Bulletin of the American Physical Society
50th Annual Meeting of the Division of Plasma Physics
Volume 53, Number 14
Monday–Friday, November 17–21, 2008; Dallas, Texas
Session NO4: Laser-Plasma Interactions |
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Chair: David S. Montgomery, Los Alamos National Laboratory Room: Reunion B |
Wednesday, November 19, 2008 9:45AM - 9:57AM |
NO4.00001: Saturation of backward stimulated Raman scattering in kinetic regime: Wavefront bowing and trapped particle self-focusing of electron plasma waves L. Yin, B.J. Albright, K.J. Bowers, W. Daughton, H.A. Rose Backward stimulated Raman scattering (SRS) in a single laser speckle is examined in the kinetic regime using 2D/3D PIC simulations. Wavefront bowing of electron plasma waves due to trapped electron nonlinear frequency shift and amplitude-dependent damping is observed under both NIF and Trident experimental conditions. This is followed by the trapped particle modulational instability which evolves nonlinearly into self-focusing. Self-focusing leads to rapid transverse electron plasma wave phase variation and increased rate of loss of trapped electrons and subsequent increase of damping. In 3D this electron plasma wave turbulence may also exhibit loss of angular coherence by formation of a filament necklace, a process not possible in 2D. This reduction of electron plasma wave coherence, and hence reduction of source coherence for backscattered light, and increased damping, fundamentally limit how much backscatter can occur in a laser speckle. (KJB: Guest Scientist. Currently with D. E. Shaw Research, LLC, New York, NY 10036.) [Preview Abstract] |
Wednesday, November 19, 2008 9:57AM - 10:09AM |
NO4.00002: Single speckle SRS threshold as determined by electron trapping, collisions and speckle duration Harvey Rose, William Daughton, Lin Yin, Bruce Langdon Speckle SRS intensity threshold has been shown\footnote{L. Yin \textit{et al}., \textit{Physics of Plasmas} \textbf{15}, 013109 (2008).} to increase with spatial dimension, D, because both diffraction and trapped electron escape rate increase with D, though the net effect is to substantially decrease\footnote{D. S. Montgomery \textit{et al}., \textbf{9}, 2311(2002).} the threshold compared to 1D linear gain calculations. On the other hand, the apparent threshold appears to decrease\footnote{Bruce Langdon \textit{et al}., 38$^{th}$ Anomalous Absorption Conference (2008).} with integration time in PIC simulations. We present an optimum\footnote{Harvey A. Rose, \textit{Physics of Plasmas} \textbf{10}, 1468 (2003).}$^,$\footnote{Harvey A. Rose and L. Yin, \textit{Physics of Plasmas} \textbf{15}, 042311 (2008).}, nonlinearly resonant\footnote{Harvey A. Rose and David A. Russell, \textit{Phys. Plasma} \textbf{8}, 4784 (2001).} calculation of the SRS threshold, taking into account large fluctuations of the SRS seed reflectivity, R$_{0}$. Such fluctuations, absent in 1D, are caused by a gap in the linear reflectivity gain spectrum which leads to an exponential probability distribution for R$_{0}$. While the SRS threshold intensity is of course finite, these fluctuations lead to a decrease of apparent threshold with increasing speckle lifetime. [Preview Abstract] |
Wednesday, November 19, 2008 10:09AM - 10:21AM |
NO4.00003: Kinetic Dispersion of the Langmuir Decay Instability and its Relevance for Ignition Plasmas J.P. Palastro, L. Divol, P. Michel, E.A. Williams, D. Strozzi In indirect drive inertial confinement fusion, laser pulses must propagate through several millimeters of plasma to reach the hohlraum wall. During propagation, the pulse can drive large amplitude electron plasma waves (EPW) via Raman scattering. EPWs can severely inhibit the conversion of pulse energy to x-rays at the hohlraum wall through pump depletion and backscatter of the light. In addition, large amplitude EPWs can heat the plasma through particle trapping and wave breaking, which may result in preheat of the ignition fuel. The Langmuir decay instability (LDI), where a large amplitude EPW decays into a secondary EPW and an ion acoustic wave (IAW), may provide a saturation mechanism for the growth of EPWs, and thus limit both backscatter and plasma heating. Here we calculate a fully kinetic dispersion relation for LDI and compare it to the standard fluid dispersion relation. We find that to lowest order in primary EPW amplitude the kinetic dispersion relation predicts phenomena not captured by fluid dispersion. The kinetic dispersion does, however, reproduce the fluid dispersion for small $k\lambda _d $, where $k$ is the wave number for the incident EPW, and $\lambda _d $ is the Debye length. The relevance of kinetic dispersive effects for ignition plasmas is also presented. [Preview Abstract] |
Wednesday, November 19, 2008 10:21AM - 10:33AM |
NO4.00004: Experimental demonstration of optical mitigation techniques for simulated backscatter in ignition relevant plasmas D.H. Froula, L. Divol, T. Doeppner, R. London, R. Berger, P. Michel, S. Dixit, J.S. Ross, P. Neumayer, N. Meezan, L. Suter, S.H. Glenzer Inertial confinement fusion and high-energy density physics experiments require intense and energetic laser beams to propagate efficiently through long plasmas. A series of experiments performed at Omega will be presented that study the effects of smoothing by spectral dispersion (SSD) and polarization smoothing on the mitigating of both stimulated Brillouin scattering (SBS) and stimulated Raman scattering. We measure a factor of 1.7 reduction in the backscatter when polarization smoothing is applied; no effect on the SBS is observed when SSD with 3 {\AA} bandwidth is applied. An analytical model relevant to inertial confinement fusion plasma conditions shows that the measured reduction in backscatter is not a result of the reduction in beam contrast, but a result of the fact that, on average, only one of the polarizations is amplified over a speckle length at a given transverse location. The results from these experiments compare well to linear theory as modeled in 3 dimensions by pf3D. This work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. [Preview Abstract] |
Wednesday, November 19, 2008 10:33AM - 10:45AM |
NO4.00005: Design and analysis of high-density laser-plasma interaction experiments Richard London, Dustin Froula, Richard Berger, Laurent Divol, Paul Neumayer, Siegfried Glenzer, Laurence Suter Control of laser backscatter is required to achieve ignition with inertial confinement fusion. For indirect drive fusion at the National Ignition Facility (NIF), 351-nm laser beams must propagate through several mm of plasma at electron densities of 1--2x10$^{21}$ cm$^{-3 }$to deposit energy at the hohlraum walls. Experiments are being conducted at the Omega Laser Facility to study propagation in similar plasmas and to validate computer programs used to predict backscatter on NIF. Gas-filled hohlraums have recently been used to emulate low-density NIF plasmas (4-8x10$^{21}$ cm$^{-3}$, Froula et al., Phys. Plasmas\textbf{, 14}, 044705, 2007 and Meezan, et al., ibid, 056304). At higher densities, laser heating of the gas creates density variations that compromise the plasma uniformity. A new hohlraum design that produces more uniform plasma conditions will be described. Predictions of the stimulated Raman and Brillouin backscatter from these targets will be presented and compared to experimental measurements. [Preview Abstract] |
Wednesday, November 19, 2008 10:45AM - 10:57AM |
NO4.00006: Simulation of Stimulated Raman Scattering from Gas-filled Hohlraums Richard Berger, L. Divol, R.A. London, D.H. Froula, L.J. Suter, P. Michel, S. Dixit, S.H. Glenzer A series of experiments performed at Omega Laser Facility have measured the stimulated Raman and Brillouin backscatter and beam spray of 351nm laser light from gas-filled hohlraums at electron densities from .06 N$_{c}$ to 0.13 N$_{c}$ and electron temperature about 3 keV. Experiments with similar plasma conditions were reported recently (Froula \textit{et al.,} Phys. Plasmas\textbf{, 14}, 055705, 2007 and Meezan, \textit{et al. ibid}, 056304). The experiments were designed to have convective gains for SRS between 5 and 30. The gain was varied with the laser intensity and the fill density. Above .09 N$_{c}$ the simulated and measured backscatter is predominately SRS. PF3D simulations will be presented that compute the effects of SSD and polarization smoothing on the onset intensity for significant reflectivity (R $>$ 1{\%}) as well as the intensity and density scaling of the scatter. The simulated and measured reflectivity and beam spray will be compared. . [Preview Abstract] |
Wednesday, November 19, 2008 10:57AM - 11:09AM |
NO4.00007: Stimulated backscatter in a speckled laser beam Edward Williams, Laurent Divol High power lasers used for laser-plasma interaction experiments are commonly equipped with phase plates, which create high intensity speckles in their far field. Independent speckle models for stimulated backscatter have been proposed in which the reflectivity is determined by a convolution over the speckle intensity distribution. Such models might apply when the resonant amplification region extends over no more than a characteristic speckle length. By a variational approach, we show how these results are modified if the amplification region extends over multiple speckle lengths, reducing the contrast of the effective intensity distribution, thereby narrowing the distribution of spatial gains. For SBS, the phase conjugate mode grows at twice the average spatial growth rate. It can dominate over the other modes if the higher growth can overcome the small mode fraction. We compare these analytic estimates with those obtained from our parallel laser-plasma interaction code SLIP which solves the steady state mode coupling equations in 3D. [Preview Abstract] |
Wednesday, November 19, 2008 11:09AM - 11:21AM |
NO4.00008: Self-focusing induced reduction of Stimulated Brillouin Scattering for the case of monospeckle laser beams interacting with a plasma Paul-Edouard Masson-Laborde, Stefan Hueller, Denis Pesme, Pascal Loiseau, Christine Labaune, Heidi Bandulet The mechanism explaining the low level of Stimulated Brillouin Scattering observed in laser-plasma experiments with monospeckle laser beams, carried out at the LULI facility, is studied by means of numerical simulations. For the regime where the beam power is well above the self-focusing critical power, simulations carried out with the codes Harmony2D and HERA-ILP (in 2D and 3D geometry respectively), show time-averaged reflectivities of the order of only a few percent. Because of self-focusing and the filament resonant instability, SBS takes only place in self-focused hot spots located in the low-density front part of the plasma. The shortened hot spot sizes and the steepened flow-profile dramatically reduce SBS. This scenario may also applies to the most intense laser hot spots in a spatially smoothed laser beam. [Preview Abstract] |
Wednesday, November 19, 2008 11:21AM - 11:33AM |
NO4.00009: Three-dimensional steady-state analysis of laser-plasma instabilities in NIF hohlraum designs Pierre Michel, Laurent Divol, Ed Williams, John Palastro, Debbie Callahan, Larry Suter We have developed a three-dimensional steady-state model, SLIP, that can analyze various laser-plasma instabilities (LPI) such as Stimulated Brillouin and Raman scattering, cross-beam energy transfer and cross-beam backscattering enhancement. The electromagnetic waves are modeled with a paraxial approximation, and the laser field is initialized with the electric fields measured on NIF. The plasma waves are described in the linear kinetic regime. Our model includes both Thomson and blackbody noise sources. This tool allows modeling of the propagation of full-scale NIF laser beams over very large volumes (tens of mm$^{3}$), and can assess several important LPI issues in realistic conditions. We will present an application of our model for a detailed analysis of various LPI effects in the most current NIF hohlraum design. This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. [Preview Abstract] |
Wednesday, November 19, 2008 11:33AM - 11:45AM |
NO4.00010: Dependence of simulated laser reflectivity on gain in National Ignition Campaign targets D.E. Hinkel, A.B. Langdon, C.H. Still, D.J. Strozzi, E.A. Williams Ignition targets\footnote{D. A. Callahan, invited talk, APS-DPP, 2008.} designed for the National Ignition Campaign range in radiation temperature from 270 to 300 eV. These high-Z cylinders contain a DT capsule overcoated with either Be or CH, and are filled with H4He. The large laser spots in these designs reduce laser intensity, and hence levels of stimulated backscatter, where laser light resonantly scatters off ion acoustic or electron plasma waves. Gain exponents for backscatter\footnote{D. E. Hinkel, D. A. Callahan, A. B. Langdon, S. H. Langer, C. H. Still, and E. A. Williams, \textit{Phys. Plasmas }\textbf{15}, 056314 (2008).} have been calculated for these ignition targets. Both pF3D$^{3}$ and DEPLETE\footnote{D. J. Strozzi, E. A. Williams, D. E. Hinkel, D. H. Froula, R. A. London, and D. A. Callahan, ``Ray-based calculations of laser backscatter in ICF targets'', submitted, \textit{Phys.} \textit{Plasmas.}} have simulated laser light propagation for these targets. The correlation of reflectivity to gain exponent will be presented and discussed. [Preview Abstract] |
Wednesday, November 19, 2008 11:45AM - 11:57AM |
NO4.00011: Precision Scattered-Laser-Light Spectroscopy in Direct-Drive Implosions D.H. Edgell, W. Seka, J.A. Delettrez, R.S. Craxton, V.N. Goncharov, I.V. Igumenshchev, J.F. Myatt, A.V. Maximov, R.W. Short, T.C. Sangster, R.E. Bahr Analysis of time-resolved, scattered-light spectra from spherical target implosions on OMEGA reveals a discrepancy between the precision measurements and predictions based on hydrocode modeling. The total scattered light measured and the details of the spectral shifts in the scattered light suggest that less power is being absorbed by the target than predicted. Contradictorily, the model accurately predicts ``bang times'' suggesting that laser absorption is well modeled. Nonlinear LPI behavior that differs for portions of the beams penetrating to different depths in the corona may provide an explanation. Cross-beam power transfer and steepening of the density profile at the critical and/or quarter-critical surfaces are investigated in attempts to reconcile the observations. This work was supported by the U.S. Department of Energy Office of Inertial Confinement Fusion under Cooperative Agreement No. DE-FC52-08NA28302. [Preview Abstract] |
Wednesday, November 19, 2008 11:57AM - 12:09PM |
NO4.00012: Different Aspects of the Two-Plasmon-Decay Instability in Direct-Drive ICF W. Seka, D.H. Edgell, J.F. Myatt, A.V. Maximov, R.W. Short, D. Russell, D. DuBois The two-plasmon-decay (TPD) instability is observed in almost all OMEGA implosion experiments through its signature \textit{$\omega $}/2 and 3\textit{$\omega $}/2 emission and frequently through hard x{\-}ray emission above 40 keV. This instability presents many facets, one of which has long been known: the potential target preheat problem associated with the energetic-electron production. Recent evidence suggests that the fast-electron distribution is anisotropic and directed into the target. Progress in understanding the energetics of the instability, i.e., the relative partition of Langmuir wave energy into thermal and suprathermal particles, and the influence of profile steepening near $n_{c}$/4 on laser-energy deposition in spherical implosion experiments will be presented. This work was supported by the U.S. Department of Energy Office of Inertial Confinement Fusion under Cooperative Agreement No. DE-FC52-08NA28302. [Preview Abstract] |
Wednesday, November 19, 2008 12:09PM - 12:21PM |
NO4.00013: Two-Plasmon-Decay Hot-Electron Distributions from Bremsstrahlung Measurements J.F. Myatt, D.H. Edgell, W. Seka, A.V. Maximov, R.W. Short Preheat caused by hot electrons from the two-plasmon-decay (TPD) instability has been implicated in the reduction of fuel areal density \textit{$\rho $R} in direct-drive implosions on OMEGA.$^{ }$Hard x-ray measurements have been performed using a four-channel, hard x-ray detector (HXRD) with surrogate targets at an intensity of 5 $\times $ 10$^{14}$ W/cm$^{2}$. A bremsstrahlung model has been developed assuming that the electrons are produced within a cone of half-angle $\Theta $ and exponentially distributed in energy, characterized by $T_{hot}$. The angular-dependent signals of the HXRD channels are consistent with $T_{hot}$ = 120$\pm $20 keV and a half-angle of $\Theta $~= 10$\pm $5\r{ }. The possible origin of such a distribution and its implications will be discussed. This work was supported by the U.S. Department of Energy Office of Inertial Confinement Fusion under Cooperative Agreement No. DE-FC52-08NA28302. [Preview Abstract] |
Wednesday, November 19, 2008 12:21PM - 12:33PM |
NO4.00014: Observation of LPI Thresholds for the Nike Laser J.L. Weaver, J. Oh, B. Afeyan, M. Charbonneau-Lefort, L. Phillips, J. Seely, D. Kehne, C. Brown, S. Obenschain, A.J. Schmitt, U. Feldman, G. Holland, R.H. Lehmberg, E. Mclean, C. Manka The Nike laser is being used to study thresholds for laser plasma instabilities (LPI) at intensities (10$^{15}$-10$^{16}$ W/cm$^{2})$ relevant to advanced implosion designs for direct drive inertial confinement fusion. The combination of short wavelength (248 nm), large bandwidth (1-2 THz), and beam smoothing by induced spatial incoherence available with this krypton-fluoride laser make these experiments unique among current facilities. This talk will present an overview of results with an emphasis on the two-plasmon decay instability (2$\omega _{p})$. Measurements of x-rays and emission near $^{1}$/$_{2} \quad \omega _{o}$ and $^{3}$/$_{2}$ $\omega _{o}$ harmonics of the laser wavelength have been collected over a wide range of intensities for both solid and foam targets. Data indicate collective multiple-angle driven excitation compatible with previous observations using solid planar targets. [Preview Abstract] |
Wednesday, November 19, 2008 12:33PM - 12:45PM |
NO4.00015: Laser Plasma Instability (LPI) Driven Light Scattering Measurements with Nike KrF Laser J. Oh, J.L. Weaver, D.M. Kehne, S.P. Obenschain, E.A. McLean, R.H. Lehmberg With the short wavelength (248 $nm$), large bandwidth ($1\sim2 $ $THz$), and ISI beam smoothing, Nike KrF laser is expected to have higher LPI thresholds than observed at other laser facilities. Previous measurements using the Nike laser [J. L. Weaver et al, Phys. Plasmas 14, 056316 (2007)] showed no LPI evidence from CH targets up to $I\sim2\times10^{15}$ $W/cm^{2}$. For further experiments to detect LPI excitation, Nike capabilities have been extended to achieve higher laser intensities by tighter beam focusing and higher power pulses. This talk will present results of a recent LPI experiment with the extended Nike capabilities focusing on light emission data in spectral ranges relevant to the Raman (SRS) and Two-Plasmon Decay (TPD) instabilities. The primary diagnostics were time-resolved spectrometers with an absolute-intensity-calibrated photodiode array in $(0.4\sim0.8)\omega_0$ and a streak camera near $0.5\omega_0$. The measurements were conducted at laser intensities of $10^{15}\sim10^{16}$ $W/cm^{2}$ on planar targets of CH solids and RF foams. [Preview Abstract] |
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