Multi-Physics Analysis of the Fermilab Booster RF Cavity Page: 2 of 3
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to get realistic losses and temperature profile. The overall
quality factor of the cavity ranges from 280 at the
injection frequency to 1100 at the extraction frequency.i i
(a)
(b) (c)
Figure 2: Geometry of the Booster cavity.
RF MODEL AND ANALYSIS
A full 3D cavity model with all required details has
been created in Comsol Multiphysics [5], as shown in
Figure 3. Utilizing the symmetry of the structure by
enforcing perfect magnetic conductor boundary
conditions on the symmetry planes allows simulating just
one quarter of the structure, thus reducing the complexity
of the computation.
Figure 4 shows the electric field calculated on the
cavity surface at the injection frequency (37.77 MHz) for
a gap voltage of 55 kV (27.5kV per gap), indicating a
maximum field of 3.28 MV/m in the gap. It is quite far
from the Kilpatrick break down criteria -10 MV/m.
Therefore, in principal, sparking in vacuum area shouldn't
be a concern if the cavity surface is relatively clean. On
the other hand, the maximum electric field in air occurs
nearby the edges of the tuner connection. The
corresponding maximum field is about 17 kV/cm, which
is 57% of the field breakdown limit in air (30kV/cm) as
shown in Figure 4(b). It is worth noting that, the
maximum field value depends largely on the blend radius
of the cavity edges, as shown in Figure 4(c).
Given that the material properties change during the
frequency sweep from the injection frequency (37.77
MHz) to the extraction frequency (52.81 MHz), we have
tried to match our simulation model to the actual
measured quality factor of the cavity during normal
operation. Figure 5 shows the simulated curve (solid blue)
compared to the measured actual value (dotted red). Good
agreement between the simulated values and the
measured values was obtained by adjusting the material
properties in the model. This step was imperative in orderp
ItFigure 3: 3 -D simulation model for a quarter of the
Booster cavity.
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(b)
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Blend Raduis [in]
(c)
Figure 4: Simulated electric field (55 kV gap voltage):
(a) on the surface of the Booster cavity, (b) on the tuner
connection, (c) maximum electric field in air versus edge
blend radius.U
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Awida, M.; Reid, J.; Yakovlev, V.; Lebedev, V.; Khabiboulline, T. & Champion, M. Multi-Physics Analysis of the Fermilab Booster RF Cavity, article, May 14, 2012; Batavia, Illinois. (https://digital.library.unt.edu/ark:/67531/metadc845356/m1/2/: accessed July 16, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.