Vacuum Insulator Development for the Dielectric Wall Accelerator Page: 4 of 33
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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)I'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 radiography3 and 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
material. 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 occurs-i. 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 surface. 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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Harris, J. R.; Blackfield, D.; Caporaso, G. J.; Chen, Y.; Hawkins, S.; Kendig, M. et al. Vacuum Insulator Development for the Dielectric Wall Accelerator, article, March 17, 2008; Livermore, California. (https://digital.library.unt.edu/ark:/67531/metadc900190/m1/4/?rotate=180: accessed July 16, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.