Bulletin of the American Physical Society
63rd Annual Meeting of the APS Division of Plasma Physics
Volume 66, Number 13
Monday–Friday, November 8–12, 2021; Pittsburgh, PA
Session WI02: Post-Deadline Invited SpeakersInvited Live
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Chair: Richard Town, Lawrence Livermore Natl Lab Room: Ballroom C |
Thursday, November 11, 2021 3:00PM - 3:30PM |
WI02.00001: Measurements of specular reflections (“glint”) of the inner beams in a hohlraum Invited Speaker: Nuno Lemos At the National Ignition Facility (NIF) indirect-drive inertial confinement fusion (ICF) experiments aim to obtain controlled thermonuclear burn by firing 192 laser beams, grouped into 4 beams per quad and 4 cone angles, onto the inner surface of a high-Z hohlraum. The goal is a sufficiently uniform and intense radiation bath generated at the hohlraum wall to compress a spherical capsule containing the fuel at the center of the hohlraum. There are several processes that can absorb or re-direct the laser energy, reducing the x-ray drive and symmetry, reducing thermonuclear burn effiiciency. In this work we quantify one of these processes – inner beam specular reflection of laser light off the cylindrical hohlraum wall at peak power that escapes through the opposite laser entrance hole of the hohlraum. The amount of reflection "glint" is inferred from the x-rays generated by the bottom glinted inner beams that hit a Ti witness plate capping the top part of the hohlraum. The x-ray signal is calibrated by a second Ti witness plate hit by a known laser intensity and placed outside the hohlraum. The x-ray emission from both witness plates is imaged onto a gated x-ray camera to evaluate the inner beam glint power as a function of time and hohlraum gas fill pressure. Large variations in the glint level were observed between inner beams even within the same quad indicating that inner cone glint is largest from the most grazing incidence angle beams (21.2°) with the lowest turning point density. Further understanding of the potential roles of intraquad CBET was gained by changing the polarization arrangement of the 23.5° quad beams. |
Thursday, November 11, 2021 3:30PM - 4:00PM |
WI02.00002: Hypervelocity impact in stellar media: Spacecraft Heat Shield study in DIII-D* Invited Speaker: Dmitriy M Orlov A study of carbon ablation at high heat flux relevant to hypervelocity spacecraft entries was performed in the DIII-D tokamak as part of the Frontiers in Science campaign. Exploration missions to the Solar System’s gaseous giants and hyperbolic re-entries into the Earth’s atmosphere require spacecraft heat shields that can withstand high velocity (>10 km/s) and extreme heat flux (>10 MW/m2). Testing and modeling [1,2] material performance in this regime is challenging due to lack of adequate ground testing facilities. Conditions in DIII-D L-mode edge plasma reproduce the flow velocity and high heat flux experienced during the Galileo probe’s entry into the atmosphere of Jupiter. Three types of samples were used for the experiments: stationary graphite rods [3] protruding from the floor of the vessel, 1-mm-diameter porous carbon spheres, and 700-micron-diameter glassy carbon spheres injected from the floor into the edge plasma. In the graphite rod experiments, the mass loss rates as a function of heat fluxes determined from an extensive array of spectroscopic measurements are found to agree with semi-empirical ablation models obtained from spacecraft flight data. Experimental results for the pellet trajectories and mass loss rates of the porous and glassy carbon pellets are confirmed using the UEDGE-DUSTT simulations. These pellet experiments are also compared against simulations of carbon-based meteorite atmospheric entries [4]. It is found that due to thermo-mechanical stress the glassy pellets shatter fragments with sharp edges. Similar spallation was observed during Galileo probe’s entry into the Jovian atmosphere and is also expected to affect survivability of meteors in Earth’s atmosphere. We demonstrate that scaling between DIII-D experimental results, available flight data, and numerical models can be used to address questions ranging from optimization of heat shields for future planetary missions to understanding extraterrestrial delivery of organic material to planet surfaces. |
Thursday, November 11, 2021 4:00PM - 4:30PM |
WI02.00003: Experimental Evidence of Plasmoids in Magnetic Reconnection Invited Speaker: Jacob A Pearcy Magnetic reconnection, the process by which magnetic fields in plasmas change their topology and release magnetic energy, is a ubiquitous phenomenon with fundamental importance to many disciplines from astrophysics to laboratory plasmas. However, the physics governing the reconnection process in many parameter regimes remains controversial. The classical Sweet-Parker reconnection model failed to explain the fast reconnections observed in solar flare evolution and in laboratory experiments. Meanwhile, contemporary reconnection theories and simulations predict that the long, narrow current sheets which form in magnetic reconnection events are structurally susceptible to the plasma tearing instability and may split into isolated magnetic islands (or plasmoids), resulting in an enhanced reconnection rate. Here, we report the first experimental evidence of plasmoid formation and dynamics in laser-driven high β reconnection experiments (where β is the ratio of thermal pressure to magnetic pressure). Using advanced proton radiography and deflection-field reconstruction, the spatial structure and temporal evolution of plasmoids are measured, offering new physical insight into magnetic reconnection in plasmas. The first visualization of the ejection, growth and coalescence of secondary magnetic islands, and evidence suggestive of tertiary magnetic islands, shed light on the dynamics of the reconnection hierarchy. Additional data from time-resolved Thomson scattering measurements quantify the plasma conditions, potentially enabling investigation of the transition from collisional reconnection to collisionless reconnection. |
Thursday, November 11, 2021 4:30PM - 5:00PM |
WI02.00004: Laser Frequency Conversion in a Dynamic Plasma Grating Invited Speaker: Caterina Riconda Laser-generated plasma gratings are dynamic optical elements for the manipulation of coherent light at high intensities, beyond the damage threshold of solid-stated based materials.They can be formed by a periodic ponderomotive potential generated by two identical counterpropagating Lasers. A fluid model is presented that allows to predict the peak value and formation time of the gratings, as well as kinetic simulations that show the existence of different regimes beyond the limitation of the fluid model[1, 2]. Combining fluid and kinetic results, general criteria for the peak value and life-time of the grating can be found. A novel application[3] of this transient plasma grating exploits its finite lifetime to affect the wave dispersion relation. As a result, different set-ups are identified that can result in laser frequency downshift or spectral splitting. The former can be used for Raman amplification in plasma and the latter for dual color x-ray generation by Thomson/Compton scattering. |
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