G. Edmiston

Plasma structures observed in gas breakdown using a 1.5 MW, 110 GHz pulsed gyrotron

Regular two-dimensional plasma filamentary arrays have been observed in gas breakdown experiments using a pulsed 1.5 MW, 110 GHz gyrotron. The gyrotron Gaussian output beam is focused to an intensity of up to 4 MW/cm2. The plasma filaments develop in an array with a spacing of about one quarter wavelength, elongated in the electric field direction. The array was imaged using photodiodes, a slow camera, which captures the entire breakdown event, and a fast camera with a 6 ns window. These diagnostics demonstrate the sequential development of the array propagating back toward the source. Gases studied included air, nitrogen, SF6, and helium at various pressures. A discrete plasma array structure is observed at high pressure, while a diffuse plasma is observed at lower pressure. The propagation speed of the ionization front for air and nitrogen at atmospheric pressure for 3 MW/cm2 was found to be of the order of 10 km/s.

Interface Breakdown During High-Power Microwave Transmission

The major limiting factor in the transmission of narrowband high-power microwaves (HPM) has been the interface between vacuum-vacuum or even more severely between vacuum-air if HPM are to be radiated into the atmosphere. Extensive studies have identified the physical mechanisms associated with vacuum/dielectric flashover, as opposed to the mechanisms associated with dielectric/air flashover, which are not as well known. Due to the high electron collision frequencies (in the terahertz range) with the background gas molecules, established mitigation methods and concepts of vacuum/dielectric flashover will have to be re-evaluated. The primarily limiting factors of HPM transmission through a dielectric/air interface are presented based on recent experiments at 2.85 GHz. The physics of the involved mechanisms and their practical ramifications are discussed. The potential of surface roughness/geometry for flashover mitigation is addressed as well

Contributing Factors to Window Flashover under Pulsed High Power Microwave Excitation at High Altitude

One of the major limiting factors for the transmission of high power microwave (HPM) radiation is the interface between dielectric-vacuum, or even more severely, between dielectric-air if HPM is to be radiated into the atmosphere. Surface flashover phenomena which occur at these transitions severely limit the power levels which can be transmitted. It is of major technical importance to predict surface flashover events for a given window geometry, material and power level. When considering an aircraft based high power microwave platform, the effects on flashover formation due to variances in the operational environment corresponding to altitudes from sea level to 50,000 feet (760 Torr to 90 Torr) are of primary interest. The test setup is carefully designed to study the influence of each atmospheric variable without the influence of high field enhancement or electron injecting metallic electrodes.

Imaging of High-Power Microwave-Induced Surface Flashover on a Corrugated Dielectric Window

Dielectric window flashover is a severe pulse-shortening phenomenon limiting the power levels radiated in high power microwave (HPM) systems. This type of flashover develops in regions under high field stress coinciding with the dielectric interfaces separating the vacuum and atmospheric pressure sections of a microwave system. The formation of plasma at the exit aperture of a transmitting system can have several detrimental effects, including premature termination of the radiated pulse and/or the reflection of potentially damaging levels of radiation back toward the microwave source. Experimental studies of HPM surface flashover have been conducted under a variety of conditions in the S-band at power levels up to 5 MW with the aim of quantifying the relative impact of parameters such as gas pressure, type, and window geometry. One particular geometry variant designed with grooves perpendicular to the major electric field component at the window surface exhibited superior flashover suppression characteristics when compared with smooth window geometries. Images of HPM surface flashover evolution on this corrugated dielectric window geometry are presented.

Rotating Out-of-Plane Micromirror

Out-of-plane micromirrors have been developed for a wide range of applications including optical switching, beam steering, and precise transmission and reception of bio-optical signals. This paper focuses on the design, simulation, and testing of a rotating out-of-plane micromirror. The system consists of a polysilicon micromirror, which is erected to an out-of-plane position using a relatively simple postprocessing procedure. The mirror is mounted on a gear which has a rotational freedom of 360° and can be driven at frequencies ranging from 1 to 1000 Hz using an electrostatically actuated rotational drive. Multiple out-of-plane configurations of the mirror are possible, with each utilizing a serpentine spring that attaches the mirror to the gear and a position specific ?catch block? to allow 30 °, 45° , 60°, 75°, and 90° orientations of the mirror. This paper focuses on the 45° out-of-plane mirror, and it was tested for robustness as well as optical performance. A good correlation was found between experiment and various simulations.

Seed electron production from O− ions under high-power microwave excitation

Surface and volume breakdown formation during pulsed high-power microwave (HPM) excitation can severely limit the power densities which can be transmitted into an atmospheric medium. Recent studies in this area have focused on developing models which accurately predict flashover formation at either dielectric/air interfaces or in the gas volume directly adjacent to these interfaces. These models are typically validated through comparison with experimentally gathered data. With respect to HPM surface flashover, experiments in the S-band at 5 MW power levels have reported on the contributing factors to flashover development including the effects of gas type, pressure, and relative humidity. A Monte Carlo-type electron motion simulation code, MC, has been developed to calculate the increasing electron density during flashover formation in this case. Results from the MC code have exhibited a quantitative agreement with experimental data over a wide range of atmospheric conditions. A critical parameter to flas...

Similarities of dielectric surface flashover under atmospheric conditions for pulsed unipolar and RF excitation

Mechanisms in vacuum surface flashover caused by rf (f < 10 GHz) or unipolar voltages are virtually identical. Similarities between rf (representing high-power microwave window breakdown on the high-pressure side) and unipolar surface flashover are expected in an atmospheric environment as well. Two separate experimental setups were utilized to investigate both unipolar flashover and rf window flashover under atmospheric conditions while controlling excitation, temperature, pressure, humidity, and type of gas present, all under a similar electric field-surface geometry. The local electric field at the flashover initiating points has been numerically calculated in detail for all test geometries. For both rf and unipolar pulsed excitation, the flashover dynamics are changed by the application of UV light to the dielectric surface. A UV prepulse has a distinct impact on the arc’s path and a tendency to decrease the hold-off electric field. The effect of humidity on the hold-off electric field for both pulsed unipolar and rf excitations, along with temporally resolved emission spectroscopy of the flashover event, is discussed.

Short-Pulse High-Power Microwave Surface Flashover at 3 GHz

High-power microwave (HPM)-induced surface flashover is investigated in order to gain a better understanding of this phenomenon and reduce the limitations it imposes on transmitted power levels. This paper builds on previous testing using a magnetron producing 5 MW for 4 mus at 2.85 GHz. Both the previous and current experimental setups are designed to produce a flashover on the high-pressure side of a transmission window without the influence of a triple point. The limitations of the previous experiment included a maximum power of 5 MW and a pulse rise time of 50 ns. The current HPM source is an experimental virtual cathode oscillator (vircator), the output of which has been extensively characterized. The vircator is capable of producing 50-MW peak for 100 ns with an adjustable frequency from 3 to 5 GHz and a rise time of < 4 ns. The dominant modes of the vircator and magnetron are the circular TE11 and rectangular TE10 modes, respectively, with the major electric field component in both setups normal to the direction of propagation, yielding comparable field geometries at the transmission window. The experimental setup permits the study of factors, including gas pressure, composition, temperature, and air speed. Diagnostic equipment allows the analysis of power levels and flashover luminosity with subnanosecond resolution. Additional experimental results, including a detailed analysis of the flashover delay times under various conditions, are compared with data from literature and previous testing. A trend of increasing delay time with pressure is clearly observable, and Eeff/p versus p * r data fall within what has been previously observed in literature primarily for HPM volume breakdown.

Contributing Factors to Window Flashover Under Pulsed High Power Microwave Excitation at High Altitude

One of the major limiting factors for the transmission of high power microwave (HPM) radiation is the interface between dielectric-vacuum, or even more severely, between dielectric-air if HPM is to be radiated into the atmosphere. Surface flashover phenomena which occur at these transitions severely limit the power levels which can be transmitted. It is of major technical importance to predict surface flashover events for a given window geometry, material and power level. When considering an aircraft based high power microwave platform, the effects on flashover formation due to variances in the operational environment corresponding to altitudes from sea level to 50,000 feet (760 Torr to 90 Torr) are of primary interest. The test setup is carefully designed to study the influence of each atmospheric variable without the influence of high field enhancement or electron injecting metallic electrodes.

Similarities of Dielectric Surface Flashover at Atmospheric Conditions for Pulsed Unipolar and RF Excitation

Mechanisms in vacuum flashover caused by rf (f<10 GHz) or unipolar voltages are virtually identical. Similarities between rf (representing high power microwave window breakdown on the high pressure side) and unipolar flashover are expected in an atmospheric environment as well. Our experimental setups enable studying both unipolar flashover and rf window flashover at atmospheric conditions while controlling excitation, temperature, pressure, humidity, and type of gas present. The local electric field at the flashover initiating points has been numerically calculated in detail for all test geometries. For both rf and unipolar pulsed excitation, the flashover dynamics are changed by the application of UV light to the dielectric surface. A UV pre-pulse has a distinct impact on the arcs path and a tendency to increase the hold-off electric field. The effect of humidity on the hold-off electric field for both pulsed unipolar and rf excitations, along with temporally resolved emission spectroscopy of the flashover event, will be discussed.