Far-field sheaths due to fast waves incident on material boundaries

The problem of “far-field” sheath formation is studied with a new quantitative one-dimensional model. These radio-frequency (rf) sheaths occur when unabsorbed fast waves in the ion cyclotron range of frequencies are incident on a conducting surface not aligned with a flux surface. Use of a nonlinear sheath boundary condition gives self-consistent solutions for the wave fields and sheath characteristics, and it introduces a sheath-plasma-wave resonance which can enhance the sheath potential. The model is used to compute the parametric dependence of the far-field sheath potential. Its application to post-process the rf fields computed by a full-wave code for a typical D(H) minority heating scenario is also discussed. This work shows that two-dimensional effects (included heuristically) are essential in determining whether far-field sheath potentials are strong enough to cause significant edge interactions, such as impurity generation and reduced heating efficiency.