Abstract
The present understanding (May 1983) of current drive by lower hybrid waves in Tokamak plasmas is reviewed with particular stress on the theory. First, the "classical" model of Fisch and its variants are discussed in light of recent experimental data. It is argued that these simple models do not account for most of the features of the results obtained. Next, effects of runaways on current generation are considered. It is shown that runaways, resulting from a residual ohmic electric field, may play an important role. Further, the mechanism of runaway-current sustainment, as proposed by LIU et al. (1982b) is re-examined. It is shown that, for a realistic range of parameters, this mechanism does not allow a significant current to be sustained. Finally, some results of a recently-developed numerical code, which include the evolving electron distribution function, ray tracing and transport, are briefly noted. It is concluded that none of the existing theories can be used to interpret satisfactorily all the experimental observations.