Evidence for, non-ECR, collisionless heating in a low pressure microwave discharge

Summary form only given. It has been shown that at low pressures (<5 mTorr) collisionless heating mechanisms (not ohmic heating) are responsible for the coupling of EM energy into both parallel plate and high density inductive discharges. A question remains: are there collisionless heating mechanisms present in low pressure microwave discharges? This paper will present the status and results of a set of experiments that investigate microwave discharge heating processes at low pressure. In this investigation an argon plasma is excited by a cylindrical cavity applicator with inner diameter of 17.78 cm. Power is coupled into the cavity with a variable length (0-6 cm) probe antenna placed in the center of a variable height (5-15 cm) top plate. The power input is 100-400 W at a microwave frequency of 2.45 GHz. The discharge pressure is varied from 0.5-40 mTorr and the argon flow rates range from 2-50 sccm. The discharge chamber is 12.5 cm in diameter and 8 cm tall. No permanent or electro-magnets are used in this experiment.