Search

We present an interferometry setup and the detailed fringe analysis method for intense short pulse (SP) laser experiments. The interferometry scheme was refined through multiple campaigns to investigate the effects of pre-plasmas on energetic electrons at the Jupiter Laser Facility at Lawrence Livermore National Laboratory. The interferometer used a frequency doubled ( $\unicode[STIX]{x1D706}=0.527~\unicode[STIX]{x03BC}\text{m}$ ) 0.5 ps long optical probe beam to measure the pre-plasma density, an invaluable parameter to better understand how varying pre-plasma conditions affect the characteristics of the energetic electrons. The hardware of the diagnostic, data analysis and example data are presented. The diagnostic setup and the analysis procedure can be employed for any other SP laser experiments and interferograms, respectively.

In laser fusion, the coupling and the propagation of the laser beams in the plasma surrounding the pellet must be well controlled for to succeed in producing a high energy level. To achieve thermonuclear ignition and high gain, the coupling efficiency must be as high as possible, the uniformity of the energy deposition must be very good and the fast electron generation must be minimized. This implies a deep understanding of the laser–plasma interaction mechanisms to keep the nonlinear processes at a low level. Important advances in laser–plasma interaction physics have been achieved thanks to the converging efforts of the experimental and theoretical approaches.Among the different studies of the last few years, we will report results on three themes which are important for future fusion experiments. The first concerns the ability of plasmas to induce temporal and spatial incoherence to the laser beams during their propagation. Beam smoothing, beam spraying and increased incoherence may in turn reduce the level of backscattering instabilities.In laser fusion, multiple beams are used to irradiate the target. The effect of the overlap of the laser beams on parametric instabilities may complicate the problem. Not only is there the interplay between instabilities driven by one beam, but also the interplay between instabilities driven by different beams. In the Laboratoire pour l'Utilisation des Lasers Intenses (LULI) experiment, although the overall stimulated Brillouin scattering (SBS) reflectivity was reduced, a well-defined resonance of the amplitude of ion acoustic waves (IAWs) associated with SBS has been observed for waves propagating along the bisecting direction between two laser beams. Energy transfer between two identical laser beams has been observed and correlated with plasma induced incoherence.The nonlinear saturation of stimulated scattering instabilities is a fundamental ingredient of the understanding of the observed and future reflectivity levels. Using Thomson scattering, the decay of the primary IAWs associated with SBS in secondary IAWs has been observed and correlated with the saturation of SBS.The experiments were performed with the six-beam laser facility of the LULI at Ecole Polytechnique, using one or two 1.053 µm interaction beams in the nanosecond regime and a well-characterized preformed plasma. Multiple diagnostics, including Thomson scattering of a probe beam with spatial, temporal and spectral resolution, were used. The comparison between the experimental results and numerical simulations is used to improve the physics included in the codes.

A model for stimulated Brillouin (SBS) and Raman (SRS) backscattering of a spatially smoothed laser beam interacting with a collisional, inhomogeneous, expanding plasma is presented. It is based on the independent hot spots description [H. A. Rose and D. F. DuBois, Phys. Rev. Lett. 72, 2883 (1994)], in which the overall plasma reflectivity is assumed to be a sum of the individual speckle reflectivities. Self-focusing is taken into account in the computation of the speckle intensity profile and reflectivities. Two additions have been made to previous similar theories: (i) the thermal effects are retained along with the ponderomotive force for what concerns speckle self-focusing, and (ii) SRS (convective and absolute) is accounted for in calculations of the speckle reflectivity. The model is benchmarked against recent laser–plasma experiments at Laboratoire pour l’Utilisation des Lasers Intenses, at Ecole Polytechnique, France, with well-characterized interaction conditions. A good agreement is found between the experimental SBS levels and the model calculations using the measured plasma parameters. This agreement applies for two types of beam smoothing techniques, random phase plates, and polarization smoothing, various plasma densities, and laser energies. Self-focusing itself, and thermal effects in it, play both a fundamental role in defining the level of plasma backscattering. The absolute Raman instability in speckles dominates the SRS response. The model predictions for the SRS reflectivity are less satisfactory, although they demonstrate the same trends as the experimental data. It follows from model calculations and experimental data that the polarization smoothing technique provides an efficient method of control of parametric instabilities allowing a reduction of several times in the level of SBS and SRS reflectivities.

In this article, we present how ion acoustic waves can be unambiguously identified and can be used to definitely demonstrate the existence of the Langmuir decay instability (LDI) of electron plasma waves associated with stimulated Raman scattering. This observation is very important in the context of inertial confinement fusion because the LDI is a mechanism that may influence the stimulated Raman scattering saturation and growth. The experimental identification of those waves relies on Thomson scattering of a short wavelength probe beam. The Thomson scattered light was analyzed by two sets of spectrometer and streak camera providing information on the density fluctuations associated with the electron plasma waves and ion acoustic waves with temporal and spectral resolutions. Selection of a narrow range of angles (4° of aperture) inside the probe and scattered light apertures provided necessary resolution in wave vector. The method used for the design of the Thomson scattering diagnostic is described as well as the technique used to clearly identify the observed ion acoustic waves as one of the Langmuir decay instability products.

Significant reductions of stimulated Brillouin (SBS) and Raman (SRS) scattering are measured by adding polarization smoothing (PS) to a random phase plate smoothed beam. The associated plasma waves, as measured by Thomson scattering, are spatially and temporally modified and reveal that the interplay between SBS and SRS has to be taken into account to understand the effect of the smoothing. The results also support the numerical simulations [S. Hueller et al., Phys. Plasmas 5, 2706 (1998); R. Berger et al., Phys. Plasmas 6, 1043 (1999)] predicting a reduction of self-focusing with PS, resulting in a decrease of the maximum laser intensity and consequently of SBS and SRS gains. (c) 2000 The American Physical Society.

The inverse free-electron laser (IFEL) interaction is studied theoretically and computationally in the case where the drive laser intensity approaches the relativistic regime, and the pulse duration is only a few optical cycles long. The IFEL concept has been demonstrated as a viable vacuum laser acceleration process; it is shown here that by using an ultrashort, ultrahigh-intensity drive laser pulse, the IFEL interaction bandwidth and accelerating gradient are increased considerably, thus yielding large energy gains. Using a chirped pulse and negative dispersion focusing optics allows one to take further advantage of the laser optical bandwidth and produce a chromatic line focus maximizing the gradient. The combination of these novel ideas results in a compact vacuum laser accelerator capable of accelerating picosecond electron bunches with a high gradient (GeV/m) and very low energy spread.

This paper presents the first experimental evidence of the transverse localization of stimulated Brillouin scattering (SBS) emission to the laser beam axis, demonstrating that only a few small regions of plasma contribute to the emission. As a consequence, SBS reflectivity of these regions is much higher than the average SBS reflectivity, by a factor of {approximately}50{endash} 100 . These observations are consistent with the recent concept of SBS growing mainly in active speckles, having an intensity higher than the average laser intensity. {copyright} {ital 1998} {ital The American Physical Society}

Thomson scattering was used to measure the Langmuir wave spectrum driven by stimulated Raman scattering. The Thomson scattering signals measured in the experiment showed Langmuir waves with components both parallel and antiparallel to the incident laser’s wave vector, k0. The parallel component was attributed to stimulated Raman scattering. However, the Langmuir waves with components antiparallel to k0, which cannot be explained by stimulated Raman scattering, were attributed to the Langmuir decay instability (LDI). The relative amplitude of the two Langmuir wave features and their angular width supported the conclusion that the Langmuir waves traveling antiparallel to the incident laser’s wave vector were driven by the Langmuir decay instability.

Experimental reflectivities due to stimulated Brillouin scattering (SBS) and ion acoustic fluctuations have been successfully modelled, over four orders of magnitude of SBS reflectivity, using a newly developed theory of parametric instabilities in randomized laser beams. The observed temporal evolution of SBS has been interpreted using the hydrodynamic evolution of the plasma parameters. Location of SBS in the front part of the plasma is due partly to pump depletion in the SBS-active speckles, and partly to plasma conditions.

This Letter presents the first experimental evidence of seeding of ion acoustic waves (IAW) from stimulated Brillouin scattering along the resonant direction between two interaction beams. Using a secondary interaction beam at a reduced intensity acting as seed, a well defined resonance was observed along the bisecting direction between the two laser beams. A strong reduction of the IAW was observed along off-resonance directions, including for backscatter of the primary beam.

Direct observation of the location of ion-acoustic waves (IAWs) explains for the first time the stimulated Brillouin scattering (SBS) spectra in backward and side directions. The regions of SBS activity are limited and smaller than the scale length of the plasma. The IAWs are localized in the front part of the plasm, resulting in blue Doppler shift of the scattered electromagnetic wave, and the distance from the summit changes with the SBS angle. {copyright} {ital 1996 The American Physical Society.}

We present a technique to obtain simultaneous recording of time‐resolved Thomson scattering spectra at multiple locations in a laser‐produced plasma. The multiplexing technique, although simple, makes possible for the first time the direct correlation of the behavior of plasma waves at different locations within the interaction region in a plasma. The application of this technique to the study of parametric instabilities in laser‐produced plasmas is discussed, as well as the potential application to other diagnostics.

The frequency and wave number of ion acoustic waves (IAWs), with wave numbers corresponding to stimulated Brillouin scattering (SBS), have been measured as a function of the laser intensity, with time-resolved collective Thomson scattering. At high intensities $(Ig{10}^{13}{\mathrm{W}/\mathrm{c}\mathrm{m}}^{2})$, the scattered light contains a novel component with a large frequency shift, in addition to light frequency shifted by the amount anticipated $\left({2k}_{0}{c}_{s}\right)$ for SBS-driven ion waves. This is the first observation of large IAWs driven by the interaction laser and not necessarily associated with a parametric instability.

A Kentech x-ray streak camera was run at the LLNL compact multipulse terawatt (COMET) laser to record simultaneous space- and time-resolved measurements of picosecond laser-produced plasmas. Four different x-ray energy channels were monitored using broadband filters to record the time history of Cu targets heated at irradiances of 10(16)-10(19) W∕cm(2). Through the Cu filter channel, a time-resolution below 3 ps was obtained. Additionally, an array of 10 μm diameter pinholes was placed in front of the camera to produce multiple time-resolved x-ray images on the photocathode and time-integrated images on the phosphor with 10 and 15 times magnification, respectively, with spatial resolution of < 13 μm.

This paper presents an experimental and theoretical study of stimulated Brillouin scattering (SBS) in laser produced plasma using a laser pump with a duration of 8–10 psec. The experiments were performed in a preformed plasma to minimize the flow velocity and have the same plasma conditions over a large range of laser intensities. The reflectivity was then compared to theoretical results over an intensity range of 1013–2×1015 W/cm2. A short pulse was used so that the SBS was in the temporally growing regime and saturation was not an issue.

Second-harmonic emission during the interaction between a laser beam and an underdense preformed plasma has been studied simultaneously using time-resolved images as well as two-dimensional time-integrated images. The emission appeared with very localized structures in space and pulsed in time. These structures are consistent with the presence of filamentary structures whose presence has been confirmed by Schlieren diagnostic. The spatial dimensions of the observed second-harmonic emission is in agreement with the scale length of the filamentation instability. The characterization of second-harmonic emission appears to be a potential diagnostic for filamentation.

We show that high quality monochromatic x-ray images of an extended laser produced plasma can be obtained with a Johann spectrometer. Monochromatic x-ray shadowgraphy is also demonstrated with such a configuration. The versatility of this diagnostic is illustrated with measurements (spectroscopic and imaging) of some resonance transitions of neonlike copper.

Unstable resonators constructed of totally reflecting optics are particularly sensitive to extra-cavity feedback. This is demonstrated experimentally by reflecting the attenuated output of an injection mode-locked TEA CO(2) laser, fitted with a confocal unstable resonator, back into the laser resonator. Even after attenuation by ~10(6), significant perturbation ( greater, similar10%) could be observed in the temporal characteristics of the output train. A theory of extra-cavity feedback in the geometric limit is presented.

We report a study of upshifted and downshifted Raman backscattered light obtained from two different plasma configurations using a 0.53-μm laser pump. The results provide strong evidence of coupling between plasma waves generated by stimulated Raman scattering and ion waves caused by, or facilitating, stimulated Brillouin scattering

Signatures of filamentation have been observed in preformed plasmas using complementary diagnostics: time‐resolved images of the transmitted laser light, dark field imaging, and time‐resolved spectra of Raman light. This last diagnostic clearly shows the presence of small channels inside the plasma with temporal evolution in agreement with the formation of filaments. The filamentary structures disappeared when random phase plates were used in the laser beam. This result is in agreement with a theoretical analysis showing that filamentation does not grow when the speckle size is smaller than the perturbation wavelength which maximizes, in the coherent case, the filamentation growth.

Many different plasma instabilities and plasma modes can coexist in laser plasmas. These instabilities are usually treated as if they occur in isolation, however, more attention has recently been paid to how they interact with each other. This paper reviews three examples of experiments that have demonstrated an interaction between stimulated Raman scattering (SRS) and stimulated Brillouin scattering (SBS). In each case, the presence of SBS is seen to modify the behavior of the SRS process. In one case, SRS was totally suppressed by seeding SBS. The same mechanism might also be responsible for the gap in the Raman spectrum. Some theoretical and numerical studies of the interaction between SRS and SBS are also discussed, showing how increasingly detailed models can give improved agreement with experimental observations.

Time resolved monochromatic imaging diagnostic is used to study in lasing plasma the effects of laser irradiation non-uniformity on line focused plasma conditions. Significant large scale non-uniformities are observed with a conventional focusing system. A new focusing arrangement (SWA) is used to produce a more uniform linear plasma. Quantitative measurements of the influence of the local laser intensity variations on the ionization balance and on the recombination time history are presented and discussed.

A laser‐produced plasma has been studied in which there is a peak plasma density between critical and quarter‐critical for a substantial amount of time (∼400 psec). The behavior of temporally and spectrally resolved light near the wavelength of the interaction laser λ0, is compared to the decay with time of the peak plasma density inferred from three‐halves harmonic emission and Raman scattered light. Evidence for seeding of stimulated Brillouin scattering by the two‐plasmon decay instability is presented.

Producing a long narrow plasma column for X-ray laser studies requires focussing a laser beam into a line focus. Uniformity of intensity along the line is an important consideration. An array of wedges was used in conjunction with a cylindrical lens to provide averaging over many segments of the laser beam, resulting in a more uniform intensity distribution along the line focus. X-ray and infrared images of the resulting line plasmas are shown, to demonstrate the smoothing capability of the wedge array.

Results of our preliminary studies for a demonstration of soft X-ray laser Thomson scattering in laser-produced dense plasma are presented. The investigated plasmas are produced by single-side heated foil targets using a 300-ps pulse of 438-nm wavelength at irradiances between 1013 and 1014 Wcm-2. The Nelike zinc X-ray laser, delivering ~ 1 mJ of focused energy at 21.2 nm, is injected to the plasma as the Thomson probe. The X-ray laser pulse is timed to arrive to the plasma 0.5 or 1 ns after the peak of the optical pulse, encountering electron densities in the range of 1020–1022 cm-3. The spectrum near 21 nm, emitted at ~ 30°with respect to the incident X-ray laser, is analyzed by a flat-field spectrometer viewing through the back of the target. The results show that the choice of appropriate target material and thickness are essential to the success of this experiment. From the spectroscopic measurements using Al and polypropylene (C3H6) foils, the latter appears as a suitable candidate for Thomson scattering experiments near 21 nm. A weak spectral feature near 21.2 nm potentially indicating Thomson scattering was observed using a 1.2 μm polypropylene foil. Further data analysis is required to support this conclusion.

Backscattered light at wavelengths near the incident laser wavelength of 0.35 μm irradiated thin targets has been spectrally and temporally resolved. Low‐Z targets show significant backscattered signals even after the foils burn through when the density gradient scale length is approximately 1000 times the laser wavelength. The results show the importance of considering seeding mechanisms in the plasma.

Three‐halves harmonic emission from a plasma in which the peak plasma density is nc>n>nc/4 has been temporally and spectrally resolved. This is achieved by using 0.53 μm wavelength laser light to irradiate a plasma preformed by 0.35 μm wavelength laser light. It is observed that both the backscattered red‐shifted and blue‐shifted peaks of the 3ω0/2 emission from the 0.53 μm laser disappear at approximately the same time. This result constrains the possible production mechanism for backscattered blue‐shifted photons to scattering of the blue‐shifted photons or plasmons close to nc/4.

University research in the UK with high power lasers is carried out at the SERC's Central Laser Facility with a multi-terawatt neodymium glass laser, VULCAN, and a developmental KrF laser, SPRITE. These systems are briefly described together with the design of a new KrF laser to supersede VULCAN. The new laser design, SUPERSPRITE, is based on optical and Raman multiplexing which is being developed with the present SPRITE system. The specification of SUPERSPRITE is for 3.5 kJ in 1 ns and a peak power of 300 TW in short pulses. The new technology is seen as highly cost effective in relation to neodymium glass lasers. A resume of the development of XUV lasers in the UK in collaboration with laboratories overseas is given. The work is based on laser action through recombination in highly ionized ions and recent progress includes collaborative experiments on the GEKKO XII facility in Japan which have demonstrated laser action at the shortest wavelength to date at 45 A in Mg XII. The physics of energy transport in short pulses is fundamental to the extrapolation of recombination lasers to shorter wavelengths and is being studied from a more basic standpoint using both the VULCAN and SPRITE facilities. Some details of this work are given.

Time‐resolved spectra of backward and forward stimulated Raman scattering were measured from the interaction of 0.26 μm laser radiation with foil targets. The plastic foil targets were designed to go underdense before the peak of the 450 psec, 2×1015 W/cm2 laser pulse. The Raman spectra were quite narrow at any one time (Δω/ω∼0.05) and shifted to shorter wavelengths during the pulse. The backward Raman spectra can be well predicted by assuming that the Raman occurs in a disk‐shaped region on the top of the parabolic density profile, using well accepted threshold estimates.

Summary form only given, as follows. Application of x-ray beams as probes in Thomson scattering experiments allows investigations of plasmas in new regimes of high plasma densities. Compressed fuel in the inertial fusion experiments, ultra short laser pulse produced plasmas for X-ray sources, foam plasmas used in radiation transport studies, shock compressed plasmas for equation of state experiments are examples of ionized gases which can be only diagnosed with short wavelength probes. When fully developed, TS from high density plasmas will justify further development of radiation sources such as x-ray and free electron lasers. Imminent prospect for these advances has motivated our theoretical study. Discussion of the TS cross-section in dense laser produced plasmas will highlight specific issues of laser heated plasmas such as small volumes and inhomogeneity of a scattering region, lack of a coherence in probe beams, required spectral resolution and intensity of the X-ray probe. Sufficient brightness of the probe is necessary to overcome bremsstrahlung emission and absorption in dense plasmas. These limiting factors must be addressed before incoherent TS can be used as an effective diagnostic tool at high densities.

We report detailed experimental results on an X-band (8.547 GHz) RF photoinjector, currently being commissioned at UC Davis on the LLNL site. The device operates with an average accelerating gradient in the 50-100 MeV/m range, and produces relativistic (1.5-2 MeV) photoelectron bunches, with a measured relative energy spread < 1.8%, at the resolution limit of our energy spectrometer. The measured quantum efficiency is 2/spl times/10/sup -5/, at the maximum of the Schottky curve, and the 90% normalized emittance is measured at 1.65 /spl pi/-mm mrad for 20 pC of charge. The bunch duration is extremely short, under the 700 fs resolution of our streak camera. Timing jitter has also been measured directly from the delay between the UV light diffused by the photocathode, and Cerenkov radiation produced by the photoelectrons; a systematic delay is found, corresponding to the time-of-flight difference between the UV photons and the accelerated electrons, and the additional timing jitter is estimated at 5 ps, or 15 degrees of RF phase. Finally, coherent synchrotron experiments are underway, as the photoelectron bunches radiate coherently up to the THz region.

Experiments have been performed to study the propagation of intense laser pulses to high plasma densities. The issue of self-focusing and filamentation of the laser pulse as well as developing predictive capability of absorption processes and x-ray conversion efficiencies is important for numerous programs at the Laboratory, particularly Laser Program (Fast Ignitor and direct-drive ICF) and D&NT (radiography, high energy backlighters and laser cutting). Processes such as resonance absorption, profile modification, linear mode conversion, filamentation and stimulated Brillouin scattering can occur near the critical density and can have important effects on the coupling of laser light to solid targets. A combination of experiments have been used to study the propagation of laser light to high plasma densities and the interaction physics of intense laser pulses with solid targets. Nonparaxial fluid codes to study nonstationary behavior of filamentation and stimulated Brillouin scattering at high densities have also been developed as part of this project.

A complete, three-dimensional theory of Compton scattering is described, which fully takes into account the effects of the electron beam emittance and energy spread upon the scattered X-ray spectral brightness. This formalism is then applied to Compton scattering in a three-dimensional laser focus, and yields a complete description of the influence of the electron beam phase space topology on the X-ray spectral brightness; analytical expressions including the effects of emittance and energy spread are also obtained in the one-dimensional limit.

The modification of the optical characteristics of a laser beam by a plasma is a key issue in laser-plasma coupling. it is critical to understand how this takes place, if we are ever to understand the interaction processes in the plasma corona as well as the coupling at super-high intensities--as when laser pulses approach Petawatt intensities. Interpreting and understanding parametric instabilities in laser-produced plasmas has been a problem of increasing complexity. Improvements in diagnostic capabilities in experimental studies, as well as refinements in the modeling (using different numerical techniques), are showing a complex scenario: strong interplay among instabilities, modification of the plasma conditions caused by the instabilities, and modification to the initial distribution of laser intensity inside the plasma. Of particular interest are stimulated Brillouin scattering (SBS) and stimulated Raman scattering (SRS), instabilities which have been studied extensively during the past 20 years, both theoretically and experimentally. Until now, most studies--mainly driven by requirements associated with inertial confinement fusion (ICF)--have concentrated on backscattering instabilities. The role of forward instabilities has not received much attention, despite having the potentials for strongly modifying the overall laser-plasma interaction region. The objective of this project is to study numerically the nonlinear enhancement of large-angle,more » forward scattering of two identical laser beams propagating in a preformed plasma. it is known that filamentation instability and self-focusing are capable of modifying laser-beam geometry, altering the electromagnetic-field distribution and spectral properties. These instabilities, combined with forward SBS, apparently cause a plasma-induced smoothing (self-smoothing) of the laser light as it propagates through the plasma. The final effect may have consequences similar to the temporal smoothing introduced intentionally in many laser systems. They do not propose this phenomenon as a smoothing technique; however, they claim that the understanding of this effect is crucial to the interpretation of experimental results on parametric instabilities.« less

The nonlinear enhancement of large-angle forward scattering of two identical laser beams propagating in a preformed plasma has been observed experimentally. The spectral analysis of the forward-scattered light shows two components, one which is unshifted with respect to the initial laser light frequency, and the other which is redshifted by a few angstroms. The redshifted component is found to be strongly enhanced in the case of crossed beam interaction in comparison with that of one beam illumination. Two-dimensional numerical simulations show that this enhancement is due to large-angle forward stimulated Brillouin scattering in which each beam serves as seed for the forward scattering of the other. (c) 2000 The American Physical Society.

The phase noise and jitter characteristics of the laser and rf systems of a high gradient X-band photoinjector have been measured experimentally. The laser oscillator is a self-modelocked Titanium:Sapphire system operating at the 108th subharmonic of the rf gun. The X-band signal is produced from the laser by a phase-locked dielectric resonance oscillator, and amplified by a pulsed TWT and klystron. A comparison between the klystron and TWT amplifier phase noise and the fields excited in the rf gun demonstrates the filtering effect of the high Q structure, thus indicating that the rf gun can be used as a master oscillator, and could be energized by either a rf oscillator such as a magnetron or a compact source such as a cross-field amplifier. In particular, the rf gun can play the role of a pulsed rf clock to synchronize the photocathode laser system: direct drive of a synchronously modelocked AlGaAs quantum well laser has been achieved using the X-band gun rf fields. This novel, GHz repetition rate, lase...

Time-resolved spectra and location of electron plasma waves (EPW) produced by stimulated Raman scattering in back and in side directions have been measured using Thomson scattering of a short wavelength probe beam. Significant Raman sidescattering was observed for angles as large as 40\ifmmode^\circ\else\textdegree\fi{} from the laser axis. The Raman growth is larger and starts earlier in the front part of the density profile than at the summit. Simultaneous measurements at multiple places in the plasma provided an indication of the coherence length of the EPW.

Thomson scattering was used to measure the Langmuir wave spectrum generated by stimulated Raman scattering. This experiment detected Langmuir waves with components both parallel and antiparallel to the incident laser's wave vector ${\stackrel{\ensuremath{\rightarrow}}{k}}_{0}$. The parallel component was attributed to stimulated Raman scattering. However, the Langmuir waves moving antiparallel to ${\stackrel{\ensuremath{\rightarrow}}{k}}_{0}$, which cannot be explained by stimulated Raman scattering, were attributed to the Langmuir decay instability.

An experimental study of stimulated Brillouin scattering (SBS) with a 1 ps laser pulse in a large underdense preformed plasma is presented. It is shown that the SBS reflectivity increases from ${10}^{\mathrm{\ensuremath{-}}4}$ to ${10}^{\mathrm{\ensuremath{-}}1}$ as the intensity of the interaction beam rises from 5\ifmmode\times\else\texttimes\fi{}${10}^{14}$ to ${10}^{16}$ W/${\mathrm{cm}}^{2}$; for intensities between ${10}^{16}$ and ${10}^{17}$ W/${\mathrm{cm}}^{2}$, a saturation level at 10% is experimentally established. This experimental result is compared with theoretical estimates of time-integrated SBS reflectivity in the transient phase of the so-called modified decay regime. The latter results are seen to be consistent with the experimental data provided that there exists a nonthermal noise in the plasma depending on the laser intensity.

The first experimental measurements of near-forward scattering from a plasma at wavelengths near the pump wavelength are presented. Simultaneous, time-resolved spectra from several oblique forward angles show the characteristic wavelength shift associated with scattering from an ion wave. Energy measurements show that forward scattering is stronger than backscattering. The angular distribution of forward scattered energy is in qualitative agreement with the simple convective-instability theory of stimulated Brillouin scattering.

Summary form only given. A National Academy of Science Review Committee on inertial confinement fusion has endorsed a 12-point technical contract for the Nova program. Recent experiments have achieved a substantial number of these goals. The ten-beam laser has now been operated with the rms power balance among the ten beams as low as 8% in the foot and 5% in the peak of a high-energy, temporally shaped pulse. A decrease in the level of plasma instabilities from large scale length, low density plasma has been demonstrated by the use of random phase plates. Neutron measurements have been used to demonstrate high densities in well-understood implosions. A detailed understanding of the Rayleigh-Taylor instability at the ablation front of X-ray drive planar foil targets, with large hydrodynamic growth factors, has been demonstrated.

Summary form only given. Temperature and density homogeneity along X-ray laser line plasmas is important for the optimization of the gain. Irradiation nonuniformities are usually present in experiments and produce large-scale inhomogeneities in the plasma emission along the amplification axis. The authors have investigated the evolution of such inhomogeneities on Ne-like Ge line plasmas and their effect on the gain with a space- and time-resolved monochromatic imaging technique, for both uniform and nonuniform laser irradiation. In particular, they have studied 3d-3p lasing at 199 /spl Aring/ and the quadrupole E2 (8.031 /spl Aring/) and dipole 3A (8.371 /spl Aring/) line emissions originating from its upper and lower states, respectively.

A high brightness KrF Raman laser generating 60 GW, 40 ps pre-pulse free pulses has been focused to an 8 μm focal spot at 9x10 16 W cm -2 . Axial and lateral transport of energy has been studied by analysis of X-ray images of the focal spot and from X-ray spectra emitted by a thin layer of aluminium beneath a variable thickness of CH polymer on planar targets. The results show strong lateral transport of energy and reduced axial penetration of heating.

Measurements of reflectivity of stimulated Brillouin scattering, from an underdense, homogeneous plasma, irradiated by a 10 ps-1.06μm laser pulse, show an increase from 10 -4 to 10 -2 , as the laser intensity is varied between 10 13 to 10 15 W/cm 2 . Numerical simulations have been developed to interpret these data. At low laser intensity, ( 14 W/cm 2 ), the low reflectivity is well explained as being due to saturation of the instability in the convective regime, and good agreement with numerical simulations has been obtained. At high laser intensity, the model predicts higher reflectivities than measured in the experiment, although non-linear effects in ion sound waves evolution have been observed.

Lateral and axial thermal energy transport in laser-plasma interactions has been studied using a short (12 psec) pre-pulse free KrF pumped Raman laser pulse ( = 268 nm) of low beam divergence. A new method of producing narrow line foci using random phase plates was employed to study the plasma conditions required to generate high gain, short wavelength recombination X-ray lasers. In addition, the laser energy was focusedinto a small focal spot (8 tim) for higher intensity (1-2 x 1017 cm2) interaction studies. 1 . INTRODUCTION Recent numerical simulation studies have shown that the gain obtained in recombination X-ray laser experiments increases for shorter wavelength, higher Z materials when the pulse length of the heating laser pulse is reduced(1). It has also been shown that the gain increases when the line width is narrowed(2). It is therefore of great interest to study the lateral and axial thermal energy transport for short pulse,narrow line focus irradiation, and to determine whether lateral energy transport will

High Z exploding foil targets are used in many soft x-ray laser schemes. The foils are typically irradiated with a long ('1 ns) optical laser pulse that burns through the foil to produce a large hot plasma with long density scale lengths suitable as an amplifier. While it is expanding, and before the plasma conditions are suitable for stimulated emission, a large fraction of the heating laser energy is lost through radiation and conduction. We report the results of experiments attempting to increase the efficiency of exploding foil amplifiers through the following procedure. A laser beam ('1 2 W/cm2) is used to heat the foil sufficiently to expand it to approximately 200 pm. A high intensity (''1 5 W/cm2) short pulse (1 0 ps) laser beam is then used to raise the plasma to the desired temperature and ion state. Temporally resolved x-ray spectra from Yb foils are presented.© (1990) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

We present in this paper observations of stimulated Briiiouin backscattered light from a plasma irradiated by a 1 0 ps 1 .06 m laser pump, at intensities between 1 012 and 1 016 W cm2. During this short time duration, the observed growth of the instability can be explained only in terms of a purely temporal growth, in contrast to the convective growth normally associated with this instability.

A platform for analysis of material properties under extreme conditions, where a sample is bathed in radiation with a high temperature, is under development. Depositing maximum laser energy into a small, high-Z enclosure produces this hot environment. Such targets were recently included in an experimental campaign using the first four of the 192 beams of the National Ignition Facility [J. A. Paisner, E. M. Campbell, and W. J. Hogan, Fusion Technol. 26, 755 (1994)], under construction at the University of California Lawrence Livermore National Laboratory. These targets demonstrate good laser coupling, reaching a radiation temperature of 340 eV. In addition, there is a unique wavelength dependence of the Raman backscattered light that is consistent with Brillouin backscatter of Raman forward scatter [A. B. Langdon and D. E. Hinkel, Phys. Rev. Lett. 89, 015003 (2002)]. Finally, novel diagnostic capabilities indicate that 20% of the direct backscatter from these reduced-scale targets is in the polarization orthogonal to that of the incident light.