A Magnetostrictive Tuning System for Particle Accelerators

Abstract

Energen, Inc. has designed, built, and demonstrated several fast and slow tuners based on its magnetostrictive actuators and stepper motor. These tuners are designed for Superconducting Radio Frequency (SRF) cavities, which are important structures in particle accelerators that support a wide spectrum of disciplines, including nuclear and high-energy physics and free electron lasers (FEL). In the past two years, Energen’s work has focused on magnetostrictive fast tuners for microphonics and Lorentz detuning compensation on elliptical-cell and spoke-loaded cavities. These tuners were custom designed to meet specific requirements, which included a few to 100 micron stroke range, hundreds to kilohertz operation frequency, and cryogenic temperature operation in vacuum or liquid helium. These tuners have been tested in house and at different laboratories, such as DESY, Argonne National Lab, and Jefferson Lab. Some recent results are presented in this paper.

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A MAGNETOSTRICTIVE TUNING SYSTEM FOR PARTICLE
ACCELERATORS1
Chiu-Ying Tai2, Jordan Cormier, William Espinola, Zhixiu Han, Chad Joshi, Anil Mavanur, Livia
Racz, Energen, Inc. Lowell, MA 01854, U.S.A.
Kenneth Shepard, Argonne National Laboratory, Argonne, IL 60439, U.S.A.
Edward Daly, Kirk Davis, Jefferson Lab, Newport News, VA 23606, U.S.A
Abstract
Energen, Inc. has designed, built, and demonstrated
several fast and slow tuners based on its magnetostrictive
actuators and stepper motor. These tuners are designed
for Superconducting Radio Frequency (SRF) cavities,
which are important structures in particle accelerators that
support a wide spectrum of disciplines, including nuclear
and high-energy physics and free electron lasers (FEL).
In the past two years, Energen’s work has focused on
magnetostrictive fast tuners for microphonics and Lorentz
detuning compensation on elliptical-cell and spoke-loaded
cavities. These tuners were custom designed to meet
specific requirements, which included a few to 100
micron stroke range, hundreds to kilohertz operation
frequency, and cryogenic temperature operation in
vacuum or liquid helium. These tuners have been tested
in house and at different laboratories, such as DESY,
Argonne National Lab, and Jefferson Lab. Some recent
results are presented in this paper.
INTRODUCTION
Background
Superconducting radio frequency (SRF) cavities, which
include both elliptical and spoke types, are devices inside
particle accelerators that are used to bring the charged
particles to the required energy and speed. The
performance of these cavities can be affected by many
different sources, such as manufacturing defects,
asymmetries in the cooling process, and noise from the
surrounding environment. Cavity tuners - devices used to
physically change the shape of the cavities - are used to
correct for these non-idealities and tune the resonant
frequency of the structure to match the frequency of the
input energy. There are two kinds of tuners – “slow” or
“coarse”, and “fast” or “fine”. Slow tuners provide large
forces and hundreds of kHz tuning range to compensate
for static defects, while fast tuners respond quickly but
only over a few hundreds to a few thousand Hz. They are
used to compensate the frequency shift caused by
microphonics (from mechanical sources) and Lorentz
detuning (from electromagnetic sources).
Since 1997, Energen has been working with engineers
and scientists at Jefferson Laboratory, the National
Superconducting Cyclotron Laboratory, the Spallation
Neutron Source, Deutsches Elektronen–Synchrotron
(DESY), and Argonne National Laboratory (ANL) to
develop and demonstrate tuning mechanisms based on
____________________________________________
1This work is supported by DOE SBIR Program DE-FG02-03ER83648
2ctai@energeninc.com
magnetic “smart” materials (MSM) for a variety of SRF
cavities. Details of the tuner designs and a subset of the
test results have been reported previously [1][2][3]. In the
past two years, Energen has focused on fast tuners for
active microphonics control and Lorentz detuning
compensation. The present paper reports on the detailed
design and testing of these tuners.
Magnetic “Smart” Materials (KelvinAll®)
Magnetic “smart” materials (MSM) change their shape
in a predictable and reversible manner when exposed to a
magnetic field. Energen has developed the MSM
KelvinAll® to have considerable advantages over other
smart materials for SRF cavity tuners. Other candidate
materials include TbDyZn, developed by the U.S. Navy
for cryogenic applications to approximate the properties
of the common room-temperature material Terfenol-D
(Tb0.3Dy0.7Fe2). This material has a Curie temperature
around 170 K – the maximum temperature at which it will
respond to a magnetic field – forcing even preliminary
tests to be done at cryogenic temperatures. Further, this
alloy is only available in small quantities through custom
fabrication, making it very expensive. The most
commonly-used smart material for SRF cavity tuners is
the piezoelectric lead zirconate titanate (PZT). Its main
disadvantage is that its elongation drops drastically with
decreasing temperature. Figure
temperature-dependent elongation of the materials
discussed [4].
0.6
1 compares the
0
0.1
0.2
0.3
0.4
0.5
0 50 100 150200250300350
Temperature (K)
Max. Elongation (%)
KelvinAll® $$
TbDyZn $$$$$
Terfenol-D $
PZT $$

Figure 1: Temperature-dependent elongation for several
smart materials and their relative costs.
FAST TUNER DESIGN
Specifications
Energen has custom designed and built several different
fast tuner prototypes for different facilities based on the
requested specifications listed in Table 1.
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Table 1: Specifications of Energen’s fast tuners
ANL–1st
generation
Operation
environment
vacuum
Max
compressive
load
Stroke
50 to 100 µm 20 µm
Bandwidth < 160 Hz > 60 Hz
Waveform
Continuous
sine wave rise time, 1.6 ms
pulse length
DESY JLab
4.2 K in LHe 2.1 K to 18 K in 2 K in LHe
445 N 1500 N 15000 N
8 µm
> 1 k Hz
Continuous
sine wave
Square wave: 250 µs
Actuator Designs
The core technology underlying all of these devices is
KelvinAll® material in rod form. However, in order to
achieve the different requirements, the material may be
used in monolithic (bulk) or composite (laminated) form,
together with different rod and electromagnetic coil
geometries and device designs. The geometry of the
KelvinAll rod is determined by the specified stroke and
load. A bulk rod with certain diameter has an upper
operating frequency limit due to eddy current effects. To
exceed this limit, alternating sheets of KelvinAll® and
epoxy can be used. In Table 1, such laminated rods were
used in both the DESY and JLab actuators.
Different superconducting coils were used in these
actuators. The superconductor NbTi has its critical
temperature at 9 K, and is able to drive the MSM rod at
4.2 K or below in liquid helium (ANL and JLab designs)
without special cooling concerns. In the DESY actuator
design, a Nb3Sn coil with higher critical temperature
(18 K) was used. In order to ensure normal operation,
extra thermal strips were used to connect the coil and the
heat sink for enhanced cooling in vacuum.
Another way to increase the coil efficiency without
quenching it is to use field concentrators such as
laminated silicon steel housing, as was done in both the
DESY and JLab actuators. The purpose of the lamination
in the field concentrators is to reduce eddy currents and
heat generation.
Each actuator system underwent a mechanical design
process for the mass-spring system to ensure the specified
bandwidth, which was dependent on the natural frequency
of the structures. Figure 2 shows some actuators and

Figure 2: Energen’s fast tuners – DESY actuator (upper
left); central portion of DESY actuator (lower left); and
ANL actuator (right).
components; more details can be found in previous
publications [3][5].
TUNER TESTS
Laboratory Tests at Energen
All the fast tuners mentioned above have been tested at
Energen under actual operating conditions. Figure 3
shows stroke vs. input current for the 1st generation ANL
tuner. Data were measured at 4.2 K in liquid helium.
More than 70 µm of full stroke under different loading
forces was obtained in DC operation, and 80 to 90% of
full stroke is available in AC operation up to 160 Hz.
A 2nd generation tuner for ANL is being developed,
discussed elsewhere in this paper.
100
0
20
40
60
80
05101520
I (A)
253035 40
Stroke (micron)
445N
310N

Figure 3: Stroke vs. input current for ANL tuner under
two loads, 310 and 445 N.
Test Preparation at ANL
The 1st generation ANL tuner is currently ready for final
test. It is attached to an adjustable holder (see Figure 4),
and will be installed in the beta 0.5 triple-spoke cavity.
The tuner assembly is inside the helium vessel. The turner
will be slightly pulled back from the cavity during the
cooling process, and adjusted to contact when ready for
tuning. Figure 5 shows the location of the tuner in the
system.


Figure 4: ANL tuner and adjustable holder.


Figure 5: Location (inside dashed line) of the fast tuner in
the beta 0.5 triple spoke cavity (drawing from ANL).
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Finite element analysis of the cavity (provided by
Advanced Energy Systems) indicates that a 20 µm
actuation at this location can generate a 120 Hz frequency
shift. Based on this analysis, Energen’s MSM tuner with
up to 70 µm stroke could provide a tuning range of more
than 400 Hz.
Blocking Force Test at DESY
A blocking force test of the tuner was recently
conducted at DESY using the apparatus shown in
Figure 6. In this setup, the MSM tuner pushes against a
very stiff piezostack, which measures displacement and
pushing force of the MSM tuner by monitoring change in
output voltage as the piezostack is compressed. A pre-load
force of ~1 kN was applied using the top screw. The test
was performed at 4.2 K in vacuum. Some results are
shown in Figure 7. It is very promising that the MSM
actuator can still move and provide significant pushing
force under 1000 N pre-load against a very stiff structure
(the piezostack).

MSM tuner
Piezostack
Pre-load
screw

Figure 6: Test apparatus at DESY.
0
50
100
150
200
250
300
0123456789
Coil input current (A)
MSM output force (N)

Figure 7: Output force vs. input current for DESY tuner.
Tuner Tested at Jefferson Laboratory (JLab)
The fast tuner developed for JLab was tested on a
prototype 12 GeV 7-cell cavity system (see Figure 8). The
JLab tuning system has a stepper motor for slow tuning of
the cavity, and the load on the MSM actuator depends
upon the distance moved by the stepper motor. The total
frequency shift due to the MSM tuner was measured using
an RF network analyzer at different compressive loads.
Test results show that the tuning range of Energen’s tuner
was more than twice the required 1200 Hz shift under all
loading conditions. The smallest controllable frequency
shift was found to be 1.2 Hz, implying an actuator
resolution of 4 nm. More details have been presented [5].
Figure 8: Magnetostrictive actuator/tuner JLab cavity
CONCLUSIONS AND FUTURE WORK
Several different SRF cavity tuners built to varying
specifications based on KelvinAll® magnetic smart
material have been tested under different conditions,
successfully demonstrating Energen’s fast tuner design
and development capabilities. Future work includes on-
site testing of the 1st generation ANL tuner, development
of a 2nd generation ANL tuner with improved bandwidth,
on-site tests at DESY, and development of a low-cost
combined slow-fast tuning system.
ACKNOWLEDGMENTS
The authors would like to thank Stefan Simrock, Lutz
Lilje, and Przemyslaw Sekalski at DESY for their help in
tuner testing; also Tom Shultheiss at Advanced Energy
Systems for the FEA results on spoke cavities, and Joel
Fuerst at Argonne National Laboratory for discussions
and drawings.
REFERENCES
[1] C.H. Joshi, B. Bent, M. Drury, J. Preble, V. Nguyen,
“A Magnetostrictive Tuning Mechanism For SRF
Cavities,” Proceedings of the Particle Accelerator
Conference, New York, New York, March 29-April
2, 1999.
[2] C.H. Joshi, A. Pappo, D. Upham, J. Preble, “Progress
In The Development of SRF Cavity Tuners Based On
Magnetic Smart Materials,” Proceedings of the
Particle Accelerator Conference, Chicago, Illinois,
June 18-22, 2001.
[3] A. Mavanur, C.Y. Tai, C.H. Joshi, T. Grimm,
“Magnetostrictive Tuners
Proceedings of the Particle Accelerator Conference,
Portland, Oregon, May 12-16, 2003.
[4] Energen, Inc. website, http://www.EnergenInc.com,
2005.
[5] A. Mavanur, C.Y. Tai, C.H. Joshi, E. Daly, and K.
Davis, “Magnetostrictive Fast Tuners for SRF
Cavities,” Proceeding
Superconducting Conference, Jacksonville, Florida,
Oct. 4-8, 2004.
for SRF Cavities,”
of the Applied
Proceedings of 2005 Particle Accelerator Conference, Knoxville, Tennessee
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