Aspect ratio dependence of bootstrap current and RF current driving efficiency

The normalized efficiency of generating non-inductive current with a radio frequency wave decreases relatively slowly, but the neo-classical orbit effects (viscosity) increase rapidly with lowering the aspect ratio, indicating far-reaching beneficial effects on the economical steady-state generation with a low aspect ratio configuration. Indeed, due to the viscosity, the plasma self-induced bootstrap current flows with a large fraction of the equilibrium current in the neo-classical reversed field pinch (RFP) with the low aspect ratio of A = 2.0, then significantly reduces the rf-driven current required for generating its steady-state configuration. As a result, the engineering reactor power gain (QE) is enhanced since the circulation power required for the steady-state operation is minimized. (authors)