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 prf(W/cm³)[nₑ(10¹³ cm^-3)]²0.5, electron-cyclotron current-drive efficiency (and 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.