Bulletin of the American Physical Society
57th Annual Meeting of the APS Division of Plasma Physics
Volume 60, Number 19
Monday–Friday, November 16–20, 2015; Savannah, Georgia
Session JO5: Laser Plasma Interactions I |
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Chair: David Turnbull, Lawrence Livermore National Laboratory Room: 200 |
Tuesday, November 17, 2015 2:00PM - 2:12PM |
JO5.00001: Measurements of the Conduction-Zone Length and Mass Ablation Rate to Study the Hydrodynamic Coupling in Cryogenic Direct-Drive Implosions on OMEGA D.T. Michel, A.K. Davis, V.N. Goncharov, S.P. Regan, T.C. Sangster, R. Epstein, S.X. Hu, I.V. Igumenshchev, D.D. Meyerhofer, W. Seka, D.H. Froula The ablation-front trajectory and the averaged mass ablation rate is measured in direct-drive cryogenic target implosions on the OMEGA Laser System by imaging the soft x rays emitted by the coronal plasma. The length of the conduction zone is determined by coupling x-ray and scattered-light measurements. These measurements are compared to hydrodynamic simulations to study the modeling of the hydrodynamic coupling for various beam and target radii. Reducing the beam focal-spot radius relative to the target radius is a method that is being studied to reduce cross-beam energy transfer and increase the hydrodynamic efficiency. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0001944. [Preview Abstract] |
Tuesday, November 17, 2015 2:12PM - 2:24PM |
JO5.00002: Heat-Flux Measurements from Collective Thomson-Scattering Spectra R.J. Henchen, S.X. Hu, J. Katz, D.H. Froula, W. Rozmus Collective Thomson scattering was used to measure heat flux in coronal plasmas. The relative amplitude of the Thomson-scattered power into the up- and downshifted electron plasma wave features was used to determine the flux of electrons moving along the temperature gradient at three to four times the electron thermal velocity. Simultaneously, the ion-acoustic wave features were measured. Their relative amplitude is used to measure the flux of the return-current electrons. The frequencies of these ion-acoustic and electron plasma wave features provide local measurements of the electron temperature and density. These spectra were obtained at five locations along the temperature gradient in a laser-produced blowoff plasma. These measurements of plasma parameters are used to infer the Spitzer$-$H\"{a}rm flux $\left( {q_{\mbox{SH}} =-\kappa \nabla T_{\mbox{e}} } \right)$ and are in good agreement with the values of the heat flux measured from the scattering-feature asymmetries. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0001944. [Preview Abstract] |
Tuesday, November 17, 2015 2:24PM - 2:36PM |
JO5.00003: Plasma photonics in ICF {\&} HED conditions Pierre Michel, David Turnbull, Laurent Divol, Bradley Pollock, Cecilia Y. Chen, Eleanor Tubman, Clement S. Goyon, John D. Moody Interactions between multiple high-energy laser beams and plasma can be used to imprint refractive micro-structures in plasmas via the lasers' ponderomotive force. For example, Inertial confinement fusion (ICF) experiments at the National Ignition Facility already rely on the use of plasma gratings to redirect laser light inside an ICF target and tune the symmetry of the imploded core. More recently, we proposed new concepts of plasma polarizer and waveplate, based on two-wave mixing schemes and laser-induced plasma birefringence. In this talk, we will present new experimental results showing the first demonstration of a fully tunable plasma waveplate, which achieved near-perfect circular laser polarization. We will discuss further prospects for novel ``plasma photonics'' concepts based on two- and four-wave mixing, such as optical switches, bandpass filters, anti-reflection blockers etc. These might find applications in ICF and HED experiments by allowing to manipulate the lasers directly in-situ (i.e. inside the targets), as well as for the design of high power laser systems. [Preview Abstract] |
Tuesday, November 17, 2015 2:36PM - 2:48PM |
JO5.00004: Diagnosing Cross-Beam Energy Transfer Using Beamlets of Unabsorbed Light from Direct-Drive Implosions D.H. Edgell, R.K. Follett, V.N. Goncharov, I.V. Igumenshchev, J. Katz, J.F. Myatt, W. Seka, D.H. Froula A new diagnostic is now being fielded to record the unabsorbed laser light from implosions on OMEGA. Unabsorbed light from each OMEGA beam is imaged as a distinct ``spot'' in time-integrated images. Each spot is, in essence, the end point of a beamlet of light that originates from a specific region of a beam profile and follows a path determined by refraction. The intensity of light in the beamlet varies along that path because of absorption and cross-beam energy transfer (CBET) with other beamlets. This diagnostic allows for the detailed investigation of the effects of CBET on specific locations of the beam profile. A pinhole can be used to isolate specific spots, allowing the time-resolved spectrum of the beamlet to be measured. A fully 3-D CBET hydrodynamics code postprocessor is used to model the intensity and wavelength of each beamlet as it traverses the coronal plasma to the diagnostic. The model predicts that if a single beam in a symmetric implosion is turned off, the recorded intensity of nearby spots will decrease by $\sim$ 15\% as a result of loss of CBET from the dropped beam. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0001944. [Preview Abstract] |
Tuesday, November 17, 2015 2:48PM - 3:00PM |
JO5.00005: Improved Wavelength Detuning Cross-Beam Energy Transfer Mitigation Strategy for Polar Direct Drive at the National Ignition Facility J.A. Marozas, T.J.B. Collins, P.W. McKenty, J.D. Zuegel Cross-beam energy transfer (CBET) reduces absorbed light and implosion velocity, alters time-resolved scattered-light spectra, and redistributes absorbed and scattered light. These effects reduce target performance in both symmetric direct-drive and polar-direct-drive (PDD) experiments on the OMEGA Laser System and the National Ignition Facility (NIF). The CBET package (\textit{Adaawam}) incorporated into the 2-D hydrodynamics code \textit{DRACO} is an integral part of the 3-D ray-trace package (\textit{Mazinisin}). The CBET exchange occurs primarily over the equatorial region in PDD, where successful mitigation strategies concentrate. Detuning the initial laser wavelength (d$\lambda _{\mathrm{0}})$ reduces the CBET interaction volume, which can be combined with other mitigation domains (e.g., spatial and temporal). By judiciously selecting the ring and/or port $\pm \mbox{d}\lambda_{0} $ in each hemisphere, using new \textit{DRACO} diagnostic abilities, improved wavelength detuning strategies trade-off overall energy absorption for improved hemispherical energy balance control. These balanced-wavelength detuning strategies improve performance for high-convergence implosions. Simulations (2-D \textit{DRACO}) predict improved implosion performance and control in both the shell trajectory and morphology for planned intermediate PDD experiments on the NIF. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0001944. [Preview Abstract] |
Tuesday, November 17, 2015 3:00PM - 3:12PM |
JO5.00006: Inline Modeling of Cross-Beam Energy Transfer and Raman Scattering in NIF Hohlraums David Strozzi, D.S. Bailey, C.A. Thomas, S.M. Sepke, G.D. Kerbel, P. Michel, L. Divol, O.S. Jones Inline models of cross-beam energy transfer (CBET) and stimulated Raman Scattering (SRS) have been added to the radiation-hydrodynamics codes Hydra and Lasnex. Both processes are important in hohlraums with high gas fill density, particularly for implosion symmetry. Coupled-mode equations are solved along laser ray paths for both models. The inline model shows the SRS gain rate exceeds that of SRS light absorption along most of the laser ray path, and most SRS light escapes the target. Most SRS-driven Langmuir wave power is deposited slightly inside the laser entrance hole (LEH), which reduces how much inner-beam power reaches the equator. This also makes the LEH hotter, which affects CBET. Compared to removing SRS power from the incident laser, the inline SRS model does not change total x-ray drive but makes the drive stronger from the poles than the equatorial waist. This reduces the need to artificially clamp CBET in order to match implosion shape data, which has historically been needed for high gas fill hohlraums. We are applying the models to a set of NIF shots with varying gas fill densities. [Preview Abstract] |
Tuesday, November 17, 2015 3:12PM - 3:24PM |
JO5.00007: Cross-Beam Energy Transfer Driven by Incoherent Laser Beams with Frequency Detuning A. Maximov, J.F. Myatt, R.W. Short, I.V. Igumenshchev, W. Seka In the direct-drive method of the inertial confinement fusion (ICF), the coupling of laser energy to target plasmas is strongly influenced by the effect of cross-beam energy transfer (CBET) between multiple driving laser beams.\footnote{J. F. Myatt \textit{et al}., Phys. Plasmas \textbf{21}, 055501 (2014).} The laser$-$plasma interaction (LPI) model of CBET is based on the nonparaxial laser light propagation\footnote{A. V. Maximov \textit{et al}., Phys. Plasmas \textbf{11} 2994 (2004).} coupled with the low-frequency ion-acoustic-domain plasma response. Common ion waves driven by multiple laser beams play a very important role in CBET. The effect of the frequency detuning (colors) in the driving laser beams is studied and it is shown to significantly reduce the level of common ion waves and therefore the level of CBET. The differences between the LPI-based CBET model and the ray-based CBET model used in hydrocodes\footnote{I. V. Igumenshchev \textit{et al}., Phys. Plasmas \textbf{19}, 056314 (2012).} are discussed. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0001944. [Preview Abstract] |
Tuesday, November 17, 2015 3:24PM - 3:36PM |
JO5.00008: Evaluation of Wavelength Detuning to Mitigate Cross-Beam Energy Transfer Using the Nike Laser P.W. McKenty, J.A. Marozas, J. Weaver, S.P. Obenschain, A.J. Schmitt Cross-beam energy transfer (CBET) has become a serious threat to the overall success of direct-drive experiments, and especially for polar-direct-drive (PDD) ignition experiments. CBET redirects incident laser light before it can be absorbed into the target, thereby degrading overall target performance. CBET is particularly detrimental over the equator of the target, which is hydrodynamically very sensitive to such losses in the PDD configuration. A promising solution uses laser wavelength detuning between beams to shift the resonance, thereby reducing the interaction cross section between them. Testing this process for direct drive is now underway at the Nike laser at the Naval Research Laboratory. Calculations evaluating the effect CBET has on the scattered-light signals indicate such an experiment will demonstrate the benefits of wavelength detuning for direct-drive implosions. Two-dimensional simulation results will be presented, predicting the effect for both spherical and cylindrical experiments. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0001944. [Preview Abstract] |
Tuesday, November 17, 2015 3:36PM - 3:48PM |
JO5.00009: Signatures of Stimulated Raman and Brillouin Scattering in Direct-Drive National Ignition Facility Experiments W. Seka, M.J. Rosenberg, M. Hohenberger, J.F. Myatt, J.G. Shaw, A.A. Solodov, R.W. Short, P.B. Radha, S.P. Regan Stimulated Raman and Brillouin scattering (SRS and SBS, respectively) have been identified in polar-direct-drive (PDD) implosion experiments at the National Ignition Facility. The SBS spectra primarily represent cross-beam energy transfer with low SBS gain and good agreement between experiments and simulations have been obtained. The SRS spectra are also multibeam interactions and reflect the evolution of density scale length in the corona. Two-dimensional \textit{DRACO} simulations of these experiments have been used to generate simulated SBS spectra and SRS gains. A quantitative comparison of the simulations with the experimental results will be presented. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0001944. [Preview Abstract] |
Tuesday, November 17, 2015 3:48PM - 4:00PM |
JO5.00010: Modifying the Kinetic Behavior of Stimulated Raman Scattering with External Magnetic Fields B.J. Winjum, A. Tableman, F.S. Tsung, W.B. Mori We show the effect of an external magnetic field ($B_0$) on stimulated Raman scattering (SRS) in the kinetic regime using particle-in-cell simulations. 1D simulations (with three velocity components for particle motion) are sufficient to show that orienting $B_0$ perpendicular to the laser propagation direction can reduce SRS reflectivity. We show the effect of $B_0$ on trapped particle motion and on local heating. In 2D simulations of single- and multi-speckled laser beams, trapped particles can be restricted to, or freed from, speckles and local bursts of SRS activity by $B_0$. $B_0$ collinear with the laser propagation direction acts to align trapped particles with the daughter electron plasma wave (EPW) in SRS, which can both limit collective speckle interactions and make 2D SRS more 1D-like. On the other hand, $B_0$ perpendicular to the laser propagation direction acts to deflect trapped particles transversely across the daughter EPW and to dynamically change the population of particles that are resonant with the EPW, disrupting the nonlinear wave-particle effects on EPWs. This acts to decrease SRS reflectivity. Hot electron motion is restricted for either orientation, but to different effect with regard to local heating, SRS recurrence, and speckle interactivity. [Preview Abstract] |
Tuesday, November 17, 2015 4:00PM - 4:12PM |
JO5.00011: Exploiting the self-similar nature of Raman and Brillouin amplification R. Trines, E.P. Alves, R.A. Fonseca, L.O. Silva, E. Webb, F. Fiuza, R.A. Cairns, R. Bingham, P. Norreys Raman and Brillouin amplification are two schemes for amplifying and compressing short laser pulses in plasma. Depending on the laser and plasma configurations, these schemes could potentially deliver the high-energy high-power pulses needed for inertial confinement fusion, especially fast-ignition fusion. Analytical self-similar models for both Raman and Brillouin amplification have already been derived, but the consequences of this self-similar behavior are little known and hardly ever put to good use. In this talk, we will give an overview of the self-similar laws that govern the evolution of the probe pulse in Raman and Brillouin amplification, and show how these laws can be exploited, in particular regarding the parameters of the initial probe pulse, to control the properties of the final amplified probe and improve the efficiency of the process. [Preview Abstract] |
Tuesday, November 17, 2015 4:12PM - 4:24PM |
JO5.00012: Kinetically driven Raman scattering in short, bi-speckle laser-plasma interaction experiments Kevin Glize, Christophe Rousseaux, Sophie Baton, Vincent Dervieux, Livia Lancia In order to investigate collective speckles behavior in laser-plasma interaction, bi-speckle experiments have been performed using the ELFIE facility (LULI). Two independent laser pulses (1.06 nm, 1.5 ps FWHM) interact with preformed He plasma (0.06 nc, 300 eV). The first beam drives stimulated Raman scattering, while the second, which its intensity is set near SRS threshold, is focused near the first one (typically 90 $\mu$m). The interaction, with crossed and parallel polarization, was studied for both variation of the time delay and the lateral distance between the two pulses, featuring a highly resolved Thomson-scattering diagnostic and backward Raman imaging. It is shown that the kinetic perturbations are of primary importance on triggering SRS in the weak speckle, which exhibits SRS instability up to an expectedly long time delay after the interaction of the strong one. The experimental results will be discussed with the help of 2D PIC simulations (CALDER code). [Preview Abstract] |
Tuesday, November 17, 2015 4:24PM - 4:36PM |
JO5.00013: Lowering the risk of stimulated Brillouin backscatter from NIF hohlraums by re-pointing beams Richard Berger, K.L. Baker, C.A. Thomas, J.L. Milovich, A.B. Langdon, D.J. Strozzi, M. Michel The 64 beams that make a 50 degree angle with the hohlraum axis have been measured to reflect by Stimulated Brillouin Backscatter (SBS) enough laser light to cause optical damage and limit design parameter space. The amount of backscatter has been seen to depend on the initial plasma density filling the hohlraum, the hohlraum wall material, and the laser pulse length. The most important parameter causing SBS is the laser intensity on the hohlraum wall. In previous hohlraum designs, the intensity of the 50 degree beams has been controlled by cross-beam energy transfer (CBET). [\textit{P. Michel, et al. Phys. Rev. Lett. {\bf 102} 025004 (2009)}] Recent designs with reduced CBET have experienced an increase in SBS. Here we show that repointing beams can reduce the laser intensity at the wall and still maintain good beam smoothing. The reduction in intensity is achieved by separating the 44 and 50 degree cones of beams along the hohlraum axis and then repointing beams within each cone to reduce overlap while preserving polarization smoothing. PF3D simulations show dramatic reductions of SBS are possible. Experiments will determine whether increased laser entrance hole sizes will be required and whether this technique will open up new design options. [Preview Abstract] |
Tuesday, November 17, 2015 4:36PM - 4:48PM |
JO5.00014: ABSTRACT WITHDRAWN |
Tuesday, November 17, 2015 4:48PM - 5:00PM |
JO5.00015: Stimulated Brillouin Scattering in Shock Ignition Liang Hao, Jun Li, Wenda Liu, Rui Yan, Chuang Ren We study laser-plasma interactions and hot electron generation for shock ignition using both fluid and PIC simulations. Typical parameters for OMEGA experiments are used with a density scale length of 170 $\mu $m and a pulse length of $\sim$ 15 ps. A series of simulations with laser intensities between 2 $\times$ 10$^{15}$ and 5 $\times$ 10$^{16}$ W/cm$^{2}$ finds that stimulated Brillouin scattering (SBS) increases significantly with the incident intensity, limiting the transmitted intensity at the 0.17n$_{\mathrm{c}}$ to be under 3 $\times$ 10$^{15}$ W/cm$^{2}$. It is also found that proper modeling of the SBS reflectivity requires realistic flow profiles and seed levels for the electromagnetic fields. The majority of the hot electrons are found to be from stimulated Raman scattering and of moderate energies. However, high energy electrons of preheating threat can still be generated from the two-plasmon-decay instability. [Preview Abstract] |
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