ELECTROMAGNETIC AND THERMAL SIMULATIONS FOR THE SWITCH REGION OF A COMPACT PROTON ACCELERATOR Page: 3 of 5
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ELECTROMAGNETIC AND THERMAL SIMULATIONS FOR THE
SWITCH REGION OF A COMPACT PROTON ACCELERATOR*
L. Wang#, G. J. Caporaso, J. S. Sullivan, LLNL, Livermore, CA, U.S.A.
A compact proton accelerator for medical applications
is being developed at Lawrence Livermore National
Laboratory. The accelerator architecture is based on the
dielectric wall accelerator (DWA) concept. One critical
area to consider is the switch region. Electric field
simulations and thermal calculations of the switch area
were performed to help determine the operating limits of
SiC switches. Different geometries were considered for
the field simulation including the shape of the thin Indium
solder meniscus between the electrodes and SiC. Electric
field simulations were also utilized to demonstrate how
the field stress could be reduced. Both transient and
steady-state thermal simulations were analyzed to find the
average power capability of the switches.
A compact proton accelerator based on the concept of
dielectric wall accelerator (DWA) with field gradients as
high as 100 MV/m is being developed [1,2]. The
objective of this project is to determine the feasibility of
making a compact proton accelerator for cancer therapy
treatment. The existing proton treatment facilities are very
large and costly. A compact proton machine will not only
reduce the cost, but also can be used in most hospitals.
One critical area to consider in the design is the switch
region. The switches considered for this application are
SiC photoconductive switches. They are compact
switches capable of operating at high electric field and
high peak current at elevated temperature with long
lifetime. Maximum switch critical field is limited by bulk
breakdown strength of SiC and field enhancement of
switch electrodes. In order to operate at 100 MV/m field
gradient in SiC switch material, field enhancement at
electrode edges needs to be minimized. In this study,
electric field stresses on different geometries were
simulated to demonstrate how the field stress could be
reduced. In addition to the field simulations, thermal
calculations were performed to help determine the
operating limits of SiC switches.
ELECTRIC FIELD SIMULATION
Figure 1 shows the two-dimensional view of a SiC
switch. A SiC layer is sandwiched between two
electrodes. Between the electrode and SiC layer is a very
thin layer of Indium solder meniscus.
* This work was performed under the auspices of
the U.S. Department of Energy by University of
California Lawrence Livermore National Laboratory
under contract No. W-7405-Eng-48.
Case Indium solder meniscus
Case2 pressed Indium solder meniscus
Figure 1: Two-dimensional configuration of a SiC switch.
The bottom pictures show an enlarged view of the area
between the electrode and SiC layer.
Since during the manufacturing process, Indium solder
meniscus could be pressed to have curved edges (Case 2
in Figure 1), electric field simulations were performed to
study the effects on the field enhancement. In the
simulation, the top electrode is charged to 11 kV while
the lower electrode is at ground. Magnitude of electric
field along the interface of SiC and Indium solder
meniscus for Case 1 and 2 are displayed in Figure 2 and
3, respectively. For both cases, the two peaks occur at the
edges as expected. For case 1, the peak field is about 80
MV/m and is below the breakdown strength of SiC
selected (around 200 MV/m). However, for Case 2, the
peak field stress is three times the peak field stress of
Case 1 and is above the breakdown threshold of SiC
selected. Therefore, SiC switches can fail at a lower
voltage than expected. The results explain what has been
observed in the experiment.
Figure 2: Magnitude of electric field along the interface of
SiC and Indium solder meniscus for Case 1.
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Wang, L; Caporaso, G J & Sullivan, J S. ELECTROMAGNETIC AND THERMAL SIMULATIONS FOR THE SWITCH REGION OF A COMPACT PROTON ACCELERATOR, article, June 15, 2007; Livermore, California. (https://digital.library.unt.edu/ark:/67531/metadc877564/m1/3/: accessed April 23, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.