Traveling Wave RF Systems for Helical Cooling Channels Page: 1 of 4
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FERMILAB-CONF-09-204-AD-APC
TRAVELING WAVE RF SYSTEMS FOR HELICAL COOLING CHANNELS*
K. Yoneharat, A. Lunin, A. Moretti, M. Popovic, G. Romanov, Fermilab, Batavia IL, USA
M. Neubauer, R.P. Johnson, Muons, Inc., Batavia, IL, USA
L. Thorndahl. CERN. Geneva. SwitzerlandAbstract
The great advantage of the helical ionization cooling
channel (HCC) is its compact structure that enables the
fast cooling of muon beam 6-dimensional phase space.
This compact aspect requires a high average RF gradient,
with few places that do not have cavities. Also, the muon
beam is diffuse and requires an RF system with large
transverse and longitudinal acceptance. A traveling wave
system can address these requirements. First, the number
of RF power coupling ports can be significantly reduced
compared with our previous pillbox concept. Secondly,
by adding a nose on the cell iris, the presence of thin
metal foils traversed by the muons can possibly be
avoided. We show simulations of the cooling
performance of a traveling wave RF system in a HCC,
including cavity geometries with inter-cell RF power
couplers needed for power propagation.
INTRODUCTION
A muon collider has been proposed as the next
generation collider to explore the energy frontier beyond
the LHC. Because the muon is unstable, a compact muon
accelerating and phase space cooling system is required.
To this end, a Helical ionization beam phase space
Cooling Channel (HCC) has been proposed [ 1 ] and
studied [ 2 ]. It consists of a helical dipole, helical
quadrupole, and solenoid magnetic components, and a
dense hydrogen gas is continuously filled in a helical
beam path for ionization beam cooling. A high pressure
hydrogen gas filled RF (HPRF) cavity [3,4] will be used
for compensation of the energy loss in the ionization
beam cooling process.
Practical RF structures are now being modeled for the
HCC, which is inherently compact yet requires relatively
low frequencies. Here we consider a traveling wave
structure, which seems preferable to the pillbox approach
that has been studied before because the number of power
coupler ports can be significantly reduced.
RF FIELDS IN HCC SIMULATIONS
Figure 1 shows a typical muon beam phase space
evolution of transverse and longitudinal planes in the
phase space cooling channels for muon colliders. Solid
lines represent simulated results with realistic helical
magnetic fields. Broken lines represent cooling channel
sections in which the simulations have not yet been done
with any realistic fields.
*Work supported in part by USDOE STTR Grant DE-FG02-08ER86350
tyonehara@fnal.govAfter the 6D HCC sections, even smaller emittances
may be achieved using techniques such as Parametric-
resonance Ionization Cooling or Reverse Emittance
Exchange with very high field magnets [5].5 .0-
~E1.0.
S0.5
-Reverse Emitiance
\ FxchangeChannel
'; ~ ~ _na iemcc
,ic
4 __- ^L -Icc
- -
Trai'nsri f[nniratjnCoinn Ceh: nmmad]I
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Transverse etnrittance [trm rad]Fig. 1: Fernow-Neuffer plot of emittance evolution for a
muon collider. Dashed lines represent parts yet to be
simulated, solid lines are simulated with realistic
magnetic fields generated by practical current conductors
but not practical RF solutions.
HCC simulation studies have been done in simple RF
structures, where the RF field distribution is represented
as an ideal pillbox cell oriented along the axis of the HCC
structure. This RF field direction is then at an angle
relative to the helical beam path, and only the strong
coupling between transverse and longitudinal momenta in
the helical magnet prevents this situation from causing
unwanted beam heating. Table 1 shows the designed RF
field parameters in a series of HCCs simulation with
discrete RF frequencies.
Table 1: Designed RF parameter in simulation
parameter unit Value
Mean energy loss rate MeV/m 10.3
Field gradient (Eo) MV/m 16.1
Tentative frequency MHz 200, 400, 800, 1600
Tentative cell length mm 200, 100, 50, 25
HCC TRAVELING WAVE RF DESIGN
In this study, large iris-loaded RF cells have been
investigated as shown in Figure 2. The RF traveling wave
is injected from one end of the helical RF structure. The
phase velocity (0.92c) is tuned by adjusting the cell outer
radius (- 25 cm for 400 MHz). Unfortunately an
excessive propagating RF power of - 2.5 GW is needed
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Yonehara, K.; Lunin, A.; Moretti, A.; Popovic, M.; Romanov, G.; Neubauer, M. et al. Traveling Wave RF Systems for Helical Cooling Channels, article, May 1, 2009; Batavia, Illinois. (https://digital.library.unt.edu/ark:/67531/metadc934904/m1/1/: accessed April 23, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.