Advanced electromagnetic design of cavities for high current accelerators Page: 4 of 5
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1 st3rt harmonic vs. thncma b d tal d defflei
defletod ah st aa. in.
d004almoe 0.6e4sos oiiddelco ht
.s aI badditiona d d.
0.8YM 01 0.2liemin 0^10 soonde
Figure. 5. Comparison of the change in deflecting field
with time for the fundamental and the combined deflection.
The two deflecting surfaces nearly coincide, the combined
deflection shows the maximal effect.
damental mode. Figure 4 shows a modified deflector that
has an additional deflecting mode at 1.05 GHz. The cavity
shape variation has been chosen to (1) minimally affect the
fundamental mode and (2)add surfaces at locations with
low magnetic field amplitude to minimize the introduction
of additional wall losses. The cavity represented here is not
yet optimized but already indicates a significant improve-
ment in terms of rf losses (see Table 2).
Table 2. Comparison of Some Data for the 20 MeV and
12 MeV Deflectors
20 MeV Cavity 12 MeV Cavity
Gap Field 24 MV/in 20 MV/in
RF-Losses 48 kW 34 kW
Peak Loss-Dens. 68 W/cm2 60 W/cm2
In Fig. 5 the deflecting pulses from the fundamental and
the combined harmonics are compared. The combined de-
flection pulse stays at a high level for a longer portion of
the rf period and then drops off faster than the fundamen-
tal. The third harmonic field has a phase difference of 1800
at the center of the gap and an amplitude of 1/10 with re-
spect to the fundamental.
III. SUPERCONDUCTING CAVITIES FOR
MEDIUM ENERGY PROTONS
Cost-saving issues in the design of high current proton
accelerators raise the question of using superconducting
cavities. Low risk and the need for only little technical
development favor the use of elliptical cavities at high en-
ergies. At lower energies a good candidate seems to be
the spoke structure, first proposed by J. Delayen et al.
. Figure 6 shows an eight-cell spoke cavity with a cross-
bar structure at 7 MeV for 350 MHz. This is still a non-
Figure. 6. Eight-cell spoke cavity with cross-bar structure
at 350 MHz and 7 MeV. This is one quarter of the cavity as
discretized with MAFIA. The spoke shape can be further
improved, but did already help to evaluate some general
properties of this new structure. The beam-axis coincides
with the z-axis.
optimized geometry that needs improvements. But it al-
ready hints that it seems suitable for several reasons:
. It has a higher mechanical stability at low 1 than an
. The bore size can be freely chosen, without affecting
. The mode spectrum indicates that there are no prob-
lematic modes that should mix with the accelerating
. It has a high ZT2/Q (800 fl/m or more).
Ratios of Epea.k /EoT of the order of 5 seem to be achievable
at the energy of 7 MeV. This will improve with higher 0.
Our spoke model especially needs further improvement to
achieve more reasonable ratios of H,0k /EoT. These are
predominantly determined by the geometry of the spoke
base, where it meets the outer cavity wall.
 M. Bartsch et al. "Solution of Maxwell's Equations",
Computer Physics Comm. 72, 22-39 (1992)
 James H. Billen et al. "A New Rf Structure for
Intermediate-velocity Particles", Proc. of the 1994 In-
ternational LINAC Conference, Vol I, p. 341
 Frank L. Krawczyk, Nathan K. Bultman, K.C. Dominic
Chan, Rick L. Martineau, Subrata Nath, Lloyd M.
Young "Design of RF-Cavities in the Funnel of Accel-
erators for Transmutation Technologies", Proc. of the
ADTT Conference, Las Vegas, Nevada, July 1994
 Frank L. Krawczyk, "Comments on the ESS Funnel
Cavity Design", LANL Memo AOT-1:94-171, 1994
 Jean R. Delayen, Cort L. Bohn and C. T. Roche, Pro-
ceedings of the 1990 Linear Accelerator Conference, p
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Krawczyk, F.L. Advanced electromagnetic design of cavities for high current accelerators, article, May 1, 1995; New Mexico. (digital.library.unt.edu/ark:/67531/metadc710211/m1/4/: accessed September 19, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.