Impact of a charge sheet on neoclassical transport

Abstract

Perpendicular electron heating in cyclotron waves accumulates resonant particles on the low magnetic field side, where an electron charge sheet may result (Hsu et al 1984 Phys. Rev. Lett. 53 564). Electrostatic fields are produced on both sides of the charge sheet. The impact of a charge sheet on transport is investigated based on neoclassical theory. A full particle simulation is performed using the Boris algorithm. The electrostatic field in the R-direction traps circulating particles or makes trapped particles circulate depending on the field direction. Transport coefficients are rigorously derived for such an electrostatic field. The electrostatic trapping or de-trapping changes the trapped particle population, bootstrap current and thermal conductivity. The positive electrostatic field corresponding to the negative biasing reduces transport, and then a thermal barrier is produced. Therefore, a strong positive electrostatic field can improve confinement. The transport behavior qualitatively agrees with a polarity study of biasing in TEXTOR, where the particle confinement time is three times lower in positive biasing.