CURRENT DRIVE BY SYNERGY OF SLOW MODES IN TOKAMAKS

Enhancement of lower hybrid current drive in the presence of wave conv erted ion-ion slow modes is discussed using a one-dimensional quasi-li near equation. The special case in which the slow mode resulting from conversion in a deuterium-hydrogen mixture is absorbed by low energy e lectrons is considered in detail. In this scheme, the effect of the io n-ion modes is to increase the electron tail density, and the current enhancement is given by the ratio of the parallel velocity distributio ns with and without mode converted slow modes, calculated at the lower p(parallel-to) limit of the lower hybrid (LH) tail. The method of syn ergy is suited for remote control of the LH fast tail population, and is useful for current drive optimization. The computation also gives s ome insight into current drive by ion-ion slow modes in the absence of LH waves, a situation of relevance in a high temperature plasma where LH accessibility is severely reduced.