OBSERVATION AND ANALYSIS OF A 2-PARALLEL-TEMPERATURE ELECTRON TAIL DURING LOWER-HYBRID CURRENT-DRIVE ON A TOKAMAK

A ''two-parallel-temperature'' feature of the fast-electron distributi on function has been observed during lower-hybrid current-drive experi ments on the Versator-II tokamak (B. Richards, Ph.D. thesis, Massachus etts Institute of Technology, 1981). Computational modeling predicts t his feature and indicates that the colder tail is initiated by low-pow er, high-N(parallel-to) waves from the sidelobes of the antenna N(para llel-to) spectrum. The modeling indicates these waves bridge the ''spe ctral gap,'' enabling current drive even though the wave parallel phas e velocity greatly exceeds the electron thermal velocity for the major ity of the wave power.