Characterizing and modeling the statistics associated with the initiation of gas breakdown has proven to be difficult due to a variety of rather unexplored phenomena involved. Experimental conditions for high power microwave window breakdown for pressures on the order of 100 to several 100 torr are complex: there are little to no naturally occurring free electrons in the breakdown region. The initial electron generation rate, from an external source, for example, is time dependent and so is the charge carrier amplification in the increasing radio frequency ͑RF͒ field amplitude with a rise time of 50 ns, which can be on the same order as the breakdown delay time. The probability of reaching a critical electron density within a given time period is composed of the statistical waiting time for the appearance of initiating electrons in the high-field region and the build-up of an avalanche with an inherent statistical distribution of the electron number. High power microwave breakdown and its delay time is of critical importance, since it limits the transmission through necessary windows, especially for high power, high altitude, low pressure applications. The delay time distribution of pulsed high power microwave surface flashover has been examined for nitrogen and argon as test gases for pressures ranging from 60 to 400 torr, with and without external UV illumination. A model has been developed for predicting the discharge delay time for these conditions. The results provide indications that field induced electron generation, other than standard field emission, plays a dominant role, which might be valid for other gas discharge types as well.

A reduction in the rise time of a 2.85 GHz high power microwave (HPM) pulse is achieved by implementing an overvoltaged spark gap inside a waveguide structure. The spark gap is oriented such that when triggered, the major electric field component of the dominant TE(10) mode is shorted. The transition from a transmissive to a highly reflective microwave structure in a relatively short period of time (tens of nanoseconds) creates a means to switch multimegawatt power levels on a much faster timescale than mechanical switches. An experimental arrangement composed of the waveguide spark gap and a high power circulator is used to reduce the effective rise time of a HPM pulse from a U.S. Air Force AW/PFS-6 radar set from 600 ns down to 50 ns. The resulting HPM pulse exhibits a much more desirable excitation profile when investigating microwave induced dielectric window flashover. Since most theoretical discussions on microwave breakdown assume an ideal step excitation, achieving a "squarelike" pulse is needed if substantial comparison between experiment and theory is sought. An overview of the experimental setup is given along with relevant performance data and comparison with computer modeling of the structure.

In an effort to investigate the physics involved in the initiation of high power microwave surface flashover, a strong direct current (dc) electric field was introduced to the flashover region. The primary objective of this study is to demonstrate that an external electric field can have a significant impact on the delay time for surface flashover. It has been observed experimentally that the statistical and formative delay times for surface flashover can be varied depending on the polarity of the applied dc field. This external electric field may sweep possible breakdown initiating electrons away from the flashover region prior to the application of the high power microwave pulse. A distinct increase in the average statistical delay was observed primarily with a superimposed dc field in a positive polarity geometry as well as a decrease in the formative delay for either polarity in 167 mbar of nitrogen.

High power microwave (HPM) dielectric surface flashover can be rapidly induced by providing breakdown initiating electrons in the high field region. An experimental setup utilizing a 2.85 GHz HPM source to produce a 4.5 MW, 3 µs pulse is used for studying HPM surface flashover in various atmospheric conditions. If flashover is to occur rapidly in an HPM system, it is desirable to provide a readily available source of electrons while keeping insertion loss at a minimum. The experimental results presented in this paper utilize a continuous UV source (up to 0.3 mW/cm 2 ) to provide photo-emitted seed electrons from the dielectric surface. Similarly, electrons were provided through the process of field emission by using metallic points deposited on the surface. Initial experiments utilizing 0.2 mm 2 aluminum points with a spatial density of 25/cm 2 have increased the apparent effective electric field by a factor of ~1.5 while keeping the insertion loss low (<0.01 dB). The field enhancements have sharply reduced the delay time for surface flashover. For an environment consisting of air at 2.07x10 4 Pa (155 Torr), for instance, the delay time is reduced from 455 ns to 101 ns. Two radioactive sources were also used in an attempt to provide seed electrons in the high field regions. Presented in this paper is a comparison of various field-enhancing geometries and how they relate to flashover development along with an analysis of time resolved imaging and an explanation of experimental results with radioactive materials.

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.