Investigation of absorption mechanisms in helicon discharges in conducting waveguides

Abstract

This paper investigates the mechanisms by which the helicon and associated Trivelpiece–Gould waves are absorbed in helicon discharges produced in conducting chamber; the experiments were based on a recent theory of damping and absorption of helicon modes in conducting waveguides (Ganguli et al 2007 Phys. Plasmas 14 113503). In particular, it was also planned to investigate the manner in which the absorbed energy is utilized for the production of warm electrons that are needed for ionization because helicon discharges are high density, low Te discharges and the tail of the bulk electron population may not have sufficient high-energy electrons. To this end, two separate regimes were considered. The first was a low pressure (≈0.2–0.3 mTorr), low magnetic field (≈16–20 G) regime where both wave absorption and warm electron production are shown to proceed through Landau damping. The second was a moderate pressure (≈10 mTorr), moderate magnetic field (≈60–65 G) regime, where both power absorption (which is collisional) and warm electron production proceed via high-energy electrons produced by acceleration of bulk electrons (from neighboring regions) across large potential gradients.