L.L. Hatfield

Initiation of high power microwave dielectric interface breakdown

A simple model of vacuum/dielectric/vacuum interface breakdown initiation caused by high power microwave has been developed. In contrast to already existing models, a spatially varying electron density normal to the interface surface has been introduced. Geometry and parameter ranges have been chosen close to the conditions of previously carried out experiments. Hence, physical mechanisms have become identifiable through a comparison with the already known experimental results. It is revealed that the magnetic field component of the microwave plays an important role. The directional dependence introduced by the magnetic field leads to a 25% higher positive surface charge buildup for breakdown at the interface downstream side as compared to the upstream side. This and the fact that electrons are, in the underlying geometry, generally pulled downstream favors the development of a saturated secondary electron avalanche or a saturated multipactor at the upstream side of the dielectric interface. The previousl...

Electric current in dc surface flashover in vacuum

Dielectric surface flashover in vacuum is characterized by a three-phase development, as shown by current measurements covering the range from 10−4 to 100 A, assisted by x-ray emission measurements, high speed photography, and time-resolved spectroscopy. Further information is gained from a comparison of the flashover dynamics at 77 and 300 K. Phase one comprises a fast (several nanoseconds) buildup of a saturated secondary electron avalanche reaching current levels of 10 to 100 mA. Phase two is associated with a slow current amplification, with a duration on the order of 100 ns, reaching currents in the ampere level. The final phase three is characterized again by a fast (nanoseconds) current rise up to the impedance-limited current on the order of 100 A in this specific apparatus. The development during phase two and three is described by a zero-dimensional model, where electron-induced outgassing leads to a Townsend-like gas discharge above the surface. The feedback mechanism towards a self-sustained d...

Window breakdown caused by high-power microwaves

Physical mechanisms leading to microwave breakdown on windows are investigated for power levels on the order of 100 MW at 2.85 GHz. The test stand uses a 3-MW magnetron coupled to an S-band traveling wave resonator. Various configurations of dielectric windows are investigated. In a standard pillbox geometry with a pressure of less than 10/sup -6/ Pa, surface discharges on an alumina window and multipactor-like discharges starting at the waveguide edges occur simultaneously. To clarify physical mechanisms, window breakdown with purely tangential electrical microwave fields is investigated for special geometries. Diagnostics include the measurement of incident/reflected power, measurement of local microwave fields, discharge luminosity, and X-ray emission. All quantities are recorded with 0.21-ns resolution. In addition, a framing camera with gating times of 5 ns is used. The breakdown processes for the case with a purely tangential electric field is similar to DC flashover across insulators, and similar methods to increase the flashover field are expected to be applicable.

Phenomenology of subnanosecond gas discharges at pressures below one atmosphere

Volume breakdown and surface flashover in quasi-homogeneous applied fields in 10-5 to 600 torr argon and dry air are investigated, using voltage pulses with 150 ps risetime, <1ns duration, and up to 150 kV amplitude into a matched load. The test system consists of a transmission line, a transition to a biconical section, and a test gap, with gap distances of about 1mm. The arrangement on the other side of the gap is symmetrical. Diagnostics include fast capacitive voltage dividers, for determination of voltage waveforms in the gap, and conduction current waveforms through the gap. X-ray diagnostics use a scintillator-photomultiplier combination with different absorber foils yielding coarse spectral resolution. Optical diagnostics include use of a streak camera to get information on the discharge channel geometry and dynamics, and temporally resolved measurements with photomultipliers. Breakdown delay times are on the order of 100-400 ps, with minima occurring in the range of several 10torr. X-ray emission extends to pressures >100 torr, indicating the role of runaway electrons during breakdown. Maximum X-ray emission coincides with shortest breakdown delay times at several 10 torr. Simple modeling using the average force equation and cross sections for momentum transfer and ionization supports the experimental results

Electrode Erosion Phenomena in a High-Energy Pulsed Discharge

The erosion rates for hemispherical electrodes, 2.5 cm in diameter, made of graphite, copper-graphite, brass, two types of copper-tungsten, and three types of stainless steel, have been examined in a spark gap filled with air or nitrogen at one atmosphere. The electrodes were subjected to 50 000 unipolar pulses (25¿s, 4-25 kA, 5-30 kV, 0.1-0.6 C/shot) at repetition rates ranging from 0.5 to 5 pulses per second (pps). Severe surface conditioning occurred, resulting in the formation of several spectacular surface patterns (craters up to 0.6 cm in diameter and nipples and dendrites up to 0.2 cm in height). Surface damage was limited to approximately 80 ¿m in depth and was considerably less in nitrogen gas than in air. Anode erosion rates varied from a slight gain (a negative erosion rate), for several materials in nitrogen, to 5 ¿cm3/C for graphite in air. Cathode erosion rates of 0.4 ¿cm3/C for copper-tungsten in nitrogen to 25 ¿cm3/C for graphite in air were also measured.

Concepts for Optical Control of Diffuse Discharge Opening Switches

Optical control of diffuse discharges is discussed as opening mechanism for rep-rated switches. Diffuse discharges can be sustained or terminated by making use of optogalvanic effects, that means resonant interaction of laser radiation with diffuse plasma. Independent of control mechanisms, the performance of diffuse discharge opening switches is strongly affected by such fill gas properties as attachment and electron mobility.

Breakdown Delay Times for Subnanosecond Gas Discharges at Pressures Below One Atmosphere

With a RADAN 303-A pulser (a rise time of 150 ps and a maximum voltage of 150 kV into matched load), fast breakdown in argon and air is investigated. An oil-filled coaxial transmission line is coupled with a lens to a biconical section and a radial millimeter-size gap operated at subatmospheric pressure. Diagnostics include capacitive voltage dividers which allow the determination of voltage across and current through the gap with a temporal resolution defined by the digitizer (20 Gs/s, 6 GHz) used. A scintillator-photomultiplier combination with different metal absorber foils and a temporal resolution of 2 ns is used as X-ray detector to obtain a rough energy spectrum of the X-rays and electrons in the range of 10-150 keV. Discharges are characterized by runaway electrons over much of the pressure range, with a strong excitation and ionization layer at the cathode surface, and ldquofree-fallrdquo conditions with negligible gaseous ionization for the rest of the gap. High-energy electrons (> 60 keV) are observed up to atmospheric pressure. Time-to-breakdown curves versus pressure have been measured for different applied voltage rise times. They resemble Paschen curves with a steep increase toward low pressure and a slow increase toward high pressure. The major experimental findings and particularly the time-to-breakdown curves are confirmed using simple force-equation modeling. Monte Carlo calculations simulating collisional ionizations and developing electron avalanches in three dimensions have been used to verify and explain the experimental results.

Methods of increasing the surface flashover potential in vacuum

The effects of surface coating and magnetic fields on dielectric surface breakdown in vacuum are described. Coatings of metals and meta oxides increase the pulsed surface hold-off voltage by factors of up to three, depending on the material. Measurements of the secondary emission coefficient show substantial differences between coated and uncoated samples at high energies. The influence of magnetic fields on dielectric surface breakdown for uncoated samples is investigated in the pressure range of 10/sup -5/ to 1 Pa, for both DC and pulsed electric and magnetic fields. Insulation effects (increase in breakdown voltage of up to 2* at magnetic field amplitudes of 0.5 T) are observed when the magnetic field is parallel to the surface of the dielectric and perpendicular to the applied electric field. Magnetic insulation shows a strong material dependence and decreases with increasing pressure and surface roughness. >

Current, luminosity, and X-ray emission in the early phase of dielectric surface flashover in vacuum

With high-speed electrical and optical diagnostics, an attempt is made to elucidate the physical mechanisms leading to surface flashover. The experimental device uses a cable discharge to study self-breakdown along the surface of an insulator in vacuum. Preflashover current, breakdown voltage, luminosity, and soft X-ray emission are measured in temporal correlation with a resolution of 1 ns. The results show a linearly increasing current in the subampere range, and a corresponding linearly increasing luminosity, before an exponential increase of both signals takes over. The linear phase is accompanied by X-ray emission which ceases at the onset of the exponential phase. The strong influence of externally applied magnetic fields on the linear phase points to the existence of free electrons above the surface during the early phase of flashover. A linear current rise without magnetic field and the formation of a current plateau with an insulating magnetic field indicate a saturation of the current amplification mechanism in the early phase. >

Microwave magnetic field effects on high-power microwave window breakdown

Microwave window breakdown in vacuum is investigated for an idealized geometry, where a dielectric slab is located in the center of a rectangular waveguide with its normal parallel to the microwave direction of propagation. An S-band resonant ring with a frequency of 2.85 GHz and a power of 60 MW is used. With field enhancement tips at the edges of the dielectric slab, the threshold power for breakdown is observed to be dependent on the direction of the microwaves; i.e., it is approximately 20% higher for the downstream side of the slab than it is for the upstream side. Simple trajectory calculations of secondary electrons in an RF field show a significant forward motion of electrons parallel to the direction of microwave propagation. Electrons participating in a saturated secondary avalanche on the upstream side are driven into the surface, and electrons on the downstream side are driven off the surface, because of the influence of the microwave magnetic field. In agreement with the standard model of dielectric surface flashover for dc conditions (saturated avalanche and electron-induced outgassing), the corresponding change in the surface charge density is expected to be proportional to the applied breakdown threshold electric field parallel to the surface.