Strong Local Interaction of Microwave Discharges With Solid Dielectrics in Vacuum

Excitation of microwave discharges by pulsed microwave radiation (≤2 MW, 1.95 GHz, 1-10 μs) on dielectric surfaces in vacuum (10-6 torr) was studied experimentally. Different stages of a surface microwave discharge were observed: secondary-electron-emission microwave discharge (multipactor), surface microwave breakdown (filamentary microwave discharge), and plasma-flare microwave discharge. It is found that, in the stage of microwave breakdown (which lasts for ~0.1 μs), ≥70% of the incident microwave power is absorbed by a dense plasma filament with a diameter of ~100 μm, an electron density of up to 2 × 10¹⁸ cm^-3, and an electron temperature of about 2 eV. Strong interaction of the dense plasma of the filamentary microwave discharge with the dielectric leads to local destruction of the dielectric surface in the form of a long thin erosion track (l ≈ 6 cm, d ~ 100 μm). The coefficient of microwave power absorption by a plasma filament in a rectangular waveguide is calculated using an electrodynamic model that does not involve expansion in the waveguide modes and takes into account reflections of the scattered wave from the waveguide walls by introducing an array of filament mirror images. It is shown that the coefficient of microwave power absorption by the filamentary discharge plasma can reach 70% and more, which agrees well with the experimental data.