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Author: Thomas G Jenkins
Requested Type: Poster
Submitted: 2016-02-15 10:50:58

Co-authors: D. N. Smithe

Contact Info:
Tech-X Corporation
5621 Arapahoe Avenue, Suite A
Boulder, CO   80303

Abstract Text:
The formation of RF-enhanced plasma sheaths near antenna hardware poses particular challenges for finite-difference time-domain (FDTD) modeling of ICRF effects in fusion plasmas. Such sheaths, though orders of magnitude smaller than the wavelengths of the driven RF waves, nevertheless influence plasma evolution. Ions accelerated through the sheath, for example, may sputter high-Z materials from antenna surfaces, leading to subsequent radiative cooling in the reactor core. We have recently [Jenkins/Smithe, PSST 24, 015020 (2015)] developed a sub-grid kinetic sheath boundary condition that enables the accurate computation of sheath potentials near metal or dielectric-coated walls, such that physical effects of DC/RF sheath dynamics can be included in macroscopic FDTD simulations that need not explicitly resolve sheath scale lengths. This model allows us to explore the role of the sheath in complex, experimentally relevant antenna geometries. Test particles, with energies suitably modified as they pass through the time-varying sheath potential to strike the wall, can be added to enable the simulation of sputtering processes.

We will present simulations/animations of C-Mod’s field-aligned ICRF antenna operation, examining the physics of localized RF sheaths at antenna surfaces (including impurity sputtering processes) and of wave launch. The use of dielectric coatings, applied to selected antenna components to alter local electromagnetic/surface properties and mitigate such sputtering, is also explored. High-performance computing capabilities (Titan, at ORNL) enable features of the launched waves, antenna hardware, and localized sputtering effects to be resolved at sub-millimeter scales.

Supported by U.S. DoE (OFES), Award DE-SC0009501. This research used resources of the Oak Ridge Leadership Computing Facility at the Oak Ridge National Laboratory, which is supported by the Office of Science of the U.S. DoE under Contract No. DE-AC05-00OR22725.