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 VI3: Invited Post-Deadline Talks |
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Chair: Cary Forest, University of Wisconsin, Madison Room: OCC Oregon Ballroom 204 |
Thursday, November 8, 2018 3:00PM - 3:30PM |
VI3.00001: Generation of ultrahigh field by micro-bubble implosion Invited Speaker: Masakatsu Murakami Breaking the 100-MeV barrier for proton acceleration will help elucidate fundamental physics and advance practical applications from inertial confinement fusion to tumor therapy. Herein we propose a novel concept of bubble implosions [1]. A bubble implosion combines micro-bubbles and ultraintense laser pulses of 1020 – 1022 W/cm2 to generate ultrahigh fields and relativistic protons. The bubble wall protons undergo volumetric acceleration toward the center due to the spherically symmetric Coulomb force and the innermost protons accumulate at the center with a density comparable to the interior of a white dwarf. Then an unprecedentedly high electric field is formed, which produces an energetic proton flash. Three-dimensional particle simulations confirm the robustness of Coulomb-imploded bubbles, which behave as nano-pulsars with repeated implosions and explosions to emit protons. [1] M. Murakami, A. Arefiev, M. A. Zosa, Generation of ultrahigh field by micro-bubble implosion, Scientific Reports 8 (1) (2018) 7537. doi:10.1038/s41598-018-25594-3. |
Thursday, November 8, 2018 3:30PM - 4:00PM |
VI3.00002: Micron-cubed Particle Beam Induced Plasma Dynamics on Femto to Microsecond Timescales Invited Speaker: Roxana Tarkeshian High performance accelerators can produce particle beams that are femtosecond in duration and have transverse sizes on the micron scale. To address the challenge of measuring and optimizing such micron-cubed particle beams we propose and have carried out an in-depth analysis of an ionization based technique [1]. In contrast to photo-ionized processes, when using intense particle beams to ionize a gas, very different physics is manifested due to the large unipolar fields of the charged particle beam that impart a significant momentum to the plasma electrons. As they escape with high radial velocities, they leave the ions unshielded. This non-neutral plasma undergoes Coulomb explosion and the resulting dynamics offers new avenues for direct particle beam characterization. By characterizing the tunnel ionization induced species, bunch properties can be retrieved. In addition, the exponential dependency of tunnel ionization on the electron beam's radial space-charge field, and thus its charge density, results in a sensitive monitor that can measure micron cubed electron bunches with unprecedented resolution (few 10’s of nm level). Achieving such resolution is challenging or simply not possible with today’s methods, let alone on a single shot basis that is minimally invasive. Simulations using the WARP particle-in- cell code will be presented of the plasma dynamic phenomena on timescales ranging from the femto- to microsecond. Progress on experimentally implementing this novel diagnostic at the BELLA Petawatt laser facility will be discussed that rely on active plasma lenses to focus GeV level beams to reach the required intensities. [1] R. Tarkeshian et al, PRX 08, 021039, 2018 |
Thursday, November 8, 2018 4:00PM - 4:30PM |
VI3.00003: First principles analysis of interactions between fast ions and microturbulence Invited Speaker: George J Wilkie By taking advantage of a high-energy expansion in gyrokinetics, several reduced models are found for the effect of energetic particles on microturbulence and vice versa. For example, fast ions have been known to have a strong stabilizing impact on thermal ion-scale turbulence [1]. However, a theoretical explanation consistent with all known features of this effect has until recently been lacking. A physically transparent reduced model has now been developed which explains fast ion stabilization in terms of a direct mapping to the ion-electron temperature ratio [2]. This first principles model quantitatively and qualitatively describes several known aspects of the stabilization phenomenon, including: reduction of the linear growth rates, enhancement of the nonlinear effect, the destabilization of electron-driven modes, the relatively weak effect of alpha particles in burning plasmas, and more. |
Thursday, November 8, 2018 4:30PM - 5:00PM |
VI3.00004: The role of kinetic instabilities in post-disruption runaway electron beam formation after argon injection in DIII-D Invited Speaker: Andrey Lvovskiy Kinetic instabilities in the MHz range driven by relativistic runaway electrons (REs) have been observed for the first time during the current quench of a tokamak disruption and are well-correlated with intermittent losses of REs that can suppress sustained RE beam formation. When the instabilities exceed a threshold power RE beam formation is no longer observed, enabling a first understanding of the critical argon (Ar) quantity required to form a RE beam. Improvements to the DIII-D gamma ray imaging system to measure high gamma fluxes enabled the first diagnosis of the post-disruption RE seed energy distribution function and its dependence on pre-disruption parameters. The instabilities are observed when RE energy (ERE) exceeds 2.5 MeV, the number of modes grows linearly with maximum ERE, and their frequencies lie in the range 0.1–3 MHz, below the ion cyclotron frequency. Possible plasma waves excitable by REs in this region are proposed. Increasing the quantity of injected Ar results in a strong dissipation of REs, a reduced number of high-energy REs, and correspondingly a smaller amplitude of kinetic instabilities that then enables RE beam formation. Increasing the pre-disruption plasma current increases the available flux and is found to raise the fraction of high-energy RE seeds and further destabilizes the kinetic instabilities. Thus, a higher Ar quantity was necessary to form the RE beam at high pre-disruption plasma current. No such kinetic instabilities are observed after the injection of Ar pellets, resulting in reliable production of RE beams. This work opens new directions to understand RE beam formation and suppression as well as to develop validated models to design the disruption mitigation system of future tokamaks such as ITER. |
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