Observation and analysis of a two-parallel-temperature electron tail during lower-hybrid current-drive on a tokamak

A ‘‘two-parallel-temperature’’ feature of the fast-electron distribution function has been observed during lower-hybrid current-drive experiments on the Versator-II tokamak (B. Richards, Ph.D. thesis, Massachusetts Institute of Technology, 1981). Computational modeling predicts this feature and indicates that the colder tail is initiated by low-power, high-N∥ waves from the sidelobes of the antenna N∥ spectrum. The modeling indicates these waves bridge the ‘‘spectral gap,’’ enabling current drive even though the wave parallel phase velocity greatly exceeds the electron thermal velocity for the majority of the wave power.