Abstract
Recent advances in finite-difference time-domain (FDTD) modeling techniques allow plasma-surface interactions such as sheath formation and sputtering to be modeled concurrently with the physics of antenna near- and far-field behavior and ICRF power flow. Although typical sheath length scales (micrometers) are much smaller than the wavelengths of fast (tens of cm) and slow (millimeter) waves excited by the antenna, sheath behavior near plasma-facing antenna components can be represented by a sub-grid kinetic sheath boundary condition, from which RF-rectified sheath potential variation over the surface is computed as a function of current flow and local plasma parameters near the wall. These local time-varying sheath potentials can then be used, in tandem with particle-in-cell (PIC) models of the edge plasma, to study sputtering effects. Particle strike energies at the wall can be computed more accurately, consistent with their passage through the known potential of the sheath, such that correspondingly increased accuracy of sputtering yields and heat/particle fluxes to antenna surfaces is obtained. The new simulation capabilities enable time-domain modeling of plasma-surface interactions and ICRF physics in realistic experimental configurations at unprecedented spatial resolution. We will present results/animations from high-performance (10k-100k core) FDTD/PIC simulations of Alcator C-Mod antenna operation.
Highlights
Physical processes occurring in the edge and scrape-off layer (SOL) of magnetically confined fusion plasma can dramatically reduce the effectiveness of the RF sources used for heating or current drive within the core plasma
Sheath formation on plasma-facing antenna components, for instance, may induce sputtering of high-Z ions from these surfaces; such impurity components induce radiative cooling as transport processes carry them toward the reactor core, and may quench the fusion reaction altogether
Higher densities near plasma-facing antenna components are associated with higher material sputtering rates and sheath potential drops, and reduced densities and sheath potential values can be achieved by widening the gap between the antenna and the plasma's last closed flux surface, such a change may open additional, undesired channels into which RF wave power may flow
Summary
Physical processes occurring in the edge and scrape-off layer (SOL) of magnetically confined fusion plasma can dramatically reduce the effectiveness of the RF sources used for heating or current drive within the core plasma. Parametric decay instabilities or other resonant phenomena may be excited in the plasma edge, reducing RF antenna efficiency as injected RF power is partially diverted away from the core into undesired edge oscillations. In these interactions, the plasma edge density plays a significant role. We will discuss a process for modeling sputtering impurity generation in these discharges using VSim's particle-in-cell (PIC) capabilities These complex numerical simulations are possible only due to the increasing capabilities afforded by massively parallel supercomputing platforms, and the computations we describe here exercise these capabilities at large scales (tens of thousands of processor cores)
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