Bulletin of the American Physical Society
55th Annual Meeting of the APS Division of Plasma Physics
Volume 58, Number 16
Monday–Friday, November 11–15, 2013; Denver, Colorado
Session BO4: Laser-Plasma Instabilities |
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Chair: John Sethian, Naval Research Laboratory Room: Plaza D |
Monday, November 11, 2013 9:30AM - 9:42AM |
BO4.00001: LPI studies with grazing incidence irradiation at the Nike laser J. Weaver, D. Kehne, A. Schmitt, S. Obenschain, V. Serlin, J. Oh, R. Lehmberg, J. Seely Studies of laser plasma instabilities (LPI) at the Nike laser facility at NRL have previously concentrated on planar targets irradiated with their surface normal aligned to the central axis of the laser. Shots with planar targets rotated up 60$^{\circ}$ to the laser have shown changes in thresholds for the two-plasmon decay instability and stimulated Raman scattering near the quarter critical region. In the case of rotated low-Z targets, spectra were observed to shift to lower wavelength and were substantially stronger in the visible and ultraviolet spectral ranges. The low-Z target data show growth at an incident intensity slightly below ($\sim$30{\%}) the threshold values observed at normal incidence. A rapid rise in signal level over the same laser intensities was also observed in the hard x-ray data which serve as an overall indicator of LPI activity. Shots with rotated planar high-Z targets showed that the visible and ultraviolet emissions dropped significantly when compared to low-Z targets in the same geometry. This presentation will include results from upcoming experiments to determine the LPI signal for low-Z, high-Z, and high-Z coated targets at lower laser intensities for several angles of target rotation. Shots with widely separated laser beams are also planned to explore cross beam energy transport at Nike. [Preview Abstract] |
Monday, November 11, 2013 9:42AM - 9:54AM |
BO4.00002: Measurements of Electron Density Profiles of Plasmas Produced by Nike KrF Laser for Laser Plasma Instability (LPI) Research Jaechul Oh, J.L. Weaver, S.P. Obenschain, A.J. Schmitt, D.M. Kehne, M. Karasik, L-Y. Chan, V. Serlin, L. Phillips Knowing spatial profiles of electron density ($n_e$) in the underdense coronal region ($n < n_c/4$) of plasma is essential to understanding LPI initiation in inertial confinement fusion research. In the recent Nike LPI experiment, a side-on grid imaging refractometer (GIR)\footnote{R.S. Craxton, et al, Phys. Fluids B 5, 4419 (1993)} was deployed for measuring the underdense plasma profiles. Plasmas were produced from flat CH targets illuminated by Nike KrF laser with total energies up to 1 kJ of 0.5 $\sim$ 1 nsec FWHM pulses. The GIR resolved $n_e$ up to $3\times 10^{21}/cm^3$ in space taking 2D snapshot images of probe laser ($\lambda = 263 nm$, $\Delta t = 10 ps$) beamlets ($50\mu m$ spacing) refracted by the plasma at a selected time during the laser illumination. The individual beamlet transmittances were also measured for $T_e$ estimation. Time-resolved spectrometers with an absolute-intensity-calibrated photodiode array and a streak camera simultaneously detected light emission from the plasma in spectral ranges relevant to Raman (SRS) and two plasmon decay instabilities.\footnote{J. Oh, et al, GO5.4, APSDPP (2010)} The measured spatial profiles are compared with simulation results from the FAST3D radiation hydrocode and their effects on the LPI observations are investigated. [Preview Abstract] |
Monday, November 11, 2013 9:54AM - 10:06AM |
BO4.00003: Thermal effects on the electron density fluctuations in ICF plasmas Wojciech Rozmus, T. Chapman, R. Berger, A. Brantov, V. Bychenkov, M. Tzoufras We have examined modifications of the electron distribution functions (EDF) due to thermal gradients in the ignition-scale ICF plasmas. In particular, given the high background temperatures of such plasmas the heat-carrying electrons have energies (20 -- 40 keV) that are close to kinetic energies of the electrons that are resonant with Langmuir waves produced by parametric instabilities, such as stimulated Raman scattering. We have found that under these conditions the modifications of the EDF introduce anisotropy in the plasma response that manifests itself in the significant reduction (increase) of the Landau damping of Langmuir waves propagating along (against) the temperature gradient. Similarly there is strong anisotropy in the fluctuation spectra of the electron plasma waves that modifies Thomson scattering cross-section. The EDF have been calculated and compared using the standard Spitzer-Harm theory, numerical solutions to the Fokker-Planck equations and analytical solutions of the kinetic equation. An impact of this theory on the observations of scattering instabilities and Thomson scattering experiments in ICF plasmas will be discussed. [Preview Abstract] |
Monday, November 11, 2013 10:06AM - 10:18AM |
BO4.00004: Nonlinearity of the Two--Plasmon Decay Instability as Reflected in the Half-Harmonic Emission from Imploding Direct-Drive Targets W. Seka, J.F. Myatt, J. Zhang, R.W. Short, D.H. Froula, D.T. Michel, A.V. Maximov, D.F. DuBois, D.A. Russell, H.X. Vu The half-harmonic spectra from imploding direct-drive targets show clear evidence of the absolute two-plasmon--decay (TPD) instability near $n_{\mathrm{c}}$/4. This is followed by a rapid nonlinear evolution to lower densities covering the convective TPD regime all the way to the Landau cutoff. This evolution is not temporally resolved by our diagnostics but the spectral signatures are clear evidence for the nonlinearity. Simulations using a 3-D Zakharov code support these conclusions. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0001944. [Preview Abstract] |
Monday, November 11, 2013 10:18AM - 10:30AM |
BO4.00005: Three-Dimensional Modeling of the Two-Plasmon--Decay Instability and Stimulated Raman Scattering Near the Quarter-Critical Density in Plasmas H. Wen, A.V. Maximov, R. Yan, C. Ren, J.F. Myatt, W.B. Mori Three-dimensional simulations of laser--plasma interactions have been performed in the plasma region near the quarter-critical density using the particle-in-cell (PIC) code \textit{OSIRIS}. For parameters\footnote{R. Yan, A. V. Maximov, and C. Ren, Phys. Plasmas \textbf{17}, 052701 (2010).} relevant to the direct-drive inertial confinement fusion experiments, two-plasmon decay (TPD) and stimulated Raman scattering (SRS) have been observed to coexist in the same region. Both instabilities contribute to the generation of fast electrons. A new fluid model including both TPD and SRS has been developed. The growth rates of absolutely unstable TPD and SRS modes in the linear regime of this model are in agreement with analytical theory.\footnote{A. Simon\textit{ et al.}, Phys. Fluids \textbf{26}, 3107 (1983).}$^{\mathrm{,}}$\footnote{J. F. Drake and Y. C. Lee, Phys. Rev. Lett. \textbf{31}, 1197 (1973).} This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0001944. [Preview Abstract] |
Monday, November 11, 2013 10:30AM - 10:42AM |
BO4.00006: Two-Plasmon Decay Driven by Multiple Finite Bandwidth Laser Beams J. Zhang, J.F. Myatt, A.V. Maximov, R.W. Short, D.F. DuBois, D.A. Russell, H.X. Vu Multiple laser beams can cooperatively drive two-plasmon decay (TPD) through the sharing of common Langmuir waves, leading to low-instability thresholds. This is potentially important for inertial confinement fusion because of the ability of TPD to produce hot electrons. The effect of finite laser bandwidth and multiple beam colors on cooperative TPD has been investigated with an extended Zakharov model\footnote{D. F. DuBois, D. A. Russell, and H. A. Rose, Phys. Rev. Lett. \textbf{74}, 3983 (1995); D. A. Russell and D. F. DuBois, Phys. Rev. Lett. \textbf{86}, 428 (2001).} with a view to mitigating the instability for ignition-scale (National Ignition Facility) polar-drive interaction parameters. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0001944. [Preview Abstract] |
Monday, November 11, 2013 10:42AM - 10:54AM |
BO4.00007: Measurements of the Two-Plasmon-Decay Generated, Hot-Electron Fraction as a Function of the Quarter-Critical Density Scale Length D.H. Edgell, D. Haberberger, S.X. Hu, D.T. Michel, J.F. Myatt, D.H. Froula Simulations suggest that direct-drive implosions on the National Ignition Facility (NIF) will develop density scale lengths of the order of 500 $\mu $m that lead to large two-plasmon--decay (TPD) common-wave gains. The scale length was varied on OMEGA EP experiments using targets with different radii of curvature, including some negative curvature (concave) targets that achieved NIF density scale length. By varying the target curvature, the density scale length (L) was increased from 100 $\mu $m to 500 $\mu $m while maintaining a nearly constant ratio of laser intensity (I) to electron temperature (T). The TPD threshold is predicted to scale as T/(IL). Over this range, the fraction of laser energy converted to hot electrons is measured to increase rapidly from 0.005{\%} to 2.1{\%}. These results will be compared to the gains calculated by a 3-D multibeam TPD model. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0001944. [Preview Abstract] |
Monday, November 11, 2013 10:54AM - 11:06AM |
BO4.00008: Ultraviolet Thomson Scattering from Two-Plasmon--Decay Driven Electron Plasma Waves at Quarter-Critical Densities R.K. Follett, D.T. Michel, S.X. Hu, J.F. Myatt, R.J. Henchen, J. Katz, D.H. Froula Thomson scattering (TS) was used to probe electron plasma waves (EPW's) driven by the two-plasmon--decay (TPD) instability near quarter-critical density. TPD-driven EPW's were observed at densities consistent with the common-wave TPD model. Five laser beams $\left( {\lambda {\kern 1pt}_{3\omega } =351\mbox{\thinspace nm}} \right)$produced 400-$\mu $m-diam (FWHM) laser spots with overlapped intensities up to $3\times 10^{14}\;\mbox{W/cm}^{2}.$ A 263-nm TS beam was used to probe densities ranging from 0.18 to 0.26 $n_{\mathrm{c}}$, where $n_{\mathrm{c}}$ is the critical density for 351-nm light. The experimental geometry was chosen to match the five-beam TPD common wave $k$ vector. The TS spectrum shows a large amplitude, narrow ($\sim $1.6-nm FHWM) feature centered around 423.4 nm. This wavelength corresponds to scattering from EPW's with a normalized wave vector $k \mathord{\left/ {\vphantom {k {k_{3\omega } }}} \right. \kern-\nulldelimiterspace} {k_{3\omega } }=1.3,$ a density of $n_{\mathrm{e}}$/$n_{\mathrm{c}} =$ 0.243, and a temperature of $T_{\mathrm{e}} =$ 2 keV. This is consistent with the predicted values given by the dispersion relations and TPD maximum growth hyperbola. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0001944. [Preview Abstract] |
Monday, November 11, 2013 11:06AM - 11:18AM |
BO4.00009: The Effects of Beam Geometry and Polarization on Two-Plasmon Decay Driven by Multiple Laser Beams R.W. Short, J.F. Myatt, J. Zhang It is now well established that in direct-drive geometries, two-plasmon decay (TPD) is a collective process, in which a given set of decay waves is driven by several laser beams.\footnote{ C. Stoeckl\textit{ et al.}, Phys. Rev. Lett. \textbf{90}, 235002 (2003).}$^{\mathrm{,}}$\footnote{ D. T. Michel\textit{ et al.}, Phys. Rev. Lett. \textbf{109}, 155007 (2012).} The single-beam decay is maximized on a hyperbola in $k$ space, so that maximum convective gain for the multibeam process occurs near the intersection of the hyperbolas corresponding to the beams involved. These hyperbolas intersect at the origin in $k$ space, so TPD is most strongly driven in this region and can be absolute there. This small-$k$ absolute instability is expected to dominate the linear phase of TPD growth, which is found to be consistent with Zakharov simulations.\footnote{ J. Zhang\textit{ et al.}, ``Two-plasmon decay driven by multiple finite bandwidth laser beams,'' this conference.} This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0001944. [Preview Abstract] |
Monday, November 11, 2013 11:18AM - 11:30AM |
BO4.00010: Analysis of nonlinear stimulated Raman scattering for NIF hohlraum with Diamond ablators Lin Yin, Brian Albright, Richard Berger, Ben Bergen, Kevin Bowers New capsule designs with Diamond ablators have recently been considered for NIF experiments. The new designs with diamond capsule require much shorter laser pulses ($\sim 12$ns) and more mass is ablated from Diamond capsule, resulting in higher electron temperature $T_e$ and density $n_e$ at peak laser power and leading to different hohlraum plasma conditions (i.e., different $k \lambda_{D}$ values where $k$ is the wave number of the electron plasma waves and $ \lambda_{D}$ is the Debye length). Multi-speckled VPIC simulations at mm-scale size are used to assess the stimulated Raman scatter (SRS) risk of new diamond-ablator designs on the NIF by modeling SRS in the hohlraum using plasma conditions from the Hydra simulations. We will present VPIC results of SRS reflectivity, hot electron energy and flux from regions with various $k \lambda_{D}$ values, and SRS spectra that peak at wavelengths below 600 nm. [Preview Abstract] |
Monday, November 11, 2013 11:30AM - 11:42AM |
BO4.00011: Mitigating Stimulated Raman Scattering in Hohlraum Plasmas Using Magnetic Insulation D.S. Montgomery, B.J. Albright, J.L. Kline, L. Yin, P.Y. Chang, J.R. Davies, G. Fiksel, D.H. Froula, R. Betti, M.J. MacDonald Controlling stimulated Raman scattering (SRS) in hohlraum plasmas is important for achieving high-gain inertial fusion using indirect drive. Experiments at the National Ignition Facility (NIF) suggest that coronal electron temperatures in NIF hohlraums may be cooler than initially thought due to efficient thermal conduction from the under dense low-Z plasma to the dense high-Z hohlraum wall [1]. This leads to weaker Landau damping and stronger growth of SRS. Magnetic insulation of the heat conducting electrons can occur when the Hall parameter $\omega_{ce}\tau_{ei}$ $\gg$ 1, where $\omega_{ce}$ is the electron-cyclotron frequency, and $\tau_{ei}$ is the electron-ion collision time. For NIF laser-plasma conditions, it is shown that a 10-T external magnetic field may substantially reduce cross-field transport and may increase coronal plasma temperatures, thus increasing linear Landau damping and mitigating SRS. We will present calculations and simulations supporting this concept, and will present initial results from Omega experiments using gas-filled hohlraums with external B-fields up to 10-T.\\[4pt] [1] M.D. Rosen et al., High Energy Density Phys. \textbf{7}, 180 (2011). [Preview Abstract] |
Monday, November 11, 2013 11:42AM - 11:54AM |
BO4.00012: Effects of Early Time Stimulated Brillouin Backscatter on Recent NIF Ignition Experiments Joseph Ralph, John Moody, Debra Callahan, Denise Hinkel, Omar Hurricane, Richard Town, David Strozzi, Jose Milovich, Bruce Remington, Laura Berzak Hopkins, Edward Dewald, Thomas Dittrich, Hye-Sook Park, Harry Robey, Laurent Divol, Pierre Michel, Steven Ross, David Turnbull, Sebastien Le Pape, Robin Benedetti, Brian MacGowan, Otto Landen, John Kline Recent NIF ignition hohlraum experiments are focusing on two indirect-drive platforms, higher adiabat CH capsule implosions and high density carbon capsule implosions These designs require both a higher density hohlraum gas-fill as well as an early-time increase in laser power compared to the low adiabat CH designs. Experiments on these recent designs have shown backscatter within the first 2 ns of the laser pulse which is primarily Stimulated Brillioun Scatter (SBS). This SBS is observed from the outer laser cone beams incident near the laser entrance hole with very little SBS scatter from the inner cone beams. The backscattered power level reaches about 30{\%} in some cases and is observed to be a function of the wavelength difference between the inner cones and the outer cones used to tune the implosion symmetry suggesting a dependence on early time cross beam energy transfer. We will present the backscatter results and discuss the implications of the scatter on capsule implosion behavior. 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] |
Monday, November 11, 2013 11:54AM - 12:06PM |
BO4.00013: The numerical study of nonlinear instabilities in ICF plasmas Chunyang Zheng, Liang Hao, Zhanjun Liu Convective Raman or Brillouin amplification and competition between them in inhomogeneous, drifting plasmas are discussed. Based on a five-wave model, relations of the backscattering reflectivity between Raman and Brillouin are deduced, and the influence of inhomogeneity, drift, damping to the pattern of competition is analyzed. For the given plasma conditions and noise source, the backscatter spectrum can be constructed. Recent gas-filled hohlraum experiments at the SHEN-GUANG laser facility show good agreement between measured reflectivity and our model predictions. The features of side-scatter in overlapping laser beams are also studied. We observe that backward Raman scattering level of one pump beam can be significantly enhanced when the second beam crosses it. This can be explained as the induced stimulated Raman side scattering, and the scattering wave of the first beam is as the seed light of the second beam. The variance of Raman scattering level with the different crossing angles is investigated. An extended five-wave model including nonlinear damping and detuning of plasma wave is under consideration. [Preview Abstract] |
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