| Helicon waves have been shown to be an efficient means to produce high density (1019 m-3) plasmas at powers below 1 kW. However, they have been observed to reach a density limits with increasing power. The two possible causes for this limit, wave coupling at higher densities and neutral depletion, are examined experimentally in this work. The neutral depletion aspect appeared first in steady state operation; a reduction in the neutral pressure to 10% of the initial fill pressure was measured during plasma generation. The wave coupling aspect is addressed through the use of two antennae with differing kz spectra: a longer (18 cm) antenna operated at 800 W to initiate a helicon plasma, and a shorter (5 cm) one operated up to 3 kW to couple to higher densities at shorter wavelengths as predicted by the dispersion relation. A maximal plasma density increase of 2.5 times only lasted during the first 10 msec of the high power pulse before decaying back to the low power levels. Time scales for the decay corresponded to those estimated from a model of neutral and ion particle balances developed for this experiment. The effects of neutral depletion were thus seen to dominate in the plasma generation process for the helicon source. |
