Electron cyclotron resonance heating in tandem-mirror end cells

Abstract

A relativistic Fokker–Planck code is used to study the formation of non-Maxwellian electron velocity distributions in the end cell of a tandem mirror from the combined effects of Coulomb collisions and electron cyclotron radio-frequency (rf) heating. Axial electrostatic potential profiles consistent with thermal-barrier operation are assumed which lead to reasonable electron density profiles. The presence of the potentials results in four populations of electrons occupying different regions of phase space, three of which are solved for self-consistently; the fourth is the passing electron distribution from the center cell that is fixed as a Maxwellian. It is found that the second-harmonic barrier rf near the bottom of the magnetic well, together with rf cavity fields, give a good correspondence to the behavior of the hot electrons in the TMX-U experiment [IEEE Trans. Plasma Sci. PS-16, 1 (1988)]. The behavior of the fundamental rf in the plug region part way up the magnetic well shows poor correlation with experimental results.