Power dependence of electron-cyclotron current drive for low- and high-field absorption in tokamaks.

A new relativistic, bounce-averaged Fokker-Planck--quasilinear code shows that for experimentally relevant rf power p/sub rf/(W/cm/sup 3/) /(n/sub e/(10/sup 13/ cm/sup -3/))/sup 2/approx. >0.5, electron-cyclotron current-drive efficiency (absorptivity) is power dependent; at given plasma radius the current-drive efficiency increases above linear values for absorption of the rf on the low-field side of the cyclotron resonance layer, but decreases and even passes through zero for absorption on the high-field side. Thus, electron-cyclotron current-drive by high-field launch will generally not be suitable.