High power microwave surface flashover seed electron production methods

Abstract

Surface flashover imposes a fundamental limitation to the magnitude of high power microwaves which can be radiated from the vacuum environment of the source into atmospheric conditions. Providing seed electrons through various methods allows for initiatory conditions to be more closely controlled and the delay time variations to be reduced so that developmental mechanisms can be more closely examined. The experiment uses a coaxial magnetron capable of producing a ~4.5 MW, 3 µs pulse, at 2.85 GHz propagating in the TE 10 mode. The pulse rise time measured at the window is reduced using a spark gap pulse steepening technique. The fast rise time pulse propagates through the dielectric into an atmospheric test chamber where various conditions such as gas pressure, type of gas, UV illumination, and charged particle creation by radioactive sources can be controlled. Previous research has shown the significant impacts of UV radiation on the delay time averages and statistical distributions. Surface distributed seeding sources and volume distributed sources will be discussed while the primary focus of this paper will address use of alpha radiation as an ionizing agent. Thus far, a reduction in average delay time by as much as 60% has been achieved at sub-microsecond time scales, which also significantly affected the width of the statistical distributions of the delay time. Alpha particles have a short penetration distance in air which makes them a good candidate for study since the number of electron-ion pars created along the path is large. Analysis of the alpha particles influences will be discussed along with a statistical analysis of breakdown delay in the presence of ionization.

Rapid formation of dielectric surface flashover due to pulsed high power microwave excitation

“…SOURCES Two radioactive sources were utilized in an attempt to provide seed electrons in the high field region [13]. The first radioactive method was 0.1 mCi/ltr, 85 Kr (Krypton-85) gas in a 7 ppb mixture with Argon.…”

“…Delay Time Distributions for various pressures in Argon, without UV (top graph) and with UV (bottom graph)[13]; 40 trials per line.…”

“…Delay time comparison of Argon/ 85 Kr mixture (solid line), τ av = 379 ns, with Argon (dashed line), τ av = 382 ns, at 400 torr[13].…”