Radio frequency power deposition in a high-density helicon discharge with helical antenna coupling

Abstract

A high-density helicon discharge ${\mathrm{(n}}_e{\text{⩽10}}^{20} {\mathrm{m}}^{\text{−3}})$ produced through a new $\text{m=1}$ helical antenna is investigated. Various diagnostics are applied to measure the discharge parameters and the radio frequency (rf) quantities like the plasma resistance and the rf field distribution. Special attention is paid to the axial asymmetry of the discharge, which is characteristic for helicon devices with helical antennas. The axial profiles of the rf wave fields, as well as the energy deposition profiles, reveal that the rf power is mainly transferred and absorbed via the $\text{m=+1}$ helicon mode traveling in the positive magnetic-field direction. The experimental findings are compared with numerical results obtained from a fully electromagnetic model, which takes into account the rf current distribution of the launching antenna, as well as the finite size of the plasma column. The antenna–plasma coupling, as well as the total rf power deposited in the plasma, can be explained satisfactorily if the measured profiles are taken in the computations. In particular, the axial asymmetry of the helicon discharge can be understood in terms of the radial inhomogeneity of the plasma column. Furthermore, the calculations show that the small-scale Trivelpiece–Gould waves may be excited near the plasma edge. These waves would carry a considerable fraction of the absorbed rf power and may thus be important for the rf power coupling.