Experimental Investigation of Material and Geometry Effects on Microwave Breakdown of Evanescent-Mode Cavity Resonators

This article presents an experimental investigation of microwave breakdown in evanescent-mode cavity resonators with different surface materials and post geometries, aiming at unveiling the role of secondary electron emission (SEE), attachment, and diffusion on microwave breakdown in inhomogeneous electric fields. The breakdown power of six cavities resonating at 2.6 GHz was measured at pressures ranging from 1 to 100 mbar in ambient air. The cavity with lower secondary emission yield surface material has a higher breakdown power on the left side of the breakdown curves, indicating the importance of SEE in microwave breakdown, especially at low pressures. Meanwhile, except for the cavity with hemisphere post, it was found that the quality factor has little effect on the breakdown power since electric field distributions inside these cavities are not significantly linked to post geometry. Two distinct regimes are observed in the breakdown curves with a transition pressure of 10 mbar. Namely, the breakdown curves present an emission-free branch above the transition, where microwave breakdown occurs in the gap, and a diffusion branch below the transition, where the microwave plasma expands out of the gap. We performed a theoretical calculation and found that the transition occurs when the attachment length equals the post radius.