Time-Domain Modeling of Plasmas at RF-Time Scales David Smithe, Tech-X

Results from tokamak experiments such as PPPL's NSTX device indicate that significant anomolous power absorption can occur in the edge of a fusion plasma. Understanding of this phenomenon is a critical issue for analysis of RF heating scenarios on the ITER fusion experiment. Two probable edge absorption candidates, RF sheath losses and parametric decay instability, are both inherently non-linear, and likely to depend significantly on non-axisymmetric geometric detail in the vicinity of the antenna structures. Analysis of these phenomena is beyond the capabilities of existing axisymmetric frequency-domain linear-solvers used for analysis of heating and current drive in core fusion plasma, and so we are augmenting our analysis capability with the time-domain 3-D general-geometry electromagnetic and particle-in-cell simulation framework, Vorpal [1]. This modern, object-oriented framework has demonstrated scalability to over 2000 cpu's, a necessity for this type of calculation. We have successfully introduced into this framework an implicit plasma solver [2], in order to accurately treat electromagnetic plasma wave characteristics in the wide range of plasma conditions occurring from edge plasma to core plasma, including situations where the plasma frequency is not resolvable at the RF time-scales of interest, and including sharp plasma resonances and cutoff behaviors common in the RF regime. We present benchmarking of this new plasma solver for 1-D, 2-D, and 3-D scenarios. We also discuss implementation plans for non-linear sheath boundary models, non-linear edge-plasma conditions leading to parametric decay, and also tracking of high-energy particles in core-heating scenarios, where issues of finite-banana-width effects and superadiabaticity remain outside the scope of the existing frequency-domain solvers.

[1] Nieter and Cary, JCP 196 (2004) 448-473.
[2] Smithe, accepted for publication, PoP (2007).