Vacuum insulator development for the dielectric wall accelerator

Abstract

At Lawrence Livermore National Laboratory, we are developing a new type of accelerator, known as a dielectric wall accelerator, in which compact pulse-forming lines directly apply an accelerating field to the beam through an insulating vacuum boundary. The electrical strength of this insulator may define the maximum gradient achievable in these machines. To increase the system gradient, we use “high-gradient insulators” composed of alternating layers of dielectric and metal for the vacuum insulator. In this paper, we present our recent results from experiment and simulation, including successful testing of a high-gradient insulator in a functioning dielectric wall accelerator cell. Our results indicate that proper high-voltage conditioning of the insulators can delay the onset of flashover, that the observed conditioning consists of both a permanent and a temporary part, and that the insulators’ voltage-holding capability increases with increasing dielectric layer thickness.

Full text

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LLNL-JRNL-402445
Vacuum Insulator Development
for the Dielectric Wall Accelerator
J. R. Harris, D. Blackfield, G. J. Caporaso, Y.-J. Chen,
S. Hawkins, M. Kendig, B. Poole, D. M. Sanders, M.
Krogh, J. E. Managan
March 20, 2008
Journal of Applied Physics

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Vacuum Insulator Development for the Dielectric Wall Accelerator
J.R. Harrisa), D. Blackfield, G.J. Caporaso, Y.-J. Chen, S. Hawkins, M. Kendig, B. Poole,
and D.M. Sanders
Lawrence Livermore National Laboratory, Livermore CA 94551
M. Krogh
Complete Compact Aero Operation Systems, Inc., Lees Summit, MO 64081
J.E. Managan
Department of Physics and Astronomy, Vanderbilt University, Nashville TN 37235
Abstract
At Lawrence Livermore National Laboratory, we are developing a new type of
accelerator, known as a Dielectric Wall Accelerator, in which compact pulse forming
lines directly apply an accelerating field to the beam through an insulating vacuum
boundary. The electrical strength of this insulator may define the maximum gradient
achievable in these machines. To increase the system gradient, we are using "High
Gradient Insulators" composed of alternating layers of dielectric and metal for the
vacuum insulator. In this paper, we present our recent results from experiment and
simulation, including the first test of a High Gradient Insulator in a functioning Dielectric
Wall Accelerator cell.
a) Electronic mail: harris89@llnl.gov

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I. Introduction
At Lawrence Livermore National Laboratory, we are developing a new type of linear
induction accelerator known as the Dielectric Wall Accelerator (DWA)1,2. DWAs use
stacked pulse-forming lines to directly apply an accelerating field to the beam through an
insulating vacuum boundary, or "dielectric wall" (Fig. 1). Variants of this technology
will be suitable for a number of applications, including radiography3and radiation
therapy4. The desire for compact systems and high accelerating gradients in these
machines has driven improvements in several areas, especially transmission lines5,
dielectric materials6, and fast photoconductive switching7. One of the ultimate technical
limitations on the DWA concept is likely to be the electrical strength of the dielectric
wall itself. The limiting factor in the design of high voltage vacuum insulators is
generally vacuum surface flashover, rather than the bulk strength of the insulating
material8. The most widely accepted theory of surface flashover holds that an avalanche
of secondary electrons occurs along the insulator surface, desorbing gas through which
the breakdown occurs8-11. The gradients envisioned for DWAs, and especially the 100
MV/m gradient envisioned for proton radiotherapy, are well beyond the capabilities of
conventional, straight-walled vacuum insulator materials. A number of techniques, such
as angled insulators or applied magnetic fields, can increase the voltage at which
flashover occurs by making it more difficult for secondary electrons to return to the
insulator surface8. However, insulators in the DWA will be subjected to voltage
reversals, preventing optimized use of angled insulators which have a preferred polarity,
and the strong magnetic fields needed for magnetic flashover inhibition are not desirable
as they would complicate beam transport. In support of DWA development, we are

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currently studying multilayer High-Gradient Insulators (HGIs)12. HGIs are vacuum
insulating structures composed of alternating layers of metal and dielectric (Fig. 2). This
concept was originally identified by several researchers based on the observation that the
electric field strength needed to initiate flashover of conventional vacuum insulators
increased with decreasing length8, and pioneering experiments were performed by
Smith13, Gray14, and Pillai and Hackam15. When properly designed, these structures have
shown the capability to withstand higher voltages than conventional (un-angled)
insulators by a factor from 1.5 to 4 14, an improvement which is comparable to that
obtained by use of angled insulators8, and gradients of 100 MV/m have been
demonstrated for small structures subjected to nanosecond pulses16.Despite these
promising results, no comprehensive explanation of the operating mechanism of HGIs
has emerged, and experiments carried out by different researchers have yielded
contradictory results. In this paper we report the results of our recent experiments with
HGIs, including testing in an integrated DWA structure, and compare our results to those
in the literature. We also show how displacement current may affect the flashover
process in short-pulse, high-gradient DWAs.
II. Small Sample Testing
To investigate the conditionability of HGIs, and to determine what effect the choice of
insulator and metal layer thicknesses has on their strength, we tested a number of HGIs
consisting of thin Rexolite and stainless steel layers, hot pressed and machined to the
final 2.54 cm diameter (Fig. 3). These samples are listed in Tables I - III. In this section,
we describe our testing procedures and results, and compare these to results reported in
the literature.