Characterization of the ion beam formed in a low magnetic field helicon mode

An experimental characterization of the ion beam formed during a low magnetic field (<3 mT) helicon mode has been performed using a number of retarding field energy analysers, for pressures between 0.03 Pa < p ₀ < 0.4 Pa in argon. The low-field helicon mode is formed over a narrow magnetic field range (1 mT < B₀ < 3 mT) using a single solenoid giving a strong diverging field, and is characterized by a distinct density peak. For pressures below about 0.3 Pa, an ion beam is observed in the downstream region of the plasma reactor, and its presence is directly correlated with that of the low-field mode, suggesting that the ion beam formation is induced by the appearance of the helicon mode. For the same pressure and rf power input, as the magnetic field is further increased, a second ion beam regime is observed for magnetic fields above 5 mT. In this second regime the plasma density is less than half that for the low-field mode, and the system seems most likely to be inductively coupled. A simple analytical model is developed to describe the general features and behaviour of the observed ion energy distribution functions as a function of the plasma potential profile and neutral gas pressure.