Bulletin of the American Physical Society
60th Annual Meeting of the APS Division of Plasma Physics
Volume 63, Number 11
Monday–Friday, November 5–9, 2018; Portland, Oregon
Session JO6: Laser Plasma Interactions |
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Chair: Russell Follett, University of Rochester Room: OCC B115-116 |
Tuesday, November 6, 2018 2:00PM - 2:12PM |
JO6.00001: A Brief History of Backscatter on NIF Nuno Lemos, Jaebum Park, James Ross, Clement S Goyon, Laurent Divol, John D Moody, Pierre A Michel In indirectly-driven inertial confinement fusion (ICF) experiments, understanding how the laser energy is transported and coupled into the hohlraum and capsule is critical for achieving ignition. Laser-plasma instabilities (LPI) play a major role in energy transport, implosion symmetry and nuclear fuel preheat, and have had a significant impact on ICF experiments carried out at the National Ignition Facility since the beginning of the ignition campaign in 2009. The primary LPIs at NIF are cross-beam energy transfer, which moves power between laser cones, and backscatter, in the form of stimulated Brillouin and Raman scattering (SBS and SRS). Precise measurement of the backscattered light will provide information about LPI and quantify the amount of energy that is coupled into the hohlraum. In this presentation we summarize the evolution of backscatter throughout years of operation at NIF and the various designs and platforms used for ICF experiments; we identify the key changes that have reduced backscatter and increased energy coupling into the target. |
Tuesday, November 6, 2018 2:12PM - 2:24PM |
JO6.00002: Nonlinear Transformation of Convective Raman Backscattering Instability to Absolute and Inflation of Laser Reflectivity Chuan Sheng Liu Nonlinear trapped electron effects to transform convective Raman Backscattering Instability to Absolute with greatly enhanced reflectivity is demonstrated. Its relevance to experiments and other nonlinear effects will be discussed. |
Tuesday, November 6, 2018 2:24PM - 2:36PM |
JO6.00003: Saturation of Stimulated Raman Scattering in Inhomogeneous Plasma A. V. Maximov, J. G. Shaw, R. W. Short, J. P. Palastro Stimulated Raman scattering (SRS) in one of the most important laser-plasma interaction processes for the inertial confinement fusion (ICF) because it determines the balance between scattering and absorption, and also because SRS generates fast electrons that can propagate to the core of ICF target.1 SRS can grow as an absolute instability and the instability threshold is well exceeded in ICF experiments at the National Ignition Facility (NIF). A crucial question in ICF research has been how much laser light can propagate through the instability region to higher plasma densities. The SRS instability has been simulated in inhomogeneous plasmas with the laser-plasma simulation environment for the parameters relevant to NIF experiments. The SRS absolute instability has been found to undergo the dynamic saturation in which the electrostatic waves with a broad spectrum are coupled to the Raman and laser light waves, thereby increasing their incoherence. This allows a large fraction of the incident laser power to propagate through the SRS instability region to higher plasma densities. 1 J. F. Myatt et al., Phys. Plasmas 21, 055501 (2014). |
Tuesday, November 6, 2018 2:36PM - 2:48PM |
JO6.00004: Stimulated Brillouin Scattering of Outer Beams in Gas-filled Hohlraums Dong Yang The goal of laser plasma instability (LPI) study in the indirect-drive inertial confinement (ICF) fusion is to minimize its uncertainty during the coupling of laser energy to capsule. Shenguang-III is built to operate at an energy of 180 kJ using 48 UV lasers. Initial 100kJ scale experiments using gas-filled hohlraums exhibits evident beam-to-beam backscatter difference even in the same cone. The focal spot as well as the SRS spectra of multiple beams are studied to elucidate whether the beam condition or the plasma condition might be distinct between different laser beams of the same cone. The SRS spectra suggest the beam-to-beam backscatter difference often correlates with the plasma condition in some shots. Thus a gas-filled hohlraum with one side LEH truncated is employed to study the beam propagation in large, hot and well-defined plasmas. The SRS and SBS backscatter is studied when the outer beam intensity is from 5×1014 W/cm2 to 1.5×1015 W/cm2 while keeping other heaters beams energy constant. The beneficial effects of beam smoothing, the competition of instabilities in large-scale plasmas, the influence of LEH size are also explored in combination with a rad-hydro and LPI simulation. |
Tuesday, November 6, 2018 2:48PM - 3:00PM |
JO6.00005: Multi-beam polarization mixing and its impact on other laser-plasma instabilities in ICF experiments at the National Ignition Facility Pierre Michel, Brian James MacGowan, Malcolm Lazarow, Thomas D Chapman, Laurent Divol, Nuno Lemos, Jaebum Park, John D Moody, Richard L Berger, David Jerome Strozzi, Jonathan S Wurtele Wave mixing between lasers in plasma can lead to power exchange as well as polarization rotation of the overlapping beams. In ICF experiments at the National Ignition Facility (NIF), 96 laser beams with mm-spot-size overlap in a ~mm^3 plasma at the entrance of the “hohlraum” targets, leading to a complex evolution of each beam’s polarization state and power. Calculations including full polarization effects from individual NIF beams can show strong power imbalances between beams after leaving the overlap region, depending on the laser and plasma conditions. The beams calculated to have the highest intensity can subsequently trigger high levels of backscatter as they propagate further inside the target. In particular, stimulated Brillouin scattering (SBS) is of high concern as it is not only deleterious to implosion performance but can also damage optical components in the laser beam line. We will show calculations and experimental observations of high SBS caused by multi-beam interactions and discuss possible mitigation strategies. |
Tuesday, November 6, 2018 3:00PM - 3:12PM |
JO6.00006: Mitigating cross-beam energy transfer in directly-driven inertial-confinement-fusion targets using broadband laser light Jason Bates, Jason Myatt, John Shaw, Russell Follett, James Weaver, Robert Lehmberg, Stephen Obenschain Cross-beam energy transfer (CBET) is a significant energy-loss mechanism in directly-driven inertial-confinement-fusion (ICF) targets. Simulations performed with the wave-based code LPSE suggest that Gaussian laser bandwidths of 2 – 5 THz (corresponding to normalized bandwidths of 0.2% – 0.6%, respectively, at a laser wavelength of 351 nm) are effective at suppressing CBET under realistic plasma conditions [J. Bates et al., Phys. Rev. E 97, 061202(R) (2018)]. Although such values exceed those currently available with high-energy Nd:glass lasers used in ICF research today, effective CBET mitigation could likely be achieved by employing excimer laser drivers, which have native bandwidths in the multiterahertz regime. An alternate approach for suppressing CBET might be to pass narrow-band, Nd:glass laser light through diatomic gas cells and to rely on the phenomenon of stimulated rotational Raman scattering to augment the laser spectrum with additional discrete-wavelength components [D. Eimerl, D. Milam and J. Yu, Phys. Rev. Lett. 70, 2738 (1993)]. In this presentation, we review our modeling of these approaches with LPSE to date and assess their respective efficacies for suppressing CBET in ICF plasmas.
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Tuesday, November 6, 2018 3:12PM - 3:24PM |
JO6.00007: Laser-Absorption Profile Measurements on OMEGA to Verify Cross-Beam Energy Transfer Mitigation Using a Variable Wavelength Beam Dana H Edgell, Russell Follett, Joe Katz, John Palastro, David Turnbull, Dustin H Froula A variable wavelength beam (TOP9) is being deployed on OMEGA to study cross-beam energy transfer (CBET) mitigation and increased absorption with wavelength detuning. Laser light scattered from a target is the most-direct measurement for diagnosing laser absorption in a direct-drive implosion. The beamlets diagnostic images unabsorbed light from all beams as distinct “spots” onto a gated optical imager. The intensity of each spot is a measure of the effective absorption for a light “beamlet” originating from a specific point on the beam profile, including CBET along its path. In symmetric‑drive implosions, CBET transfers energy from beamlets near the center of the beam profile to beamlets near the edge of the beam profile. CBET mitigation can be diagnosed by a change in the beamlet spot intensity pattern. The intensity of the TOP9 beamlet will be used to diagnose changes in absorption as the TOP9 wavelength varies. |
Tuesday, November 6, 2018 3:24PM - 3:36PM |
JO6.00008: Design of a high-bandwidth probe laser for LPI and plasma photonics experiments Joshua Ludwig, Wojciech Rozmus, Pierre Michel Pump-probe laser-plasma experiments have recently demonstrated that the refractive index of a laser-plasma system could be arbitrarily modified, enabling the design of plasma-based optical elements such as polarizers and Pockels cells. In this presentation, we will present a new design for a probe laser with high, tunable bandwidth, to be built at the optical science laboratory (OSL) laser at LLNL. The goal is to achieve single-shot probing of plasma photonics structures. Our design is a variation on smoothing by spectral dispersion (SSD). In this study we are varying several key parameters (e.g. modulation frequency, modulation depth, color-cycling and angular dispersion) while keeping the bandwidth fixed, and look at the impact on laser-plasma interactions for single-shot probing of plasma photonics structures, and mitigation of LPI in ICF experiments. |
Tuesday, November 6, 2018 3:36PM - 3:48PM |
JO6.00009: Saturation of Cross-Beam Energy Transfer for Multi-Speckled Laser Beams L. Yin, B. J. Albright, D. J. Stark, W. D Nystrom, R. F. Bird Cross-beam energy transfer (CBET) is the process by which two crossing laser beams transfer energy between one another through stimulated Brillouin scattering (SBS). Understanding the nonlinear saturation of CBET, including the effects of wave-particle interaction, the excitation of secondary instabilities such as forward stimulated Raman scattering (FSRS), and speckle geometry, is important to controlling low-mode asymmetry in ICF implosions. In this work, VPIC simulations of the nonlinear saturation of CBET for multi-speckled laser beams crossing at arbitrary angles are discussed. In contrast to the CBET saturation dynamics by stochastic ion heating examined previously [1] (a process occurring on ~ns-time scales), these simulations show CBET saturating on a faster (~10s ps) time scale. The nonlinear saturation dynamics, including the excitation of FSRS, may limit the efficacy of CBET for symmetry control and contribute to capsule preheat from FSRS hot electrons. [1] P. Michel, et al., Phys. Plasmas 20, 056308 (2013). |
Tuesday, November 6, 2018 3:48PM - 4:00PM |
JO6.00010: Hot-electron generation and preheat in direct-drive experiments at the National Ignition Facility A.A. Solodov, M.J. Rosenberg, W. Seka, R. Epstein, R.W. Short, R.K. Follett, A.R. Christopherson, R. Betti, P.B. Radha, S.P. Regan, D.H. Froula, V.N. Goncharov, J.F. Myatt, J.W. Bates, A.J. Schmitt, P. Michel, M. Hohenberger, T. Chapman Laser–plasma instabilities, such as stimulated Raman scattering, can degrade the performance of direct-drive implosions by generating hot electrons that preheat the target. To assess the extent of hot-electron generation at ignition scales and conditions, planar and spherical target NIF experiments have been designed using the radiation–hydrodynamic codes DRACO and LILAC. Planar-target experiments, with Te ~5 keV and density gradient scale lengths of Ln ~600 μm, exhibited hot-electron temperatures of ~50 keV and conversion efficiencies ranging from ~0.5 to 3% as the laser intensity at the quarter-critical surface increased from ~6 to 15 ×1014 W/cm2. Spherical target experiments will be fielded in September 2018. The target will consist of an outer plastic ablator and an inner Ge-doped plastic layer. The difference in hard x-ray signals between the mass-equivalent plastic and multilayer implosions and the Ge Ka emission will be used to infer the hot-electron energy deposition in the unablated shell. The experiments will demonstrate how the divergence of hot electrons and the extent to which they slow down in the ablator reduce the preheat. |
(Author Not Attending)
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JO6.00011: Magnetic field generation in ICF target corona and its effect on hot electron generation by the two-plasmon-decay instability Wenda Liu, Chuang Ren The large amplitude (>10 T) and long scale length (>10μm) magnetic field observed in the coronal plasma of inertial confinement targets can be generated from the Rayleigh-Taylor instability and further amplified by the magneto-thermal instability (MTI). We will discuss the range of the magnetic field strengths that can be amplified by MTI. Particle-in-Cell simulations show that B field on the order of 100T would not affect linear growth of the two-plasmon-decay instability but can significantly reduce hot electron generation by interrupting the staged acceleration process |
Tuesday, November 6, 2018 4:12PM - 4:24PM |
JO6.00012: The Generation of Collimated Moderate Temperature Electron Beam in Shock Ignition Relevant Experiments on OMEGA Lasers Shu Zhang, Christine M Krauland, Jun Li, Jonathan L Peebles, Farhat N Beg, Sarah Muller, Neil B Alexander, Chuang Ren, Wolfgang R. Theobald, Riccardo Betti, Daniel J Haberberger, Edward Michael Campbell, Rui Yan, Eli B Borwick, Jocelain Trela, Dimitri Batani, Philippe Nicolaï, Robert Scott, Mingsheng Wei Understanding of laser-plasma instabilities (LPI) and the resultant hot electrons are critical for the design of the shock ignition (SI). We have conducted a series of experiments on OMEGA-EP and OMEGA-60 to characterize the hot electrons and signatures of LPI from the interaction of the kilo-joule infrared (IR) and ultraviolet (UV) lasers with long-scalelength keV plasmas at SI-relevant peak laser intensities (1016 W/cm2). The hot electrons were found to have temperatures in the range of 45 – 90 keV with 2.0 – 3.5% energy conversion efficiency. The hot electron beam divergence is less than 22˚, which is indicated by the size of the Cu K-alpha spot. The fraction of the high energy hot electrons (> 120 keV) has also been quantified. The time-resolved spectrum of the SRS light shows pump depletion in the low-density plasma (0.01 nc– 0.20 nc) during the first 0.5 ns of the UV spike pulse. The observed directional and moderate temperature hot electrons are encouraging for the electron-assisted SI. Details of the experiments and comparisons with the PIC simulations will be presented. |
Tuesday, November 6, 2018 4:24PM - 4:36PM |
JO6.00013: Enhancing and optimizing laser-plasma instabilities as x-ray sources at NIF Patrick Poole, Robert K Kirkwood, Scott Wilks, Brent Blue Experiment and simulation efforts are detailed that develop a high fluence, high photon energy continuum x-ray source by optimizing laser plasma instabilities on NIF. Such instabilities are typically minimized in fusion interactions to prevent drive-symmetry-breaking beam energy transfer and target preheat due to hot electron acceleration. Here instability mitigation efforts developed over previous decades are reversed to instead promote hot electron generation which leads to isotropic bremsstrahlung x-ray emission in a photon energy range that is otherwise inaccessible for high fluence materials effects testing. Experiments on Omega and NIF will be presented demonstrating selection of dominant laser plasma interactions via target and laser combinations, allowing control over both the x-ray spectral content and time of emission, as well as >10x increase in yield over typical setups and the most efficient 50 keV x-ray source to date on NIF. |
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