Coupling of RF waves to a plasma with incomplete damping

Abstract: A theoretical model for the radio frequency (RF) heating system of a two current strap antenna in a tokamak is presented, considering a finite reflection of plasma waves and taking passive conducting components, e.g. conducting limiters, close to the antenna into account. Specifically, scenarios resulting in undesirable effects of the coupling, such as a lowering of the coupling resistance or the current drive, and variational structures of quantities in continuous parameter intervals are being investigated. A plane slab geometry is used, neglecting poloidal variations and using an equidistant discretization of toroidal coordinates. It is shown that most notable effects of passive components are found in the regime of low single pass damping. The quality factor (fraction of resistive power to apparent power transmitted to the plasma) and the plasma directivity averaged with respect to frequency is lowered when taking passive components into account. The presence of passive components also affects the spectrum of coupling resistance, but with no frequency averaged effects of the coupling resistance being observed. There are heavy oscillations of the coupling resistance, quality factor and plasma directivity for small variations of antenna frequency in the case of low single pass damping, which in turn relates to small variations of plasma density.