MT-20 Paper 4L-03, to be published in IEEE Transactions on Applied Superconductivity 18. No. 2, LBNL-63481
1
Abstract— The RF coupling coil (RFCC) module of MICE is
where muons that have been cooled within the MICE absorber
focus (AFC) modules are re-accelerated to their original
longitudinal momentum. The RFCC module consists of four
201.25 MHz RF cavities in a 1.4 meter diameter vacuum vessel.
The muons are kept within the RF cavities by the magnetic field
generated by a superconducting coupling solenoid that goes
around the RF cavities. The coupling solenoid will be cooled
using a pair of 4 K pulse tube cooler that will generate 1.5 W of
cooling at 4.2 K. The magnet will be powered using a 300 A two-
quadrant power supply. This report describes the ICST
engineering design of the coupling solenoid for MICE.
Index Terms—Niobium Titanium, Passive Quench Protection,
Superconducting Solenoid
. INTRODUCTION
HE muon ionization cooling experiment (MICE) will be
located at the Rutherford Appleton Laboratory [1]. MICE
consists of a proton target station, a pion collection system, a
muon decay channel, and a muon cooling channel with
spectrometers at each end. The muon cooling channel
contains two spectrometer modules to analyze muons and
other particles that enter and leave the MICE cooling channel
[2], three absorber focus coil (AFC) modules that focus and
ionization cool the muons in an absorber inside the focusing
magnet [3], and two RF coupling coil (RFCC) modules that
reaccelerate the muons back to their original momentum [4].
The RFCC module consists of a 1.9 m long vacuum vessel
that contains four 201.25 MHz RF cavities that are bounded
by thin beryllium windows [5]. Each of the cavities is
separately powered through an RF coupler. The coupling
magnet is located outside of the vacuum vessel that contains
the RF cavities. The coupling magnet is a superconducting
magnet that produces enough magnetic field (up to 2.2 T on
the magnet centerline) to guide the muons and keep them
within the iris of the thin RF-cavity windows. Fig. 1 shows an
RFCC module of MICE. Shown in Fig. 1 are the RF cavity
vacuum chamber, the RF cavities, and the coupling magnet.
Manuscript received 27 August 2007. This work was supported by the
Lawrence Berkeley Laboratory and the Office of Science, U.S. Department of
Energy under Contract No. DE-AC02-05CH11231.
M. A. Green is from the Lawrence Berkeley National Laboratory, Berkeley
CA 94720, USA, e-mail: magreen@lbl.gov. D. Li, S. O. Prestemon and S. P.
Virostek are also from the Lawrence Berkeley National Laboratory.
L. Wang, F. Y. Xu, H. Wu, L. K. Li, X. L. Guo, C. S. Liu, G. Han, L. X.
Jia are from the Institute of Cryogenics and Superconductive Technology,
Harbin Institute of Technology, Harbin 150080, P. R. China.
Fig. 1. The RFCC Module showing the RF Cavities, the RF Vacuum Vessel,
the Vacuum Pump, The Couplers, a Cavity Tuners and the Coupling Magnet.
I. THE MICE COUPLING MAGNET
The Institute of Cryogenics and Superconductive
Technology at the Harbin Institute of Technology in
partnership with the Lawrence Berkeley National Laboratory
will build the coupling magnets for MICE and the MUCOOL
experiment [6] at Fermilab. MICE requires two coupling
coils; the 201 MHz RF cavity breakdown and dark current
experiment at Fermilab requires a single coupling coil. The
three magnets are identical except for the way the magnet
cryostats are attached to the stand (see Fig. 1).
The coupling magnet consists of a single superconducting
coil that carries up to 3.35 MA-turns [7]. The magnet cryostat
is determined by the spacing between the RF couplers of the
two center cavities within the RF cavity vacuum vessel. The
vacuum pumping ports (see the vacuum pumps at the bottom
of Fig. 1) for the RF cavities have the same spacing as the two
center RF couplers. The left side of Fig. 2 shows a 3D view of
the MICE coupling magnet cryostat. The right side of Fig. 2
shows a top view of the coupling magnet cryostat. The
magnet coolers are shown in both views. The magnet cryostat
is bounded by the center cavity couplers and vacuum ports. In
order to maximize the coupling magnet coil length, the magnet
vacuum vessel is designed to fit around the couplers and
vacuum pump out ports.
The Engineering Design of the 1.5 m Diameter
Solenoid for the MICE RFCC Modules
L. Wang, M. A. Green, F. Y. Xu, H. Wu, L. K. Li, X. L. Guo, C. S. Liu, G. Han, L. X. Jia, D. Li, S. O.
Prestemon and S.P Virostek
T
Magnet Cryostat
RF Vacuum Vessel
RF Cavity
RF Coupler
RF Tuner
RFCC Stand
Vacuum Pump
Force Gusset
MT-20 Paper 4L-03, to be published in IEEE Transactions on Applied Superconductivity 18. No. 2, LBNL-63481
2
Fig 2. A View of the Coupling Magnet Cryostat. (The left side shows a 3 dimensional view of the magnet cryostat that shows the service stack and the two
pulse tube coolers. The right view shows the magnet cryostat from the top. Note; the baseline magnet design is based on the use of two pulse tube coolers.)
Fig. 2 shows the indentations in the coupling magnet
cryostat that fit around the 130 mm OD couplers for the two
center RF cavities. There is an indentation in the cryostat wall
at the bottom of the magnet cryostat that fits around a 130 mm
OD vacuum pumping post for the RF cavities and the vacuum
space around the RF cavities. The depth of the indentations in
the magnet cryostat wall is about 40 mm.
The cryostat shown in Fig. 2 has four cold-mass supports at
each end. The 300 K ends of the cold-mass supports are at
angles of 45, 135, 225 and 315 degrees with respect to the RF
coupler ports. The cold mass supports carry a longitudinal
force of 500 kN [8]. This force is transmitted to the ends of
the magnet cryostat through a cone shaped structure around
the support bands. There is an structure on the cryostat wall to
carry the 135 kN longitudinal force between the cold mass
supports, that are at the same angular orientation.
Pulse tube coolers will be used to cool the coupling magnet.
The reason for selecting a pulse tube cooler is the magnetic
field that is present at the magnet service neck [9]. The
coolers shown in Fig. 2 have the rotary valve, its motor, and
the ballast tanks attached to the cold head that goes down into
the service neck for the magnet cryostat. The rotary valve
motor for the cooler can be shielded with iron. Alternatively,
the valve motor and ballast tanks can be moved p to 1-meter
from the cooler cold head assembly. The pulse tube coolers
shown in Fig.2 must be oriented vertically with the cold heads
in the down position. The position of the magnet coolers and
HTS leads is determined by the magnetic field generated by
the coupling magnet and the rest of MICE [10, 11].
III. THE COUPLING MAGNET CONDUCTOR
The magnet conductor selections depended on a number of
factors. First, the maximum current in the coupling magnet is
limited to 300 A. Having a larger magnet current is positive
from a quench protection perspective. A larger magnet
current means that the heat flow into the first stage of the
cooler is increased. A larger first stage heat flow translates to
a higher first stage temperature and a larger heat leak at 4 K.
The quench analysis showed that the magnet will quench
safely even at a lower current, so the conductor decision
favored reducing the heat leak into both stages of the cooler.
A lower current is also better for the HTS leads, which must
operate in a magnetic field as high as 0.4 T. The same
conductor as used for the tracker magnet was selected [2].
The conductor cross-section is shown in Fig. 3.
Fig. 3. Coupling Magnet Conductor Cross-section with 222 Nb-Ti Filaments
in a Copper Matrix (I
c
= 760 A @5 T & 4.2 K).
Pulse Tube Cooler
Cold Mass Support
Bayonet
Current Leads
RF Pumping Port Indent
Vacuum Vessel
Pulse Tube Cooler
RF Coupler Indent
Service Stack
Service Stack
Cold Mass Support
Cryostat Pumping Port
Force Structure
Nb-Ti
Cu
MT-20 Paper 4L-03, to be published in IEEE Transactions on Applied Superconductivity 18. No. 2, LBNL-63481
3
Fig. 4. A 3D View of the Coupling Magnet Cold Mass Assembly and the 60 K Shield Assembly (On the left side of the figure is the magnet cold mass with its
Shield. On the right side of the figure is the magnet cold mass. Shown as part of the cold mass assembly are the coolers, the leads, and the cold mass supports.)
The superconductor in the coupling coil conductor is 47 wt
percent titanium and niobium. The Nb-Ti has been processed
and heat-treated for a j
c
> 2750 A mm
-2
at 5 T and 4.2 K. The
conductor for the coupling magnet has insulated dimensions of
1.65 mm by 1.00 mm. (The bare dimensions are 1.60 mm by
0.95 mm.) The copper to superconductor ratio is about 4. The
nominal RRR for the copper is 70; the conductor n value is
greater than 35. The conductor has 222 filaments that are
nominally 41 µm in diameter. The nominal twist pitch for the
conductor is 19 mm. The I
c
is >760 A at 5 T and 4.2 K.
Using this conductor, the magnet margin is expected to be
>0.8 K when the induction at the high field point is 7.44 T, the
current is 210 A, and the cold mass temperature is 4.2 K
IV. THE COUPLING MAGNET DESIGN PARAMETERS
The coupling magnet is the largest of the three types of
magnets in MICE both in terms of diameter and stored
magnetic energy at full current. The coupling magnet is also
the largest magnet in the MUCOOL as well. The coupling
magnet is designed to fit around the RF cavity vacuum vessel.
The two halves of the RF cavity vacuum vessel will slide into
the bore of the coupling magnet and they will be welded
together. The force carrying gussets will connect the magnet
cold mass supports to the RF cavity vacuum vessel, which in
turn will connect to the rest of MICE. Table 1 shows the
coupling magnet parameters.
TABLE 1. COUPLING MAGNET SPECIFICATIONS
Parameter
Flip
Non-flip
Cryostat Length (mm)
489
Cryostat Inner Radius (mm)
694.4
Cryostat Thickness (mm)
~390
Coil Length (mm)
285
Coil Inner Radius (mm)
750
Coil Thickness (mm)
102.5
Number of Layers
96
No. Turns per Layer
166
Magnet Self Inductance (H)
592.5
Magnet J (A mm
–2
)*
114.6
108.1
Magnet Current (A)*
210.1
198.2
Magnet Stored Energy (MJ)*
13.1
11.6
Peak Induction in Coil (T)
~7.40
~7.12
Coil Temperature Margin (K)
~0.8
~1.0
* Worst case design based on p = 240 MeV/c and
β
= 420 mm
Each coupling magnet will be powered using a single two-
quadrant power supply with a controlled voltage ±10 V and a
controlled current from 0 to 300 A. At the full design voltage
of the power supply, the magnet will charge to 210.1 A in
about 13140 s (at an average voltage of 9 V). In a rapid
discharge mode, the magnet will discharge in about 5400 s.
Cold Mass Support
Support Band
Cold Mass Support
Pulse Tube Cooler
Quench System
60 K Service Neck Shield
Vacuum Port Indent
4 K Cooling Tube
Coupler Indent
60 K Magnet Shield
Cold Mass
Cooler 1
st
Stage
Cooler 2
nd
Stage
MT-20 Paper 4L-03, to be published in IEEE Transactions on Applied Superconductivity 18. No. 2, LBNL-63481
4
V. MAGNET QUENCH PROTECTION, COLD MASS SUPPORTS,
CURRENT LEADS, AND COOLING
The coupling magnet will be passively quench protected by
sub-dividing the coil into eight parts. Each coil sub-division
will have a diode and resistor across it. The quench protection
diodes will permit the coil to be charged and discharged with
as much as 22 V across the coil. The resistors and diodes limit
the voltages to ground to <2000 V and the layer-to-layer
voltages to within the coil to <320 V. Quench back in
combination with coil sub-division ensures that the hot spot
temperature in the coil will be <120 K. The coupling magnet
quench protection system details are described in [12].
The coupling magnet will use an oriented tension band
support system similar to the support bands used in helium
cryostats used for MRI magnets and in long life helium tanks
used in space. The use of tension bands ensures that the heat
flow into the coil cold mass will be minimized. The cold mass
supports for the coupling magnet are described in [13].
The current into the coupling magnet will enter the magnet
through a pair of 220 A leads. The lower part of the leads will
be HTS leads. The upper leads will be conduction cooled
copper leads. The heat flow down these leads is carried to the
first stage of the magnet coolers. The performance of the HTS
leads is affected by the magnetic field in the region between
the first and second stages of the magnet coolers [10]. The
leads and the connection of the leads to the magnet cooling
system are described in [13].
Figures 2 and 4 show a pair of pulse tube coolers on the
magnet service neck. It is assumed that the coolers will be
Cryomech PT415 pulse tube coolers that generate 1.5 W at 4.2
K and 40 W at 40 K. It is expected that the magnet can
operate while fully charged on a single cooler. The second
cooler will be needed to cool the magnet while charging. The
use of two PT410 (1.0 W at 4.2 K with 40 W at 40 K) is also
an option. The coolers will be connected to the magnet using
a thermal-siphon loop [14]. The details of the coupling
magnet cooling system are described in [13]. Table 2 shows
the expected temperature and heat loads when the magnet is
operated on two PT415 coolers.
TABLE II. PROJECTED COUPLING MAGNET HEAT LOADS
Heat Load (W)
Source of the Heat Load
1
st
Stage
2
nd
Stage
Cold Mass Support
3.0
0.2
MLI Radiation Heat Load
~8.5
~0.7
Pipes and Necks
6.0
0.14
Instrumentation Wires
1.0
0.12
Heat Shield Supports
1.0
----
Current Leads
19.3
0.13
Superconducting Joints
----
0.01
Total Stage Heat Load (W)
38.8
1.30
Stage Temperature (K)
~40
~4.05
VI. CONCLUDING COMMENTS
The MICE and MUCOOL coupling solenoid magnets have
a maximum stored energy of 13.1 MJ. These magnets will
operate at currents at or below 210.1 A, so the magnet self-
inductance is high. The design of the passive quench
protection system is dictated by the magnet self-inductance
and stored energy.
The magnets will be cooled using 4.2 K pulse tube coolers.
This choice of cooler is dictated by the magnetic field outside
of the solenoid. Because the magnets will be cooled using
small 4.2 K coolers, they must be engineered so that the heat
leak into both stages of the cooler is minimized.
REFERENCES
[1] G. Gregoire, G. Ryckewaert, L. Chevalier, et al, “MICE and
International Muon Ionization Cooling Experiment Technical Reference
Document,” http://hep04.phys.itt.edu/cooldemo
[2] M. A. Green, C. Y. Chen, T. Juang et al, “Design Parameters for the
MICE Tracker Solenoid,” to be published in IEEE Transactions on
Applied Superconductivity 17, No. 2, (2007)
[3] S. Q. Yang, M. A. Green, G. Barr, U. Bravar, J. Cobb, W. Lau, et al,
“The Mechanical and Thermal Design for the MICE Focusing Solenoid
Magnet System,” IEEE Transactions on Applied Superconductivity 15,
No. 2, p 1259, (2005).
[4] M. A. Green, D. Li, S. P. Virostek and H. Witte, “Progress on the
Coupling Coil for the MICE Channel,” Proceedings of 2005 Particle
Accelerator Conference Knoxville TN, p 3468, (2005).
[5] D. Li, M. A. Green, S. P. Virostek, M. S. Zisman, “Progress on the RF
Coupling Module for the MICE Channel,” Proceedings of 2005 Particle
Accelerator Conference Knoxville TN, p 3417, (2005)
[6] D. Li and M. S. Zisman, “201-MHz NCFR Cavity Program,” (at
MUCOOL in Fermilab), Neutrino Factory and Muon Collider
Collaboration Meeting (March 2006), see the following web site:
http://www.mice.iit.edu/nfmcc06/
[7] M. A. Green, D. Li, S. P. Virostek, L. Wang, H. Wu, L. K. Li, et al,
“Progress on the Design of the Coupling Coils for MICE and
MUCOOL,“ to be published 2007 proceedings of the Particle
Accelerator Conference in Albuquerque NM, (2007)
[8] S. Q. Yang, D. E. Baynham, P. Fabbricatore, S. Farinon, M. A. Green, et
al, “The Physical Connection and Magnetic Coupling of the MICE
Cooling Channel Magnets and the Magnet Forces for Various MICE
Operating Modes,” IEEE Transactions on Applied Superconductivity 17,
No. 2, p 1251, (2007).
[9] M. A. Green and H. Witte, “The Use of Small Coolers in a Magnetic
Field,” to be published in Advances in Cryogenic Engineering 53, AIP
Press, Melville NY (2008),
[10] M. A. Green and H. Witte, “Using High Temperature Superconducting
leads in a Magnetic Field,” to be published in Advances in Cryogenic
Engineering 53, AIP Press, Melville NY (2008)
[11] M A. Green, S. Q. Yang, D. E. Baynham, T. W. Bradshaw, J. H. Cobb,
P. Lau, W. W. Lau, and H. Witte “The Effect of Magnetic Field on the
Position of the HTS Leads and the Cooler in the Service Tower of the
MICE Focusing Magnet,” submitted to IEEE Transactions on Applied
Superconductivity 18, (this publication) (2008)
[12] M. A. Green, L. Wang, and X. L. Gou, “Quench Protection for the
MICE Cooling Channel Coupling Magnet” submitted to the proceedings
of EUCAS-07 in Brussels Belgium (2007)
[13] L. Wang, H. Wu, L. K. Li, C. S. Liu, M. A. Green and S. P. Virostek,
“Helium Cooling System and Cold Mass Support System for the MICE
Coupling Solenoid,” submitted to IEEE Transactions on Applied
Superconductivity 18, (this publication) (2008)
[14] M. A. Green, “How the Performance of a Superconducting Magnet is
affected by the Connection between a Small Cooler and the Magnet,”
IEEE Transactions on Applied Superconductivity 16, No. 1, p 1330,
(2006)
MT-20 Paper 4L-03, to be published in IEEE Transactions on Applied Superconductivity 18. No. 2, LBNL-63481
5
DISCLAIMER
This document was prepared as an account of work sponsored by the United States Government.
While this document is believed to contain correct information, neither the United States
Government nor any agency thereof, nor The Regents of the University of California, nor any of
their employees, makes any warranty, express or implied, or assumes any legal responsibility for the
accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed,
or represents that its use would not infringe privately owned rights. Reference herein to any specific
commercial product, process, or service by its trade name, trademark, manufacturer, or otherwise,
does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United
States Government or any agency thereof, or The Regents of the University of California. The views
and opinions of authors expressed herein do not necessarily state or reflect those of the United States
Government or any agency thereof, or The Regents of the University of California.
