Gradient effects on intense, pulsed electron-cyclotron-wave absorption and current drive

Gradients along the magnetic field of the parallel index of refraction N∥ or the magnetic-field strength B can substantially alter the dynamics of electrons subjected to intense, pulsed electron-cyclotron waves. Electrons trapped in a phase-space island, or ‘‘bucket,’’ about a cyclotron resonance tend to remain trapped and be accelerated with the bucket as N∥ and B change. The process can result in a substantial increase in the opacity (with the optical thickness possibly exceeding the linear value) and an improvement in the efficiency with which current can be driven in a tokamak. In this paper, a semiquantitative theory for the absorption and current-drive efficiency is developed and compared with numerical calculations from a multiparticle orbit code. Applications to heating and current drive in present and future tokamaks are discussed.