Bulletin of the American Physical Society
2005 47th Annual Meeting of the Division of Plasma Physics
Monday–Friday, October 24–28, 2005; Denver, Colorado
Session BO1: Laser-Plasma Coupling |
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Chair: Hector Baldis, University of California, Davis Room: Adam's Mark Hotel Governor's Square 10 |
Monday, October 24, 2005 9:30AM - 9:42AM |
BO1.00001: Electron Transport Modeling in Inertial Confinement Fusion Experiments V.N. Goncharov, G. Li, P.B. Radha, J.A. Delettrez, A.V. Maximov, R.L. McCrory Thermal transport plays an important role in inertial confinement fusion. The Spitzer--Harm model has conventionally been used in hydrocodes. Such a model, however, breaks when the electron mean free path exceeds a few percent of the spatial scale length in a plasma. To extend the validity of the model, a flux limiter $f $is introduced and the Spitzer flux $q_{SH}$ is replaced by a fraction of the free-stream flux \textit{fq}$_{FS}$ in regions where $q_{SH} \quad >$ \textit{fq}$_{FS}$. Alternatively, a convolution form\footnote{ J. F. Luciani \textit{et al}., Phys. Rev. Lett. \textbf{51}, 1664 (1983); E. M. Epperlein and R. W. Short, Phys. Fluids B \textbf{3}, 3092 (1991).} and multigroup diffusion model\footnote{ G. P. Schurtz \textit{et al}., Phys. Plasmas \textbf{9}, 4238 (2000).} of the thermal flux have been proposed in the past. In this talk, a new nonlocal model is presented which takes into account the finite deposition range of electrons. Such a model is based on the solution of a simplified kinetic equation. Heat flux, calculated with the model, is used in the hydrocode \textit{LILAC} to simulate ICF experiments. Comparison of the results of such simulations with both the experimental data and Fokker--Planck simulations will be presented. This work was supported by the U.S. Department of Energy Office of Inertial Confinement Fusion under Cooperative Agreement No. DE-FC52-92SF19460. [Preview Abstract] |
Monday, October 24, 2005 9:42AM - 9:54AM |
BO1.00002: Electron Distribution and Transport in a Laser Field in Direct-Drive ICF Plasmas A.V. Maximov, J. Myatt, R.W. Short In studies of direct-drive inertial confinement fusion (ICF) physics, the modeling of electron heat transport has a major role. In the region close to the critical-density surface, the distribution of electrons is influenced simultaneously by steep density and temperature gradients and by multiple laser beams which have their turning points and are largely absorbed in this region. The effect of the laser field on the electron distribution function and on the electron heat transport in the near-critical-density region is considered. The modifications in the electron distribution function are obtained by solving the kinetic equation for electrons with the Fokker--Planck--Landau collision integral. The importance of these changes in the electron distribution and transport for the numerical modeling of direct-drive ICF experiments with hydrodynamic codes and Fokker--Planck codes\footnote{ E. M. Epperlein and R. W. Short, Phys. Fluids B \textbf{3}, 3092 (1991).}$^{,}$\footnote{ A. Sunahara \textit{et al}., Phys. Rev. Lett. \textbf{91}, 095003 (2003).} is discussed. This work was supported by the U.S. Department of Energy Office of Inertial Confinement Fusion under Cooperative Agreement No. DE-FC52-92SF19460. [Preview Abstract] |
Monday, October 24, 2005 9:54AM - 10:06AM |
BO1.00003: Effect of Ponderomotive Terms on Heat Flux in Laser-Produced Plasmas G. Li, V.N. Goncharov A laser electromagnetic field introduces ponderomotive terms\footnote{V. N. Goncharov and G. Li, Phys. Plasmas \textbf{11}, 5680 (2004).} in the heat flux in a plasma. To account for the nonlocal effects in the ponderomotive terms, first, the kinetic equation coupled with the Maxwell equations is numerically solved for the isotropic part of the electron distribution function. Such an equation includes self-consistent electromagnetic fields and laser absorption through the inverse bremsstrahlung. Then, the anisotropic part is found by solving a simplified Fokker--Planck equation. Using the distribution function, the electric current and heat flux are obtained and substituted into the hydrocode \textit{LILAC} to simulate ICF implosions. The simulation results are compared against the existing nonlocal electron conduction models\footnote{G. P. Schurtz, P. D. Nicola\"{\i}, and M. Busquet, Phys. Plasmas \textbf{9}, 4238 (2000).} and Fokker--Planck simulations. This work was supported by the U.S. Department of Energy Office of Inertial Confinement Fusion under Cooperative Agreement No. DE-FC52-92SF19460. [Preview Abstract] |
Monday, October 24, 2005 10:06AM - 10:18AM |
BO1.00004: Stimulated Brillouin Scattering in Plasmas Relevant to Direct-Drive Laser Fusion W. Seka, J. Myatt, A.V. Maximov, R.W. Short, R.S. Craxton, R.E. Bahr, T.C. Sangster, H. Baldis Recent experimental and theoretical work on stimulated Brillouin scattering in plasmas of interest to direct-drive ICF has resulted in improved understanding of previous experimental data. Extrapolations to NIF-scale direct-drive plasmas can now be made with greater confidence. The region close to critical density has been identified as the region where the interplay of hydrodynamic evolution, SBS gain, and electromagnetic seeding is particularly complex and important. This work was supported by the U.S. Department of Energy Office of Inertial Confinement Fusion under Cooperative Agreement No. DE-FC52-92SF19460. [Preview Abstract] |
Monday, October 24, 2005 10:18AM - 10:30AM |
BO1.00005: Laser Beam Propagation in Large-Scale NIF Plasmas Siegfried Glenzer, D.H. Froula, L. Divol, M. Dorr, R.L. Berger, S. Dixit, C. Haynam, J.P. Holder, O.S. Jones, D. Kalantar, S. Langer, B.J. MacGowan, N. Meezan, C. Niemann, B. Still, R. Wallace, B.A. Hammel, E. Moses The propagation of intense, high-energy laser beams through large-scale length plasmas has been measured with the first experiments on the National Ignition Facility. X ray imaging data show that smoothed beams propagate through the full length of a 7 mm long fusion-scale plasma burning through at t=1.5 ns. However, unsmoothed beams show whole beam self-focusing, beam spray and stalled propagation until the end of the 3.5 ns-long experiment. Integrated calculations that use realistic beams verify the effectiveness of beam smoothing which lowers the power fraction of high intensity speckles in the laser beam that are above threshold for filamentation. This ability to model the results quantitatively allows us to specify the laser beam intensity and smoothing for future laser fusion experiments. [Preview Abstract] |
Monday, October 24, 2005 10:30AM - 10:42AM |
BO1.00006: Simulation of Laser Beam Propagation Through NIF Gaspipes Milo Dorr, Richard Berger, Laurent Divol, Charles Still, Edward Williams, Dustin Froula, Siegfried Glenzer, Ogden Jones, Nathan Meezan The fully three-dimensional simulation of laser beam propagation and interaction with large-scale NIF plasmas poses a formidable computational challenge even with the largest available computers. A new strategy is therefore being employed to enable the laser plasma interaction (LPI) code pF3d to model NIF gaspipe experiments, requiring the simulation of LPI in several millimeters of plasma for a few nanoseconds. Using a mixed 2D/3D approach, the light propagation and hydrodynamic equations are solved in a 2D plane while heat conduction and energy deposition are computed on a 3D coarser grid (called the collar grid) that extends far beyond the laser beam. By assuming an axisymmetric beam, the 3D conduction source is obtained by rotating the 2D laser intensity about the symmetry axis. A dynamic renormalization of laser intensity is performed to account for beam spreading in the omitted third dimension. This approach has been used to show that the use of polarization smoothing and smoothing by spectral dispersion allows efficient propagation through ignition scale plasmas on NIF. [Preview Abstract] |
Monday, October 24, 2005 10:42AM - 10:54AM |
BO1.00007: Scaling of laser interactions with plasmas from high-Z gasbags Richard Berger, C. Constantin, L. Divol, N. Meezan, C. Niemann, L. Suter Gasbags filled with high-Z gases (Krypton and Xenon) illuminated with 38 heater beams of 351nm wavelength and two probe beams at the Omega Laser Facility showed remarkably little stimulated Brillouin backscatter and even less stimulated Raman backscatter of the 351nm and 527nm probe beams. The probe beams were focused with an f/6.7 lens to a 250 micron diameter spot near the center of the gasbag to produce a vacuum intensity of 8 x 10$^{14}$ W/cm$^{2}$. Both the heater and probe beams were 1ns in duration but the probe beams were delayed 500ps with respect to the heater beams. For the duration of the probe beam interaction with the plasma, the electron temperature at the center reaches 4 keV for the lowest fill pressures (N$_{e}$ = 4 x 10$^{20}$ cm$^{-3}$ where N$_{e}$ is the electron density) and 3 keV for the higher fill pressures (N$_{e}$ = 1 x 10$^{21}$ cm$^{-3})$. Because Z Te/Ti $>>$ 1, ion acoustic waves driven by the SBS interaction are weakly damped, and expectations, based on experiments with CO$_{2}$ fills where Z Te/Ti is also large, are that SBS should exceed 25{\%} rather than the measured 2-5{\%}. With a combination of linear gain calculations and pF3d simulations, we show that the reflectivity can be explained by a combination of the effects of inverse bremsstrahlung absorption of the probe beam and the backscattered light, velocity and density gradients, and the large Debye length of the hot plasma. [Preview Abstract] |
Monday, October 24, 2005 10:54AM - 11:06AM |
BO1.00008: Reduction of backscatter in long plasma with beam smoothing in the moderate gain regime Laurent Divol, E.A. Williams, R.L. Berger, D. Hinkel In typical NIF ignition targets, the laser beams propagate through underdense regions many millimeters long. In large regions of these hot underdense plasmas, stimulated Brillouin and Raman scattering instabilities are in a strongly damped regime with linear amplification gains usually below 20 (for intensity). Under these conditions of long plasmas with moderate gains, phase conjugation can greatly enhance the simulated scattering processes by doubling the effective gain. We'll show that polarization smoothing and smoothing by spectral dispersion, as they reduce the beam contrast and coherence, can strongly reduce phase conjugation and thus lower the reflectivity. We'll quantitatively assess the effect for NIF parameters using an analytical approach (with a variational method) and fluid simulations of the backscattering using the laser plasma interaction code pF3d. This work was performed under the auspices of the U.S. Department of Energy by the University of California, Lawrence Livermore National Laboratory under contract No. W-7405-Eng-48. UCRL-ABS-213869 [Preview Abstract] |
Monday, October 24, 2005 11:06AM - 11:18AM |
BO1.00009: Laser-plasma interactions in NIF ignition targets D.E. Hinkel, E.A. Williams, P.A. Amendt, D.A. Callahan, O.S. Jones, S.M. Pollaine Recent ignition point designs for achieving ignition in 2010 at the National Ignition Facility (NIF) have been developed at Lawrence Livermore National Laboratory. These four designs use approximately 1 MJ of input laser energy, have ``cocktail'' walls, and contain some combination of gas fill, liner, foam fill or shine shields in the interior of the target. An analysis of the laser-plasma interactions in these targets is presented. Levels of laser scatter and beam spray are estimated from post-processing radiation-hydrodynamics simulations of the various point designs. NIF beams propagating through the generated plasma blow-off of these designs will be simulated, using pF3D, to further quantify the degree of laser scatter and spray. The role of nonlinear saturation of laser scatter via wave-wave and wave-particle interactions will be addressed. [Preview Abstract] |
Monday, October 24, 2005 11:18AM - 11:30AM |
BO1.00010: Cavity Formation and Collapse in Stimulated Brillouin Scattering Jay Fahlen, F.S. Tsung, W.B. Mori Linear theory of stimulated Brillouin scattering (SBS) predicts higher laser reflectivities than are observed in experiments. Recently, Weber, Riconda, and Tikhonchuk (PRL \textbf{94}, 055005 (2005)) showed that one dimensional particle-in-cell (PIC) simulations of SBS in the strongly coupled regime develop long-lived density cavities after the occurrence of X-type wavebreaking. These density cavities are several laser wavelengths long and are capable of trapping laser energy within them. This poster presents one and two dimensional OSIRIS simulation results on strongly coupled SBS. The 1D results are in general agreement with the results of Weber et al.; however the 2D results indicate that in multi-dimensions the cavities exist for shorter times and their collapse strongly heats the plasma. These simulations demonstrate that cavity formation and collapse from SBS generates fast electrons and may be partly responsible for the reduction in laser reflectivity. The importance of electron kinetics to SBS for NIF conditions will also be discussed. [Preview Abstract] |
Monday, October 24, 2005 11:30AM - 11:42AM |
BO1.00011: Vlasov simulations of Raman scattering from homogeneous and inhomogeneous plasmas D.J. Strozzi, A. Bers, E.A. Williams, A.B. Langdon We have performed kinetic simulations of stimulated Raman scattering (SRS) using the 1-D Vlasov code ELVIS [D. J. Strozzi et al., \emph{Comput. Phys. Comm.} \textbf{164}, 156 (2003)]. For electron plasma waves (EPWs) with $k\lambda_{D} > 0.3$ electron trapping increases the backward SRS reflectivity over linear values, as reported by others [H. X. Vu et al., \emph{Phys. Rev. Lett.}, \textbf{86}, 4306 (2001)]. The enhancement takes place for both mobile or fixed ions. The electric field $(k,\omega )$ spectrum shows the plasma waves are down-shifted in $\omega$ from the linear dispersion curve. This downshift is correlated with large EPW amplitude and phase-space vortices in the electron distribution, and is likely due to trapping. The scattered light comes in temporal bursts. Finite-extent pulses of plasma waves are generated near the laser entrance and propagate in the direction of the laser. Forward SRS and Raman re-scatter of back SRS also occur. In an inhomogeneous plasma, the damping reduction due to trapping allows the plasma waves to propagate along the density gradient, rather than developing only near the resonance point. The detuning due to inhomogeneity does not prevent high reflectivity once trapping occurs. \\ $^{\ast}$Work at LLNL performed under auspices of U.S. Dept. of Energy by University of California, LLNL contract W-7405-Eng-48. [Preview Abstract] |
Monday, October 24, 2005 11:42AM - 11:54AM |
BO1.00012: Evidence of a Nonlinear Dispersion Relation for Langmuir Waves D.S. Montgomery, J.L. Kline Langmuir waves (LW) are driven to large, nonlinear levels by stimulated Raman scattering (SRS) in experiments using an intense diffraction-limited laser beam to interact with a preformed plasma. The LW frequency and wave number ($\omega$, k) are inferred both from the SRS scattered light spectrum, and from Thomson scattering of the driven LW. In addition, Thomson scattering from thermal-level ion wave fluctuations is used to infer the time-dependent plasma conditions. In the experiment, the plasma temperature drops from 700 to 500 eV while the LW is driven by SRS, thus providing a time-dependent change of k$\lambda_{D}$. Further evidence indicates that an assumption of constant electron density during this time is valid. Using these plasma conditions, the inferred LW dispersion is compared to the linear kinetic dispersion relation. At k$\lambda_{D}$ $\sim$ 0.34, the LW frequency is $\sim$ 6.5\% lower than that predicted by the usual linear dispersion relation. However, ($\omega$, k) inferred from the experimental data compare favorably to the nonlinear dispersion relation of Rose and Russell [Phys. Plasmas 8, 4784 (2001)] for a LW average amplitude e$\Phi /T_{e} \sim$ 0.6. [Preview Abstract] |
Monday, October 24, 2005 11:54AM - 12:06PM |
BO1.00013: Control of collective FSBS and backscatter SRS through plasma composition Harvey Rose, Pavel Lushnikov Nominal NIF parameters are near the collective forward SBS (FSBS) threshold (P. M. Lushnikov and H. A. Rose, Phys. Rev. Lett. \textbf{92}, 255003 (2004), ``L{\&}R''). It will be shown that being on this instability edge can be used as a control lever: a small amount of high Z dopant may lead to qualitative change in FSBS regime at fixed laser intensity, possibly reducing backscatter instability losses (Such results have already been observed, but absent SSD, a key aspect of our theory: R. M. Stevenson et al., Phys. Plasmas \textbf{11}, 2709 (2004); L. J. Suter et al., 2738, ib.). P\textit{onderomotive }FSBS regimes are determined by the parameter $\tilde {I}=F^2\left( {{\mbox{v}_{\mbox{osc}} } \mathord{\left/ {\vphantom {{\mbox{v}_{\mbox{osc}} } {\mbox{v}_\mbox{e} }}} \right. \kern-\nulldelimiterspace} {\mbox{v}_\mbox{e} }} \right)^2{\left( {{n_e } \mathord{\left/ {\vphantom {{n_e } {n_c }}} \right. \kern-\nulldelimiterspace} {n_c }} \right)} \mathord{\left/ {\vphantom {{\left( {{n_e } \mathord{\left/ {\vphantom {{n_e } {n_c }}} \right. \kern-\nulldelimiterspace} {n_c }} \right)} \nu }} \right. \kern-\nulldelimiterspace} \nu $, with \textit{$\nu $} the dimensionless ion acoustic damping coefficient and $F$ the optic f/{\#}. Analytical results will be presented which show a decrease of $\tilde {I}{\kern 1pt}\mbox{'s}$ threshold value through the addition of high Z dopant to low Z plasma, owing to increased \textbf{thermal contribution} to FSBS. Alternatively, one may raise the threshold by managing the value of \textit{$\nu $}$_{ }$by, $e.g.$, adding He to SiO$_{2}$. For nominal NIF parameters, a range of He fraction in SiO$_{2}$ plasma is predicted to suppress backscatter SRS while maintaining control of forward SBS. [Preview Abstract] |
Monday, October 24, 2005 12:06PM - 12:18PM |
BO1.00014: Observation of amplification of a 1ps pulse by SRS of a 1 ns pulse in a plasma with conditions relevant to pulse compression Robert Kirkwood, E Dewald, S.C. Wilks, N. Meezan, C. Niemann, L. Divol, R.L. Berger, O.L. Landen, J. Wurtele, A.E. Charman, R. Lindberg, N.J. Fisch, V.M. Malkin We have observed the amplification of a 1 ps, 1200 nm, probe pulse when counter propagating with a 1ns, 1 x 10$^{15}$ W/cm$^{2}$, 1064 nm pump pulse, in a He gas plasma created by the pump. When the gas and plasma density is adjusted to match the resonance condition for the probe to seed the stimulated Raman scattering (SRS) of the pump ($\sim $ 1 x 10$^{19}$ e/cm$^{3})$ the transmitted probe energy is enhanced by 20x to 30x its value off resonance, and as much as 4 mJ of energy is transferred. This is the first demonstration that a 1ns pump beam can significantly amplify an ultra short pulse by SRS in a plasma that can survive irradiation by the pump, and is therefore attractive for compression of the pump when the interaction length is increased. Experiments both at reduced pump intensity, and with an 1124 nm wavelength probe interacting in a 2.5 x 10$^{18}$ e/ cm$^{3}$ plasma, show a strong scaling of amplification with the resonant density and probe wavelength, and a weaker scaling with pump intensity. This work was performed under the auspices of the U.S. Department of Energy by Univ. of California, Lawrence Livermore National Laboratory under Contract W-7405-Eng-48. [Preview Abstract] |
Monday, October 24, 2005 12:18PM - 12:30PM |
BO1.00015: Relevance of the adiabatic approximation to kinetically model the stimulated Raman scatter Didier B\'enisti Modeling of the stimulated Raman scatter, at a nonlinear kinetic level, and in a 3D geometry, is investigated theoretically. It is shown that the adiabatic approximation yields a very accurate value of the real part of the plasma susceptibility, $\Xi$, provided that the longitudinal and transverse gradient scale lengths of the fields are not smaller than the laser wavelength. The plasma wave dispersion relation, derived using the adiabatic approximation, is compared, and found different, to that of other models (\textit{e.g.} H.A. Rose and D.A. Russel, Phys. Plasmas, vol. 11, 4787 (2001)). The adiabatic approximation yields $Im(\Xi)=0$. This seems to be wrong for small amplitude electrostatic waves which are expected to experience Landau damping. However, it is shown that the electronic distribution function, and hence $Im(\Xi)$, are non local quantities. Their values depend on the maximum amplitude of the electrostatic field experienced by the electrons. This non local property, and the fact the laser light is made of lots of speckles, can be used together to show the reduction of the non collisional dissipation rate of energy for small amplitude waves, once the maximum electrostatic field amplitude is large enough. This renders the adiabatic approximation valid in the whole plasma, even in those regions where the electrostatic field amplitude is small. [Preview Abstract] |
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