We present initial performance studies for beamlet, a single-beam prototype for megajoule- class neodymium-glass laser fusion drivers using a multipass main amplifier, adaptive optics, and efficient, high-fluence conversion to the third harmonic. The beamlet final amplifier uses Brewsters-angle glass slabs with a square 39 by 39 cm2 aperture and a full-aperture plasma-electrode Pockels cell switch. The laser has been tested at the fundamental wavelength over a range of pulselengths from 1 - 10 ns up to energies of 5.8 kJ at 1 ns and 17.3 kJ at 10 ns at a beam area of 35 by 35 cm2. A 39-actuator deformable mirror system corrects the beam to a Strehl ratio of 0.4.
The optical pulse-shaping system for the 60-beam 30-kJ (UV) OMEGA fusion laser is capable of producing complex temporally shaped optical pulses for amplification and delivery to fusion targets. The pulse-shaping system consists of optical modulators driven by an optically activated electrical waveform generator. The electrical waveform generator consists of Si photoconductive switches, and variable impedance microstrip lines. Complex optical pulse shapes with 50 to 100 ps structure have been produced.
The propagation and compression of a broad-band pulse in a chirped-pulse-amplification laser is studied. A numerical model is used to predict the effects of gain narrowing, gain saturation and self-phase modulation on final pulse recompression. A compact size double-pass amplifier scheme with spectral filtering is proposed to reduce the effect of self-phase modulation.
The amplification of 300 fs pulses in a Pr3+-doped fluoride fiber is investigated experimentally. We found that for these short inapt pulses the high normal group velocity dispersion (GVD) of the fiber leads to a self induced chirped pulse amplification which prevents spectral pulse distortions due to nonlinearities. As a consequence the amplified pulses can be recompressed to their original duration in an external delay line for gain values in the order of 15 - 20 dB. These findings are important for applications in communication and ultrafast spectroscopy.
We present and explore an idea of creating a source of extremely short (10-15 s) coherent radiation tunable in the XUV wavelength range. The underlying physical mechanism consists in nonlinear properties of rapidly ionized atoms to efficiently convert the spectra of laser radiation in the 1015 - 1016 W/cm2 intensity range. We demonstrate that under certain conditions the nonlinear effects of high-order harmonic generation, spectrum broadening and blueshifting can be simultaneously engaged and favorably combined. This mechanism offers an attractive possibility to enter the attosecond duration range by optimizing the process of high-order harmonic excitation at the ionizing fronts of ultrashort laser pulses.
Semiclassical approach has been developed to investigate above-threshold ionization of atoms in high intensity laser field. Influence of Coulomb potential of parent ion and relativistic effects on electron motion in the continuum and electron wavepacket spreading were taken into account. Numerical simulations show that these factors lead to modification of cut-off frequency law from the well known formula (Omega) equals 3.17 Up plus Ip to (Omega) equals f(I) Up plus Ip (atomic units e equals m equals $HBAR equals 1 are used throughout this paper), where Up is electron ponderomotive energy and Ip is atom ionization potential. Function f(I) exceeds the 3.17 value at relatively low laser intensities and decreases with laser intensity growth to almost zero. It is shown that a maximum cut-off frequency (Omega) max should exist. The critical laser intensity corresponding to this frequency Icr on the order of (omega) 2, where (omega) is laser frequency. Also, we present an explanation of processes resulting in the formation of behind cut-off region of high harmonic spectra.
Both analytical and numerical analysis of weakly coupled backward stimulated Raman scattering (BSRS) of a modulated laser pulse in an underdense plasma is made. In that regime the spectrum of a backscattered field contains only the usual Stokes lines from respective spectral components of a laser pulse, each narrow compared to (omega) pe. Additionally, the intensity of radiation, backscattered from a given spectral component of a laser pulse, dramatically lowers in appearing in the pulse spectrum of another component, red-shifted by 2(omega) pe with respect to the former. It is proved that a long-wavelength plasma wave with a relativistic phase velocity, propagating in plasma, affects the mode structure of weakly coupled BSRS. It is shown via direct numerical modeling that the strong pulse modulation visibly modifies the spectrum of strongly coupled BSRS. The linear theory clarifies some details of numerically modeled evolution of BSRS spectra due to nonlinear pulse evolution.
Two types of scattering of the laser radiation arising when an intense (1014 - 1016 W/cm2) subpicosecond ionizing pulse propagates in an atmospheric-density gas are considered: the stimulated Raman scattering (SRS) of the laser pulse by the electron plasma wave and the scattering as a result of the spatio-temporal instability in the self-produced plasma. The contributions of the spatio-temporal instability and the SRS to the laser pulse spectrum are compared.
Electron bunch acceleration in the space-charge field generated by laser pulse in an underdense plasma is investigated. The analytical method is developed and used to describe the energy spectrum of ultra-relativistic electrons accelerated by the one-dimensional fast intense plasma wave of the arbitrary form. The peculiarities of electron energy spectra for different length of acceleration in the linear and nonlinear plasma wave are considered.
In the present work the interaction of single and series of laser pluses with laser energy to 0.5 J (for different light polarizations and incident angles) with aluminum target was investigated. It was found out, that p-polarized laser light is absorbed much better than s-polarized light. It is a result of resonance absorption. The main part of absorbed laser energy is spent on heating and fast particle generation. X-ray intensity is higher in the case of p-polarized laser light than in the case of s-polarized laser light. Results of numerical calculations by codes 'SKIN' and 'ION' are in good agreement with the experimental ones.
We describe the generation of optical harmonics of high order during the interaction of intense femtosecond laser pulses with the surface of insulating and metallic solid targets. With a p- polarized fundamental beam from a titanium sapphire, chirped pulse amplification laser system ((lambda) equals 800 nm) we have observed even and odd harmonics up to the eighteenth order (lambda approximately equals 45 nm) for fundamental laser intensities of about 1017 to 1018 W/cm2.
X-ray K- and L-shell emission from aluminum and carbon plasmas produced by subpicosecond UV-laser pulses has been investigated using various spectrographs. The spectra have been measured as a function of laser intensity in the range from 1014 W/cm2 to 1018 W/cm2. A computer simulation including reabsorption of the resonance lines has been performed and the observed and calculated spectra compared. Plasma parameters have been deduced by line emission and continuous emission observations. It has been found that for high laser intensities the resonance line emission and that from the satellites comes from the overdense region. A scaling of the electron temperature with the irradiance has been obtained. From the x-ray spectra measured at different laser intensities, the thresholds for the formation of hydrogen-like and helium-like ions in aluminum and carbon plasmas have been determined.
The heating of the solid targets by ultrashort intense laser pulses is investigated using both analytical and numerical methods. The regularity of skin-effect regimes changing in accordance with pulse and plasma parameters is analyzed. The laser energy absorption and energy deposition in the different skin-effect regions is studied. The self similar solution describing heating of a plasma in the 'sheath inverse bremsstruhlang' skin-effect region is obtained and discussed.
We have considered, for the first time to our knowledge, the plasmon-polaritons surface waves into femtosecond laser-induced plasma. We have shown that plasmon-polaritons can exist into the surface of expanded femtosecond laser-induced plasma and have found out their basic characteristics.
We have found out that in the case of the laser pulse with pico- or subpicosecond duration its energy part, scattered via SBS in the strongly inhomogeneous short plasma, reaches the value of a fraction of a percent for the intensity of less than or equal to 1016 W/cm2, if there is no prepulse, and for the intensity of less than or equal to 1015 W/cm2, if such a prepulse is presented. This fact may be of use in the experimental evaluation of the pulse contrast value and influence.
Simulations of a linearly polarized laser wave in the range I(lambda) 2 is contained in (1018 - 5 multiplied by 1019) Wmicrometers 2/cm2 normally incident on a slightly overdense plasma have been performed with a 1.5 D relativistic particle-in-cell code. These kinetic simulations support the existence of relativistic self induced transparency, and complement past analytical work on the subject. Electron heating, laser absorption and ion motion are considered.
Significant success in the field of creation of ultra high power (I(lambda) 2 greater than or equal to 1018 W(DOT)micrometer2/cm2) short-pulse (less than or equal to 1 ps) lasers results in strong interest in such a laser radiation interaction with matter. In the case of such intensity dense (ne approximately equals 1023 cm-3) and hot (Te approximately equals 10 keV) plasma is produced practically immediately (just several periods of light oscillation) and further interaction of the pulse occurs with this plasma. One of the results of such an interaction is generation of fast particles and of rather intense hard x-ray emission. X-ray pulse duration is determined by a free path of the produced fast electrons (approximately equals 0.1 ps). This pulse intensity is sufficient for registration by the common methods. Laser pulse repetition rate can be very high -- up to approximately equals 100 MHz. Hence, it is possible to obtain the instantaneous x-ray photographs of the fast mechanical, chemical and biological processes; the stage of these processes is fixed on the temporal level of approximately equals 1 ps. High energy of the x-ray quanta produced (up to 100 keV) makes it possible to study the objects of small size -- even the separate large molecules. Ultrashort x-ray pulse can be also applied for fast decoupling of molecular bonds with monitoring of the processes by the optical laser. Laser spot diameter on the target is small (approximately equals 10 micrometers); depth of thermal wave penetration into the target is also small (approximately equals 10-3 cm). That is why the very object or deposed onto it thin layer of heavy metal can be used as the target. In the latter case the object is irradiated only by x-ray pulse. Noteworthy, that the high power pulse can be produced by the comparatively cheap and small, table-top laser. Numerous papers, both experimental and theoretical were devoted to the study of short laser pulse interaction with plasma. The function of electrons distribution in the laser field was calculated, absorption coefficient was determined as well as structure of electromagnetic field inside the plasma. In this paper we determine the distribution function and parameters of the fast particles flux in plasma. In the nonrelativistic approximation our results are in agreement with those previously published. A novel feature is calculation of the energy and spectrum parameters of the hard x-ray pulse, produced by the intense laser pulse interaction with the solid state target.
The dependence of laser-induced damage threshold of K8 (Russia) and BK7 (USA) glasses on laser pulse duration in the wide region from 4 (DOT) 10-11 to 3 (DOT) 10-8 s has measured. Strong attention is paid to methods of measurements. It is indicated to strong influence of laser radiation statistics on the results of the threshold measurements. It is shown that intrinsic laser-induced damage threshold of glasses does not depend on pulse duration when there is not self-focusing. However the time dependence appears when spot size has become more than the wavelength of laser radiation. It is determined by Kerr nonlinearity in the region of pulse durations less than 10-9 s. For longer pulses the other types of nonlinearity are important as well. It is established that the critical power of the self-focusing is not constant for the same glass, and one depends on the concrete focusing conditions. Besides it is shown that the vector character of electric field strength of laser radiation must take into account under calculation of this value in the focal region under tight focusing conditions.
Composed of sp3 bonded nodules of carbon, amorphic diamond films are deposited in vacuum onto almost any substrate by condensing carbon ions carrying keV energies. These multiply charged ions are obtained from the laser ablation of graphite at intensities in excess of 1011 W cm-2. The high energy of condensation provides both for the chemical bonding of such films to a wide variety of substrates and for low values of residual compressive stress, 0.6 - 0.8 GPa. On selected films hardness cannot be measured because of deformation of the diamond indenter and only a lower limit of 78 GPa can be reported. Coatings of 2 - 5 micrometer thicknesses have extended lifetimes of materials such as Si, Ti, ZnS, ZnSe, Ge and stainless steel against the erosive wear from high-speed particles by factors of tens to thousands. The mechanical properties of amorphic diamond films are further enhanced by a low coefficient of friction of about 0.1. The combination of these mechanical properties seems to make amorphic diamond an attractive material for use as a protective coating in current industrial applications. Deposited upon silicon, quartz or sapphire, amorphic diamond films have interesting electrical properties including a very high coefficient of emissivity.
The restrictions on density, dimension and temperature of ignitor are obtained which provide the thermonuclear burn wave generation with high efficiency of energy release. Calculations are carried out by means of TERA code based on Monte-Carlo simulation of kinetics of thermonuclear particles and radiation for different values of temperature inhomogeneity parameter with taking into account the energy transport from relatively cold fuel to the ignitor.
It is necessary to reach large gains in laser target (G) to get useful energy in a fusion reactor. Using the fission blanket in a reactor allows one to increase the energy gain. The first Russian projects of laser fission-fusion reactor were studied. It was suggested to use a two-cascade hybrid reactor scheme, allowing one to reach gain in fission blanket more than 1000. As a result it will be possible to use laser target with G on the order of 0.1 - 1 in such type of a reactor. It was suggested to target the design for the 'ignition' experiment at the laser energy 0.1 - 0.3 MJ. It was a high aspect ratio cryogenic direct driven target. But it is necessary to use a lot of laser beams around the target to provide spherical symmetry of laser irradiation. Large surface 'will be lost' for fission blanket. In addition the laser pulse should have a sharp time profile. It is necessary to use about five-to-ten times more laser energy to get 'ignition' in indirect driven target. The new target design, named 'Greenhouse target,' was suggested. It is a very important problem to compensate the negative influence of holes on compression symmetry. It was suggested to use the target with 'relief' to compensate this effect. This target design is studied now. We are studying four alterative approaches to the laser target design for hybrid reactor: (1) the target with inner laser energy input; (2) high aspect ratio target for laser with pulse duration about 100 ns; (3) conic target; (4) exploding pusher spherical or conic targets for carbon-dioxide laser. The numerical simulations were made by using Lagrange code 'ATLANT'.