Modeling far-field RF sheaths in Alcator C-Mod

It is shown that a theory of far-field sheath formation can qualitatively explain the observation on Alcator C-Mod of large plasma potentials (> 100 V) on field lines not directly connected to a powered antenna. The theory describes rf sheath formation when unabsorbed fast ICRF waves are incident on a conducting boundary possibly far away from the antenna. The resulting rf sheath drive is sensitive to the angle between the surface normal and the equilibrium B field. The main conclusion of this work is that the rapid tangential variation in the B field-limiter geometry at the tip of the limiter leads to the formation of large sheath potentials of the same order as the measured ones in C-Mod.