Power requirements for current drive

Abstract

General formulas for the efficiency of current drive in toroidal plasmas are derived using entropy arguments. The highest possible efficiency for current drive in which a high‐energy electron tail is formed is shown to be p=E_rj, with p and j the power and current densities and E_r≊0.09n₁₄ V/m with n₁₄ the electron density in units of 10¹⁴/cm.³ The electric field required to maintain the current in a runaway discharge is also shown to equal E_r. If the plasma current is carried by near‐Maxwellian electrons, waves that have a low phase velocity, compared to the energy of the electrons with which they interact, can drive a current with Ohmic efficiency, p=ηj². Such waves were first discussed in the context of current drive by Fisch [Rev. Mod. Phys. 5 9, 175 (1987)].