F. Hegeler

Plasma development in the early phase of vacuum surface flashover

The primary physical mechanism responsible for charge-carrier amplification, in a developing surface discharge, has eluded conclusive identification for decades. This paper describes the results of experiments to directly detect charge-carriers, above the dielectric surface, within the developing discharge. Free electrons are detected by measuring the deflection of a laser beam, focused to a 20 /spl mu/m 1/e diameter, with an angular sensitivity of 0.18 mVspl mu/rad and a risetime of 6 ns. The estimated detection threshold for electrons in the developing discharge is 10/sup 16/ cm/sup -3/ to 10/sup 17/ cm/sup -3/. A streak camera is used to gather spatial information regarding luminous processes with a maximum resolution of 25 /spl mu/m and 0.6 ns. Current measurements have a sub-nanosecond response time and a detection threshold of 100 mA. Laser deflection measurements demonstrate the rapid development of a particle gradient, generally within 10 /spl mu/m of the surface near the cathode and in the range of 75 to 175 /spl mu/m from the surface near the anode, during the developing discharge. Streak camera measurements demonstrate the formation of an intense, visible emission, 25 to 50 /spl mu/m in diameter, located near the insulator surface, during the formation of the discharge. These results imply that charge-carrier amplification occurs above the surface of the insulator, in a region of neutral particles desorbed or otherwise ejected from the insulator surface. >

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. >

Fast Electrical and Optical Diagnostics of the Early Phase of Dielectric Surface Flashover

With a new experimental setup we attempt to acquire information on the physical mechanisms involved in the early phase, of dielectric surface flashover in vacuum. The device uses a cable discharge into an impedance matched, coaxial, surface flashover test chamber in a micro-torr vacuum, operated in self-breakdown mode with a dc charging voltage. Novel techniques for current measurement provide resolutions in the milliampere and subnanosecond range for the preflashover current. They are supplemented by voltage measurements, measurements of the self-luminosity, and soft X-ray diagnostics. Preliminary results support the existence of a secondary electron avalanche above the surface and electron stimulated outgassing before flashover occurs.

Insulator surface breakdown in a simulated low Earth orbit environment

Surface flashover on insulators are investigated under UV irradiation or with a plasma background. A DC voltage up to 50 kV or a voltage pulse (up to 70 kV with 200 ns duration) is applied to the test gap, and the breakdown current, breakdown voltage, and soft X-ray emission are recorded by highly sensitive sensors with risetimes in the order of one nanosecond. Preliminary results with a plasma background have shown a more rapid development in the breakdown initiation compared to measurements in vacuum with no plasma. With a magnetic shielding technique using permanent magnets, the duration of an applied voltage pulse can be increased by a factor of 2 to 3 without causing flashover. UV illumination on the electrodes decreased the flashover voltage (for the DC case) or the voltage pulse duration without breakdown (for the pulsed case), whereas UV illumination on the dielectric surface enhanced the flashover potential.

Real-time detection of outgassing and plasma buildup during the early phase of dielectric surface flashover

Employing a newly developed refractive index sensor, we gather additional information about the processes involved in dielectric surface flashover. The sensor detects gradients in the refractive index above the dielectric surface, time-correlated with other signals of interest, by measuring the deflection of a focused laser beam. It utilizes a 10 mW HeNe laser beam incident on a bi-cell solid state photodetector to provide a signal with angular sensitivity on the order of 0.18 mV/μrad, and temporal resolution of 6 ns. This new diagnostic component is combined with current, voltage, luminosity, and x-ray sensors that have risetimes ranging from 0.4 to 3 ns. To the knowledge of the authors, this is the first application of this diagnostic principle to the problem of dielectric surface flashover. Preliminary measurements with the refractive index diagnostic demonstrate the existence of a time-changing refractive index above the dielectric surface accompanying the exponential rise of the flashover current. These measurements can be interpreted to indicate the formation of a plasma channel a few tens of microns above the dielectric surface, which expands with a velocity on the order of 106 cm/s . A fraction of these measurements display an indication of neutral gas desorption, within the threshold of detection, prior to flashover.

The early phase of dielectric surface flashover

Results of high-temporal-resolution current, luminosity, and X-ray measurements indicate the existence of free electrons during the first phase of dielectric surface flashover. The linearly rising current points to a saturation mechanism of the carrier amplification during this phase. In a subsequent phase, the exponentially rising current, the termination of the X-ray emission, and the build-up of plasma above the surface indicate gaseous ionization processes. The observations are compatible with the standard model for dielectric surface flashover-the saturated secondary electron emission avalanche and electron induced gas desorption.<<ETX>>

EXPERIMENTAL INVESTIGATION OF THE EARLY PHASE OF DIELECTRIC SURFACE FLASHOVER IN A SIMULATED LOW EAR

An experimental apparatus has been designed and constructed to acquire information on the physical mechanisms involved in the early phase of dielectric surface flashover in a simulated Low Earth Orbit (LEO) environment. The setup consists of a cable discharge pulser connected to the dielectric test gap via a spark gap in selfbreakdown mode. The geometry of all interconnecting lines and of the discharge chamber is coaxial and the impedances are closely matched to the cable pulser. As a first stage of the simulated LEO environment a low energy argon plasma is used. A xenon UV source, a low energy electron gun, and a combination of UV radiation and plasma will be applied to the dielectric test sample in the near future. Preliminary results with plasma have shown a change in the duration and development of the breakdown process compared to the case with vacuum.

High speed and high resolution diagnostics for the early phase of dielectric surface flashover

With a novel experimental setup, emphasizing high-speed and high-resolution electrical and optical diagnostic methods, the authors attempted to elucidate the physical mechanisms leading to surface flashover in vacuum. The device used a cable discharge into an impedance matched, coaxial, surface flashover test chamber in a micro-torr vacuum, operated in self-breakdown mode with a DC charging voltage. Preflashover current, breakdown voltage, luminosity, and soft X-rays were measured for different dielectric materials. In addition, DC magnetic fields were applied. Preliminary results indicated the existence of a secondary electron avalanche above the surface and electron stimulated outgassing before flashover occurred.<<ETX>>

The early phase of dielectric surface flashover in a simulated Low Earth Orbit environment

In Low Earth Orbit (LEO) the environment space plasma and UV radiation influences the surface charging and the surface flashover voltage of insulators, and thus the performance of high voltage systems. Understanding the mechanisms leading to surface flashover will make it possible to apply certain shielding techniques (e.g. electric and magnetic shielding) which can increase the dielectric flashover voltage drastically. In our experimental apparatus, a dc voltage up to 50 kV or a voltage pulse (up to 100 kV with 200 ns duration) is applied to the test gap. The geometry of all interconnecting lines and of the discharge chamber is coaxial and the impedances are closely matched. Breakdown current and voltage are recorded by highly sensitive sensors with risetimes of less than 0.8 ns. Preliminary results with a plasma background have shown a change from a surface dominated flashover to a plasma dominated breakdown compared to measurements in vacuum with zero plasma density. With applied UV radiation the dielectric flashover voltage amplitude decreases and the voltage and current waveform is altered, compared to results without UV, indicating a different current amplification mechanism in the early phase of dielectric surface breakdown.

Insulator surface flashover with UV and plasma background and external magnetic field

Surface flashover often sets the limit on the maximum voltage of a system in vacuum. UV irradiation and/or plasma will further decrease the flashover potential. This experiment investigates the UV and plasma induced dielectric surface flashover, emphasizes the early phase of breakdown, and uses external magnetic fields to increase the surface flashover potential. In our experimental apparatus, a dc voltage up to 60 kV or a voltage pulse (up to 100 kV with 200 ns duration) is applied to the test gap. The geometry of all interconnecting lines and of the discharge chamber is coaxial and the impedances are closely matched. The current, voltage, and soft X-rays are recorded. The plasma is generated by an electron cyclotron resonance plasma source and the UV radiation originates from a UV enhanced dc Xenon arc lamp. External magnetic fields influence the current in the pre-flashover phase. With a magnetic flux density of only 30 mT, the flashover potential of a UV induced breakdown increases by up to a factor of 4. With a plasma background, the duration of an applied voltage pulse can be increased by a factor of 2 without causing flashover.