Advanced ponderomotive description of electron acceleration in ICRF discharge initiation

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 E_z 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 (0^th, 1^st and 2^nd order). The efficiency for electron acceleration by E_z (z, t) is then assessed by comparing the values of these terms at the slow varying coordinate z₀. When (i) the 0^th order term is not significantly larger than 1^st order term at the reflection point, or when (ii) the 2^nd order term is negative and not sufficiently small compared to the 1^st 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.