Time-domain modeling of plasma sheaths on RF antenna surfaces

A nonlinear, multi-parameter subgrid model for plasma sheaths [1] has been implemented in the Vorpal finite-difference time-domain code. Physically, such sheaths form near plasma-material interfaces; our focus will be on the plasma-facing surfaces of RF antennas (used for heating or profile control) in fusion devices. Large sheath potentials on antenna surfaces may arise due to rectification of the sheath by RF electromagnetic fields, and these high potentials are associated with undesired plasma-surface interactions such as sputtering (wherein high-Z atoms are ejected from metallic antenna surfaces by bombarding ions falling through the sheath).



We demonstrate that the new sheath model reproduces published benchmarks [2], and enables the study both of nonlinear sheath behaviors on realistic 3D RF antenna structures, and of the associated impurity production via sputtering from these structures. For the latter study, we discuss initial efforts to couple Vorpal with the GITR (Global Impurity TRansport) code and simulate impurity production in Alcator C-Mod discharges. Longer-term efforts will focus on developing the predictive capability of the coupled codes for impurity production modeling and mitigation.



[1] Myra, PoP 24, 072507 (2017), [2] Kohno & Myra, CPC 220, 129 (2017)