Bulletin of the American Physical Society
58th Annual Meeting of the APS Division of Plasma Physics
Volume 61, Number 18
Monday–Friday, October 31–November 4 2016; San Jose, California
Session UO9: Laser Plasma Interactions I |
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Chair: David Turnbull, University of Rochester Room: 211 CD |
Thursday, November 3, 2016 2:00PM - 2:12PM |
UO9.00001: pF3D Simulations of SBS and SRS in NIF Hohlraum Experiments Steven Langer, David Strozzi, Peter Amendt, Thomas Chapman, Laura Hopkins, Andrea Kritcher, Scott Sepke We present simulations of stimulated Brillouin scattering (SBS) and stimulated Raman scattering (SRS) for NIF experiments using high foot pulses in cylindrical hohlraums and for low foot pulses in rugby-shaped hohlraums. We use pF3D [R. L. Berger et al., \textit{Phys. Plasmas} \textbf{5}, 4337 (1998)], a massively-parallel, paraxial-envelope laser plasma interaction code, with plasma profiles obtained from the radiation-hydrodynamics codes Lasnex and HYDRA. We compare the simulations to experimental data for SBS and SRS power and spectrum. We also show simulated SRS and SBS intensities at the target chamber wall and report the fraction of the backscattered light that passes through and misses the lenses. Work performed under the auspices of the U.S. Department of Energy by LLNL under Contract DE-AC52-07NA27344. Release number LLNL-ABS-697482. [Preview Abstract] |
Thursday, November 3, 2016 2:12PM - 2:24PM |
UO9.00002: Modeling Laser--Plasma Interactions at Direct-Drive Ignition-Relevant Plasma Conditions at the National Ignition Facility A.A. Solodov, M.J. Rosenberg, J.F. Myatt, R. Epstein, W. Seka, M. Hohenberger, R.W. Short, J.G. Shaw, S.P. Regan, D.H. Froula, P.B. Radha, J.W. Bates, A.J. Schmitt, P. Michel, J.D. Moody, J.E. Ralph, D.P. Turnbull, M.A. Barrios Laser--plasma interaction instabilities, such as two-plasmon decay (TPD) and stimulated Raman scattering (SRS), can be detrimental for direct-drive inertial confinement fusion because of target preheat by generated high-energy electrons. The radiation--hydrodynamics code \textit{DRACO} has been used to design planar-target experiments that generate plasma and interaction conditions relevant to direct-drive--ignition designs ($I_{\mbox{L}} \sim 10^{15}{\mbox{W}} \mathord{\left/ {\vphantom {{\mbox{W}} {\mbox{cm}}}} \right. \kern-\nulldelimiterspace} {\mbox{cm}}^{2},\mbox{\thinspace }T_{\mbox{e}} >3\mbox{\thinspace keV},$ density gradient scale lengths of $L_{\mbox{n}} \sim 600\,\mu \mbox{m})$. The hot-electron temperature of $\sim 40\mbox{\thinspace to\thinspace 50\thinspace keV}$ and the fraction of laser energy converted to hot electrons of $\sim 0.5\mbox{\thinspace to\thinspace 2.3\% }$ were inferred based on comparing the simulated and experimentally observed x-ray emission when the laser intensity at the quarter-critical surface increased from $\sim 6\mbox{\thinspace to\thinspace }15\mbox{\thinspace }\times \mbox{\thinspace }10^{14}{\mbox{W}} \mathord{\left/ {\vphantom {{\mbox{W}} {\mbox{cm}^{2}}}} \right. \kern-\nulldelimiterspace} {\mbox{cm}^{2}}.$ The measured SRS energy was sufficient to explain the observed total energy in hot electrons. Implications for ignition-scale direct-drive experiments and hot-electron preheat mitigation using mid-$Z$ ablators will be discussed. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0001944. [Preview Abstract] |
Thursday, November 3, 2016 2:24PM - 2:36PM |
UO9.00003: Stimulated Raman Scattering in Direct-Drive Inertial Confinement Fusion Plasmas W. Seka, M.J. Rosenberg, W. Theobald, J.F. Myatt, A.V. Maximov, R.W. Short, S.P. Regan, P. Michel, C.S. Goyon, J.D. Moody Stimulated Raman scattering (SRS) is clearly visible in all planar and spherical direct-drive National Ignition Facility experiments. They are also visible in high-intensity OMEGA experiments with comparable \textit{IL}$_{\mathrm{n}}$ products ($I$ and $L$ are the intensity and density scale length near the quarter-critical surface). The two experimental platforms are complementary and provide information regarding single-beam and multibeam SRS. Experimental evidence and rough estimates of the levels of SRS will be shown and discussed. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0001944. [Preview Abstract] |
Thursday, November 3, 2016 2:36PM - 2:48PM |
UO9.00004: SRS analyses of direct-drive ICF experiments at the National Ignition Facility P. Michel, M. Rosenberg, J. Myatt, A. Solodov, W. Seka, T. Chapman, M. Hohenberger, L. Masse, C. Goyon, D. Turnbull, S. Regan, J.D. Moody A series of planar target experiments was recently conducted at the National Ignition Facility (NIF) to study the laser-plasma interactions processes responsible for the production of suprathermal electrons, and their scaling from experiments at the Omega facility to full-scale ICF experiments at the MJ level on the NIF [1]. We will present experimental analyses and simulations of Stimulated Raman Scattering (SRS) in these planar target experiments. Our work indicates the presence of purely backscattered SRS refracted off nearly one-dimensional density gradients, as well as more complicated features such as side-scatter and scattering from non-1D features (e.g. edges) in the target. Simulations using ray- and paraxial-wave- based simulation codes are used to extrapolate the hot electron fraction from the SRS measurements, and point to SRS being the primary mechanism for the generation of suprathermal electrons in these experiments. We will also present analyses of spherical implosions experiments and provide extrapolations and implications for future full-scale direct-drive experiments at NIF. [1]: M. Rosenberg et al., this conference. [Preview Abstract] |
Thursday, November 3, 2016 2:48PM - 3:00PM |
UO9.00005: Relative Significance of the Stimulated Raman Scattering and Two-Plasmon--Decay Instabilities at Quarter-Critical Density R.W. Short, H. Wen, A.V. Maximov, J.F. Myatt, W. Seka In direct-drive experiments on OMEGA, correlated signals of half-harmonic light and hot-electron production have usually been ascribed to two-plasmon decay (TPD). However, as scale lengths and temperatures increase, absolute stimulated Raman scattering (SRS) is expected to play a larger role in generating hot electrons and half-harmonic light. This may already be occurring in more-recent OMEGA experiments.\footnote{W. Seka \textit{et al}., Phys. Rev. Lett. \textbf{112}, 145001 (2014). } Both instabilities occur at quarter-critical density, and for obliquely incident light, they can merge into a ``hybrid'' instability with a threshold differing from SRS and TPD thresholds considered separately.\footnote{ B. B. Afeyan and E. A. Williams, Phys. Plasmas \textbf{4}, 3845 (1997).} This talk analyzes how the thresholds of the quarter-critical instabilities vary with the incidence angle and polarization of the incident light, as well as the plasma parameters, and the expected significance for direct-drive experiments. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0001944. [Preview Abstract] |
Thursday, November 3, 2016 3:00PM - 3:12PM |
UO9.00006: Laser--Plasma Interaction Near the Quarter-Critical Density in Direct-Drive Inertial Confinement Fusion A.V. Maximov, H. Wen, J.F. Myatt, R.W. Short, C. Ren The laser--plasma interaction (LPI) near the quarter-critical density in direct-drive inertial confinement fusion (ICF)\footnote{ J. F. Myatt\textit{ et al}., Phys. Plasmas \textbf{21}, 055501 (2014).}$^{\mathrm{\thinspace }}$plasmas strongly influences the coupling of laser energy to the target and the generation of fast electrons capable of preheating the target fuel. The full modeling of LPI near the quarter-critical density includes the interplay between two-plasmon decay and stimulated Raman scattering instabilities as well as ion-acoustic perturbations. The results of the kinetic particle-in-cell simulations are in agreement with the simulation results from the fluid-type code.\footnote{ H. Wen\textit{ et al}., Phys. Plasmas \textbf{22}, 052704 (2015).\par } The fast-electron flux and the $\omega $/2 half-omega light spectra are calculated for the parameters relevant to direct-drive ICF experiments on the OMEGA Laser System and at the National Ignition Facility. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0001944. [Preview Abstract] |
Thursday, November 3, 2016 3:12PM - 3:24PM |
UO9.00007: Three-dimensional simulations of the angular and polarization dependence of stimulated Brillouin backscattering from NIF hohlraums Richard Berger, A. B. Langdon, C. A. Thomas, K. L. Baker, C. S. Goyon, D. P. Turnbull The National Ignition Facility (NIF) groups its 192 beams in 48 quads, 2/3 of which are 'outer' beams and 1/3 'inner' beams. Half of the outer quads are focused at the laser entrance hole (LEH) at an mean angle of $44^{\circ} $ and the other half at $50^{\circ}$ with respect to the hohlraum axis. The majority of the stimulated Brillouin scatter (SBS) is reflected into the $50^{\circ} $ quads, and most of that into the $52^{\circ}$ beams. That observation we reproduce with our simulations that use the wave propagation code, pF3D.[\textit{Berger et al., Phys.Plasmas {\bf5}, 4337 (1998)}] Simulations considered a number of different pulse shapes, wall materials, capsule materials, and initial fill gas density with the plasma properties taken from 2D cylindrically-symmetric, radiation-hydrodynamic simulations of the hohlraum, capsule included. The simulations predict that different hohlraum designs have different fractions, between 20\% and 50\%, of the total SBS reflected into the backscattered light collection optics (the so-called FABS). The amount of light backscattered outside of FABS is not currently measured but is assumed to be 70\% of the light backscattered. That assumption is a reasonable but not accurate estimate. [Preview Abstract] |
Thursday, November 3, 2016 3:24PM - 3:36PM |
UO9.00008: The Effect of Cross-Beam Energy Transfer on Two-Plasmon Decay in Direct-Drive Implosions D.H. Froula, R.K. Follett, R.J. Henchen, A.K. Davis, V.N. Goncharov, D.H. Edgell, A.A. Solodov, D.T. Michel, J.F. Myatt, J.G. Shaw, C. Stoeckl Mitigation of cross-beam energy transfer (CBET) in direct-drive implosions was shown to increase the hot electrons generated by two-plasmon decay. Reducing the diameter of the laser spots by 30{\%} significantly reduces CBET and the laser absorption was measured to increase from 75{\%} to nearly 90{\%}.\footnote{D. H. Froula \textit{et al}., Phys. Rev. Lett. \textbf{108}, 125003 (2012). } The reduced CBET leads to higher intensity at the quarter-critical density surface, increasing the hot-electron production by a factor of $\sim {\kern 1pt}7.$ Adding a thin layer (0.6 to 1.1 $\mu $m) of Si to the target ablator reduced the hot-electron fraction by a factor of $\sim {\kern 1pt}2.$ Spatially resolved Thomson-scattering measurements show an $\sim {\kern 1pt}15\% $ increase in the electron temperature and an increase in the Si fraction at the quarter-critical surface when the Si layer is added. Three-dimensional laser--plasma interaction simulations of hot-electron production using the code \textit{LPSE} show that in addition to the reduced gain (smaller ${IL_{\mbox{n}} } \mathord{\left/ {\vphantom {{IL_{\mbox{n}} } {T_{\mbox{e}} }}} \right. \kern-\nulldelimiterspace} {T_{\mbox{e}} })$, the observed reduction in hot electrons results from increased electron--ion collision frequencies and reduced Landau damping of ion-acoustic waves.\footnote{R. K. Follett\textit{ et al.}, Phys. Rev. Lett. \textbf{116}, 155002 (2016).\par } This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0001944. [Preview Abstract] |
Thursday, November 3, 2016 3:36PM - 3:48PM |
UO9.00009: Experimental Investigation of Cross-Beam Energy Transfer Mitigation via Wavelength Detuning in Directly Driven Implosions at the National Ignition Facility M. Hohenberger, J.A. Marozas, P.W. McKenty, M.J. Rosenberg, P.B. Radha, D. Cao, J.P. Knauer, S.P. Regan Cross-beam energy transfer (CBET) affects directly driven, inertial confinement fusion implosions by reducing the absorbed light and the coupling of driver energy to the target. A mitigation strategy is to detune the laser wavelength of interacting beams $\left( {\Delta \lambda \ne 0} \right)$ to reduce the CBET interaction volume. In polar-direct-drive (PDD) experiments at the National Ignition Facility (NIF) the CBET-imposed energy losses occur predominantly in the equatorial region. The NIF does not support a hemispheric wavelength detuning but does have $\Delta \lambda $ capabilities between inner and outer quads. Using a north--south asymmetric beam pointing, it is therefore possible to introduce a hemispheric wavelength difference of up to $\Delta \lambda =4.6\mbox{\thinspace }{\AA}$ in the UV. We report on experiments to test this CBET mitigation scheme in PDD experiments on the NIF. Using this asymmetric beam pointing, we have completed experiments with both $\Delta \lambda =0$ and 4.6 {\AA}. The effect of CBET on the driver--target coupling is diagnosed via implosion velocities, implosion shape, and scattered-light spectra and by comparing experimental data to 2-D \textit{DRACO} simulations. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0001944. [Preview Abstract] |
Thursday, November 3, 2016 3:48PM - 4:00PM |
UO9.00010: Comparing Ray-Based and Wave-Based Models of Cross-Beam Energy Transfer R.K. Follett, D.H. Edgell, J.G. Shaw, D.H. Froula, J.F. Myatt Ray-based models of cross-beam energy transfer (CBET) are used in radiation--hydrodynamics codes to calculate laser-energy deposition. The accuracy of ray-based CBET models is limited by assumptions about the polarization and phase of the interacting laser beams and by the use of a paraxial Wentzel--Kramers--Brillouin (WKB) approximation. A 3-D wave-based solver (\textit{LPSE}-CBET) is used to study the nonlinear interaction between overlapping laser beams in underdense plasma. A ray-based CBET model\footnote{I. V. Igumenshchev\textit{ et al.}, Phys. Plasmas \textbf{19}, 056314 (2012).} is compared to the wave-based model and shows good agreement in simple geometries where the assumptions of the ray-based model are satisfied. Near caustic surfaces, the assumptions of the ray-based model break down and the calculated energy transfer deviates from wave-based calculations. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0001944. [Preview Abstract] |
Thursday, November 3, 2016 4:00PM - 4:12PM |
UO9.00011: Three-Dimensional Modeling of Polarization Effects on Cross-Beam Energy Transfer in OMEGA Implosions D.H. Edgell, R.K. Follett, J. Katz, J.F. Myatt, J. Shaw, D.H. Froula Beamlet spot images are used to diagnose cross-beam energy transfer (CBET) during OMEGA direct-drive implosions. The spots are, in essence, the end point of beamlets of light originating from different regions of each beam profile and following paths determined by refraction. The intensity of each spot varies because of absorption and CBET along that path. When each beam is linearly polarized, the image is asymmetric in terms of spot intensities. A 3-D CBET postprocessor for hydrodynamics codes is used to model the intensity, wavelength, and polarization of light from each beam. Rotation of polarization caused by CBET is tracked. The model is benchmarked using a 3-D wave-based solver for simplified CBET geometries. For linearly polarized beams in OMEGA implosions, the model predicts that polarization effects will result in asymmetric polarization and unabsorbed light profiles that are different for each beam. An asymmetric beamlet spot image similar to that recorded is predicted by the CBET model for linearly polarized beams. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0001944. [Preview Abstract] |
Thursday, November 3, 2016 4:12PM - 4:24PM |
UO9.00012: A new criterion to describe crossed-beam energy transfer in laser-plasma interactions R. Trines, H. Schmitz, E.P. Alves, F. Fiuza, J. Vieira, L.O. Silva, R. Bingham Crossed-beam energy transfer (CBET) between laser beams in underdense plasma is ubiquitous in both direct-drive and indirect-drive inertial confinement fusion. To understand the impact of this process on the final shape of the laser beams involved, as well as their imprint on either hohlraum walls or target surface, a detailed spatial and temporal description of the crossing beams is needed. We have developed an analytical model and derived new criteria describing both the spatial structure and temporal evolution of the beams after crossing. Numerical simulations have been carried out justifying the analytical model and confirming the criteria. The impact of our results on present and future multi-beam experiments in laser fusion and high-energy-density physics, in particular the "bursty" nature of beams predicted to occur in NIF experiments, will be discussed. [Preview Abstract] |
Thursday, November 3, 2016 4:24PM - 4:36PM |
UO9.00013: Cross-beam energy transfer to a single f-20 beam: simulations of previous and upcoming experiments Thomas Chapman, David Turnbull, Robert Kirkwood, Pierre Michel, Scott Wilks, Richard Berger, Denise Hinkel, John Moody, Steve Langer, Bruce Langdon, David Strozzi Motivated by materials research applications, cross-beam energy transfer can be used to transfer energy from one or more quads of beamlets at the NIF, which have an effective $f$-number of 8, to a single $f$-20 beam. Using plasma comprised of a preheated C$_5$H$_{12}$ gasbag, a preliminary experiment at the NIF demonstrated amplification of a 750 J $f$-20 beam by a factor of 2 in both power and energy. A witness plate providing gated x-ray images was used to obtain total energies and transmitted spot intensities for the pump quad, seed beamlet, and a calibration quad. These experimental diagnostics offer the opportunity to perform quantitative comparisons with simulations. We use the laser-plasma interaction code pF3D to simulate the energy transfer process, using plasma conditions obtained from the plasma hydrodynamics code HYDRA. Our simulations of the completed single-pump quad experiment recover the measured seed amplification and transmitted spot power distributions. We also show simulation results for the upcoming two-pump quad experiment. [Preview Abstract] |
Thursday, November 3, 2016 4:36PM - 4:48PM |
UO9.00014: Characterization of the hot electron population with bremsstrahlung and backscatter measurements at the National Ignition Facility Felicie Albert, Matthias Hohenberger, Pierre Michel, Laurent Divol, Tilo Doeppner, Edward Dewald, Benjamin Bachmann, Joseph Ralph, David Turnbull, Clement Goyon, Cliff Thomas, Otto Landen, John Moody In indirect-drive ignition experiments, the hot electron population, produced by laser-plasma interactions, can be inferred from the bremsstrahlung generated by the interaction of the hot electrons with the target. At the National Ignition Facility (NIF), the upgraded filter-fluorescer x-ray diagnostic (FFLEX), a 10-channel, time-resolved hard x-ray spectrometer operating in the 20- to 500-keV range, provides measurements of the bremsstrahlung spectrum. It typically shows a two-temperature distribution of the hot electron population inside the hohlraum. In SRS, where the laser is coupled to an electron plasma wave, the backscattered spectrum, measured with the NIF full-aperture backscatter system (FABS), is used to infer the plasma wave phase velocity. We will present FFLEX time-integrated and time-resolved measurements of the hot electron population low-temperature component. We will correlate them with electron plasma wave phase velocities inferred from FABS spectra for a range of recent shots performed at the National Ignition Facility. [Preview Abstract] |
Thursday, November 3, 2016 4:48PM - 5:00PM |
UO9.00015: Evaluation of the Fast-Electron Source Function for Two-Plasmon Decay from Temporal Hard X-Ray Emission J.A. Delettrez, R.K. Follett, J.F. Myatt, C. Stoeckl The modeling of the fast-electron transport in the 1-D hydrodynamic code \textit{LILAC} requires the description of the source electrons as a function of time. The particle-in-cell code \textit{OSIRIS} and the interaction code \textit{FPSE} provide some guidance but have not provided an algorithm for the energy fraction from the laser pulse as the coronal parameters change with time. The original algorithm, based on the measured hard x-ray (HXR) emission as a function of laser intensity, depended exponentially on the two-plasmon--decay threshold parameter up to about 0.9 and saturates above it. This algorithm along with \textit{FPSE} simulations produced HXR emissions much earlier than observed. Analysis of the measured HXR emissions from implosions with near-constant threshold parameter values show that the rise time of the emission can be described with an exponential curve with roughly a rise time of 200 ps. Trial and error set the start of the rise at the threshold value of 0.75. Causes for this rise time will be discussed. Comparison between measured and computed HXR emissions for different implosion scenarios will be presented, including those for cryogenic targets. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0001944. [Preview Abstract] |
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