Microwave plasma generation by the fast rotation and slow pulsation of resonant fields in a cylindrical cavity

A digitally controlled solid-state microwave generator allowing variable frequency operation and precise phase control is adopted for plasma generation. In this study, a resonant cylindrical cavity is used as a microwave applicator in place of conventional waveguides. In order to improve the plasma uniformity, the TE111 mode is agitated by injecting microwaves into the cavity from two spatially orthogonal directions, with a temporal phase difference.. Theoretical analyses and finite-difference time-domain simulations derive the following effects of the phase control. In the case of phi = +/-pi/2, fast rotation of the cavity field takes place with a rotational frequency of omega/2 pi (= 2.4-2.5GHz), where. denotes the microwave angular frequency. On the other hand, when. is linearly modulated in time with a low frequency of Omega/2 pi (= 0.1-1000 Hz), slow pulsation takes place, in which the cavity field alternately excites a circular rotation and a standing oscillation at the modulation frequency. These effects are experimentally confirmed in microwave discharges in argon at 0.1-20 Torr with total injection powers from 50 to 800W. Two-dimensional images of the optical emission from the generated plasma show that both the fast rotation and slow pulsation improve azimuthal plasma uniformity. (C) 2017 The Japan Society of Applied Physics