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The effects of pre-plasmas on the electron beam directionality was experimentally and numerically investigated. Single material and layered targets made of Ti and/or CH were used to simultaneously measure high-energy (≥3 MeV) electrons along two directions, pre-pulse energy and pre-plasma density. The electron directionality is quantified by using a new parameter, the electron energy ratio of the total kinetic energies along the two directions. Measurements and radiation–hydrodynamic (RH) simulations show that a large (≥3.5 μm) plasma scale length at the critical surface enhances electrons along the laser axis, and such pre-plasma conditions could only be achieved with the CH targets. Particle-in-cell simulations were performed on the RH generated pre-plasmas from Ti and CH targets, and the results show that the CH target provided conditions for higher forward momentum gains by electrons. First, the CH target allowed longer distances for electrons to interact with laser. Second, the intense laser pulse modified the critical surface, but the resulting surface differed. The CH target resulted in a smooth surface where a retro-reflection was observed while the Ti target resulted in a rippled surface that scattered the reflected light. As results, the CH electrons gained higher forward momentum via a direct-laser-acceleration in the counter propagating laser fields. The results presented in this article show a way of controlling the high-energy electron directionality.

An experimental study of the stimulated Brillouin scattering (SBS) instability has investigated the effects of velocity gradients and kinetic effects on the saturation of ion-acoustic waves in a plasma. For intensities less than I

Targets with a layer of aluminum buried beneath a plastic overlay have been irradiated with 0.53 1/4m wavelength, 20-ps pulse length light at (12)×1016 W cm-2. The widths of Stark broadened Al xii 1s21S01s3p P11 spectral lines and the ratios of satellite line intensities to the Al xii 1s2m S011s2p P11 spectral line emitted from the aluminum layer imply electron densities up to (68)×1022 cm-3. This result is a direct observation of significant energy transport to densities close to solid. © 1989 The American Physical Society.

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.

Thomson-scattering measurements of the growth of ion-acoustic waves are obtained from well-characterized multi-ion-species plasmas consisting of gold and beryllium. Simultaneously, the ion temperature is accurately measured by comparing the relative amplitude of thermal ion-acoustic modes. A twofold increase in ion temperature was measured when ion-acoustic waves are excited to large amplitudes by stimulated Brillouin scattering (SBS). This increase in ion temperature is a strong indication of hot ions due to trapping. The measured SBS reflectivity is explained by nonlinear detuning of the SBS instability due to these trapping effects.

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.

Modification of the growth of scattering processes in the case of multiple beam irradiation compared to single beam irradiation has been investigated in a preformed plasma using Thomson scattering of a short wavelength probe beam, and spectral and temporal analysis of reflected and transmitted light. First observations of the reduction of the amplitude of ion acoustic waves associated with simulated Brillouin scattering, amplification of the amplitude of electron plasma waves associated with stimulated Raman scattering, and transfer of energy between crossing beams with same frequency in a flowing plasma under crossed beam irradiation are reported.

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}

Direct observations of secondary Langmuir waves produced by the parametric decay instability of primary Langmuir waves are presented. The measurements have been obtained using Thomson scattering of a short-wavelength probe laser beam and are resolved in time, space, frequency, and wave number. The primary Langmuir waves were driven by stimulated Raman scattering (SRS) of a smoothed laser beam in a preformed plasma. Measurements of the amplitude of the density fluctuations associated with primary and secondary Langmuir waves show that the threshold of the Langmuir decay instability (LDI) is close to the threshold of the Raman instability. This is in agreement with theoretical predictions. However, the ratio of amplitudes of the density fluctuations associated with both secondary and primary Langmuir waves does not agree with existing theories of SRS saturation due to LDI cascading and/or strong Langmuir turbulence in homogeneous plasmas. An explanation based on the interaction beam intensity distribution produced by the random phase plate in the plasma is discussed.

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.

This paper presents recent experimental results that clearly demonstrate the complex interplay between parametric instabilities in a laser-produced plasma. These results include: (a) the interplay between ion acoustic waves (IAW) and electron plasma waves (EPW) associated with stimulated Brillouin scattering (SBS) and stimulated Raman scattering (SRS), (b) the interplay between IAW from two independent SBS decays when two interaction beams are present and (c) the observation of a secondary process, the Langmuir decay instability (LDI), which is the further decay of EPWs driven by SRS, into an IAW and a secondary EPW.

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.

Direct evidence is presented of the temporal and spatial interplay between ion acoustic waves (IAWs) associated with stimulated Brillouin scattering (SBS) and electron plasma waves (EPWs) associated with stimulated Raman scattering (SRS) in conditions of interest to inertial confinement fusion (ICF). The two types of waves grow over a limited region in the front part of the plasma, but at different times. Inhibition of the EPWs by the IAWs has been observed in the early part of the laser pulse, and this inhibition can be strengthened by increasing the level of the IAWs.

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.

X-ray laser action has been studied in a laser-produced plasma composed of germanium atoms ionized to the neonlike state. Laser gains of up to 4.1 cm−1 at 232 Å (1 Å = 10−10 m) were measured. The effect of driver laser parameters such as intensity on target, pulse duration, and pulse rise time have been studied so as to understand the requirements for increased efficiency.

Space–time‐resolved Thomson scattering was used to characterize the electron and ion temperatures in laser produced plasmas of various atomic numbers (Z=6, 13, 32, 73, 92). A Nd:glass laser with intensity between 1013 and 5×1014 W/cm2 was used to produce plasmas of relevance to laser fusion and x‐ray laser studies. A comparison of the electron temperature and density deduced from Thomson scattering, x‐ray spectroscopy, and lasnex hydrocode simulations is presented.

Time‐ and two‐dimensional (2‐D) space‐resolved monochromatic imaging is used to study the dynamics of Ne‐like populations in the electron collisionally excited germanium x‐ray laser. Large‐scale variations in the n=3 level population, specifically the upper level of the 2s2p63d‐2s2p63p laser transition, are observed in plasmas created in uniform and nonuniform irradiation conditions, and the degree of homogeneity along the amplification axis is related to the x‐ray gain. The influence of inhomogeneities on the core‐excited 3d‐3p x‐ray lasers is discussed.

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.

Near‐forward scattering of laser light in a plasma, in regimes relevant to future inertial‐confinement‐fusion (ICF) experiments, has been investigated experimentally. The intensity scaling of stimulated Raman scattering (SRS) forward scattering at one angle was measured from a low‐Z plasma. It was found that the intensity threshold and scaling were consistent with convective theory. Forward SRS saturated at a relatively low intensity. In a second experiment using Ti plasmas, absolutely calibrated, time‐resolved stimulated Brillouin scattering (SBS) spectra from up to seven different angles were measured simultaneously. It was found that the angular dependence of the scattered energy was qualitatively consistent with convective theory. The spectra provided the first evidence of near‐forward SBS. Contrary to previous estimates, near‐forward SBS was shown to be more efficient than oblique backscattering and may be an important effect for ICF.

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.

Monochromatic imaging techniques are used to study quantitatively the large‐scale structures produced in line‐focused plasma by the intensity inhomogeneities of the driving laser. Pronounced perturbations of the mean electron density of the region of the plasma where the heliumlike emission takes place are observed along the line focus axis. These variations indicate the presence of local changes in the plasma hydrodynamics that could strongly affect the time history of the lasing action in x‐ray laser recombination schemes.

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.

An experimental study of stimulated Compton scattering (SCS) from resonant electrons in a laser-produced plasma is reported. Up to 1.9 kJ of 350-nm laser light was used to irradiate 3-\ensuremath{\mu}m-thick CH targets in 1.3 to 3.3-ns pulses at intensities up to 1.3\ifmmode\times\else\texttimes\fi{}${10}^{15}$ W/${\mathrm{cm}}^{2}$. SCS was detected by making time-resolved measurements of the emission near 420 nm. The measurements are compared to the results of theoretical modeling.

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.