A new relativistic, bounce-averaged Fokker-Planck-quasilinear code shows that for experimentally relevant rf power $\frac{p_{\mathrm{rf}}(\mathrm{W}/{\mathrm{cm}}^3)}{{\left[n_e\right({10}^{13} {\mathrm{cm}}^{-3\mathrm{}}\left)\right]}^2}\gtrsim 0.5$, 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.

• Received 22 February 1988

DOI:https://doi.org/10.1103/PhysRevLett.62.426