Helicon sources with<b><i>m</i></b> 1 and<b><i>m</i></b> 2 helical antenna coupling*

Abstract

Pulsed helicon discharges produced through m = 1 or m = 2 helical antennas are investigated. Special attention is paid to the axial asymmetry, which is characteristic for helicon sources with helical antenna coupling. The axial profiles of the RF wave fields as well as the energy deposition profiles reveal that the RF power is predominantly absorbed via helicon modes with positive azimuthal mode numbers m (m = +1, +2) travelling in positive magnetic field direction. This can be attributed to the fact that the helicon modes with negative m are strongly damped or even evanescent if the radial plasma density gradient is steep enough. In particular, we examined the RF power absorption in the m = 1 helicon discharge, that cannot be understood in terms of collisions. Various non-collisional absorption mechanisms as well as absorption via excitation of Trivelpiece–Gould waves are discussed. We also present first measurements carried out on the large-volume plasma injected from the m = 1 helicon source into a diffusion chamber.