Microwave magnetic field effects on high-power microwave window breakdown

Microwave window breakdown in vacuum is investigated for an idealized geome try, where a dielectric slab is located in the center of a rectangular wave guide,vith its normal parallel to the microwave direction of propagation, A n S-band resonant ring with a frequency of 2.85 GHz and a power of 60 MW is used. With field enhancement tips at the edges of the dielectric slab, the threshold power for breakdown is observed to be dependent on the direction of the microwaves; i.e., it is approximately 20% higher for the downstream side of the slab than it is for the upstream side, Simple trajectory calcu lations of secondary electrons in an RF field show a significant forward mo tion of electrons parallel to the direction of microwave propagation. Elect rons participating in a saturated secondary avalanche on the upstream side are driven into the surface, and electrons on the downstream side are drive n off the surface, because of the influence of the microwave magnetic field , In agreement with the standard model of dielectric surface flashover for de conditions (saturated avalanche and electron-induced outgassing), the co rresponding change in the surface charge density is expected to be proporti onal to the applied breakdown threshold electric field parallel to the surf ace.