule (LMJ) projects. Amplab will demonstrate the cornerstone mechanical, electrical and optical concepts that support the NTF and LMJ amplifier schemes. Here we address the optical diagnostics that will be used to characterize optical performance ofthe amplifiers. We describe, the apparatus that will be used in pulsed measurements ofgain distribution and wave-front distortions. The large aperture diagnostic system or LADS, is now being built through a collaborative effort between CEL-V and LLNL. The LADS will provide measurements ofgain and wave front distortions over the full extracting aperture ofthe NIF and LMJ prototype amplifiers. The LADS will be able to address each of eight apertures via motorized stages and following semi-automated alignment, take data on the aperture ofinterest. The LADS should be operational in mid-'97 at LLNL and will be used to characterize the optical performance ofthe veiy first full scale prototype 4 x 2 NIF and LMJ amplifiers. It will be transported to Bordeaux, France to make similar measurements during activation ofthe first 8-aperture LMJ-like facility (LIL) that is planned to start in the near future. The gain measurement will map the gain distribution ofeach ofeight 40 by 40 cm apertures. Small signal gain of 5 %-per-cm is the nominal operating point (lamps fired at 20% of their explosion energy). It is desired to measure the small signal gain ofthe amplifier with a resolution ofO. 1% (0.005 %per-cm) at the center ofthe aperture, 0.5% at the corners, or better. The amplifier pump distribution is tailored in order to counteract the effects of amplified spontaneous emission that tends to deplete gain in areas near the edges. This diagnostic will be useful in amplifier optimization experiments. Subtle effects of shaping reflector surfaces, the tarnishing of silver or the damage to reflector coatings could be found correlated to the gain data and thus, will be readily monitored. To be useful, the wavefront measurement must resolve features that calculations and Beamlet data show will have fourth order components with peak to valley excursions of about one twentieth wave per amplifier pass (A 1 .053 tm). Even smaller effects are expected from gas stratification/motion inside beam tubes. To chart these subtle wavefront distortions with fidelity it will be necessary to resolve the measured wavefront to A /100. The wavefront and gain measurements will be performed simultaneously. A TwymanGreen interferometer set up will present reference and sample beams to a pulsed, phase shifting interferometer for wavefront analysis. A wavefront map ofthe quiescent state will be acquired 66 milliseconds before the flashlamps fire. In the quiescent state the wavefront data contains the static distortions and will be used as the reference wavefront from which the dynamic distortions will be differentially obtained. Both prompt and delayed maps of wavefront distortions accompanying a shot will be stored. The prompt effects of the firing of the lamps are important at extraction time. Sampling delayed interferograms at say 1 to 5minuteintervals, will be used to monitor the evolution ofwaste heat and to monitor the effects ofcooling flashlamps, edge claddings, convection in the different sections ofthe beam transport. .. etc. The main components for the LADS are the optical relay telescopes, the probe laser, the alignment system, the gain diagnostics cameras and the pulsed, phase shifting interferometer.

Some of the major components of laser systems used for Inertial Confinement Fusion (ICF) are the large aperture mirrors which direct the path of the laser. These mirrors are typically supported by systems which consist of mirror mounts, mirror enclosures, superstructures, and foundations. Stability design considerations for the support systems of large aperture mirrors have been developed based on the experience of designing and evaluating similar systems at the Lawrence Livermore National Laboratory (LLNL). Examples of the systems developed at LLNL include Nova, the Petawatt laser, Beamlet, and the National Ignition Facility. The structural design of support systems of large aperture mirrors has typically been controlled by stability considerations in order for the large laser system to meet its performance requirements for alignment and positioning. This paper will discuss the influence of stability considerations and will provide guidance on the structural design and evaluation of mirror support systems in ICF lasers so that this information can be used on similar systems.

We demonstrate the beam smoothing by polarization control plate (PCP) produced by liquid crystal. The liquid crystal PCP using photoisomerization of the azo-dye was developed. The PCP is constructed by 14 X 14 array of 2 mm X 2 mm square element. The damage threshold is about 12 J/cm² (532 nm, 0.9 ns). The PCP suppressed the speckles produced by a random phase plate. The contrast of the speckles is reduced at 30% by a PCP.

The National Ignition Facility (NIF) is a high energy glass laser system and target chamber that will be used for research in inertial confinement fusion. The 192 beams of the NIF laser system are pumped by over 8600 Xenon flashlamps. The power conditioning system for NIF must deliver nearly 300 MJ of energy to the flashlamps in a cost effective and reliable manner. The present system design has over 200 capacitive energy storage modules that store approximately 1.7 MJ each and deliver that energy through a single switch assembly to 20 parallel sets of two series flashlamps. Although these are many possible system designs, few will meet the aggressive cost goals necessary to make the system affordable. Sandia National Laboratory and Lawrence Livermore National Laboratory are developing the system and component technologies that will be required to build the power conditioning system for the National Ignition Facility. This paper will describe the ongoing development activities for the NIF power conditioning system.

We describe a model which gives the effects of magnetic fields on a plasma electrode Pockels cell. The fields arise from the return currents to the cathode as well as from neighboring devices such as amplifier flashlamps. In effect, electrons are treated as a static, planar fluid moving under the influence of magnetic fields, the electric field of the discharge, electron pressure gradients, and electron-atom elastic collisions. This leads to a closed 2D equation for the electron density, which is solved subject to appropriate boundary collisions. The model is applied to four cases: the baseline NIF configuration with magnetic fields due to balanced return currents; a case with unbalanced return currents; the reverser configuration containing an external field parallel to the main plasma current; and a configuration with a field perpendicular to both the current and the optical direction.

We use a 32 cm plasma electrode Pockels cell (PEPC) prototype at Lawrence Livermore National Laboratory to determine switching performance in the presence of external magnetic fields. The interaction with external magnetic fields is important because of the B-fields generated by the high current flow through amplifier flashlamp arrays, and their proximity to the PEPC. We have experimentally determined what is the maximum allowable magnetic induction for good PEPC operation, and then we calculate the magnetic induction generated by a flashlamp array to determine the minimum PEPC to amplifier spacing. We have also experimentally determined the effect of a tandem PEPC placement. We consider several cathode designs. We revisit the hollow cathode design and we investigate the tradeoffs between the hollow cathode and planar magnetron. The recent development of a metallic body for the future 1 X 2 PEPC has led us to do experiments with a biased boundary in the PEPC. Experimental results of various biasing potentials and dielectric coating materials for the PEPC body are discussed.

Beam control and diagnostic systems are required to align the National Ignition Facility laser prior to a shot as well as to provide diagnostics on 192 beam lines at shot time. A design that allows each beam's large spatial filter lenses to also serve as objective lenses for beam control and diagnostic sensor packages helps to accomplish the task at a reasonable cost. However, this approach also causes a high concentration of small optics near the pinhole plane of the transport spatial filter (TSF) at the output of each beam. This paper describes the optomechanical design in and near the central vacuum vessel of the TSF.

We have designed an economical device to measure the power time history in the National Ignition Facility's 192 beam laser. The heart of the system is a commercial, high-speed, four-channel digitizer with a 15,000 point record length. Samples of several beams are taken with fiberoptic pickoffs, separated in time with appropriate fiberoptic delays and presented to high-speed vacuum photodiodes, which convert the samples to electrical signals for the digitizer. Amplitude and time multiplexing are used to cover the required dynamic range and to record 12 samples on the digitizer, making the cost per sample competitive with alternative approaches. Forty-eight digitizers can record the required three samples from each of the 192 beams. An additional similar but lower bandwidth system is used to record the backreflected light from the main laser amplifiers and elsewhere. The recording electronics are discussed in detail.

The electro-optic deflector is analyzed and compared to smoothing by spectral dispersion for efficacy as a beam smoothing method for ICF. It is found that the electro-optic deflector is inherently somewhat less efficient when compared either on the basis of equal peak phase modulation or equal generated bandwidth.

The smoothing of the spatial illumination of an inertial confinement fusion target is examined by its spatial frequency content. It is found that the smoothing by spectral dispersion method, although efficient for glass lasers, can yield poor smoothing at low spatial frequency. The dependence of the smoothed spatial spectrum on the characteristics of phase modulation and dispersion is examined for both sinusoidal and more general phase modulation. It is shown that smoothing with non-sinusoidal phase modulation can result in spatial spectra which are substantially identical to that obtained with the induced spatial incoherence or similar method where random phase plates are present in both methods and identical beam divergence is assumed.

Recent simulations and experiments on Nova indicate that some level of smoothing may be required to suppress filamentation in plasmas on the National Ignition Facility, resulting in the addition of 1D smoothing capability to the current baseline design. Control of stimulated Brillouin scattering and filamentation is considered essential to the success of laser fusion because they affect the amount and location of laser energy delivered to the x-ray conversion region (holhraum wall) for indirect drive and to the absorptive region for direct drive. Smoothing by spectral dispersion (SSD), reduces these instabilities by reducing nonuniformities in the focal irradiance when averaged over a finite time interval. We have installed SSD on Nova to produce beam smoothing on all 10 beam lines. A single dispersion grating is located in a position common to all 10 beam lines early in the preamplifier chain. This location limits the 1(omega) bandwidth to 2.2 angstroms with sufficient dispersion to displace the speckle field of each frequency component at the target plane by one half speckle diameter. Several beam lines were modified to allow orientation of the dispersion on each arm relative to the holhraum wall. After conversion to the third harmonic the beam passes through a kinoform phase plate (KPP) designed to produce an elliptical spot at best focus. The KPPs produce a focal spot having an elliptical flat-top envelope with a superimposed speckle pattern. Over 93% of the energy is contained in the central 400 micrometers . Calculations indicate a 16% rms intensity variance will be reached after 330 ps for a single beam.

The impact of smoothing method on the performance of a direct drive target is modeled and examined in terms of its l-mode spectrum. In particular, two classes of smoothing methods are compared, smoothing by spectral dispersion (SSD) and the induced spatial incoherence (ISI) method. It is found that SSD using sinusoidal phase modulation results in poor smoothing at low l-modes and therefore inferior target performance at both peak velocity and ignition. This disparity is most notable if the effective imprinting integration time of the target is small. However, using SSD with more generalized phase modulation can result in smoothing at low l-modes which is identical to that obtained with ISI. For either smoothing method, the calculations indicate that at peak velocity the surface perturbations are about 100 times larger than that which leads to nonlinear hydrodynamics. Modeling of the hydrodynamic nonlinearity shows that saturation can reduce the amplified nonuniformities to the level required to achieve ignition for either smoothing method. The low l-mode behavior at ignition is found to be strongly dependent on the induced divergence of the smoothing method. For the NIF parameters the target performance asymptotes for smoothing divergence larger than approximately 100 (mu) rad.

The laser irradiation uniformity on the target surface has been improved using a 2D scheme of the smoothing by spectral dispersion. The laser beam was dispersed in both orthogonal directions by installing the series of pair of phase modulator and diffraction gratings. A polarization- preserving single mode fiber was utilized as the first phase modulator by which the modulation frequency and depth are easily controlled by using the cross-phase modulation. The second phase modulator was a quasi-velocity matched LiTaO₃ modulator driven by 9.4-GHz microwave. Using these modulators, the combination of modulation frequencies was varied in a range from 5/9.4 GHz to 14/9.4 GHz. The advanced phase modulation was obtained by adopting the self-phase modulation in conjunction with the cross-phase modulation in the optical fiber. It has been confirmed that the temporally changing modulation frequency (the chirped phase modulation) is favorable for better smoothing in comparison with the ordinary scheme with fixed modulation frequencies.

Design optimization of the 1 .8 MJ, 500 TW National Ignition Facility (NIF) laser has proceeded with the use of a suite of new computational models. Cost-effectiveness of alternative fundamental architectures was considered using CHAINOP. A very fast, lumped-element energetics code, CHA1NOP includes an extensive cost database, a runtime choice of optimization algorithm, and a set of heuristic rules for diffraction and nonlinear effects and for operational constraints. Its ability to flexibly consider many alternative configurations at a few seconds per chain made it the ideal "first-cut" tool for narrowing the investigation to the switched, multi-pass cavity architecture that was chosen.

Preliminary results of investigation of modified oscillator of the D2ML Nd glass laser facility are presented. Using so called Wood's selector modification of oscillator have been done to broad a bandwidth generation. The diagnostic techniques for the oscillator investigations are described. Some considerations to study temporal and spatial evolution of the broad bandwidth laser pulse during D2ML path propagation are formulated.

To achieve high compression of direct drive targets it is principally important to guarantee highly homogeneous illumination of the target. Compressions of approximately 10⁴ require degree of homogeneity of 1 - 2%. It is suggested that in the high-power laser facilities being developed and projected now (Omega Upgrade, NIF, LMJ, Iskra- 6) various methods of intensity distribution smoothing will be used to achieve such homogeneity. In this report the efficiency of some smoothing methods is considered using 1D numerical modeling.

A large aperture deformable mirror for correcting the wavefront distortion and controlling the intensity profile of laser beam is presented. The mirror diameter and thickness are 400 mm and 8 mm, respectively. 37 actuators are glued hexagonally on the back surface of the mirror. The mechanical actuator has been designed and fabricated for producing strong force of 300 N in both pushing and pulling process. The initial reflective wavefront of the deformable mirror is (lambda) /6 ((lambda) is 633 nm) in rms value. The application to x-ray laser experiment of the deformable mirror is successful for a uniform line focus pattern by the wavefront control.

We discuss aspects of adaptive optics optimization for large fusion laser systems such as the 192-arm National Ignition Facility (NIF) at Lawrence Livermore National Laboratory. By way of example we considered the discrete actuator deformable mirror and Hartmann sensor system used on the Beamlet laser. Beamlet is a single-aperture prototype of the 11-0-5 slab amplifier design for NIF, and so we expect similar optical distortion levels and deformable mirror correction requirements. We are now in the process of developing a numerically efficient object oriented C++ language implementation of our adaptive optics and wavefront sensor code, but this code is not yet operational. The results shown below are based instead on the prototype algorithms, coded-up in an integrated array processing computer language.

The near field irradiance parameters at the interface between the one micron laser, the UV generation, and transport subsystem will be discussed. The test results obtained from the Beamlet and Nova lasers used to validate the mathematical models will be presented.

We discuss the partitioning of effects on the fundamental focal irradiance distribution from whole beam self-focusing, and PSDs of the optical components in the laser chain, and the resulting third harmonic focal distribution. Beamlet and NIF simulation results are discussed.

We discuss the partitioning of effects on the fundamental focal irradiance distribution from whole beam self-focusing, and PSDs of the optical components in the laser chain. Beamlet and NIF simulation results are discussed.

We are presently adding the capability to irradiate indirectly-driven Nova targets with two rings of illumination inside each end of the hohlraum for studies of time-dependent second Legendre (P₂) and time-integrated fourth Legendre (P₄) flux asymmetry control. The rings will be formed with specially designed kinoform phase plates, which will direct each half of each beam into two separate rings that are nearly uniform azimuthally. The timing and temporal pulse shape of the outer rings will be controlled independently from those of the inner rings, allowing for phasing of the pulse shapes to control time dependent asymmetry. Modifications to the incident beam diagnostics will enable us to verify that acceptable levels of power balance among the contributing segments of each ring have been achieved on each shot. Current techniques for precision beam pointing and timing are expected to be sufficiently accurate for these experiments. We present a design for an affordable retrofit to achieve beam phasing on Nova, results of a simplified demonstration, and calculations highlighting the anticipated benefits.

First wall Laser Megajoule target chamber has to hold out against several aggressions such as neutrons, x-rays or laser scattered light during fusion experiments. In this paper, two material families: Boron Carbide (B₄C) plasma-sprayed coatings and Carbon/Carbon composites (CCC), which might be suitable for the application are presented. The development of the best quality materials was oriented to satisfy various tests. Among these tests, x-ray exposure and outgassing behavior were first investigated, and the obtained results are presented here.

The Laser Megajoule facility will chiefly be used to study the ignition of DT fuel capsules. All the elements contained in the chamber (diagnostics insertion system, target holder, first wall, debris shields) will undergo X-ray fluences and debris emissions. Numerical simulations using the 1D codes FSCATT and DELPOR (hydrodynamics and thermodynamics model) have been performed in order to characterize the mechanical behavior of different candidate materials under X-ray fluence: aluminum, boron carbide, pyrolytic graphite and fused silica. Two spectra have been utilized for these simulations: a 350 eV blackbody spectrum and the gold X-ray conversion Phebus spectrum. Vaporized and melt thicknesses are assessed as well as pressure in the materials for X-ray fluences ranging from 0.3 J/cm² to 100 J/cm². For aluminum, comparisons with LLNL simulations and with experiments carried out on Phebus and Nova facilities are given.

A series of 1D numerical simulations of X-ray emission of a Laser Megajoule baseline target has been performed using the radiation-hydrodynamics code FCI1. The results are presented in term of X-ray spectra, energies flowing through the Laser Entrance Holes (LEH) and through the hohlraum walls and in term of temporal analysis of burnthrough for a laser energy of 1.8 MJ and a total yield of 20 MJ. Parametric studies with wall thickness (20 to 100 micrometers ) and with 2 models of closing LEH are presented. Most of results are compared with LLNL numerical simulations.

A standard third harmonic frequency converter consists of a single doubler crystal and a subsequent mixer crystal. This converter configuration has optimum performance for monochromatic temporally flat pulses. Significant degradation of conversion efficiency occurs when the input pulses have large intensity variations or applied bandwidth. Calculations have shown that dynamic range can be increased and bandwidth sensitivity reduced by adding a second doubler crystal and second mixer crystal, respectively.

A new design of a multipass amplification system which adopt stimulated Brillouin scattering (SBS) phase conjugation mirror is presented in this paper. By application of active SBS mirrors and four-color laser beams, this system has the potential ability on pulse shaping and beam smoothing for target uniform illumination. The preliminary simulation result shows this 7-disk amplifier system will generate 1.5 kJ 3(omega) pulses.

The Mégajoules Laser (LMJ ) is comprised of 240 separate beam lines that each produces about 20 kJ of 1 .053 microns light. Simply stated the architecture consists of 240 optical pulse generation system that each delivers a 10 nJ shaped and modulated pulse. Next, 240 preamplifier modules amplify the tailored pulses to 1 J, whereupon they are transported to the large amplifier chains where the laser energy is increased to the 20 kJ level. The preamplifier modules contain a regenerative amplifier ( regen ), a spatial beam shaping module and a fourpass amplifier. The specifications for the regen include a total gain of i0, an output energy of 10 mJ, operating at a 1 Hz rate, a square-pulse distortion < 2, a signal-to noise-ratio > i0, and a pulse-to-pulse output energy stability ofbetter than 3%.

Weare in the midst of constructing an amplifier laboratory (Amplab) that will be the physics and engineering proving ground for full sized segmented glass amplifiers ofdesigns that will outfit the National Ignition Facility (NIF) and Laser Megajoule (LMJ) projects. Amplab will demonstrate the cornerstone mechanical, electrical and optical concepts that support the NTF and LMJ amplifier schemes. Here we address the optical diagnostics that will be used to characterize optical performance ofthe amplifiers. We describe, the apparatus that will be used in pulsed measurements ofgain distribution and wave-front distortions. The large aperture diagnostic system or LADS, is now being built through a collaborative effort between CEL-V and LLNL. The LADS will provide measurements ofgain and wave front distortions over the full extracting aperture ofthe NIF and LMJ prototype amplifiers. The LADS will be able to address each of eight apertures via motorized stages and following semi-automated alignment, take data on the aperture ofinterest. The LADS should be operational in mid-'97 at LLNL and will be used to characterize the optical performance ofthe veiy first full scale prototype 4 x 2 NIF and LMJ amplifiers. It will be transported to Bordeaux, France to make similar measurements during activation ofthe first 8-aperture LMJ-like facility (LIL) that is planned to start in the near future. The gain measurement will map the gain distribution ofeach ofeight 40 by 40 cm apertures. Small signal gain of 5 %-per-cm is the nominal operating point (lamps fired at 20% of their explosion energy). It is desired to measure the small signal gain ofthe amplifier with a resolution ofO. 1% ( 0.005 %per-cm) at the center ofthe aperture, 0.5% at the corners, or better. The amplifier pump distribution is tailored in order to counteract the effects of amplified spontaneous emission that tends to deplete gain in areas near the edges. This diagnostic will be useful in amplifier optimization experiments. Subtle effects of shaping reflector surfaces, the tarnishing of silver or the damage to reflector coatings could be found correlated to the gain data and thus, will be readily monitored.

Abstract not available.

Sfim ODS is involved in the French Megajoule Project managed by the Commissariat a l'Energie Atomique. It is a high power UV-laser using 240 square beams (410 mm X 410 mm). Sfim ODS achieved a technical and economical analysis concerning the production of flat-parallel debris-shields in silica (430 X 430 X 6 mm, transmitted wavefront: (lambda) /2, 2 500 shields/year) using large and specific dual-side fine- grinding and polishing machines. This analysis was supported by trials using half-scale windows (same stiffness equals> thickness equals 1.7 mm) showing the ability to reach the specifications with low production costs.

In the framework of the Megajoule Laser project driven by the French Atomic Energy Board, one of the most valuable optical programs in term of material volume as well as in term of component size, the actual glasses and production means appear to be inconsistent with the economical objectives. Corning proposed an alternative, based on the use of a low cost glass, together with an evolution of the production process. Combining its experience in quality optical glasses manufacturing and its mastery of forming processes, Corning conducted a production cost reduction program; the objective of this program was to validate the concept of large slab melting (about 2000 kg each), where blocks are cut off, versus the conventional single block (about 100 kg) melting. Economical improvements are based on a reduction of lost time (mold change) and production lead- time by increasing the feeding yield, on a better glass utilization, and a reduced number of molds. The technical issues were: increase the feeding yield maintaining a given glass quality level, reduce the glass allowance, improve the materials of the molds, reinforce the thermal process control, automatism of critical operations, especially at the start-up and at the end of the mold feeding. Despite the long production cycle, about 3 months including melting and annealing, the first results carry the technological options set-up.

Problems of creating high-efficiency technology of crystal KDP, DKDP blanks production for high-power lasers using the elaborated laboratory rapid-growth technology are considered. The laboratory technology enables one to grow crystal samples of the sizes and quality close to the requirements of the ICF laser drivers. The improved technology will provide samples completely satisfying the ICF demands. It is expected that the productivity of the developed technology will exceed the traditional one by an order, while the cost of samples will be essentially lower than in other technologies.

The next generation of high energy laser systems for ICF research demands an unprecedented volume of laser glass to be produced over a limited manufacturing period while still meeting ambitious targets of internal quality and overall cost. To meet this challenge, Schott has conceived a continuous manufacturing unit capable of producing 5,000 meter class PH 4 slabs of platinum particle-free phosphate laser glass within a three-year time period. This manufacturing unit concept draws on years of prior production experience with phosphate laser glass and other high quality optical materials but still represents a significant departure from the proven discontinuous manufacturing technology successfully employed over the last ten years for platinum-free phosphate laser glass. In addition, Schott has developed a new phosphate laser glass that simultaneously offers improvements in properties that relate to both laser performance and to characteristics related to forming the glass into large, high quality slabs. In this paper we will describe the key technology issues addressed in the manufacturing development and present a brief description of the planned manufacturing method to be employed. Lastly, the status of the development will be reviewed including characterization of pilot production melts of the new laser glass and the schedule for completion of the development program.

The manufacture of modern optical components requires the use of high performance interferometers, usually based on phase-shifting techniques. However, there is currently no commercial phase-shifting interferometer having the capacity to measure large parts, such as those found in Inertial Confinement Fusion Lasers (NIF and Méga-Joule), and other large systems. Standard interferometers lack the simultaneous qualities of accuracy and spatial resolution required for the measurement of such components. Indeed, it has been shown that surface ripples with wavelengths of around 1 to 10 millimeters are extremely dangerous for large ICF optics, even at low amplitude, because of the process of non-linear ripple amplification present in high power laser systems. In order to circumvent the restrictions on size and performance of standard interferometers, we have designed and built a scanning interferometer, using a standard "small" diameter phase shifting interferometer. A PC computer is used to control the measurement process, acquire the interferograms and stitch the measurements together to produce the original large surface. The measurement sequence is completely automated. The advantages ofthis technique are low cost, small size, and no loss ofspatial resolution. One system has been in actual use for the characterization of large size mirrors (approx. 400 x 600 mm) since december 1994. In this presentation, we shall look over the design of the system, produce actual measurements, and discuss the technical implications of the stitching process in relation to specifications such as those currently being derived for ICF large components. This work is supported by CEA/CEL-V, as part of the Laser MégaJoule Program.

TheNIP and LMJ programs will require large rectangular laserglass slabsof unprecedented magnitude, in terms of size and quantity. Several methods exist formelting glass, but to effectively melt a large volume with production ease, while maintaining low cost and consistent quality, the best method is continuous melting. The phosphate type laser glass to be produced must be of high precision, in addition to homogeneous and having stringentspecs for bubbles, indusiors, water content and other impurities which can adversely influence laser performance. As a Fe-melt for the forthcoming full production, we have performed continuous melting demonstratior running tests using a sub-scale furnace. Jn this system, not only is the melting technique important, but the forming of the extruded bar (instead of the traditional casting blocks) is very important for the completion of the total melting system. Melting of laser glass in a continuous fashion has not previously been achieved worldwide until now. Herein, we briefly present our concept of the system and the quality of the Iass material produced, alone a summary of the achievements from our development work.

Large aspherical focusing and beam deviating square lenses will be used in the framework of the Laser Megajoules project developed by the French Atomic Energy Commission. In order to validate the associated manufacturing processes a half scale prototype lens has been manufactured and tested by REOSC (SFIM subsidiary). Specific aspherical generating process and computer controlled micro-polishing technology have been used in order to demonstrate the faisability of a mass production on an industrial basis (approximately 1300 lenses in 6 years). A 250 mm square lens in Fused Silica with a convex hyperboloid profile (250 microns difference with respect to the best sphere) has been manufactured and REOSC reached a transmitted wavefront better than 130 nm Peak-peak, 11 nm rms. Moreover residual micro-oscillations amplitudes (spatial frequency 0.5 to 30 mm) have been limited to 60 nm PTV and 8 nm rms. The total work duration for this exploratory lens remained below 60 hours.

The manufacture of modern optical components requires the use of high performance interferometers, usually based on phase-shifting techniques. However, there is currently no commercial phase-shifting interferometer having the capacity to measure large parts, such as those found in Inertial Confinement Fusion Lasers (NIF and Méga-Joule), and other large systems. Standard interferometers lack the simultaneous qualities of accuracy and spatial resolution required for the measurement of such components. Indeed, it has been shown that surface ripples with wavelengths of around 1 to 10 millimeters are extremely dangerous for large ICF optics, even at low amplitude, because of the process of non-linear ripple amplification present in high power laser systems. In order to circumvent the restrictions on size and performance of standard interferometers, we have designed and built a scanning interferometer, using a standard "small" diameter phase shifting interferometer. A PC computer is used to control the measurement process, acquire the interferograms and stitch the measurements together to produce the original large surface. The measurement sequence is completely automated. The advantages ofthis technique are low cost, small size, and no loss ofspatial resolution. One system has been in actual use for the characterization of large size mirrors (approx. 400 x 600 mm) since december 1994. In this presentation, we shall look over the design of the system, produce actual measurements, and discuss the technical implications of the stitching process in relation to specifications such as those currently being derived for ICF large components. This work is supported by CEA/CEL-V, as part of the Laser MégaJoule Program.

A two layer coating was deposited by sol-gel dip-coating method on phosphate laser glass. First silica film was coated from SiO₂ colloidal suspension derived from ammonia catalyzed hydrolysis of tetraethoxysilane in ethanol. Then a methyl containing silicate film was covered. It was fabricated from a solution prepared by the hydrolysis of methyltriethoxysilane in the presence of hydrogen chloride catalyst. A 130 degree(s)C, 16h heat-treatment was done after coating process. It is confirmed by optical microscope that the two layer coating improves obviously the chemical durability of phosphate laser glass. The laser damage threshold of this coating is measured to be 31 J/cm² at 1060 nm wavelength of 10 ns pulse duration. It is found that silica film contributes to the improvement on both film adhesion to phosphate glass substrate and laser damage threshold of film. The methyl containing silicate single film has only 18 J/cm² laser damage threshold and can be easily dissolved in ethanol solution when it was coated on the phosphate laser glass substrate.

The realization of lasers emitting at very high peak powers mostly include performant diffractive optical elements with specific dispersive properties and high efficiency. In particular, this arises for the generation of ultrashort femtosecond pulses where technics of pulse stretching and compression are now commonly used. It thus results in a demand of high efficiency diffraction grating producing both high quality wavefronts and high threshold for laser damage. The main objective of this paper is to highlight new technologies of diffractive grating for ultrashort pulse lasers using phase volume holography in photosensitive materials. Preliminary experimental results concerning the laser damage threshold will be presented both for dichromated gelatin and photopolymer materials.

It is now accepted that absorption phenomena are not directly responsible for the flux resistance of multilayer optical components exposed to pulsed lasers. However, such studies involved `overall' absorption by components, which is now known does not characterize an intrinsic property of thin film materials. If the absorption of thin film materials is mapped by exciting different zones less than 100 micrometers in diameter, a clear disparity in absorption thresholds is shown up which must be attributed to heterogeneity in the imaginary refractive index or the extinction coefficient. The heterogeneity of absorption may be related to the incorporation of foreign matter in the materials or to dislocations, nodules, etc... Under these circumstances a key issue is whether the damage thresholds can be correlated with these microscopic absorption sites. We present the equipment used for carrying out the different types of characterization in this study: absorption and diffusion mapping, damage threshold measurements, and analysis by atomic force microscopy. An attempt is made to correlate these different results.

Nonabsorbing bulk defects can initiate laser damage in transparent materials. Defects such as voids, microcracks and localized stress concentrations can serve as positive or negative lenses for the incident laser light. The resulting interference pattern between refracted and diffracted light can result in intensity increases on the order of a factor of 2 some distance away from a typical negative microlens, and even larger for a positive microlens. Thus, the initial damage site can be physically removed from the defect which initiates damage. The parameter that determines the strength of such lensing is (Ka)² (Delta) (epsilon) where the wavenumber K is 2(pi) /(lambda) , 2a is the linear size of the defect and (Delta) (epsilon) is the difference in dielectric coefficient between matrix and scatterer. Thus, even a small change in refractive index results in a significant effect for a defect large compared to a wavelength. Geometry is also important. 3D (eg. voids) as well as linear and planar (eg. cracks) microlenses can all have strong effects. The present paper evaluates the intensification due to spherical voids and high refractive index inclusions. We wish to particularly draw attention to the very large intensification that can occur at inclusions.

One of the concerns with the Megajoule Laser design is the transport mirrors laser damage threshold. Earlier studies have shown that the main constraint on laser damage threshold comes from nodules at the mirror surface. It is therefore important to restrict both the number of such nodules and their dimensions: the larger they are, the greater is the resulting damage. SFIM-ODS, in close collaboration with CEL-V, has started a special-purpose study to characterize these nodules as precisely as possible. The objective of the study is twofold: to determine the origin of the nodules and subsequently to adapt the mirror fabrication process so as to restrict their formation, and to analyze their shapes and dimensions so as to determine which nodules are critical for laser damage. To understand the origin of the nodules and their effect on laser damage threshold, the mirrors are characterized by different methods: (1) absorption and scattering mapping: does the presence of nodules result in specific absorption?, (2) surface analysis by atomic force microscopy: to characterize nodule shape and dimensions, (3) Focused Ion Beam cutting of nodules: to locate the seed initiating the nodule (on the substrate or in the stack), and characterize the seed shape and composition (contamination, material flaw during evaporation, etc...), and (4) laser damage threshold measurements: to determine the mirror laser damage threshold and study the behavior of nodules under laser in accordance with their dimensions and shape.

The upgrade of the actual large laser facilities (GEKKO XII in our case) implies an increase of filling factor avoiding generation of high spatial-frequencies. To reach that purpose we intend to construct the beam using serrated aperture with optimized shape. The performances of our system lead after simulation to 70% filling factor with less than 2% fluctuation on laser pattern intensity map. As only a very few articles were written on that subject although it is not very new, this paper includes a theoretical approach of the serration effects. It presents mainly results from simulation and some experimental data.

Back-reflections from a target, lenses, etc. can gain energy passing backwards through a laser just like the main beam gains energy passing forwards. Unless something blocks these back-reflections early in their path, they can seriously damage the laser. A Mechanical Beam Isolator is a device that blocks back-reflections early, relatively inexpensively, and without introducing aberrations to the laser beam.

It is proposed to use laser crystals Co:MgF₂ pumped by laser radiation of 1.3 micrometers wavelength. This crystal is a vibronic laser material having a pump band with a maximum at 1.3 micrometers and a broad gain band covering the wavelength range from 1.5 up to 2.3 micrometers . The inversion lifetime is 1.3 ms at 80 K. So, it is possible to use pumping lasers operated in a free running mode. There are well-developed powerful lasers having 1.3 micrometers wavelength output. These are iodine lasers (photolytic and chemically pumped) which operated at I(²P_1/2) - I(²P_3/2) transition and neodymium solid state lasers (glass, garnets) which operated at ⁴F_3/2 - ⁴I_13/2 transition of the Nd⁺³ -ion. They have high output energy and efficiency. A conceptual scheme of the ultrashort pulse laser system of the Petawatt output power level is considered.

The use of self phase modulation for spectrum broadening with subsequent pulse compression by diffraction gratings allow to compress high energy laser pulses in wide range of pulse lengths. We have discussed different feasibility of such method for pulse shortening.

Professor Wang Ganchang first put forward a proposal of laser fusion independently in 1 964 . After this, wehave done a lot of effort on this area. Besides ICF theory simulation and experiment research, one beam solidstate laser and six beam solid-state laser were constructed. Using solid deuterium and CD2, neutron emission by laser irradiation of targets was raelized in1973;and the pellet compression by laser was realized in 1976. In 1 985, we have constructed a solid-state laser Shen-Guang-I(SG-I), and Xing-Guang-I(XG-I) Nd:glass laser in 11 992.Table 1 shown the parameters of these two laser and a constracting laser SG-II.

For years, contamination has been known to degrade the performance of optics and to sometimes initiate laser-induced damage to initiate. This study has started to quantify these effects for fused silica windows used at 355 nm. Contamination particles (Al, Cu, TiO₂ and ZrO₂) were artificially deposited onto the surface and damage tests were conducted with a 3 ns Nd:YAG laser. The damage morphology was characterized by Nomarski optical microscopy. The results showed that the damage morphology for input and output surface contamination is different. For input surface contamination, both input and output surfaces can damage. In particular, the particle can induce pitting or drilling of the surface where the beam exists. Such damage usually grows catastrophically. Output surface contamination is usually ablated away on the first shot but can also induced catastrophic damage. Plasmas are observed during illumination and seem to play an important role in the damage mechanism. The relationship between fluence and contamination size for which catastrophic damage occurred was plotted for different contamination materials. The results show that particles even as small as 10 micrometers can substantially decrease the damage threshold of the window and that metallic particles on the input surface have a more negative effect than oxide particles.

The paper describes the french ICF program.