Introduction to the Physics of HF Discharges

Abstract

This last chapter, dedicated to the mechanisms involved in high-frequency sustained discharges, put up an entirely new and original approach to this topic. The key element is θ _l , the average power lost per electron through its collision with heavy particles, in this way supplying power to the plasma. It is shown that θ _a , the power taken on average per electron from the HF field, adjusts so that θ _l =θ _a =θ, i.e. to compensate finally for the loss of charged particles. This implies, for instance, that the intensity of the E field in the plasma is not set by the operator, but by this balance requirement. The parameter θ is also instrumental in demonstrating that, contrary to common belief, the E-field intensity goes through a minimum at electron cyclotron resonance. The influence of varying the field frequency on the EEDF, and ultimately on plasma properties, is documented both theoretically and experimentally, in the case of low-pressure (<10 torr) plasmas. The case of high-pressure plasmas (including atmospheric pressure) is centred on the phenomena of discharge contraction and filamentation in rare gases with low thermal conductivity, emphasising the role of molecular ions in these monoatomic gas discharges. Interrupting the cycle leading to dissociative recombination by introducing traces of rare gases with an ionisation potential lower than that of the carrier gas leads to the disappearance of discharge contraction and filamentation.