Development of a Compact Radiography Accelerator Using Dielectric Wall Accelerator Technology Page: 4 of 6
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During this phase of the project, we focused on tech-
base development and demonstrating the utility of the
structure at high voltage. For the tech-base development,
we focused on (1) insulators, (2) pulse-forming lines and
dielectric materials, and (3) switching. By necessity, we
also focused on code and full 3-d time dependent model
development to speed up the design process with a
minimum of fabrication [2]. And lastly, we performed
high voltage demonstration experiments in preparation for
accelerator cell construction.
There are multiple configurations for the Dielectric
Wall Accelerator (DWA). The first implementation was
based on the asymmetric Blumlein [3]. Although this
configuration does not require magnetic material for
suppression of a reflected back wave, energy transfer to
the load is inefficient. For the symmetric Blumlein (fig
1), energy transfer to the beam can approach 100%, but a
backward wave must be suppressed to ensure full voltage
erection of the structure.
II. ACCELERATOR DEVELOPMENT
The overall layout of the accelerator is shown in Figure
2 and consists of an injector composed of multiple
symmetric Blumleins. Ferrite isolation is unnecessary in
the injector as the geometry allows the beam current to
dominate. In the present design, the injector provides an
energy gain of 1.5 MeV and the remaining 13 cells
provide 6.5 MeV for a final end-point energy of 8 MeV.
Overall length from the cathode to the end of the
accelerator is approximately 2-3 m. The drift section for
our particular application is approximately 3 m.Injector 5
CathodeAlthough our initial high voltage tests indicate a (dE/E)
of 8-9%, stable transport and minimum corkscrew can be
achieved with as much as (dE/E)~14% because of the
high acceleration gradient. Even at this higher limit, the
time integrated spot size is 0.6 mm radius at the final
focus magnet. And the on axis dose is 20.5 Rad in air at
1 m [4, 5].
Implementation of the DWA had several technical
objectives: achieving the necessary gradients in the
vacuum interface and materials, developing low leakage,
high specific energy dielectrics, and switching. Because
of the topology of the accelerator, the gradients in these
components were required to exceed the accelerator
gradient; charging was the most stressful condition.
These results were achieved and are summarized below.
A. Multi-layer High Gradient Insulators
A high-gradient insulator (HGI) consists of a series of
very thin (<1mm) stacked laminations interleaved with
conductive planes. This insulator technology was
originally conceived and disclosed by Eoin Gray in the
early 1980's and resulted from experimental observations
that the threshold electric field for surface flashover
increases with deceased insulator length. Pulse testing
these structures showed a significant improvement in the
breakdown electric field compared with conventional
insulators (Fig. 3).E
7 1000
U
U
07100
LU
3
0
m00 kV Cell
Conversion
TargetFigure 2. 8 MeV, 2 kA DWA design.
10
100 1a
2
13
UAnduron, 1976
Ohki. 1982
Thomp on,1996
An.1er$n, 1900
GIock, 1969
Wason. 18670P 102 10 10
Pulse Width - tp, nsFigure 3. Comparison of multilayer HGI structures with
conventional 00 insulators [6].
Several have postulated that surface flashover results
from charge injection into the insulator bulk at an
electrode interface and near to the surface. These models
predict a D-5 scaling, where D is the overall insulator
length. Coincidentally, from our earlier work, we
established that these multilayer structures also follow a
d-5 trend, where d is the insulator period length. Thus it
appears that these high gradient multilayer structures
behave as an ensemble of independent conventionalHGI (t-0 m scaling)
N 0
t-1+iscairng
I 116I scaling N'
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Sampayan, S.; Caporaso, G.; Chen, Y.; Hawkins, S.; Holmes, C.; Krogh, M. et al. Development of a Compact Radiography Accelerator Using Dielectric Wall Accelerator Technology, article, June 2, 2005; Livermore, California. (https://digital.library.unt.edu/ark:/67531/metadc878520/m1/4/: accessed July 16, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.