Advanced ponderomotive description of electron acceleration in ICRF discharge initiation

Abstract

This contribution proposes a new approach for the ponderomotive description of electron acceleration in ICRF discharge initiation. The motion of electrons in the parallel electric field Ez is separated into a fast oscillation and a slower drift around the oscillation centre. Three terms are maintained in the Taylor expansion of the electric field (0th , 1st and 2nd order). The efficiency for electron acceleration by Ez (z, t) is then assessed by comparing the values of these terms at the slow varying coordinate z0 . When (i) the 0th order term is not significantly larger than 1st order term at the reflection point, or when (ii) the 2nd order term is negative and not sufficiently small compared to the 1st order term at the reflection point, then the electron will gain energy in the reflection. An example for plasma production by the TOMAS ICRF system is given. Following the described conditions it can be derived that plasma production is (i) most efficient close to the antenna straps (few cm's) where the field gradient and amplitude are large, and (ii) that the lower frequency field accelerates electrons more easily for a given antenna voltage.