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 YO8: Laser Plasma Interactions II |
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Chair: Sasikumar Palaniyappan, Los Alamos National Laboratory Room: 212 CD |
Friday, November 4, 2016 9:30AM - 9:42AM |
YO8.00001: Classical Heat-Flux Measurements in Coronal Plasmas 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 was 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. Additional experiments probed plasma waves perpendicular to the temperature gradient. The data show small effects resulting from heat flux compared to probing waves along the temperature gradient. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0001944. [Preview Abstract] |
Friday, November 4, 2016 9:42AM - 9:54AM |
YO8.00002: Anomalous absorption of laser light on ion acoustic fluctuations Wojciech Rozmus, Valery Yu. Bychenkov Theory of laser light absorption due to ion acoustic turbulence (IAT) is discussed in high Z plasmas where ion acoustic waves are weakly damped. Our theory applies to the whole density range from underdense to critical density plasmas. It includes an absorption rate for the resonance anomalous absorption due to linear conversion of electromagnetic waves into electron plasma oscillations by the IAT near the critical density in addition to the absorption coefficient due to enhanced effective electron collisionality. IAT is driven by large electron heat flux through the return current instability. Stationary spectra of IAT are given by weak plasma turbulence theory and applied in description of the anomalous absorption in the inertial confinement fusion plasmas at the gold walls of a hohlraum. This absorption is anisotropic in nature due to IAT angular anisotropy and differs for p- and s-polarization of the laser radiation. Possible experiments which could identify the resonance anomalous absorption in a laser heated plasma are discussed. [Preview Abstract] |
Friday, November 4, 2016 9:54AM - 10:06AM |
YO8.00003: FOI-PERFECT code: 3D relaxation MHD modeling and Applications Gang-hua Wang, Shu-chao Duan One of the challenges in numerical simulations of electromagnetically driven high energy density (HED) systems is the existence of vacuum region. FOI-PERFECT code adopts a full relaxation magnetohydrodynamic (MHD) model. The electromagnetic part of the conventional model adopts the magnetic diffusion approximation. The vacuum region is approximated by artificially increasing the resistivity. On one hand the phase/group velocity is superluminal and hence non-physical in the vacuum region, on the other hand a diffusion equation with large diffusion coefficient can only be solved by implicit scheme which is difficult to be parallelized and converge. A better alternative is to solve the full electromagnetic equations. Maxwell's equations coupled with the constitutive equation, generalized Ohm's law, constitute a relaxation model. The dispersion relation is given to show its transition from electromagnetic propagation in vacuum to resistive MHD in plasma in a natural way. The phase and group velocities are finite for this system. A better time stepping is adopted to give a 3rd full order convergence in time domain without the stiff relaxation term restriction. Therefore it is convenient for explicit {\&} parallel computations. Some numerical results of FOI-PERFECT code are also given. [Preview Abstract] |
Friday, November 4, 2016 10:06AM - 10:18AM |
YO8.00004: Development Of Hard X-Ray Sources With High Radiative Power Output At The National Ignition Facility Utilizing Molybdenum and Silver Cavities Klaus Widmann, Russ Benjamin, Mark May, Daniel Thorn, Jeff Colvin, Maria Barrios, G. Elijah Kemp, Kevin Fournier, Brent Blue In our on-going x-ray source development campaign at the National Ignition Facility, we have recently extended the energy range of our laser-driven cavity sources to the 20 keV range by utilizing molybdenum-lined and silver-lined cavity targets. Using a variety of spectroscopic and power diagnostics we determined that almost 1{\%} of the nearly 1 MJ total laser energy used for heating the cavity target was converted to Mo K-shell x rays using our standard cavity design. The same laser drive for silver-lined cavities yielded about 0.4{\%} conversion efficiency for the Ag K-shell emission. Comparison with HYDRA simulations are used to further optimize the x-rays conversion efficiency. The simulations indicate that minor changes in the aspect ratio of the cavity and the layer thickness may double the radiative power of the K-shell emission. [Preview Abstract] |
Friday, November 4, 2016 10:18AM - 10:30AM |
YO8.00005: Optically Levitated Targets as a Source for High Brightness X-rays and a Platform for Mass-Limited Laser-interaction Experiments Samuel Giltrap, Nick Stuart, Tim Robinson, Chris Armstrong, George Hicks, Sam Eardley, Ed Gumbrell, Roland Smith Here we report on the development of an optical levitation based x-ray and proton source, motivated by the requirement for a debris free, high spatial resolution, and low EMP source for x-ray radiography and proton production. Research at Imperial College has led to the development of a feedback controlled optical levitation trap which is capable of holding both solid (Glass beads) and liquid (silicon based oil) micro-targets (\textasciitilde 3-10um). The optical levitation trap has been successfully fielded in a high-intensity laser interaction experiment at Imperial College London and at the Vulcan Petawatt Laser system at the Rutherford Appleton Laboratory (RAL). Here we report on the results from that RAL run including; an x-ray source size of 10-15um with very good spherical symmetry when compared to wire targets, secondly very low EMP signal from isolated levitated targets (9 times less RF signal than a comparable wire target). At Imperial College we were also able to record an x-ray energy spectrum which produced an electron temperature of 0.48KeV, and performed interferometry of a shock evolving into a blast wave off an optically levitated droplet which allowed us to infer the electron density within the shock front. [Preview Abstract] |
Friday, November 4, 2016 10:30AM - 10:42AM |
YO8.00006: Laser Backscatter and Propagation in Low-Density Ta2O5 and SiO2 Foams Derek Mariscal, Siddarth Patankar, Clement Goyon, Kevin Baker, Stephan Maclaren, Jim Hammer, Ted Baumann, Peter Amendt, Joseph Menapace, Robert Berger, Bedros Afeyan, Max Tabak, Sung Ho Kim, Sham Dixit, John Moody, Ogden Jones Recent experiments at the Jupiter Laser Facility at LLNL have investigated the propagation and backscatter of a laser in low-density foams (2-30 mg/cc) comprised of Ta2O5 and SiO2. The foams fill the volume of thin polyimide tubes (2 mm diameter, 0.5-2 mm length), while the laser is directed down the axis of the tubes. Time-resolved Stimulated Brillouin Scattering (SBS) spectrum, time-integrated Stimulated Raman Scattering (SRS) spectrum and power were measured in the focusing cone. In addition Near Backscatter Imaging (NBI) assessed SBS outside the focusing cone while X-ray diagnostics were used to assess laser propagation through the foams. While this experiment uses a 2-omega laser drive, the pulse shape, irradiance, and the ratio ne/nc are scaled to be similar to future tests using Ta2O5 foams at the NIF. Experimental results are directly compared to calculations of laser propagation and backscattered spectra. [Preview Abstract] |
Friday, November 4, 2016 10:42AM - 10:54AM |
YO8.00007: High Power Laser-Plasma Interaction under a Strong Magnetic Field Takayoshi Sano, Yuki Tanaka, Tomohito Yamaguchi, Masakatsu Murakami, Natsumi Iwata, Masayasu Hata, Kunioki Mima We investigate laser-plasma interactions under a strong magnetic field by one-dimensional Particle-in-Cell (PIC) simulations. A simple setup is considered in our analysis, in which a thin foil is irradiated by a right-handed circularly polarized laser. A uniform magnetic field is assumed in the direction of the laser propagation. Then the whistler wave can penetrate the overdense plasma when the external field is larger than the critical field strength $B_c = m_e \omega_0 / e$. In this situation, key parameters of the system are the plasma density and the size of the external field. We performed various models in the density-field strength diagram, which is actually the so-called CMA diagram, to evaluate the efficiency of the energy conversion from the laser to plasma and the reflectivity and transmittance of the laser. It is found that there are two important processes in the interaction between the whistler wave and overdense plasma, which are the cyclotron resonance of relativistic electrons and the parametric (Brillouin) instability. Because of the high temperature of electrons, ions can be accelerated dramatically by a large sheath field at the target surface. [Preview Abstract] |
Friday, November 4, 2016 10:54AM - 11:06AM |
YO8.00008: Energy deposition of quasi-two temperature relativistic electrons in fast-shock ignition scenario Seyed Abolfazl Ghasemi, Amir Hossein Farahbod Previous calculations from Solodov et al. (2008) indicate that classical stopping and scattering dominate electrons energy deposition and transport when the electrons reach the dense plasma in FSI inertial confinement fusion concept [1]. Our calculations show that, by using quasi- two temperature electrons energy distribution function [2] in comparison with exponential [3] or monoenergetic distribution function and also increasing fast electrons energy to about 7 MeV, the ratio of beam blooming to straggling definitely decreases. Our analytical analysis shows that for fuel mass more than 1 mg and for fast ignitor wavelength$\lambda_{if} >0.53\mu m$, straggling and beam blooming increases. Meanwhile, by reducing fast ignitor wavelength from 0.53 to 0.35 micron, and for fuel mass about 2 mg, electron penetration into the dense fuel slightly increases. Therefore, reduction of scattering (blooming and straggling) of electrons and enhancement of electron penetration into the dense fuel, can be obtained in relativistic regime with high energy fast electrons of the order of 5 Mev and more. Such derivations can be used in theoretical studies of the ignition conditions and PIC simulations of the electron transport in fast ignition scenario. [Preview Abstract] |
Friday, November 4, 2016 11:06AM - 11:18AM |
YO8.00009: First Experimental Comparisons of Laser-Plasma Interactions between Spherical and Cylindrical Hohlraums at the SGIII Laser Facility. Ke Lan We report the first experimental comparisons of laser-plasma interactions (LPI) between the spherical hohlraums and the cylindrical hohlraums at SGIII laser facility. The sphere is 1.8 mm in radius, and the cylinder is 1.2 mm in radius and 4.3 mm in length. Three kinds of fillings are considered for the hohlraums: vacuum, gas-filling without or with a capsule inside. A flat-top laser pulse of up to 92.73 kJ energy with 3 ns duration is used. As observed, the LPI level of the laser beams inside the gas-filled spherical hohlraums, with or without capsule, is very close to that of the outer laser beams inside the cylindrical hohlraums with the same filling while much lower than that of inner beams. These results provide important experimental references for the choice between the octahedral spherical hohlraum and the cylindrical hohlraum of an ignition target design, which decides quite different configuration of an ignition facility. Our 2D simulations with post-process calculations have evident differences from the data, and it indicates that the development of a 3D radiation hydrodynamic code, with more accurate physics models, is mandatory for spherical hohlraum study.\\ \\Keywords: Inertial Confinement Fusion, Spherical Hohlraum, Cylindrical Laser Entrance Hole, Laser transportation, SGIII prototype. [Preview Abstract] |
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