The influence of desorption gas to high power microwave window multipactor

Abstract

Gas desorbed by electrons plays a key role in multipactor saturation and final plasma breakdown at the vacuum side of the window in high power microwave systems. A multipactor model involving electron-neutral collision and ionization is established. When desorption gas pressure reaches 1Torr, the electron impact energy apparently decreases, multipactor saturates at a lower surface charging field, and multipactor saturation meets easier, compared to vacuum. Experiments for different material windows of high power microwave, with the power of 1GW, frequency of 9.4GHz, and 20ns single and multiple pulses was conducted. It was discovered that gas pressure rose prominently in the horn, when breakdown occurred. The increase in local gas pressure during the 20ns pulse is estimated from the experiment to reach several Torr, a pressure that is consistent with the presented dynamic model.

Multipactor susceptibility on a dielectric with a bias dc electric field and a background gas

“…(12), we consider only the regime of low gas pressure, where the diffusion loss and other collisions, such as excitation and recombination, can be neglected. 22 In Ref. 22, only the analytic solution to this problem was constructed under similar simplifying assumptions as in Ref.…”

“…4 Roughly, the lower (upper) boundary corresponds to electron impact energy on the dielectric surface equal to the first (second) crossover point in the secondary electron yield curve 4 and multipactor saturation occurs at the lower boundary. 5 The effects of space charge, 19 external magnetic field, 20,21 oblique rf electric fields, 20 wave reflection, 21 desorption gases, 22 and external perpendicular dc bias electric field 23 on multipactor discharge on a dielectric have been investigated. The transition of window breakdown from vacuum multipactor discharge to rf plasma has also been studied, by both PIC simulations 17,18 and volume-averaged global model (GM).…”

The influence of space charge shielding on dielectric multipactor

“…However, simulation can not essentially give the physical mechanism of the differences. As a matter of fact, for vacuum multipactor on a dielectric with strong secondary electron emission, the ambient pressure due to desorption gas is lower than several torrs, 3 and the positive space charge potential ͑referring to Fig. 1͒ can be formed.…”

“…It is the ambient gas, desorbed by the multipactor electrons and ionized quickly, that leads to the final plasma breakdown. 3,4 Experts 5, 6 have used particle-in-cell ͑PIC͒ simulations to investigate space charge field under different gas pressure during microwave dielectric breakdown. It can be discovered in Ref.…”

Design and Experiments of the Gw High-Power Microwave Feed Horn

“…With the development of narrowband multi-giga-watt HPM sources operated under vacuum, the power capacity of the HPM horn has become the major factor of limiting HPM transmission and radiation [2][3][4][5]. When the electric field amplitude at the aperture of the horn is higher than the breakdown threshold, secondary electron multipactor and plasma discharge happen at the vacuum side of the dielectric radome, leading to HPM breakdown and pulse shortening [2][3][4][5]. Consequently, in order to prevent breakdown, the aperture field of HPM horn should be uniform and below the breakdown threshold, which is about 30-40 kV/cm for S-X band HPM [6].…”