Session BO2 - Laser Plasma Interactions.
ORAL session, Monday morning, November 15
Cascade II, The Westin Seattle

[BO2.01] Interaction Experiments Under Direct-Drive NIF Conditions

Long-scale-length, laser--plasma interaction studies are carried out using up to 30 beams of OMEGA. The goal of these experiments is to examine parametric laser--plasma interaction processes relevant to NIF direct-drive ignition targets. So far, these experiments have shown that neither SBS nor SRS should be significant. Energetic electron production is studied using time-resolved, hard x-ray detectors. Preliminary data suggest that the target preheat due to these energetic electrons will be low enough for direct-drive targets to ignite. More detailed measurements are in progress. In addition, experiments have begun to examine the possibility of multiple-beam-interaction effects. This paper will discuss the present status of these experiments and their relevance to future direct-drive ignition experiments on the NIF. This work was supported by the U.S. Department of Energy Office of Inertial Confinement Fusion under Cooperative Agreement No. DE-FC03-92SF19460, the University of Rochester, and the New York State Energy Research and Development Authority.

[BO2.02] Time Evolution of Raman and Brillouin Laser Reflectivity from Long Scale Toroidal Hohlraum Plasmas

Laser backscatter from stimulated Raman scattering (SRS) and Brillouin scattering (SBS) remains a concern in the long-scale plasmas expected within ignition hohlraums, such as those designed for the National Ignition Facility (NIF). We review the time evolution of the SBS and SRS reflectivity measurements of a Nova laser beam from toroidal hohlraum plasmas, which approximate well the plasma conditions expected within NIF hohlraums. In this shot series, the f/4.3 interaction beam is run at a wavelength of 351~nm, is kept within a narrow range of intensities (2 - 3 \times 10^15~W/cm^2), and is always spatially smoothed with a random-phase plate optic. We have varied separately within the hohlraum plasma the damping of the daughter electrostatic waves of SRS and SBS by varying, respectively, the electron density n_e and the ionic species with other parameters kept fixed. Data will be presented to illustrate three main points: (1)~A significant level of SRS is observed even at high plasma-wave damping rates, which is not theoretically understood. (2)~Transient hydrodynamic effects have a decisive influence on the evolution of SBS and SRS. (3)~The observed scalings of the SRS reflectivity versus acoustic and plasma-wave dampings can vary in time.

[BO2.03] Thomson Scattering from Ion Waves driven by SBS in large-scale length plasmas

We have performed ultraviolet Thomson scattering at the Nova laser facility to measure ion acoustic waves that are excited to large amplitudes by stimulated Brillouin scattering (SBS) from one Nova interaction beam (3 ømega). The Nova beam has been smoothed with phase plates and has been operated with intensities of 0.04 - 8 \times 10^15W cm^-2. The electron temperature and flow velocity gradients have been measured with 90^\circ Thomson scattering on thermal fluctuations and found to reach peak values of T_e = 3~keV and L_V = 0.6~mm, respectively. For the observation of stimulated ion waves, the scattering angle has been adjusted to 96^\circ using a periscope in the Nova chamber to allow k-vector matching of the Thomson scattering and the stimulated ion wave k-vectors. Since we measure the scattering spectra temporally resolved with a 1m spectrometer and a streak camera, we can infer the amplitude of the ion acoustic wave compared to the amplitude of the thermal wave on each shot. We find a strong dependency of the ion wave amplitude on the intensity of the 3 ømega interaction beam. We will discuss the spectral characteristics of the ion acoustic waves, possible effects of large-scale fluctuations, and saturation of the ion wave amplitude.

[BO2.04] Study of the Saturation of Stimulated Raman Scattering by Secondary Decays

Experiments have been conducted at the NOVA laser to characterize the saturation of stimulated Raman scattering (SRS) by secondary decays. Thomson scattering was used to measure the the time resolved spectrum of product ion waves from two processes which can limit SRS by decay of the SRS Langmuir wave into an ion wave and a third wave. This experiment detected the ion wave product of the Langmuir Decay Instability (LDI), where the third wave is a Langmuir wave. Product waves of the Electromagnetic Decay Instability (EDI), where the third wave is an electromagnetic wave, were not detected. Understanding these decay processes will help to provide a physical basis for the understanding of SRS scaling, and will improve confidence in the design of ignition experiments. The experimental results and analysis will be presented.

[BO2.05] Optical Mixing Controlled Stimulated Scattering Instabilities on Omega

Using two interaction beams 155 degrees apart, on the Omega laser system, we have generated ion acoustic waves by optical mixing in an exploding foil plasma. We have demonstrated large energy transfer (x 2.7) from the pump to the probe beam in the small signal limit, when the beams cross near the Mach -1 surface, and no such effect at the Mach 1 surface, as expected. We have also monitored the stimulated Raman and Brillouin backscattering levels of the pump beam and observed substantial reductions in the reflectivity levels when and where the two beams resonantly interact. By comparing thick targets to thin ones, where the densities could be varied, we examine the dynamics of these nonlinear processes approaching the equal intensity pump and probe interactions, beyond the small signal limit explored in the past.

[BO2.06] Field distribution and hot spot statistics in plasmas irradiated by crossed RPP beams

Interaction of two crossed RPP beams with plasma is studied in two-dimensional simulations with a non-paraxial code solving Maxwell and ion-acoustic equations [1]. When the beam power is larger than the critical power for self-focusing, plasma smoothing effects lead to the angular and frequency broadening of light.

The distribution of field intensity in a plasma is described by the characteristics of the hot spot ensemble, namely sizes of hot spots and their maximum intensities. The changes in the hot spot ensemble for different times and different beam intensities are analyzed. Hot spots evolution in the case of crossed RPP beams is different from the single beam case due to beam interference.

The changes in correlation properties of light are quantified by time-time and angle-angle correlation functions. Results of simulations for equal-frequency RPP beams are compared to the case of RPP beams with different color. Effects of crossed RPP beam statistics on parametric instabilities will be discussed.

[1] V. V. Eliseev, W. Rozmus, V. T. Tikhonchuk, and C. E. Capjack, Phys. Plasmas 3, 2215 (1996).

[BO2.07] Scaling of stimulated Raman and stimulated Brillouin scattering in single hot spot experiments

Experiments using a diffraction limited laser beam to drive parametric instabilities are reported. Measurements of stimulated Raman scattering (SRS) and stimulated Brillouin scattering (SBS) from quasi-homogeneous plasmas are obtained for varying laser and plasma conditions. The studies are performed at the Los Alamos Trident laser facility, and use a 527-nm laser to generate the plasma. The plasma conditions are well-characterized using Thomson scattering, and have typical values of T_e \sim 500 eV and n_e \sim 2 x 10^20 cm^-3. A 527-nm diffraction limited laser interacts with the preformed plasma and can drive SRS, SBS and filamentation within the single hot spot. The laser intensity is varied between 10^14 - 10^16 W/cm^2, and the plasma density, target material, and transverse flow speed are varied. A competition between SRS and SBS is observed when both instabilities are active. We will present the scaling of SRS and SBS reflectivities with varying laser intensity, ion wave damping, and transverse flow speed, and will discuss interactions between these instabilities.

[BO2.08] The influence of nonlocal electron transport on stimulated forward Brillouin scattering and filamentation.

Using the 3D wave propagation code, F3D (Berger et al., Phys. Fluids B 5,2243 (1993);Phys. Plasmas 5,4337(1998)), we have computed the transmitted and reflected light for laser and plasma conditions relevant to ignition hohlraums. We find that stimulated forward Brillouin scattering and self-focusing are affected strongly by nonlocal electron heat conduction. The frequency spectrum and the wavenumber spectrum of the transmitted light are calculated and used to identify the relative contributions of each instability. Simulations with random phase plates only(Kato and Mima, Appl Phys. Comm. 329, 186(1992)), or polarization smoothing(Lefebvre Phys. Plasmas 5, 2701(1998))and in conjunction with temporal smoothing by spectral dispersion(Skupsky, et al., J. Appl. Phys. 66, 3456(1989)) or 4-colors(Pennington, et al., ICF Quarterly 5, 130 Jan. 1995) show that the different smoothing techniques affect SBFS and self-focusing differently. Experimental data on the angular spread and spectrum of transmitted light from a probe beam shows evidence of SBFS and filamentation. We will show comparisons of simulated and experimental spectra.

[BO2.09] NLSE Filament Interactions in 2D: Collisions Fusion and ''Radiation''

Our previous work on interactions between light beams in plasmas in the inertia-free NLSE (NonLinear Schrodinger Equation) approximation [unpublished, 1993] is now being used in conjunction with our published work on energy leakage (``radiation'') from individual beams for an ongoing project. The aim here is the improvement of the understanding of interactions or close encounters between initially well-separated NLSE filaments. A novel aspect is the consideration of energy loss via ``radiation'' from the high energy interaction region and the effect on filament fusion and orbiting behavior.

[BO2.10] Stability of Self-Focused Filaments in Laser-Produced Plasmas

The stability of self-focused light filaments in laser-produced plasmas is investigated using a self-consistent cylindrical density and intensity filament model and a full wave-equation treatment for the light. It is found that, if the filament radius is small enough that only one electromagnetic waveguide mode propagates, modulational (“sausage”) perturbations are convectively unstable but the spatial growth rate is very small. In larger filaments, supporting two or more modes, the instability is much stronger and can be absolute. Consequences for laser--plasma interactions are discussed. This work was supported by the U.S. Department of Energy Office of Inertial Confinement Fusion under Cooperative Agreement No. DE-FC03-92SF19460, the University of Rochester, and the New York State Energy Research and Development Authority.

[BO2.11] SRS as a locally self-sustaining nonlinear instability

It is well known that in a uniform density gradient, linear SRS produces equal amounts of scatter per unit frequency interval, with an amplitude determined by thermal fluctuations, and the gradient limited convective power gain, G, 2\pi/ln G =\kappa^' L_0^2, with L_0 the homogeneous spatial gain length and \kappa^' the wavenumber mismatch gradient. Because Langmuir wave turbulence augments thermal fluctuations, for given G there is a threshold value for the SRS coupling coefficient, K_SRS = (v_e/c_s)^3 (\lambda_D/ømega_p)\gamma_0^2/c, above which the SRS rate increases with propagation along the gradient( 29^th) Anomalous Absorption Conference, ’99.. In general, this spatial instability requires a finite boundary seed. However, once K_SRS exceeds another threshold, the instability is nonlinearly self-sustaining.

[BO2.12] Relativistic Electron Beams, Forward Thomson Scattering, and ``Raman'' Scattering

Experiments at LLE (see abstract by D. Hicks at this meeting) show that surprisingly high potentials (+0.5 to 2.0 MV) develop in plasmas irradiated by high-energy lasers. The highly conducting plasma will be a near equipotential and should attract return-current electrons in a radial beam-like distribution, especially in the outer low-density regions. This will initiate the BOT instability, creating large plasma waves with phase velocities close to c. Coherent Thomson scattering of the interaction beam from these waves must occur primarily in the forward direction. This will appear to be ``backward SRS'' upon reflection from a critical surface. We will show that the resulting spectrum is fairly broad and at short wavelengths. Collisional absorption of the scattered EM wave limits the reflectivity to low values (depending on the density scale length). Thus, a distinct difference exists between the spectrum for thick targets (n_c surface present) and thin targets (gasbags, etc., from which primarily a narrow absolute-SRS backward emission occurs, at the peak density). The thick-target, reflected-wave angular distribution will be concentrated in the backward direction. The corresponding plasma-wave k-vector will be a fraction of k_0. The variation of the spectrum with potential and angle will be discussed. Comparison will be made with recent results at LLE and LLNL. This work was supported by the U.S. Department of Energy Office of Inertial Confinement Fusion under Cooperative Agreement No. DE-FC03-92SF19460, UR, and NYSERDA.

[BO2.13] Measurements of Hard X-ray Emission from Laser--Plasma Instabilities on OMEGA

Laser--plasma instabilities producing suprathermal electrons are potentially dangerous for both direct-drive and indirect-drive laser fusion. These energetic electrons can preheat the fuel and prevent compression of the capsule to the requisite conditions for ignition. Fast-electron generation can be inferred from the hard x-ray radiation generated by the interaction of the hot electrons with the target and surrounding material. In addition, optical signatures, like the 3/2 ømega emission from the two-plasmon-decay instability, provide insight into the generation processes. Using the signals from time-resolved detectors in an energy range from 10 keV to 500 keV, this paper will present an estimate of the amount and spectrum of the hard x-ray radiation. The hard x-ray data will be correlated with the 3/2 ømega emission, and a first attempt to infer the amount of laser energy coupled to suprathermal electrons and to the target will be made. This work was supported by the U.S. Department of Energy Office of Inertial Confinement Fusion under Cooperative Agreement No. DE-FC03-92SF19460, the University of Rochester, and the New York State Energy Research and Development Authority.

[BO2.14] A Bremsstrahlung X-ray Source Driven by Stimulated Raman Scattering (SRS) of 351 nm Light

It has long been know that infrared lasers drive large amplitude Langmuir waves by SRS which accelerate electrons by the Landau interaction to produce a non-thermal population. Recent experiments [1] studying the scattering by SRS with UV lasers (\lambda = 351 nm) also suggest that the Langmuir waves have large amplitudes [2]. We have recently begun experiments to maximize the 10 to 100 keV bremsstrahlung x-rays produced in SRS experiments with UV beams. Gas-bag targets are used on Nova with densities as high as 13% of the critical density and various ion species (C, H, Xe) which allow the secondary decay processes to be controlled, as well as with SiO2 foam targets with densities as high as 20% of critical. Measurements are made of the SRS scattering from a single beam and the x-ray radiation. The average photon energy is found to be close to the electron energy that is resonant with the Langmuir waves. 50 keV x-ray fluxes as high as 10^14 (keV/keV) are observed with Au jacketed foam targets. Data will be presented. [1] R. K. Kirkwood et. al. Phys. Rev. Lett. 77, 2706 (1996), and R. K. Kirkwood et. al. submitted to Phys. Rev. Lett. [2] see C.G.R.Geddes et. al. this conference Work performed for US DoE under contract No. W7405-ENG-48

Part B of program listing