Electron heating and static sheath enhancement in front of energized RF antennas

Abstract

An analytic model has been developed to describe the increased plasma potential and acceleration of ions in the vicinity of an ICRF Faraday shield. This model gives an estimate for the electron distribution function using a heating model for electromotive fields with decorrelation caused by reflection of the electrons from the static sheath at the point where a magnetic field line intersects a grounded wall. The loss rate of the electrons from the system can be calculated from this distribution function. The expected static potential is then estimated by equating electron and ion loss rates, assuming either that the ion loss satisfies the Bohm sheath criterion or that ion gyro-orbits strike a nearby wall. The analytic model for the electrons is in good agreement with a Monte Carlo model using realistic values of the RF electric fields. These RF near-field values have been calculated, including the linear near-field plasma response and the Faraday shield. Results include the findings that a linear model for the RF plasma response is valid for typical experimental parameters and that plasma currents must be included in the RF near-field calculation. It is also found that the static potential is sufficiently increased by electron heating so that it could easily cause impurities to be generated from the antenna structure. Results of the expected static potential are given for various ion species.