The historical descriptions on developing AVLIS laser systems for the last six years in Japan are given, and such items as some development issues, operational experiences and those further to be developed in the next step of R and D program are briefly explained. Laser-J completed its first term of R and D program at the end of June, 1992 and received a check and review procedure by its sponsors and Japanese AEC on its consequences. Laser-J has got a 'GO' signal to implement the next step of developing full scale of AVLIS hardwares, the outcome of which will be subjected to another check and review to be made by them. If Laser- J could clear the process, it would go to the last step of building a set of Demo Facility and making Enriching Demonstration Test thereof. The total span will amount to 10 years.

France has developed a very complete nuclear industry, from mining to reprocessing and radwastes management, and now has a major electro-nuclear park, with 55 power reactors, supplying 75% of the nation's electricity and representing 32% of its energy requirements. The modern multinational EURODIF enrichment plant in Pierrelatte in the south of the country supplies these reactors with enriched uranium as well as foreign utilities (30% exports). It works smoothly and has continuously been improved to reduce operating costs and to gain flexibility and longevity. Investment costs will be recovered at the turn of the century. The plant will be competitive well ahead of an aging production park, with large overcapacity, in other countries. Meanwhile, world needs will increase only slightly during the next 15 years, apart from the Asian Pacific area, but many world governments are becoming well aware of the necessity to progressively resume nuclear energy development worldwide from the year 2000 on.

A method based on polarization selectivity and three step laser photoionization is presented for separation of the odd isotopes of gadolinium. Measurements of the spectroscopic parameters needed to quantify the excitation pathway are discussed. Model results are presented for the efficiency of photoionization. The vapor properties of electron beam vaporized gadolinium are presented which show dramatic cooling during the expansion of the hot dense vapor into a vacuum. This results in a significant increase in the efficiency of conversion of natural feed into enriched product in the AVLIS process. Production of enriched gadolinium for use in commercial power reactors appears to be economically viable using technology in use at LLNL.

The multistep photoionization of uranium atoms implies choosing an irradiation scheme and this choice is only possible if the following spectroscopic parameters are known: oscillator strength, isotopic shift, hyperfine structure, lifetime, autoionization spectrum. In order to measure these parameters two kinds of experimental set-up are used: laser induced fluorescence and laser induced photoionization techniques. Since the oscillator strengths determine the laser fluences needed for an effective atomic photoionization, this parameter must be accurately measured and two different methods are used: the saturation method, and branching ratio plus lifetime.

We have examined the stepwise-resonant three-photon-ionization spectrum of neutral Zirconium atoms using three separately-tunable pulsed visible dye lasers. The ground-level (first-step) transitions were chosen on the basis of demonstrated ⁹¹Zr selectivity. Lifetimes of even-parity levels around 36000 cm^-1, measured with the delayed- photoionization technique, range from 10 to 100 nsec. Direct ionization cross sections appear to be less than 10^-17 cm²; newly-detected autoionizing levels give peak ionization cross sections (inferred from saturation fluences) up to 10^-15 cm². Portions of Rydberg series converging to the 315 and 763 cm^-1 levels of Zr⁺ were identified. 'Clumps' of autoionizing levels are thought to be due to Rydberg-valence mixing.

A computer simulation code to treat atomic excitation and laser beam propagation simultaneously in an atomic laser isotope separation system has been developed. The three- level Bloch-Maxwell equations are solved numerically to analyze the change of pulse shapes, the modification of laser frequencies and the time-varying atomic populations. The near- resonant effects on the propagation of frequency chirped and non-chirped laser pulses have been analyzed. It was found that there are serious differences in pulse shapes, frequency modifications and propagation velocities between laser pulses with and without chirping.

Ionization probability in three color excitation of uranium atoms depends on the photon flux which drives the transition to an autoionizing level. For a proper choice of the flux ratio, the ionization probability is insensitive of the actual fluxes over a considerable range. Thus, the laser intensities used may be lowered by a factor of about 3 as compared to values predicted on the basis of cross-sections of the individual transitions. A quantum-mechanical model, based on a detailed calculation of population dynamics of the levels in the ionization scheme, predicts the ionization probability as a function of the three-laser intensities. Ionization saturation measurements provide parameters for the model. The theoretical predictions were partially consistent with the experimental results. Ionization probabilities as high as 93% are achievable.

Among the mainly interesting parameters of an atomic vapor laser isotope separation process is the ionization yield. This parameter can be controlled as long as the laser beam spatial intensity distribution and temporal shape are well defined and not subjected to unexpected disturbing effects such as coherent propagation phenomena, leading to spatial and temporal reshaping with hot spots and pulse lengthening and delays. On the other hand, economical considerations require optically thick atomic columns which can favor such effects. In this work, we study the photoionization yield in atomic thulium vapor when propagation effects occur. We compare our experimental results with those of a coherent propagation code describing a four level system and including inhomogeneous broadening as well as degeneracies. We choose a three step photoionization scheme.

Experiments of technological interest for the project of laser isotope separation of uranium (Atomic Vapor Laser Isotope Separation, 'AVLIS') have yielded experimental data concerning the hyperfine structure (hfs) of levels of atomic uranium. The present paper reports on these data, their obtention and a parametric interpretation by the Condon Racah-Slater method. The experimental setup uses a Laser Induced Fluorescence technique in an atomic beam. Nevertheless, these experiments provide data for 28 low odd levels and 22 even levels, with an accuracy that is sufficient for a theoretical interpretation. Following the interpretation of the fine structure of the lower levels of atomic uranium by Guyon, it was reasonable to undertake a parametric interpretation of the hfs data concerning 22 of these levels on the basis of the configuration 5f³6d7s².

A particle-mesh method is used to simulate plasma dynamics of electrostatic ion extraction in AVLIS process. The method is being presented. Scalings laws are established in order to perform simulations at lower plasma densities than the actual ones. Simulation results are compared to experiments. Simulated currents onto product collectors are in good agreement with experimental ones. Furthermore, simulation results are in very good agreement with the experimental curve of extraction efficiency versus collectors polarization, for polarization going from zero to values such that efficiency approaches unity.

The recent developments of the components for high power Copper Vapor Laser (CVL) have been oriented towards four main goals: high quality laser beam, mainly for the CVL oscillators, increase of the extracted energy out of the amplifying stage, fully integrated and monolithic design for oscillator and amplifier, and extended lifetime and high reliability. A first step of this work, which is done under contract with CILAS (Compagnie Industrielle des Lasers) led to an injection seeded oscillator and a 100 Watt amplifier; the present step concerns development of a 400 Watts class amplifier.

Uranium enrichment by laser isotope separation uses a three step operation which requires four visible wavelengths to boost an individual U²³⁵ isotope from a low lying atomic energy level to an autoionizing state. The visible wavelengths are delivered by dye lasers pumped by copper vapor lasers (CVL). In this particular talk, a single dye chain consisting of a master oscillator and amplifier stages will be described and some of its performance given.

The Lawrence Livermore National Laboratory's (LLNL) Atomic Vapor Laser Isotope Separation (AVLIS) Program has developed a high-average-power, pulsed, tunable, visible laser system. Testing of this hardware is in progress at industrial scales. The LLNL copper- dye laser system is prototypical of a basic module of a uranium-AVLIS plant. The laser demonstration facility (LDF) system consists of copper vapor lasers arranged in oscillator- amplifier chains providing optical pump power to dye-laser master-oscillator-power-amplifier chains. This system is capable of thousands of watts (average) tunable between 550 and 650 mm.

The laser system described in the previous paper is used for experiments in which success requires tight tolerances on beam position, direction, and wavefront. Indeed, the optimum performance of the laser itself depends on careful delivery of copper laser light to the dye amplifiers, precise propagation of dye laser beams through restricted amplifier apertures, and accurate monitoring of laser power at key locations. This paper describes the alignment systems, wavefront correction systems, and laser diagnostics systems which ensure that the control requirements of both the laser and associated experiments are met. Because laser isotope separation processes utilize more than one wavelength, these systems monitor and control multiple wavelengths simultaneously.

This pilot has been set up to test the Avlis materials in conditions similar to those of a separator. It has therefore been designed as a corrosion loop where the feed is in gaseous phase and the circulation in liquid phase. The temperature is everywhere maintained above the uranium melting point. The facility includes an evaporation apparatus supplied by Leybold S.A., a set of 14 resistors controlled by a regulating system (Eurotherm software), and a cooled inner vessel aimed at the creation of a cold zone in the main vacuum vessel. A 60 kW scanning spot-focusing gun is used, set horizontally on the vacuum vessel. A coil and two magnetic arms set against the crucible create a constant and non homogeneous electromagnetic field which bends the electron beam towards the pool. The field configuration maintains a part of the back-scattered electrons in the crucible. The gun can be isolated from the vacuum vessel by a valve during the maintenance operations such as an emission filament replacement, the test materials being maintained in temperature and under vacuum. During the experiment a video camera records the condensation of the liquid metal in the upper part of the vessel and another camera gives a picture of the electron beam impact on the bath. Conclusions on the behavior of the materials are essentially post-mortem although the development of the gaseous phase is followed by mass spectrometry.

Due to its important role in uranium managements in Avlis separators, the subject of uranium flow handling, outside of vapor deposition areas, and directing uranium towards a collecting zone, is thoroughly studied. A special installation called 'IRIS' is used to generate uranium flows and perform drops and films flows.

Optics used in a plant scale SILVA laser system will support high average power laser beam in the visible spectrum (> 1 kW). The losses and resistance to optical stress of the various dielectric multilayer coatings have to be improved, thermal effects quantified and reduced, so specific test benches have been developed. The applied methods will be briefly commented and the various benches described and some examples reported.

TRIO-EF is a general purpose Fluid Mechanics 3D Finite Element Code. The system capabilities cover areas such as steady state or transient, laminar or turbulent, isothermal or temperature dependent fluid flows; it is applicable to the study of coupled thermo-fluid problems involving heat conduction and possibly radiative heat transfer. TRIO-EF is developed by the Heat Transfer and Structural Mechanics Department of the French Atomic Energy Commission CEA/DMT. It is widely used for applications in reactor design, safety analysis and final nuclear waster disposal. More recently, it has been used to study the thermal behavior of the AVLIS process separation module. In this process, a linear electron beam impinges the free surface of a uranium ingot, generating a two dimensional curtain emission of vapor. The metal is contained in a water-cooled crucible. The energy transferred to the metal causes its partial melting, forming a pool where strong convective motion increases heat transfer towards the crucible. In the upper part of the Separation Module, the internal structures are devoted to two main functions: vapor containment and reflux, irradiation and physical separation. They are subjected to very high temperature levels and heat transfer occurs mainly by radiation. Moreover, special attention has to be paid to electron backscattering. These two major points have been simulated numerically with TRIO-EF and in this paper, we present and comment the results of such a computation, for each of them. After a brief overview of the computer code, two examples of the TRIO-EF capabilities are given: a crucible thermal hydraulics model, and a thermal analysis of the internal structures.

The selective multiple photon dissociation of CF₃T in CF₃T/CF₃H mixtures by two infrared irradiation fields was studied. An extensive parametric study was done to determine the effect of wavelength and fluence of the laser field and temperature and pressure of the gas. The best results were obtained with irradiation by two closely space laser lines in the 9P branch. Under these conditions, isotopic selectivities exceeding 20000, combined with yields per pulse greater than in single frequency experiments were observed.

The current shortage of ¹⁸O has revived interest in using one step UV photodissociation of formaldehyde to enrich ¹³C, ¹⁷O and ¹⁸O. The frequency doubled output of the copper laser pumped dye laser system currently in operation at LLNL can be used to drive this dissociation. We use a simple kinetics model and our experience with Atomic Vapor Laser Isotope Separation (AVLIS) process design to examine the relative merits of different designs for a formaldehyde photodissociation process. Given values for the molecular photoabsorption cross section, partition function, spectroscopic selectivity, collisional exchange and quenching cross sections (all as parameters), we perform a partial optimization in the space of illuminated area, formaldehyde pressure in each stage, and formaldehyde residence time in each stage. We examine the effect of cascade design (heads and tails staging) on molecule and photon utilization for each of the three isotope separation missions, and look in one case at the system's response to different ratios of laser to formaldehyde costs. Finally, we examine the relative cost of enrichment as a function of isotope and product assay. Emphasis will be as much on the process design methodology, which is general, as on the specific application to formaldehyde.

Pressure dependence of threshold fluences for dielectric breakdown of pure CF₃H and CF₃H/CF₃T mixtures has been studied using 10.6 micrometers pulses from a TEA CO₂ laser. Breakdown thresholds are well above the working fluences required for laser isotope separation of trifluoromethane, allowing the use of tight focused geometries without producing nonselective processes. Analysis of the experimental results suggests that breakdown occurs as a result of a cascade process, and that recombination losses are more important than electron diffusion. In the pressure range studied, the threshold in presence of CF₃T was lower than the corresponding value in pure CF₃H, as a consequence of the preionization induced by the (beta) decay.

In this paper, we report the results of CO-laser induced photochemical reaction of UF₆ with HCl for the isotope separation of uranium hexafluoride, we also discussed that the molecular collision inducing V-T, V-V relaxation process affects on the selectivity of the isotope separation. The obtained quantum coefficiency of the reaction is about 0.34.

Numerical simulations and experimental studies have been made related to the possibility of employing an externally imposed electrostatic potential wave to separate isotopes. This wave/ion interaction is a sensitive function of the wave/ion difference velocity and for the appropriate wave amplitude and wave speed, a lighter faster isotope will be reflected by the wave to a higher energy while leaving heavier, slower isotopes virtually undisturbed in energy--allowing subsequent ion separation by simple energy discrimination. In these experiments, a set of some 200 individual electrodes, which surrounded a microamp beam of neon ions, was used to generate the wave. Measurements of the wave amplitudes needed for ion reflection and measurements of the final energies of those reflected ions are consistent with values expected from simple kinetic arguments and with the more detailed results of numeric simulations.

The design considerations for a laser system used to generate a sodium-layer guide star are presented. Laser technology developed for the Atomic Vapor Laser Isotope Separation (AVLIS) program is shown to be directly relevant to this problem and results of a demonstration using the AVLIS laser to generate such a guide star are shown. The design of a compact laser suitable for use at a large telescope such as the Keck is also presented.

Copper and dye laser systems are currently being developed at LLNL for uranium enrichment production facilities. The goals of this program are to develop low-cost, reliable and maintainable industrial laser systems. Chains of copper lasers currently operate at more than 1.5 kW output and achieve mean time between failures of more than 1000 hours. The beam quality of copper vapor lasers is approximately three times the diffraction limit. Dye lasers have near diffraction limited beam quality at greater than 1.0 kW. Diode laser pumped, Nd:YAG slab lasers are also being developed at LLNL. Current designs achieve powers of greater than 1.0 kW and projected beam quality is in the two to five times diffraction limited range. Results from cutting and drilling studies in titanium and stainless steel alloys show that cuts and holes with extremely fine features can be made with dye and copper-vapor lasers. High radiance beams produce low distortion and small heat-affected zones. We have accomplished very high aspect ratio holes in drilling tests (> 60:1) and features with micron scale (5 - 50 micrometers ) sizes.

Copper lasers have been identified as the laser of choice for AVLIS. As a result they have been developed to the point where laboratory lasers can deliver over 750 W and commercial devices are available with powers in excess of 100 W. The lasers have been developed for industrial processes and therefore have many of the attributes required for more general industrial use. Work carried out at several sites, particularly Erlangen University Germany, has demonstrated that the copper laser has the ability to drill and cut many materials which are difficult to process by other methods. The short wavelength, high average power and high repetition rate give the copper laser a number of distinct advantages over other laser types. A consortium of over 20 companies, research institutions and universities from six countries has set up a pan-European project to establish the commercial viability of copper lasers in manufacture and production. The consortium is about to embark on a 100-man year, 2 year definition study which is expected to be followed by a 3 year implementation study. The study will be under taken with approximately 50% government funding under the EUREKA- EUROLASER scheme.

A Copper Vapor Laser Power Supply has been designed using a solid state switch consisting in eighteen Isolated Gate Bipolar Transistors (IGBT),--1200 volts, 400 Amps, each--in parallel. This paper presents the Isolated Gate Bipolar Transistor (IGBTs) replaced in the Power Electronic components evolution, and describes the IGBT conduction mechanism, presents the parallel association of IGBTs, and studies the application of these components to a Copper Vapor Laser Power Supply. The storage capacitor voltage is 820 Volts, the peak current of the solid state switch is 17,000 Amps. The switch is connected on the primary of a step-up transformer, followed by a magnetic modulator. The reset of the magnetic modulator is provided by part of the laser reflected energy with a patented circuit. The charging circuit is a resonance circuit with a charge controlled by an IGBT switch. When the switch is open, the inductance energy is free-wheeled by an additional winding and does not extend the charging phase of the storage capacitor. The design allows the storage capacitor voltage to be very well regulated. This circuit is also patented. The electric pulse in the laser has 30,000 Volt peak voltage, 2000 Amp peak current, and is 200 nanoseconds long, for a 200 Watt optical power Copper Vapor Laser.

A uranium vaporization system has been developed to efficiently produce the large quantities of atomic uranium vapor that are required for an economic A VLIS process. The vapor produced is well collimated and electronically cold. Vapor is produced by high energy electrons which are magnetically steered to the melt surface. Contouring of magnetic fields helps to optimally format the primary electrons and to contain backscattered electrons. A highly compact electron beam system has been developed to facilitate modular packaging of vaporizer components. Electron beam system power will be provided by high power switching power supplies. These power supplies, which are nearing completion at LLNL, have high electrical efficiency and offer excellent protection against high-voltage arcdowns. Vapor density, composition, and quality are monitored by laser absorption spectroscopy. All laser and optical components are mounted outside the process chamber. The monitoring system is nonintrusive and is designed to survive long duration operation at high vaporization rates.