Stable modes and abrupt density jumps in a helicon plasma source

Abstract

Abrupt changes of discharge regimes in a helicon plasma source are treated proceeding from the model that assumes an rf power absorption to arise from a linear conversion of helicon waves into electrostatic waves at a plasma edge. The efficiency of conversion and thus of power absorption is found to be proportional to the squared edge amplitude of the helicon wave. The model yields a non-monotonic variation of absorbed power with the plasma density at a fixed antenna current. The specific shape of the absorption curve, that is the location, heights and widths of absorption peaks, turns out to depend on the source and antenna geometry, external magnetic field and driving frequency, whereas the absorption demonstrates extremely weak dependence on gas pressure. A power balance for the source plasma predicts a stable discharge in several density modes, and a possibility of abrupt transitions between them. The density jumps may be stimulated by the variation of absorbed power, driving frequency and/or magnetic field. The increase of magnetic field or the decrease of driving frequency may give rise to discharge disruptions. A discussion is presented of experimental data confirming the validity of the model and results obtained.