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.