Preliminary Results from the UCLA/SLAC Ultra-High Gradient CerenkovWakefield Accelerator Experiment Page: 2 of 12
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1. INTRODUCTION
The accelerating field achievable in conventional radio frequency acceler-
ators is ultimately limited by the breakdown of the metallic accelerating
structure. It is well understood that the field sustainable in a cavity in-
creases with the frequency of the cavity 1,2. The empirical relation for the
surface breakdown field of modern metallic cavities is given by
ES 220(f[GHz])1/3 MV/m, (1)
where f is the RF frequency and E5 is the maximum sustainable surface
electric field, which is about 2.5 times greater than the maximum accel-
erating field 2. It is difficult to simply follow this scaling by using ever
higher frequency cavities due to what could be called the "THz gap"; the
current lack of practical high power source between X-band microwaves (~
11 GHz) and infrared lasers (~ 28 THz).
There is considerable interest in the optical limit of break down scaling
and the building of dielectric structures powered by high intensity laser
beams. Various studies have indicated that GV/m accelerating fields should
be possible in dielectric laser accelerators s as long as the driving laser pulses
are very short 4. The difficulty with laser accelerators, however, is that they
operate at very short wavelengths (e.g. 10.6 pm at 28 THz), which greatly
complicates beam injection and phasing.
Dielectric accelerators can also be powered directly by high energy
charged particle beams via wakefield excitation 5,6, eliminating the need
to generate high power electromagnetic waves. Additionally, a particle
beam driven dielectric wakefield accelerator can operate at essentially any
wavelength since the accelerating wakefield wavelength is determine by the
dielectric geometry. This type of accelerator has been studied in depth
over the last several years, but with the maximum fields limited to 10's of
MeV/m by the lack of ultra-short drive beams. According to the scaling
laws which govern dielectric wakefield accelerators, however, the recently
achieved 20 pm pulse-length beams obtained at the Stanford Linear Ac-
celerator Center (SLAC) Final Focus Test Beam (FFTB) facility may be
sufficient to generate longitudinal fields in excess of 1 GV/m.
2. THEORY AND SIMULATION
An electron beam driven dielectric wakefield accelerator is essentially an
unform dielectric tube coated on the outside with a conductor, see Fig.
1. When an intense electron beam passes through center of the tube its
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Thompson, M. C.; Badakov, H.; Rosenzweig, J. B.; Travish, G.; Hogan, M.; Ischebeck, R. et al. Preliminary Results from the UCLA/SLAC Ultra-High Gradient CerenkovWakefield Accelerator Experiment, article, February 6, 2008; Menlo Park, California. (https://digital.library.unt.edu/ark:/67531/metadc895232/m1/2/: accessed March 28, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.