RF Sheath Models

RF sheath formation on the antennas and walls in ICRF-heated experiments can reduce the heating efficiency, limit the coupled power, and cause damage to plasma-facing structures. The sheaths are driven by a slow wave component of the rf field due to a mismatch between the magnetic field and the boundary (antenna or wall). Quantitative modeling of the highly nonlinear sheaths may now be feasible for the first time in massively-parallel-processing (MPP) codes developed in the RF SciDAC project. Recently, a new approach to sheath modeling was proposed,\footnote{D.A. D'Ippolito and J.R. Myra, Phys. Plasmas {\bf 13}, 102508 (2006).} in which the sheath physics is incorporated into the RF wave computation by using a modified boundary condition (BC) on the RF fields in both wave propagation and antenna codes. Here, we illustrate the use of the sheath BC for near-field sheaths by a model calculation that includes electromagnetic effects and a simple antenna coupling model. Properties of the model (such as the role of sheath-plasma waves) and implications for antenna codes such as TOPICA\footnote{V. Lancellotti et al., Nucl. Fusion {\bf 46}, S476 (2006).} will be discussed.