Reduced modelling of the X-B mode-conversion for RF heating and current drive applications

Mode-conversion of vacuum-launched electromagnetic waves to the electrostatic electron Bernstein wave (EBW) has gained renewed interest due to an increasing need to non-inductively drive current & heat overdense plasmas (f_pe > f_ce). One such method is the direct X-B conversion, where a vacuum-launched fast X-mode converts to the EBW via evanescent mode-coupling to the slow X-mode as an intermediary step. The efficiency of this process is typically computed with full-wave methods. However, these methods can be troublesome, as any spurious reflections due to a finite computational domain will cause non-physical interference, which affects the conversion efficiency. To avoid this issue, we re-formulate the X-B conversion problem as a boundary-value problem on a finite domain. Reflections off the domain edge are then exactly accounted for. This model is studied analytically in one dimension (1-D) and is shown to agree with the main features of previous 1-D models of the X-B conversion. The model is then used computationally to study the effect of density fluctuations on the 1-D X-B conversion efficiency. Finally, recent theoretical results suggest the possibility of incorporating mode-conversion into a ray-based code, which may help model the X-B conversion more efficiently.