Initiation of high power microwave dielectric interface breakdown

Abstract

A simple model of vacuum/dielectric/vacuum interface breakdown initiation caused by high power microwave has been developed. In contrast to already existing models, a spatially varying electron density normal to the interface surface has been introduced. Geometry and parameter ranges have been chosen close to the conditions of previously carried out experiments. Hence, physical mechanisms have become identifiable through a comparison with the already known experimental results. It is revealed that the magnetic field component of the microwave plays an important role. The directional dependence introduced by the magnetic field leads to a 25% higher positive surface charge 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 side of the dielectric interface. The previously observed emission of low energy x-ray radiation from the interface is explained by bremsstrahlung generated by impacting electrons having initially a higher energy than the average emission energy. Final breakdown is believed to be triggered by electron induced outgassing or evaporation, generating a considerable gas density above the dielectric surface and eventually leading to a gaseous breakdown.

Influence of an external magnetic field on the threshold of multipactor onset on a dielectric surface

“…The fact is that a radio-transparent window made of dielectric material is the key element of the output and input systems. Specific vacuum discharge due to the processes of the secondary electron emission, which is developed on the window surface, destructs the window (see, e.g., [1][2][3][4]) and eventually is a significant cause impeding the creation of high-power microwave sources.…”

Multipactor discharge on a dielectric surface: Statistical theory and simulation results

“…[3][4][5][6][7][8][9][10][11] Most recent studies examine the combined action of a uniform rf field, parallel to the surface, and a uniform dc electric field, normal to the surface ͑the latter will be present, e.g., if the dielectric accumulates charges͒ and are based on Monte Carlo simulations. 2,9,10 ͑An exception is the work by Neuber et al 5 and Valfells et al 11 where an exact solution for the dc field distribution was constructed for a stationary multipactor discharge.͒ However, the use of Monte Carlo simulations requires tremendous computation time since it is necessary to carry out calculations within a wide range of parameters.…”

Microwave absorption on a thin film

“…[2][3][4] The multipactoring electrons may also cause outgassing, subsequent ionization, and flashover on the dielectric window. 1,5 Measures to reduce multipactor are well-known, for example, coating a slightly conducting thin film may prevent dielectric charging which is the physical origin of multipactor. 6,7 Simple estimates show that multipactor is unlikely to be a threat to microwave windows at frequencies higher than about 30 GHz.…”