STATUS REPORT ON THE RESEARCH AND DEVELOPMENT OF
HIGH-GRADIENT NORMAL CONDUCTING RF STRUCTURES FOR
THE NEUTRINO FACTORY AND MUON COLLIDERCOLLABORATION 1J. N. Corlett, M. A. Green, D. Li, R. Rimmer, M. Zisman, R. MacGill, N. Hartman, LBNL, USA
A. Moretti, T. Jurgens, N. Holtkamp, S. Geer, M. Popovic, FNAL, USA
D. Summers, M. Booke, University of Mississippi, USA; E. Black, IIT, USA
H. Kirk, H. Wang, R. Palmer, BNL, USA
Abstract
We present a status report on the research anddevelopment of high-gradient normal-conducting RFstructures for the Neutrino Factory and Muon Collider
Collaboration. High-gradient RF structures are required inseveral areas of a neutrino factory or muon collider,including the muon bunching section, and the ionization
cooling section. In these regions the RF structures will beenclosed in strong focussing solenoidal magnets,eliminating the application of superconducting RFtechnology [1]. We discuss here the bunching andionization sections, where the required gradients are up to17 MVm-1 at 201.25 MHz, 8 MVm-1 at 402.5 MHz, and30 MVm-1 at 805 MHz. We propose using linear
acceleratingstructures,withindividualcells
electromagnetically bounded. Each cell will be poweredindependently at appropriate phase, and cell length
adjusted to optimize shunt impedance of the assembled
structure. This efficient design allows for relatively small
field enhancement on the structure walls, and peak surface
field approximately 50% greater than the effective
accelerating field. The electromagnetic boundary of eachcell may be provided by a thin Be sheet, or an assemblyof 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. We
describe the status of 805 MHz high-power test facilities,
high-gradient π-mode open cell structure, and beryllium
window or gridded-tube "pill-box" cavity studies andplans. 201.25 MHz and 402.5 MHz structure design is inprogress, and will benefit from experimental work with805 MHz components.

1 INTRODUCTION
One of the most demanding applications ofconventional conducting RF in a muon collider orneutrino factory is in ionization cooling of the muon
beam. In such schemes, muons lose momentum in a lowZ scattering material, or absorber (current designs use a
1 Work supported by the U.S. Department of Energy undercontract
No.s
DE-AC0376SF00098(LBNL),
DEAC0276CH00016 (FNAL), DE-AC0298CH10886 (BNL), DEFG0291ER40622 (University of Misissippi)

liquid hydrogen absorber), after which they regain
longitudinal momentum in a linear RF acceleratingstructure. Sections of RF and absorber are placed inside analternating solenoidal magnetic focussing field. A highRF gradient is required to rapidly restore the longitudinal
momentum to the beam, allowing closely-packed
scattering and acceleration blocks, and in a time dictatedby the short muon decay time.
The purpose of the RF systems is to match the muon
beam into the longitudinal acceptance of the coolingchannel, and replenish the longitudinal momentum lostby scattering in the hydrogen absorber. The largetransverse emittance and energy spread of the muon beamrequire a large RF bucket and a large physical aperture tocontain the beam. The upstream sections of the coolingchannel are envisaged as 201.25 MHz structures, anddownstream structures, where the emittance is reduced by
the action of previous cooling sections, at 805 MHz.
Transverse apertures in the 201.25 MHz sections arerequired to be approximately 40 cm, and 16 cm for the805 MHz sections. Such large bores lead to low shunt
impedance in conventional open-iris structures. To
increase the shunt impedance, we propose cavities withbeam-pipe apertures electromagneticaly bounded by thinwindows of beryllium, or assemblies of thin-walled tubes
of low-Z material. In addition, we will measure theproperties of a more conventional π-mode open-cellstructure 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 less expensive. Future work will
focus on the 201.25 MHz structures for the neutrino
factory. The larger structures at lower frequency present
different challenges in fabrication and control of thermaleffects.
An experimental area dedicated to the high-power
testing of 805 MHz structures for the neutrino factory andmuon collider collaboration has been built at Fermilab. A
201.25 MHz facility is also envisaged.

function of field strength and gradient. Figure 1 shows themeasured field in the magnet under various operating
conditions.

2. LAB-G TEST FACILITY
The Lab-G test facility at Fermilab allows high-power
testing of RF components at 805 MHz. The facility
comprises a concrete bunker, which acts as an x-ray shieldduring high-power operation of a cavity, asuperconducting solenoidal magnet inside which theaccelerating RF structures are placed, a high-power RFsystem with power fed into the bunker via waveguide, andcryogenics, water, controls, interlocks systems. The
facility is fully operational and awaits the installation of
the first test cavity.

3. 805 MHZ OPEN-CELL CAVITY
Figure 2 shows a drawing of a 6-cell open-cell cavity
designed to achieve the required gradient of 15 MVm-1,
with a shunt impedance of 20 MΩm -1. The cavity
operates in π-mode, with iris apertures varied to follow
the transverse beam profile, and rounded edges to reduce
electric field enhancement. The calculated peak surface
field is approximately 22 MVm-1. A 6-cell copper cavity
has been fabricated, tuned, and is about to be installed in
Lab-G to test achievable gradient and conditioning.

2.1 RF system
A klystron is used to provide a peak power of up to 10
MW in a 100 microecond pulse, at up to 10 Hz repetition
rate. Power is fed into the concrete shielded bunker
through WR975 waveguide, with a ceramic vacuum
window placed near the accelerating structure. Two
vacuum pumps may be mounted on the UHV side of thewindow. Vacuum components are shielded from themagnetic field of the solenoid. A water circuit controllable
to ± 0.1°C allows coarse control of cavity resonant
frequency through water cooling channels wrapped aroundthe cavity. The klystron bandwidth of ± 2 MHz allows afeedback system to fine control the RF source frequency
to follow the cavity resonance.

2.2 Superconducting solenoid
The superconducting solenoid magnet contains a pair ofcoils, which may be independently powered to allow
either a uniform field of up to 5T or a gradient field at thecenter of the magnet. This system allows reproduction of
the solenoidal fields in the 805 MHz section of thecooling channel, and the study of multipacting effects as aFigure 2. 6-cell open-cell structure

4. 805 MHZ BE WINDOW RFSTRUCTURE
To test the feasibility of thin beryllium foils in highfield RF structures, a low-power test cavity has been built[2], and now a high-power test cavity is being fabricated.

4.1 Low-power testing of Be windows

Figure 1. Superconducting solenoid field measurements

Low-power tests have been performed to assess thebehavior of windows formed from thin (125 µm) foils of
pre-stressed beryllium [3]. Heating of the window byradiant heat from a halogen lamp, and by RF heating in acavity, have allowed measurement of the detuning effects
caused by displacement of the foil due to thermal
expansion. The foils remain flat until the thermal
expansion exceeds the tensile pre-stress. The maximum
allowable temperature gradient across the foil isapproximately 35°C.
ANSYS modeling shows behavior similar to thatmeasured for the thin Be foils [4]. Further finite element
analysis will be used in the design of window structures
for the larger, lower-frequency sections of the coolingchannel. Figure 3 shows ANSYS calculations of thedisplacement at the center of a 38 cm diameter pre-stressedfoil of three different thicknesses as a function of power
dissipated in the foil. Similar calculations for a 16 cm foilshow reasonable agreement with measurements.
Measurements have also been performed on Be
windows in a cavity cooled to liquid nitrogen temperature,at which the electrical and thermal conductivities of Be
increase significantly. In this case the detuning of the RF
cavity due to expansion of the Be foil was notmeasurable.
Small (approximately 1 cm diameter), 125 µm thick Be
foils have been tested to high gradient in the A0 facility at
FNAL, and have survived surface fields of 35 MVm-1.
Figure 4. Be window high-power test cavity.

6. COOLING CHANNEL LAYOUTS
We have developed schemes to integrate the various
components into a cooling channel of manageable
proportions. Vacuum systems, RF power feedthroughs,
diagnostics and instrumentation have been integrated into
the assembly. Figure 5 shows a cooling channel layout
for a representative configuration for a neutrino factory
cooling channel, with 201.25 MHz RF cavities.

Figure 3. Calculated displacement of pre-stressed Be
foil as a function of power dissipated in 38 cm diameter
window thickness 127, 190.5, and 254 µm.

4.2 High-power test cavity for Be windows
A high-power test cavity has been designed to accept
demountable Be foil windows for testing to high gradientin the Lab-G facility. Figure 4 shows a sectional drawingof the cavity. The input waveguide is to the right, and theBe windows are mounted either side of the cavity center
Windows of different designs may be tested in thiscavity, and we plan to experiment with variousthicknesses, variation of the thickness as a function ofradius, different alloys, and different pre-stress. In the LabG facility we will be able to test survivability of the large
foils at high gradient, and conditioning requirements undervarious magnetic field configurations. Low secondary
emission yield coatings and conditioning tests can bemade with various surfaces and windows in the cavity.

5. GRIDDED TUBE DESIGNIn addition to the flat-foil Be window boundaries to thecavities, we are investigating an electromagnetic
termination created by an arrangement of thin-walled tubes
fabricated from low-Z metals. Such structures would havethe advantage of increased mechanical strength andstability over the flat foil, and may allow additionalcooling of the structures by passing cold gas through thetubes.

Figure 5. Cooling channel layout

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Holtkamp, PAC’99, New York, March 29-April2,1999.
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N. Corlett, N. Hartman, S. Kar, M. Leung, D.
Lozano, Mucool 110.
[3] "Deflection measurements of thin foils for the muon
cooling RF channel", J.N. Corlett, N. Hartman, D.
Li, D. Turler, Mucool 62[4] "Thin Beryllium Windows - Analysis and DesignStatus", N. Hartman, D. Li, J. Corlett, Mucool 180