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Title: Understanding and Improving High Voltage Vacuum Insulators for Microsecond Pulses

Abstract

High voltage insulation is one of the main areas of pulsed power research and development, and dielectric breakdown is usually the limiting factor in attaining the highest possible performance in pulsed power devices. For many applications the delivery of pulsed power into a vacuum region is the most critical aspect of operation. The surface of an insulator exposed to vacuum can fail electrically at an applied field more than an order or magnitude below the bulk dielectric strength of the insulator. This mode of breakdown, called surface flashover, imposes serious limitations on the power flow into a vacuum region. This is especially troublesome for applications where high voltage conditioning of the insulator and electrodes is not practical and for applications where relatively long pulses, on the order of several microseconds, are required. The goal of this project is to establish a sound fundamental understanding of the mechanisms that lead to surface flashover, and then evaluate the most promising techniques to improve vacuum insulators and enable high voltage operation at stress levels near the intrinsic bulk breakdown limits of the material. The approach we proposed and followed was to develop this understanding through a combination of theoretical and computation methods coupledmore » with experiments to validate and quantify expected behaviors. In this report we summarize our modeling and simulation efforts, theoretical studies, and experimental investigations. The computational work began by exploring the limits of commercially available codes and demonstrating methods to examine field enhancements and defect mechanisms at microscopic levels. Plasma simulations with particle codes used in conjunction with circuit models of the experimental apparatus enabled comparisons with experimental measurements. The large scale plasma (LSP) particle-in-cell (PIC) code was run on multiprocessor platforms and used to simulate expanding plasma conditions in vacuum gap regions. Algorithms were incorporated into LSP to handle secondary electron emission from dielectric materials to enable detailed simulations of flashover phenomenon. Theoretical studies were focused on explaining a possible mechanism for anode initiated surface flashover that involves an electron avalanche process starting near the anode, not a mechanism involving bulk dielectric breakdown. Experiments were performed in Engineering's Pulsed Power Lab using an available 100-kV, 10-{micro}s pulse generator and vacuum chamber. The initial experiments were done with polyethylene insulator material in the shape of a truncated cone cut at +45{sup o} angle between flat electrodes with a gap of 1.0 cm. The insulator was sized so there were no flashovers or breakdowns under nominal operating conditions. Insulator flashover or gap closure was induced by introducing a plasma source, a tuft of velvet, in proximity to the insulator or electrode.« less

Authors:
; ; ; ; ; ; ;
Publication Date:
Research Org.:
Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States)
Sponsoring Org.:
USDOE
OSTI Identifier:
902244
Report Number(s):
UCRL-TR-228713
TRN: US200717%%501
DOE Contract Number:
W-7405-ENG-48
Resource Type:
Technical Report
Country of Publication:
United States
Language:
English
Subject:
42 ENGINEERING; ALGORITHMS; ANODES; ARRAY PROCESSORS; BREAKDOWN; CLOSURES; CONES; DEFECTS; DIELECTRIC MATERIALS; ELECTRODES; ELECTRON EMISSION; ELECTRONS; FLASHOVER; PLASMA SIMULATION; POLYETHYLENES; PULSE GENERATORS; SHAPE

Citation Formats

Javedani, J B, Goerz, D A, Houck, T L, Lauer, E J, Speer, R D, Tully, L K, Vogtlin, G E, and White, A D. Understanding and Improving High Voltage Vacuum Insulators for Microsecond Pulses. United States: N. p., 2007. Web. doi:10.2172/902244.
Javedani, J B, Goerz, D A, Houck, T L, Lauer, E J, Speer, R D, Tully, L K, Vogtlin, G E, & White, A D. Understanding and Improving High Voltage Vacuum Insulators for Microsecond Pulses. United States. doi:10.2172/902244.
Javedani, J B, Goerz, D A, Houck, T L, Lauer, E J, Speer, R D, Tully, L K, Vogtlin, G E, and White, A D. Mon . "Understanding and Improving High Voltage Vacuum Insulators for Microsecond Pulses". United States. doi:10.2172/902244. https://www.osti.gov/servlets/purl/902244.
@article{osti_902244,
title = {Understanding and Improving High Voltage Vacuum Insulators for Microsecond Pulses},
author = {Javedani, J B and Goerz, D A and Houck, T L and Lauer, E J and Speer, R D and Tully, L K and Vogtlin, G E and White, A D},
abstractNote = {High voltage insulation is one of the main areas of pulsed power research and development, and dielectric breakdown is usually the limiting factor in attaining the highest possible performance in pulsed power devices. For many applications the delivery of pulsed power into a vacuum region is the most critical aspect of operation. The surface of an insulator exposed to vacuum can fail electrically at an applied field more than an order or magnitude below the bulk dielectric strength of the insulator. This mode of breakdown, called surface flashover, imposes serious limitations on the power flow into a vacuum region. This is especially troublesome for applications where high voltage conditioning of the insulator and electrodes is not practical and for applications where relatively long pulses, on the order of several microseconds, are required. The goal of this project is to establish a sound fundamental understanding of the mechanisms that lead to surface flashover, and then evaluate the most promising techniques to improve vacuum insulators and enable high voltage operation at stress levels near the intrinsic bulk breakdown limits of the material. The approach we proposed and followed was to develop this understanding through a combination of theoretical and computation methods coupled with experiments to validate and quantify expected behaviors. In this report we summarize our modeling and simulation efforts, theoretical studies, and experimental investigations. The computational work began by exploring the limits of commercially available codes and demonstrating methods to examine field enhancements and defect mechanisms at microscopic levels. Plasma simulations with particle codes used in conjunction with circuit models of the experimental apparatus enabled comparisons with experimental measurements. The large scale plasma (LSP) particle-in-cell (PIC) code was run on multiprocessor platforms and used to simulate expanding plasma conditions in vacuum gap regions. Algorithms were incorporated into LSP to handle secondary electron emission from dielectric materials to enable detailed simulations of flashover phenomenon. Theoretical studies were focused on explaining a possible mechanism for anode initiated surface flashover that involves an electron avalanche process starting near the anode, not a mechanism involving bulk dielectric breakdown. Experiments were performed in Engineering's Pulsed Power Lab using an available 100-kV, 10-{micro}s pulse generator and vacuum chamber. The initial experiments were done with polyethylene insulator material in the shape of a truncated cone cut at +45{sup o} angle between flat electrodes with a gap of 1.0 cm. The insulator was sized so there were no flashovers or breakdowns under nominal operating conditions. Insulator flashover or gap closure was induced by introducing a plasma source, a tuft of velvet, in proximity to the insulator or electrode.},
doi = {10.2172/902244},
journal = {},
number = ,
volume = ,
place = {United States},
year = {Mon Mar 05 00:00:00 EST 2007},
month = {Mon Mar 05 00:00:00 EST 2007}
}

Technical Report:

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  • High voltage insulation is one of the main areas of pulsed power research and development since the surface of an insulator exposed to vacuum can fail electrically at an applied field more than an order or magnitude below the bulk dielectric strength of the insulator. This is troublesome for applications where high voltage conditioning of the insulator and electrodes is not practical and where relatively long pulses, on the order of several microseconds, are required. Here we give a summary of our approach to modeling and simulation efforts and experimental investigations for understanding flashover mechanism. The computational work is comprisedmore » of both filed and particle-in-cell modeling with state-of-the-art commercial codes. Experiments were performed in using an available 100-kV, 10-{micro}s pulse generator and vacuum chamber. The initial experiments were done with polyethylene insulator material in the shape of a truncated cone cut at +45{sup o} angle between flat electrodes with a gap of 1.0 cm. The insulator was sized so there were no flashovers or breakdowns under nominal operating conditions. Insulator flashover or gap closure was induced by introducing a plasma source, a tuft of velvet, in proximity to the insulator or electrode.« less
  • Treatment of the surface of an alumina insulator with coatings incorporating varying amounts of Cr, Mn, and Ti can significantly increase the vacuum voltage holdoff capability of the insulator (up to 25 percent). During processing (quasimetallizing) the coating penetrates into the alumina, so it is insensitive to mechanical damage. This quasimetallize treatment is also compatible with subsequent metallizing and brazing of the alumina insulator. Quasimetallizing with appropriate formulations has been shown to change the surface characteristics of alumina in two ways: it decreases both the surface resistivity of the alumina and the secondary electron emission yield of the alumina. Eachmore » change improves the voltage holdoff characteristics of the alumina. Similar improvements in voltage holdoff capability can be obtained by doping the plain alumina to the 5 percent (by weight) level with additives of 4 parts Mn/1 part Ti or 2 Mn/1 Ti/1 Cr. Such doped ceramics have the advantage of requiring fewer processing steps than do the quasimetallized ceramics. Both doped and quasimetallized ceramics are suitable for use in ultrahigh vacuum apparatus.« less
  • Treatment of the surface of an alumina insulator with coatings incorporating varying amounts of Cr, Mn, and Ti can significantly increase the vacuum voltage holdoff capability of the insulator (up to 25%). During processing (quasimetallizing) the coating penetrates into the alumina, so it is insensitive to mechanical damage. This quasimetallizing treatment is also compatible with subsequent metallizing and brazing of the alumina insulator. A 7 Mn:1 Ti mix performed very well, being found to be as effective on a 94% Al/sub 2/O/sub 3/ alumina as on the previously investigated 95% Al/sub 2/O/sub 3/, 1% Cr/sub 2/O/sub 3/ alumina. Mixes ofmore » 6 Mn:1 Ti:1 Ca and 6 Mn:3 Cr performed about as well as the 7 Mn:1 Ti mix, but no better. Quasimetallizing with pure Mn improved the voltage holdoff capability of alumina by about half as much as when using the 7 Mn:1 Ti mix. Mixes with relatively high titanium content (4 Mn:3 Ti and 3 Mn:3 Ti:2 Cr) significantly increased the voltage holdoff capability of the alumina, but unfortunately were much more prone than the 7 Mn:1 Ti mix (or plain alumina) to suffer severe and permanent damage when a breakdown did occur. Quasimetallizing with appropriate formulations has been shown to change the surface characteristics of alumina in two ways: it decreases the surface resistivity of the alumina, and it decreases the secondary electron emission yield of the alumina. Each change improves the voltage holdoff characteristics of the alumina.« less