STABLE MODES AND ABRUPT DENSITY JUMPS IN A HELICON PLASMA SOURCE

Abrupt changes of discharge regimes in a helicon plasma source are tre ated proceeding from the model that assumes an rf power absorption to arise from a linear conversion of helicon waves into electrostatic wav es at a plasma edge. The efficiency of conversion and thus of power ab sorption is found to be proportional to the squared edge amplitude of the helicon wave. The model yields a non-monotonic variation of absorb ed power with the plasma density at a fixed antenna current. The speci fic shape of the absorption curve, that is the location, heights and w idths of absorption peaks, turns out to depend on the source and anten na geometry, external magnetic field and driving frequency, whereas th e absorption demonstrates extremely weak dependence on gas pressure. A power balance for the source plasma predicts a stable discharge in se veral density modes, and a possibility of abrupt transitions between t hem. The density jumps may be stimulated by the variation of absorbed power, driving frequency and/or magnetic field. The increase of magnet ic field or the decrease of driving frequency may give rise to dischar ge disruptions. A discussion is presented of experimental data confirm ing the validity of the model and results obtained.