Initiation of high power microwave dielectric interface breakdown

A simple model of vacuum/dielectric/vacuum interface breakdown initiation c aused by high power microwave has been developed. In contrast to already ex isting models, a spatially varying electron density normal to the interface surface has been introduced. Geometry and parameter ranges have been chose n close to the conditions of previously carried out experiments. Hence, phy sical mechanisms have become identifiable through a comparison with the alr eady known experimental results. It is revealed that the magnetic field com ponent of the microwave plays an important role. The directional dependence introduced by the magnetic field leads to a 25% higher positive surface ch arge buildup for breakdown at the interface downstream side as compared to the upstream side. This and the fact that electrons are, in the underlying geometry, generally pulled downstream favors the development of a saturated secondary electron avalanche or a saturated multipactor at the upstream si de of the dielectric interface. The previously observed emission of low ene rgy x-ray radiation from the interface is explained by bremsstrahlung gener ated by impacting electrons having initially a higher energy than the avera ge emission energy. Final breakdown is believed to be triggered by electron induced outgassing or evaporation, generating a considerable gas density a bove the dielectric surface and eventually leading to a gaseous breakdown. (C) 1999 American Institute of Physics. [S0021-8979(99)09315-9].