D. Hemmert

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...

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.

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.

Imaging of high-power microwave-induced surface flashover

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.

Field enhanced microwave breakdown in a plasma limiter

A new type of plasma limiter is being developed which can turn on in less than 1 ns. The approach taken is to initiate streamer breakdown via a micron radius needle tip. Images were taken of the gap region in argon at several pressures in order to investigate the role of the tip region.

High power microwave window breakdown under vacuum and atmospheric conditions

Microwave window breakdown is investigated in vacuum and atmospheric conditions. An S-band resonant ring with a frequency of 2.85 GHz and a power of 80 MW with a 4 MW magnetron as a source is used. Window breakdown on the vacuum side is simulated using a dielectric slab partially filling an evacuated waveguide. Various high-speed diagnostic methods yield a complete picture on the breakdown phenomenology, with far reaching similarities to dc surface flashover. During the initiation phase, free electrons are presented, which can be influenced by magnetic fields, followed by a saturated secondary electron avalanche with electron-induced outgassing. Final breakdown occurs in the desorbed gas layer above the surface. In order to simulated window breakdown on the gas-side, a segment of the resonant ring separated by two windows was filled with gas at variable pressure, and breakdown was initiated by field- enhancement tips on one of the gas-side surfaces. Threshold power densities for breakdown are measured, and first results on the phenomenology of this gas breakdown are compared with the processes of flashover in vacuum.

Gas breakdown in the subnanosecond regime with voltages below 15 kV

Gaseous breakdown in the subnanosecond regime is of interest for fast pulsed power switching, short pulse electromagnetics, and for plasma limiters to protect electronic devices from high power microwave radiation. Previous investigations of subnanosecond breakdown were mainly limited to high-pressure gases or liquids, with voltages in excess of 100 kV. In this paper, we investigate subnanosecond breakdown at applied voltages below 7.5 kV in point-plane geometries in argon, with a needle radius <0.5 /spl mu/m. The coaxial setup allows current and voltage measurements with temporal resolutions down to 80 ps. Voltages of 7.5 kV (which are doubled at the open gap before breakdown) produce breakdowns with a delay of about 1 ns. With negative pulses applied to the tip and the same amplitude, breakdown is always observed during the rising part of the pulse, with breakdown delay times below 800 ps, at pressures between 10/sup 2/ and 10/sup 4/ Pa. At lower pressure, a longer delay time (8 ns at 6 Pa) is observed. We expect the breakdown mechanism to be dominated by electron field emission, but still influenced by gaseous amplification.

Real Time Feedback Control System for an Electromagnetic Launcher

The design and implementation of a real time feedback control system for a distributed energy, bench top, electromagnetic launcher is presented. The feedback control system provides optimum pulse shaping by real time control of solid state switches. Advantages of pulse shaping control include increased energy efficiency and control of armature exit velocity. Lab VIEW 8.0 software1 is used to program a National Instruments CompactRIO programmable automation controller (PAC). This provides real time processing by use of the reconfigurable I/O (RIO) FPGA technology. The program controls switch timing from analog feedback signals supplied by B-dot probes placed along the rail length. Through signal analysis, real time armature position is derived. The program uses this data to control pulse shape and width. A dedicated B-dot probe is placed at the beginning of each stage which is the desired triggering location. A flux ruler sensor along the bore length provides a secondary velocity calculation excluded from the control system. This sensor provides velocity measurements for every centimeter of bore travel. Collected data is used to characterize the system under test for different load conditions.

Subnanosecond Breakdown in Argon at High Overvoltages

Volume breakdown and surface flashover in quasi homogeneous applied fields in 10-5 to 600 torr argon are investigated, using voltage pulses with 150 ps risetime, < 1 ns 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 one to several mm. The arrangement on the other side of the gap is symmetrical. An improved system, with oil-filled transmission lines and lens between coax and biconical section to minimize pulse distortion, is being constructed. Diagnostics include fast capacitive voltage dividers, which allow to determine voltage waveforms in the gap, and conduction current waveforms through the gap. X-ray diagnostics uses a scintillator- photomultiplier combination with different absorber foils yielding coarse spectral resolution. Optical diagnostics to obtain information about the discharge channel dynamics is in preparation. Breakdown delay times, and e-folding time constants for the conduction current during the initial breakdown phase, are on the order of 100-400 ps, with minima in the range of several 10 torr. X-ray emission extends to pressures > 100 torr, indicating the role of runaway electrons during breakdown. Maximum X-ray emission coincides with fastest current risetimes at several 10 torr, which is probably related to an efficient feedback mechanism from gaseous amplification to field enhanced electron emission from the cathode.

Gas breakdown in the sub-nanosecond regime with voltages below 15 kV

Gaseous breakdown in the sub-nanosecond regime is of interest for fast pulsed power switching, short pulse electromagnetics and for plasma limiters to protect devices from high power microwave radiation. Previous investigations of sub-nanosecond breakdown were mainly limited to high-pressure gases or liquids, with applied voltages in excess of 100 kV. In this paper, the authors investigate possibilities to achieve sub-nanosecond breakdown at applied voltages below 7.5 kV in point-plane geometries. The setup consists of a pulser (risetime between 400 ps to 1 ns), 50-/spl Omega/ transmission line, axial needle-plane gap with outer coaxial conductor, and a 50-/spl Omega/ load line. The needle consists of tungsten and has a radius of curvature below 0.5 /spl mu/m. The constant system impedance of 50 /spl Omega/ (except in the vicinity of the gap) and a special transmission-line-type current sensors enables current and voltage measurements with a dynamic range covering several orders of magnitude, with temporal resolution down to 80 ps. For pulse amplitudes of 1.7 kV (which are doubled at the open gap before breakdown) delay times between start of the pulse and start of a measurable current flow (amplitude > several milliamperes) have a minimum of about 8 ns, at a pressure of 50 torr in argon. Voltages of 7.5 kV produce breakdowns with a delay of about 1 ns. With negative pulses applied to the tip, at an amplitude of 7.5 kV, breakdown is always observed during the rising part of the pulse, with breakdown delay times below 800 ps, at pressures between 1 and 100 torr. At lower pressure, a longer delay time (8 ns at 50 mtorr) is observed. They authors expect the breakdown mechanism to be dominated by electron field emission, but still influenced by gaseous amplification.