High-gradient normal-conducting RF structures for muon cooling channels Page: 1 of 3
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HIGH-GRADIENT NORMAL-CONDUCTING RF STRUCTURES FOR
MUON COOLING CHANNELS*
J. Corlettt, M. A. Green, N. Hartman, A. Ladran, D. Li, R. MacGill, R. Rimmer, LBNL, Berkeley, CA, USA
A. Moretti, T. Jurgens, N. Holtkamp, FNAL, Batavia, IL, USA
E. Black, IIT, IL, USA
D. Summers, M. Booke, University of Mississippi, Oxford, MI, USAAbstract
We present a status report on the research and
development of high-gradient normal-conducting RF
structures for the ionization cooling of muons in a
neutrino factory or muon collider. High-gradient RF
structures are required in regions enclosed in strong
focussing solenoidal magnets, precluding the application
of superconducting RF technology [1]. We propose using
linear accelerating structures, with individual cells
electromagnetically isolated, to achieve the required
gradients of over 15 MV/m at 201 MHz and 30 MV/m at
805 MHz. Each cell will be powered independently, and
cell length and drive phase adjusted to optimise shunt
impedance of the assembled structure. This efficient design
allows for relatively small field enhancement on the
structure walls, and an accelerating field approximately 1.7
times greater than the peak surface field. The
electromagnetic boundary of each cell may be provided by
a thin Be sheet, or an assembly of thin-walled metal
tubes. Use of thin, low-Z materials will allow passage of
the muon beams without significant deterioration in beam
quality due to scattering. R&D in design and analysis of
robust structures that will operate under large electric and
magnetic fields and RF current heating are discussed,
including the experimental program based in a high-power
test laboratory developed for this purpose.
1. INTRODUCTION
One of the most demanding applications of
conventional conducting RF in a muon collider or
neutrino factory is in ionization cooling of the muon
beam. In such schemes, muons lose momentum in a low-
Z scattering material, or absorber (current designs use a
liquid hydrogen absorber), after which they regain
longitudinal momentum in a linear RF accelerating
structure. Sections of RF and absorber are placed inside an
alternating solenoidal magnetic field, which focuses the
muon beam. A high RF accelerating field is required to
rapidly restore the longitudinal momentum to the beam,
allowing closely-packed scattering and acceleration
sections, and in a time dictated by the short-lived muons.
RF systems are used for the purpose of matching the
muon beam into the longitudinal acceptance of the
cooling channel, and to replenish the longitudinal
momentum lost by scattering in the hydrogen absorber.
* Work supported by the U.S. Department of Energy under contract
No.s DE-AC0376SF00098 (LBNL), DE-AC0276CH00016 (FNAL),
DE-AC0298CH10886 (BNL), DE-FG0291ER40622 (University of
Misissippi). tjncorlett@lbl.govThe large transverse emittance and energy spread of the
muon beam require a large RF bucket and a large physical
aperture to contain the beam. Upstream sections of the
cooling channel are designed with 201.25 MHz structures,
and downstream structures, where the emittance is irduced
by the action of previous cooling sections, are designed
for 805 MHz RF systems. Bunching and matching
cavities are also used at 402.5 MHz. Transverse apertures
at the irises of the 201.25 MHz sections are required to be
approximately 40 cm, and 16 cm for the 805 MHz
sections. This aperture accounts for more than half of the
diameter of a typical cavity at these frequencies
(determined by the radius for a cylindrical TM010 mode
pillbox). Such large bores lead to low shunt impedance in
conventionally conducting open-iris structures. To
increase the shunt impedance, we propose cavities with
beam-pipe apertures electromagnetically bounded by thin
windows of beryllium, or assemblies of thin-walled tubes
of low-Z material. In addition to studies of these novel
structures, we have begun measurements of the properties
of a more conventional 7-mode open-cell structure under
the required conditions of accelerating RF field and static
solenoidal magnetic field.
Initial studies have been focused on the smaller 805
MHz structures, a frequency at which we have readily
available RF power sources, and at which structures are
relatively small and inexpensive. Current and near-term
work will focus on 201.25 MHz structures, more
appropriate for the neutrino factory, as well as the first
stage cooling section for a collider. The larger structures at
lower frequency present challenges in fabrication, control
of thermal effects, and integration into a cooling channel.
An experimental area dedicated to the high-power
testing of 805 MHz structures for the neutrino factory and
muon collider collaboration has been built in Lab-G at
Fermilab, and a 201.25 MHz facility is also planned.
2. 805 MHZ STRUCTURES
Accelerating structure designs at 805 MHz have been
developed for both a pillbox-like cavity with Be foil
windows, and for a 7-mode open cell structure. The
pillbox-like structure has a higher shunt impedance, while
the open cell structure will be used to test achievable
gradient in a more conventional design.
2.1 805 MHz Be Window Cavity
Thin Be Foils are mounted in Be-alloy rings such that
the foil is under tension after cool-down from the brazing
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Corlett, J.N.; Green, M.A.; Hartman, N.; Ladran, A.; Li, D.; MacGill, R. et al. High-gradient normal-conducting RF structures for muon cooling channels, article, June 12, 2001; California. (https://digital.library.unt.edu/ark:/67531/metadc719451/m1/1/: accessed April 24, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.