Bulletin of the American Physical Society
49th Annual Meeting of the Division of Plasma Physics
Volume 52, Number 11
Monday–Friday, November 12–16, 2007; Orlando, Florida
Session NO6: Laser-Plasma Coupling at Long Scale Length |
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Chair: Robert Kirkwood, Lawrence Livermore National Laboratory Room: Rosen Centre Hotel Salon 5/6 |
Wednesday, November 14, 2007 9:30AM - 9:42AM |
NO6.00001: LPI Risk Mitigation on NIF Using Larger Radius Hohlraums Paul Bradley, D.C. Wilson, D. Callahan, L.J. Suter, M.J. Edwards As part of the laser-plasma instability (LPI) risk mitigation strategy for ignition at the National Ignition Facility, we performed capsule/hohlraum calculations where we made the hohlraum 20{\%} larger in radius than a standard 300 eV design. The hope is the larger radius would reduce the plasma electron density and thereby reduce the LPI gains to an acceptable level. We find that although the electron density is lower, this is offset by by the increased intensity needed to heat the larger hohlraum, so there is little improvement in the LPI gains. Also, relatively more power is required in the outer cone in order to obtain an implosion symmetry good enough for ignition in our calculations. There appears to be little advantage to making the hohlraum larger in radius without taking additional steps to mitigate LPI gain. Work supported by US DOE/NNSA, performed at LANL, operated by LANS LLC under Contract DE-AC52-06NA25396. [Preview Abstract] |
Wednesday, November 14, 2007 9:42AM - 9:54AM |
NO6.00002: An Assessment of Laser-Plasma Instabilities in NIF Ignition Hohlraums with Different Capsule Ablators. R.P.J. Town, D.A. Callahan, L. Divol, M.J. Edwards, S.W. Haan, D.E. Hinkel, D.D. Ho, O.S. Jones, P. Michel, L.J. Suter, E.A. Williams The NIF ignition point design uses a cryogenic DT fuel enclosed within a copper-doped beryllium ablator. The capsule is placed in a gold-uranium cocktail hohlraum driven to a peak drive temperature of 285eV. As part of a system optimization study we are examining two alternative ablator materials: high-density carbon, and germanium-doped plastic. High-density carbon, for a given capsule size, absorbs most x-ray energy. Changing the ablator material alters the plasma conditions inside the hohlraum, consequently modifying the laser-plasma interactions (LPI). We report on LASNEX simulations of hohlraums with these three ablator materials, quantifying the bulk plasma conditions, and use them to estimate the relative risk for LPI. [Preview Abstract] |
Wednesday, November 14, 2007 9:54AM - 10:06AM |
NO6.00003: Modeling energy transfer via beam crossing in NIF hohlraums Pierre Michel, Laurent Divol, Ed Williams, Debbie Callahan The interference between two laser beams crossing in a plasma can efficiently excite an ion acoustic wave, which can drive forward Stimulated Brillouin Scattering between the beams and transfer energy from one to the other. In NIF, multiple beams will cross at the laser entrance hole of the hohlraum. The energy transfer may affect symmetry, but can be controlled through a planned tuning of the frequencies of the beams. We used our paraxial laser-plasma interaction code SLIP to study the energy transfer between laser beams for typical NIF target designs. SLIP uses a linear kinetic coupling between the laser beams and the ion acoustic wave. We will present results on the typical energy transfer between NIF beams, and on the optimum set of parameters that minimize the transfer and optimizes symmetry. [Preview Abstract] |
Wednesday, November 14, 2007 10:06AM - 10:18AM |
NO6.00004: Hohlraum Hot-Electron Production S.P. Regan, T.C. Sangster, D.D. Meyerhofer, W. Seka, B. Yaakobi, R.L. McCrory, C. Stoeckl, V.Yu. Glebov, N.B. Meezan, W.L. Kruer, L.J. Suter, E.A. Williams, O.S. Jones, D.A. Callahan, M.D. Rosen, O.L. Landen, S.H. Glenzer, C. Sorce, B.J. MacGowan The coupling of laser energy into hot $e^{-}$'s was investigated for Au hohlraums on OMEGA using the hard-x-ray diagnostic. Forty beams smoothed with phase plates and arranged in three cones irradiated vacuum, SiO$_{2}$-lined, and gas-filled targets with a 14-kJ PS26 pulse shape. Two bursts of x rays were observed from gas-filled hohlraums. The first ($T_{h} \quad \sim $ 100 keV) occurs as the LEH window explodes and is likely generated by the 2\textit{$\omega $}$_{pe}$ instability or by cooperatively driven SRS. The second ($T_{h} \quad \sim $ 50 keV) coincides with SRS during the main drive. The hot $e^{-}$ coupling increased with $n_{e}$ from 2 to 9 $\times $ 10$^{20}$cm$^{-3}$ and increases during the main drive of a CH-lined LEH hohlraum if $n_{e }\le $ 4 $\times $ 10$^{20}$ cm$^{-3}$, where $n_{e}$ is the initial $n_{e}$ of the fully ionized gas fill. Vacuum and SiO$_{2}$-lined targets (no LEH window) had a lower-level, single-x-ray burst during the main drive. Quantitative coupling estimates will be given. 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] |
Wednesday, November 14, 2007 10:18AM - 10:30AM |
NO6.00005: Simulations of gas-filled hohlraum targets on the OMEGA laser N.B. Meezan, D.J. Strozzi, R.E. Turner, K. Widmann, E.A. Williams, L.J. Suter, S.P. Regan Recent experiments on the OMEGA laser facility examined x-ray drive and hot-electron generation in gold hohlraums filled with CH gas mixtures of varying densities. Some hohlraums also had CH-lined laser entrance holes. Radiation-hydrodynamics simulations using the code HYDRA show that early-time hot electrons likely originated in the blast-wave driven by the explosion of the hohlraum windows. Hot electrons also occurred during the main drive as the hohlraum bulk plasma approached $n_e = 0.25 n_c$. The plasma waves that generate hot electrons can be driven by Stimulated Raman Scattering (SRS) or by the two-plasmon decay instability. Comparisons between simulations and SRS backscatter data can help determine the source of the hot electrons. These results can inform the design of the laser pulse for NIF ignition hohlraums. [Preview Abstract] |
Wednesday, November 14, 2007 10:30AM - 10:42AM |
NO6.00006: Kinetic modeling of Raman scattering with adiabatic electron response David J. Strozzi, Didier B\'enisti, Laurent Gremillet Nonlinearity due to electron trapping in stimulated Raman scattering (SRS) has been studied analytically and via 1-D Vlasov simulations with the ELVIS code. The adiabatic calculation of the electron susceptibility $\chi$ [D. B\'enisti, L. Gremillet, Phys. Plasmas \textbf{14}, 042304 (2007)] has been used to derive the dispersion relation of an SRS-driven plasma wave (from Re[$\chi$]), and an envelope equation (from Im[$\chi$]). Unlike earlier theories, this dispersion relation reflects both that the wave is driven, and that its phase velocity depends on amplitude. The theory agrees well with the frequency measured in simulations. The driven nature of the plasma wave initially produces a large frequency shift, after which the nonlinearity in Re[$\chi$] dominates. The overall downshift exceeds classical formulas (e.g.\ Morales and O'Neil) when $k\lambda_{De}>$ 0.35. Moreover, the frequency \textbf{and wave number} of the scattered light wave both vary with amplitude, keeping SRS closer to resonance than it would be if only the frequency varied. The detrapping of electrons when the wave amplitude decreases may give electrostatic turbulence: the distribution increases for some velocities above that of the SRS plasma wave. [Preview Abstract] |
Wednesday, November 14, 2007 10:42AM - 10:54AM |
NO6.00007: Saturation of Backward Stimulated Scattering of Laser in Kinetic Regime L. Yin, B.J. Albright, K.J. Bowers, W. Daughton, H.A. Rose Stimulated Raman (SRS) and Brillouin scattering (SBS) are examined in the kinetic regime using particle-in-cell simulations. Wavefront bowing of electron plasma waves (ion-acoustic wave) due to the trapped particle nonlinear frequency shift is observed in the SRS (SBS) regime for the first time, which increases with laser intensity. Self-focusing from trapped particle modulational instability (TPMI) [H. A. Rose, Phys. Plasmas, {\bf 12}, 12318, 2005] is shown to occur in both 2D and 3D SRS simulations. The key physics of SRS saturation is identified as a combination of wavefront bowing, TPMI and self-focusing: Bowing marks the beginning of SRS saturation and self-focusing terminates the SRS pulse, both effects resulting from cancellation of the source term for SRS. Ion acoustic wave bowing also contributes to SBS saturation. Velocity diffusion by transverse modes and rapid loss of hot electrons in regions of small transverse extent formed from self-focusing dissipate the wave energy and increase Landau damping in spite of strong electron trapping that reduces Landau damping initially. The ranges of wavelength and growth rate associated with transverse break-up of the electron plasma waves are also examined in 2D speckle simulations as well as in 2D periodic systems from BGK equilibrium and are compared with theory predictions. [Preview Abstract] |
Wednesday, November 14, 2007 10:54AM - 11:06AM |
NO6.00008: Saturation of Stimulated Raman Scatter in laser speckle by Langmuir wave self-localization Harvey Rose, L. Yin Since the trapped electron Langmuir wave (LW) frequency shift, \textit{$\delta \omega$} $<$ 0 exceeds [1] the frequency shift due to ponderomotive expulsion of plasma density for LW wavenumber \textit{k$\lambda$}$_{D} \quad >$ 0.2, self-localization effects induced by trapped electrons are dominant for short times in hot, under dense, plasma. For SRS originating in laser speckles, with daughter LW wavenumber in an intermediate wavenumber regime, $k$=0.35, we show from both 2D PIC simulations and reduced model calculations that \textit{$\delta \omega $} leads to LW phase front bowing whose curvature increases with wave amplitude, \textit{$\phi $}, and time. Once the bow radius of curvature is smaller than a speckle width, the SRS source oscillates in sign across the speckle, causing SRS saturation. This process is neither unstable nor strongly dissipative: results show reduction of SRS even while LW energy grows. However, as \textit{$\phi $} continues to grow and the trapped electron LW self-focusing threshold exceeded, the LW breaks into filaments [2], causing enhanced rate of loss of trapped electrons and associated increase of Landau damping, followed by rapid demise of the SRS pulse. \newline [1] Harvey A. Rose, Physics of Plasmas \textbf{12}, 012318 (2005) \newline [2] L. Yin, et al., Physics of Plasmas 13, 072701, (2006). [Preview Abstract] |
Wednesday, November 14, 2007 11:06AM - 11:18AM |
NO6.00009: Scattered-Laser-Light Spectroscopy in Direct-Drive Implosion Experiments D.H. Edgell, W. Seka, J.A. Delettrez, R.S. Craxton, V.N. Goncharov, I.V. Igumenshchev, J. Myatt, A.V. Maximov, R.W. Short, T.C. Sangster, R.E. Bahr The time-dependent laser absorption during spherical direct-drive implosions on OMEGA is inferred from scattered-light spectroscopy. We compare measured spectral shifts for different pulse shapes with the shifts predicted using a hydrodynamic code. The predictions vary dramatically with the electron-heat-conduction model. A nonlocal transport model provides the best match to the measurements. The modeling calculates the ``blow-by'' signal from the beam opposite the detector, improving the measurements of total scattered light. Remaining spectral discrepancies suggest nonlinear energy exchange between crossed beams due to stimulated Brillouin scattering. Analogous planar experiments test this hypothesis. This work was supported by the U.S. Department of Energy Office of Inertial Confinement Fusion under Cooperative Agreement DE-FC52-92SF19460. [Preview Abstract] |
Wednesday, November 14, 2007 11:18AM - 11:30AM |
NO6.00010: Two-Plasmon-Decay Instability Driven by Incoherent Laser Irradiation A.V. Maximov, J. Myatt, R.W. Short, W. Seka, C. Stoeckl Two-plasmon-decay (TPD) instability is an important source of hot electrons observed in direct-drive inertial confinement fusion experiments on the OMEGA Laser System.\footnote{ C. Stoeckl \textit{et al}., Phys. Rev. Lett. \textbf{90}, 235002 (2003).} A model for the TPD instability driven by incoherent laser beams in inhomogeneous plasmas has been developed that modifies the results of the three-wave TPD model\footnote{ A. Simon \textit{et al}., Phys. Fluids \textbf{26}, 3107 (1983).} for the instability thresholds. The influence of low-frequency plasma perturbations caused by the beating of electromagnetic and plasma waves on TPD through the modification of the density profile is considered. The developed TPD model is applied for conditions typical of the experiments on OMEGA. This work was supported by the U.S. Department of Energy Office of Inertial Confinement Fusion under Cooperative Agreement DE-FC52-92SF19460. [Preview Abstract] |
Wednesday, November 14, 2007 11:30AM - 11:42AM |
NO6.00011: White light Parametric Instabilities L. Silva Different techniques capable of describing the propagation and modulation instability of partially coherent/incoherent ``white'' light in nonlinear media are based on the paraxial wave approximation. In general, this approach is not valid for instabilities associated with the partially reflected backscattered radiation critical in many laser-plasma scenarios, such as ICF or fast ignition, and astrophysical scenarios. Inclusion of bandwidth/incoherence effects in laser driven parametric instabilities in plasmas can be described, for forward scattering by the standard Wigner-Moyal formalism, or by a generalized Wigner-Moyal statistical theory of radiation, or generalized photon kinetics, formally equivalent to the full wave equation, valid for partially coherent electromagnetic wave propagation in nonlinear dispersive and diffractive media. With this approach a generalized dispersion relation for Stimulated Raman Scattering driven by a partially coherent pump field has been derived, revealing a marked difference between backscattering (three-wave process), and direct forward scattering (four-wave process). This qualitative difference illustrates a fundamental difference between these two processes. The results, which generalize the classic results for plane wave pumps, demonstrate the possibility to control the growth rates of these instabilities by properly using broadband pump radiation fields. [Preview Abstract] |
Wednesday, November 14, 2007 11:42AM - 11:54AM |
NO6.00012: Collective stimulated Brillouin backscatter Pavel Lushnikov, Harvey Rose We develop the statistical theory of linear collective stimulated Brillouin backscatter (CBSBS) in spatially and temporally incoherent laser beam. Instability is collective because it does not depend on the dynamics of isolated hot spots (speckles) of laser intensity, but rather depends on averaged laser beam intensity, optic f/{\#}, and laser coherence time, T$_{c}$. CBSBS has a much larger threshold than a classical coherent beam's in long-scale-length high temperature plasma. It is a novel regime in which T$_{c}$ is too large for applicability of well-known statistical theories (RPA) but T$_{c}$ must be small enough to suppress single speckle processes such as self-focusing. Even if laser T$_{c}$ is too large for \textit{a priori} applicability of our theory, collective forward SBS$^{1}$, perhaps enhanced by high Z dopant, and its resultant self-induced T$_{c}$ reduction, may regain the CBSBS regime. We identified convective and absolute CBSBS regimes. The threshold of convective instability is inside the typical parameter region of NIF designs. Well above incoherent threshold, the coherent instability growth rate is recovered. $^{1}$ P.M. Lushnikov and H.A. Rose, Plasma Physics and Controlled Fusion, 48, 1501 (2006). [Preview Abstract] |
Wednesday, November 14, 2007 11:54AM - 12:06PM |
NO6.00013: Experimental demonstration of optical mitigation techniques for stimulated Brillouin Scattering in ignition relevant plasmas Dustin Froula, Laurent Divol, Nathan Meezan, Richard London, Richard Berger, Sham Dixit, James Ross, Paul Neumayer, Russel Wallace, Larry Suter, Siegfried Glenzer Inertial confinement fusion (ICF) 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 SSD, polarization smoothing, and laser beam defocusing on mitigating stimulated backscatter, filamentation, and beam spray. We measure a factor of 1.8 reduction in the stimulated Brillouin scattering (SBS) when polarization smoothing is applied; no effect on the SBS is observed when up to 3 angstroms of smoothing by spectral dispersion (SSD) is applied. Furthermore, we show that the SBS reflectivity is controlled by either reducing the laser beam intensity or defocusing. The results from these experiments compare well to linear theory as modeled in 3 dimensions by pf3D. [Preview Abstract] |
Wednesday, November 14, 2007 12:06PM - 12:18PM |
NO6.00014: Design of an Experiment to Observe Laser-Plasma Interactions on NIKE L. Phillips, J. Weaver, W. Manheimer, S. Zalesak, A. Schmitt, D. Fyfe, B. Afeyan, M. Charbonneau-Lefort Recent proposed designs (Obenschain et al., Phys. Plasmas 13 056320 (2006)) for direct-drive ICF targets for energy applications involve high implosion velocities combined with higher laser irradiances. The use of high irradiances increases the likelihood of deleterious laser plasma instabilities (LPI) that may lead, for example, to the generation of fast electrons. The proposed use of a 248 nm KrF laser to drive these targets is expected to minimize LPI; this is being studied by experiments at NRL's NIKE facility. We used a modification of the FAST code that models laser pulses with arbitrary spatial and temporal profiles to assist in designing these experiments. The goal is to design targets and pulseshapes to create plasma conditions that will produce sufficient growth of LPI to be observable on NIKE. Using, for example, a cryogenic DT target that is heated by a brief pulse and allowed to expand freely before interacting with a second, high-intensity pulse, allows the development of long scalelengths at low electron temperatures and leads to a predicted 20-efold growth in two-plasmon amplitude. [Preview Abstract] |
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