Bulletin of the American Physical Society
61st Annual Meeting of the APS Division of Plasma Physics
Volume 64, Number 11
Monday–Friday, October 21–25, 2019; Fort Lauderdale, Florida
Session CM9: Mini-conference: The Future of the Field II |
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Chair: Magnus Haw, NASA/Ames Room: Grand C/E |
Monday, October 21, 2019 2:00PM - 2:20PM |
CM9.00001: Characterization and Optimization of a Plasma Window for Reducing Gas Flow from High Pressure Andrew LaJoie, Jian Gao, Felix Marti The plasma window is a DC cascaded arc, in argon or helium, which restricts gas flow from a high pressure cell (order 10$^{\mathrm{2}}$ torr). This has applications in instances where a vacuum-atmosphere interface is present, for example as a component of a high pressure gas charge stripper in a high intensity ion accelerator such as the Facility for Rare Isotope Beams (FRIB) [1]. The factor by which flow is restricted is up to about 20 due to dramatically increased temperature and viscosity. A relation is developed for the flow rate from the gas cell as a function of pressure, current, and channel geometry. Insights are given on how the flow rate is related to basic plasma quantities such as electron density (in argon, about 10$^{\mathrm{16}}$ cm$^{\mathrm{-3}})$ and temperature (in argon, about 1.5 eV), determined via Stark broadening and relative emission intensities respectively. Results are compared with a cascaded arc model in PLASIMO, which has yielded comparable properties to measurements [2]. This work can serve as a guide with which the geometry of the plasma window can be optimized, maximizing gas pressure while minimizing escaping gas flow. [1] A. LaJoie et al., Trans. Plas. Sci. 2019. [2] G. M. W. Kroesen et al., Plas. Chem. {\&} Plas. Proc. 1990. [Preview Abstract] |
Monday, October 21, 2019 2:20PM - 2:40PM |
CM9.00002: A quadrature- and matrix-free discretization of the multi-species, non-relativistic, Vlasov-Maxwell system of equations James Juno We present a novel algorithm for the numerical solution of the multi-species, non-relativistic, Vlasov-Maxwell system of equations which uses high order discontinuous Galerkin finite elements to discretize the system on a phase space grid. The resulting numerical method is robust and retains a number of important properties of the continuous system, such as conservation of mass and energy. In addition, we will discuss a number of discoveries concerning the computational implementation of the algorithm which bring the cost of directly discretizing the Vlasov-Maxwell system down tremendously. We devote a portion of the presentation to the central motivation of developing a continuum discretization of the Vlasov-Maxwell system: a clean, noise-free representation of the distribution function and electromagnetic fields. We discuss a set of recent results (Skoutnev et al. ApJ Letters 2019) which disagree with particle-in-cell simulations with the same parameters and initial conditions, and demonstrate the role particle noise plays in the disagreement. We thus argue for the utility of the continuum approach, which despite its challenges and expense compared to the particle-in-cell method, nonetheless provides a complementary tool for addressing kinetic problems in plasma physics. [Preview Abstract] |
Monday, October 21, 2019 2:40PM - 3:00PM |
CM9.00003: Optimizing Laser-Plasma Interactions for Ion Acceleration using Particle-in-Cell Simulations and Evolutionary Algorithms Joseph Smith, Chris Orban, John Morrison, Kevin George, Gregory Ngirmang, Enam Chowdhury, W. Mel Roquemore The development of ultra-intense laser-based sources of high energy ions is an important goal for the field with a variety of potential applications. One of the barriers to achieving this is the need to maximize the laser to ion energy conversion efficiency. We use evolutionary algorithms to optimize laser to ion conversion efficiency by exploring variations of the target density profile and by performing thousands of one-dimensional particle-in-cell (PIC) simulations. We then use the optimal target from these one-dimensional PIC simulations in a series of two and three-dimensional PIC simulations comparing the optimal target to more conventional targets. Ions seem to accelerate from the optimal target with a TNSA-like mechanism but with increased laser coupling to electrons due to the target geometry. These results underscore the potential for this statistics-driven approach to optimizing laser-plasma simulations and experiments. [Preview Abstract] |
Monday, October 21, 2019 3:00PM - 3:20PM |
CM9.00004: Predicting QED Photon Jets from Plasma Experiments with Present-Day Lasers S. V. Luedtke, L. Yin, L. A. Labun, O. Z. Labun, B. J. Albright, D. J. Stark, R. F. Bird, W. D. Nystrom, B. M. Hegelich Discovery of quantum radiation dynamics in high-intensity laser-plasma interactions and engineering new laser-driven high-energy particle sources require accurate and robust predictions. Using QED-particle-in-cell simulations, we investigate a characteristic dipole pattern of high-energy photon emission that results when the laser pulse bores through the target, forming a channel that enhances the laser field. We observe significant stochasticity in macroscopically identical simulations and show that the stochasticity is physical in nature and expected to be present in experiments. The non-deterministic nature of the channeling phenomenon has important implications for designing an experimental campaign to detect QED photons and validate quantum radiation models, namely, experiments must produce a distribution of results to compare with predictions. We explore several ways that experiments differ from most simulations. Based on historical shot data from a petawatt-class laser, we run several simulations and predict the results and variability expected in experiments. [Preview Abstract] |
Monday, October 21, 2019 3:20PM - 3:40PM |
CM9.00005: Experimental and Radiation-Hydrodynamics Modeling Studies of Isochoric Heating at the Texas Petawatt Laser Rebecca Roycroft, Brant Bowers, Herbie Smith, Edward McCary, Frances Aymond, Gilliss Dyer, Hernan Quevedo, Erik Vold, Paul Bradley, Brian Albright, Lin Yin, Bjorn Manuel Hegelich We present experimental and simulation studies of warm dense matter produced by isochoric heating at the Texas Petawatt Laser Facility. Experimental studies of warm dense matter can provide measurements of equation of state, thermal conductivity, and other physical quantities, with the goal of more accurate modeling. This work presents results of experiments in which aluminum foils and carbon foams are isochorically heated with a laser accelerated TNSA proton beam, as well as radiation-hydrodynamics simulations of the heated targets. The brightness temperature over time of the heated target is measured by a streaked optical pyrometer. We have observed peak brightness temperatures from 1-20eV. We model the cooling and expansion of the heated target in xRAGE, an Eulerian radiation-hydrodynamics code. We find good agreement between experiment and simulation results when we include time dependence to the energy source, which we place at the rear surface of the aluminum foil. [Preview Abstract] |
Monday, October 21, 2019 3:40PM - 4:00PM |
CM9.00006: Stopping power measurements of ions in a moderately coupled and degenerate plasma Sophia Malko, W. Cayzac, V. Ospina, X. Vaisseau, J. Apinaniz, D. Batani, M. Barriga-Carrasco, R. Fedosejevs, M. Huault, P. Neumayer, G. Prestopino, C. Verona, J.A. Perez-Hernandez, R. Ramis, L. Volpe Ion stopping in dense plasmas plays a central role in ICF for the target self-heating by alpha-particles that triggers ignition. The existing experimental database is essentially limited to large projectile velocities (vp \textgreater \textgreater vth) and validates the perturbative stopping-power models in that range. The parameter region for vp \textasciitilde vth (Bragg peak), reached at low projectile velocities of few hundred keV/u is theoretically and experimentally more challenging. This work presents an experimental approach to study proton stopping at low velocity projectile in warm dense carbon at the CLPU 200 TW VEGA II, high repetition laser system. A TNSA proton beam is generated by focusing a 4 J, 30 fs laser pulse on a thin Al target with an intensity of 10$^{\mathrm{19}}$ W/cm$^{\mathrm{2}}$. A magnet-based device selects a proton energy interval of 500 \textpm 6 keV. These protons are used to probe a WDM sample created by irradiating a 1 \textmu m thick carbon target with 30 fs 10$^{\mathrm{16}}$ W/cm$^{\mathrm{2}}$ laser pulse. A magnet spectrometer coupled with an imaging MCP detector is employed to measure downshifted proton spectra. We reach a target electron temperature of 20 - 30 eV, which corresponds to $\Gamma $ \textasciitilde 0.2 and $\theta $ \textasciitilde 2 and velocity ratio of vp / vth \textasciitilde 3. [Preview Abstract] |
Monday, October 21, 2019 4:00PM - 4:20PM |
CM9.00007: Plasma Parameters in Short-Pulse-Heated Buried Tracer Layers via Fits of High-Resolution X-ray Spectra B.F. Kraus, A. Chien, Lan Gao, K.W. Hill, M. Bitter, P.C. Efthimion, M.B. Schneider, R. Shepherd, Hui Chen A quartet of high-resolution x-ray crystal spectrometers was deployed at the Titan laser to measure thermal self-emission of heated Ti and Mn layers. Solid targets were produced with thin (0.1--1 $\mu$m) layers of mid-Z tracer elements sandwiched between Al foil and a thin Al tamp (0--4 $\mu$m). When exposed to the relativistic-intensity laser pulse ($>$100 J in 1 ps), targets heat comparably to undoped Al foils, but only the thin tracer layer emits fine structure x-rays visible to spectrometers. By shooting a set of targets with varied tracer layer (Ti, MnAl, or both), tracer thickness, and tamp thickness, the time-integrated x-ray flux can be measured at many localized depths in the target. The fine structure spectra of He- and Li-like Ti and Mn is gathered by spherically-curved crystals in the focusing Johann geometry. The spectra, composed of both isolated and overlapping line emission, are fit to a multi-Gaussian model by a genetic algorithm, extracting line widths, heights and positions. These parameters are compared to atomic physics calculations, populations of excited electronic states from collisional-radiative models, and line width predictions from Stark, Doppler and opacity broadening, all of which are used to infer plasma conditions in the buried layer region. [Preview Abstract] |
Monday, October 21, 2019 4:20PM - 4:40PM |
CM9.00008: Pump-depletion Dynamics and Saturation of Stimulated Brillouin Scattering in Shock Ignition Conditions Shu Zhang, Jun Li, Sarah Muller, Wolfgang Theobald, Chuang Ren, John Palastro, Christian Stoeckl, Timothy Filkins, David Turnbull, Dan Haberberger, Michael Campbell, Riccardo Betti, Dimitri Batani, Jocelain Trela, Robbie Scott, Christine Krauland, Farhat Beg, Mingsheng Wei Shock ignition (SI) is an alternative inertial confinement fusion scheme, which requires a strong shock to ignite a pre-compressed fusion capsule. In this concept, nonlinear laser-plasma instabilities can play an important role. The recent experiments conducted on OMEGA-EP laser facility demonstrated that stimulated Brillouin scattering (SBS) can $\sim$100\% deplete the first 0.5~ns of the spike laser pump. In this period, the pump-depletion moved from $0.01-0.02$ critical density ($n_{\rm c}$) region to $0.1 - 0.2~n_{\rm c}$ region. The dynamic pump-depletion was indicated from the shape of the laser-generated blast wave and the time-resolved stimulated Raman backscattering spectra. The pump-depletion dynamics can be explained by the breaking of ion-acoustic waves in SBS. The strong laser pump depletion would reduce the collisional laser energy absorption in SI but may limit the temperature of hot electrons which could benefit the electron shock ignition. [Preview Abstract] |
Monday, October 21, 2019 4:40PM - 5:00PM |
CM9.00009: An experimental investigation of oscillating plasma bubbles and its non linear structure (evolution and effects) in a magnetized plasma system Mariammal Megalingam, Bornali Sarma This study is highlighting the experimental evidence of controlling chaos and its nonlinear behavior of fluctuations in the filamentary discharge magnetized plasma system. The cylindrical mesh grid of 80 {\%} optical transparency has been introduced in the plasma. Argon plasma is produced in the cylindrical chamber of dimension 350 mm in length and 400 mm in diameter. The cylindrical mesh grid of 90 mm in height and 120 mm in diameter placed in the bulk plasma. The chamber is evacuated down to 1.2\texttimes 10$^{\mathrm{-5}}$ mbar using both diffusion and rotary pump. Argon gas is injected by a needle valve into the chamber at working pressure of 2\texttimes 10$^{\mathrm{-4\thinspace }}$mbar. The electrical Langmuir probe has been extensively used for collecting the plasma fluctuations at various positions in and around the mesh grid. The oscillation pattern shows that at the farther most position from the grid, onset of chaos occurs at a lower value of magnetic field compared to the position which is at the center of the cylindrical grid. The dynamical transition has been explained using several nonlinear techniques such as Fast Fourier Transform, Phase Space Plot, Recurrence plot, Empirical mode decomposition etc. Therefore, it can be speculated that grid is playing a major role in controlling the chaotic behavior of the plasma oscillations. [Preview Abstract] |
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