J. P. Rayner and A. D. Cheetham

Plasma Instrumentation Laboratory, University of Canberra, Canberra, ACT 2600, Australia

andrew.cheetham@canberra.edu.au

The operation of a Helicon Plasma Source has been characterised by measuring the central number density of an Argon plasma over a parameter space defined by the input variables: vessel filling pressure, axial magnetic field and rf power input. Several distinct modes of operation have been identified in broad agreement with those observed in other Helicon Sources. In this paper we seek to explain the occurrence of the discontinuous jumps between these modes of operation. Recent work has begun to suggest that the mode jumps may be due to the existence of standing waves in the plasma with the wavelengths being set by the length of the plasma column [1]. The measurements presented here would seem to confirm this. The axial wavelengths and azimuthal mode structures in a cylindrical Helicon plasma source have been investigated for the various modes of operation. The boundaries between the modes seem to be defined by the wavelengths of standing waves within the plasma vessel that correspond to modes that would require a node in the wave fields to occur immediately under the antenna. The most favourable conditions for wave (and plasma) generation will be at wavelengths that require an anti-node under the antenna. It is suggested that the non-linear jumps that occur between the Helicon modes are associated with the standing waves requiring a node under the antenna. The results of these measurements will be presented and compared with calculations made using a simple theory of Helicon wave propagation.

1. K. P. Shamrai, V. P. Pavlenko and V. B. Taranov, Plas Phys. Cont Fus, 39 (1997) 505-529.