Effect of trapped electrons on current drive

Abstract

Trapped-particle effects on electron–cyclotron resonance heating (ECRH) and lower-hybrid current drive are studied using the adjoint formalism. A square-well model for the collision operator is used to obtain an analytic form for the Green’s function that is shown to be quite accurate. In constructing the current, the wave-heated distribution function is allowed to have an effective temperature along the velocity-space resonance curve that is different from the bulk temperature of the scattering electrons. These calculations are used to illustrate the deleterious effect of heating on the outside of a flux surface. Relativity and strong heating are each shown to increase current-drive efficiency for heating on the inside of the flux surface with Y≡harmonic number×cyclotron frequency/wave frequency less than 1, and decrease or reverse the driven current in the opposite limits. A significant limit on ECRH current drive in a mildly relativistic plasma, resulting from an upper bound on the minimum resonant energy, is demonstrated.