HIGH FORCE, PRECISION POSITIONING USING MAGNETICSMART MATERIALS 1

Chad Joshi2 and Bruce Bent, Energen, Inc., Billerica, MAMichael Drury, Joseph Preble, Viet Nguyen, Jefferson Laboratory, Newport News, VA

1 Supported by the Nuclear Physics Division of the U. S. Dept. of Energy through the SBIR Program2 E-mail: chad@EnergenInc.com

ABSTRACT

Magnetostrictive and magnetic shape memory alloysrespond to externally imposed magnetic fields in areversible and repeatable manner. These new materialswill enable more compact actuators for a wide range ofapplications such as linear motors, translation stages,robotics, and active vibration control.

Energen, Inc. has designed, built and demonstrated afine tuning mechanism for superconducting radiofrequency (SRF) cavities used in particle accelerators.This tuner is based on giant magnetostrictive materialsbeing developed by Energen, Inc. Magnetostrictorselongate when exposed to a small magnetic field. Thisextension is reversible and repeatable enabling a widerange of applications. The magnetostrictive tuner wasspecifically designed to meet the requirements of theThomas Jefferson National Accelerator Facility inNewport News, VA. The tuner consists of a high forcelinear actuator that elongates the cavity along its axisthereby changing its resonant frequency. It is installed inthe dead leg of the existing mechanical tuner. Thismechanism has a force capability of over 5000 lbs. andprovides a tuning range of 2000 Hz. Preliminary tests atJefferson Laboratory demonstrated its cavity tuningcapability.

Energen, Inc. is presently developing a newgeneration of SRF cavity tuners that can eliminate themechanical tuners that are presently employed. The basisof this new tuning mechanism is a linear stepper motorthat provides several millimeters of motion withnanometer positioning resolution. The features of thisstepper motor include high force motion and the ability tohold position when powered off. These capabilitiesenable multiple cavities to be tuned with a single set ofelectronics. Furthermore, with only electrical wiresconnecting the electronics and the stepper motor, thecomplexity of rotating vacuum and cryogenicfeedthroughs associated with mechanical tuners can beeliminated.

INTRODUCTION

The Jefferson Laboratory of Newport News, VAoperates the largest continuous beam electron acceleratorin the world. Approximately 344 superconducting RF

cavities operate at a resonant frequency of 1497 MHz toaccelerate the electrons as they pass through each cavity.To obtain a homogeneous and continuous electron beam,these cavities are designed to exacting geometrictolerances. However, small imperfections in theirmanufacture and the large dimensional change that occursas a result of cooling from room temperature to 2.0 Knecessitates active compensation for the cavity lengthduring the actual operation of the accelerator in order tomatch the resonance frequencies of multiple SRF cavitiesused in any accelerator.

SRF cavity tuning is accomplished by physicallyelongating or compressing the cavity along its axis.Jefferson Laboratory uses a mechanical system as shownin Figure 1. The system uses a stepper motor at roomtemperature to drive a wormwheel gear reductionconnected to a ball screw shaft. As the shaft turns, the twocell holders are squeezed together compressing the cavityand changing its resonant frequency. This mechanicalsystem is relatively simple in concept, however, it isdifficult to use for accurate cavity tuning. In operation,this system, like any other mechanical assembly, hasbacklash. In addition to the elastic behavior of thecomponents, split shaft couplers are used for the rotatingfeedthrough to allow for thermal contraction duringcooldown. This split assembly adds to the inherentbacklash of the system.

Figure 1 - The CEBAF superconducting RF cavityshowing its mechanical tuner.

Helium VesselWormwheel Leadscrew

SuperconductingRF Cavity

Cell HoldersDeadleg

RF Feed

Jefferson Laboratory is currently in the process ofupgrading its cryomodule to achieve a higher averagegradient, and increase the electron beam energy. As partof this upgrade project, a new cavity design is being

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developed that will result in a higher packing density.Energen is developing tuners based on magnetostrictiveactuators that will provide a total tuning range of ±200kHz with a tuning resolution of ±1 Hz. The goal of thepresent development effort is to design and demonstrate acost-effective tuning solution on a timeline consistentwith the upgrade schedule.

MAGNETOSTRICTIVE MATERIALS

Magnetostrictive materials belong to a class ofmaterials known as Òsmart materialsÓ. Magnetostrictionarises from a reorientation of the atomic magneticmoments. In ferromagnetic materials, an applied magneticfield causes rotation of the magnetization towards thefield direction within domains and/or motion of thedomain walls to increase the size of the domains withmagnetization vectors close in direction to the appliedfield. When the magnetization is completely aligned,saturation occurs and no further magnetostriction can beproduced by increasing the applied magnetic field.Magnetostriction only occurs in a material at temperaturesbelow the Curie temperature. The amount ofmagnetostriction at saturation is the most fundamentalmeasure of a magnetostrictive material. Table 1 shows acompilation of some materials and the maximum strainthey exhibit at saturation. Note that the materialsexhibiting the highest magnetostrictive strain have Curietemperatures below room temperature. These materialsshow great promise for cryogenic actuator applicationssuch as the SRF cavity tuners.

Table 1 -- Saturation strain and Curie temperatures ofselected materials. 1

Material Saturation Strain(x 10-6)

CurieTemperature(K)

Ni -50 630Fe -14 1040SmFe2 -2340 690Fe3O4 60 860DyFe2 650 630TbFe2 2630 700Tb0.3Dy0.7Fe1.9 (Terfenol-D) 1600-2400 650Tb0.6Dy0.4 @ 77 K 6300 210Tb0.5Zn0.5 5500 180Tb1-xDyxZn 5000 200

Magnetostrictive materials for cryogenic applicationsare not currently commercially available but are beingproduced at research laboratories. Energen, Inc. hasworked with the Ames LaboratoryÕs Materials PreparationCenter to obtain samples of the materials for testing andbuilding prototype devices. Energen has identified and isin the process of procuring the equipment necessary tofabricate these materials on a production basis. Thecompany intends to have these facilities in place in timeto deliver the tuner for the Jefferson Laboratory upgrade.

Several alloys from the Tb1 - xDyxZn family ofmaterials were fabricated as follows. Appropriatequantities of the elemental constituents were alloyedtogether in a sealed tantalum crucible. The crucible wasthen held just below the melting temperatures to promotecrystal growth. Once cooled, the crucible was removedand the resulting ingot was machined into rods for testingpurposes. For testing the magnetostriction, samples ofeach alloy were cut into 2-mm diameter rods of lengthsvarying from 20 to 30 mm.

Measurements of magnetostriction were made at 77K and at 4.2 K. The data for 77 K are very reproducibleand agree with available data from the literature.2 Figure2 shows the measurements on one sample of the TbDyZnalloy at 4.2 K. These data are believed to be the firstdirect measurement of magnetostriction in this materialsystem at 4.2 K and indicate that the high saturation strainremains at these low temperatures.

Figure 2 - Magnetostriction of a TbDyZn alloy at 4.2K shows high saturation strain.

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FINE TUNER PERFORMANCE

Energen, Inc. designed a magnetostrictive tunerbased on these materials that replaces the deadleg of theexisting mechanical tuner. This tuner was designed toprovide fine tuning capability in parallel with the existingtuner. The mechanical tuner would be used for coarsetuning of the resonant frequency and then themagnetostrictive tuner would be used for fine tuning.

Figure 3 shows the magnetostrictive dead leg.Because of limitations in the size of magnetostrictor rodscurrently available, the magnetostrictor consists of six 0.5inch disks of equal thickness on a 1.108" bolt circle.They are surrounded by a NbTi coil and compressedbetween two ferromagnetic end pieces that connect to thedead leg. The coil has 20 turns of superconductor with acritical current of 325 A at 4.2 K, 5.0 T. Theferromagnetic end pieces serve to focus the magnetic fluxfrom the coil onto the magnetostrictor. The peak field inthe end pieces and in the magnetostrictor is less than 1.6tesla.

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Figure 3 Ð Magnetostrictive fine tuning mechanism.

Figure 4 - Measurement of the tuner motion.

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The tuner was tested at EnergenÕs facility at 77 K andat 4.2 K. At 77 K, the current in the coil was limited to 15A to prevent potential damage to the coil and actuatorfrom overheating. Figure 4 shows the results of tests at77 K and 4.2 K

Figure 4 indicates that a total stroke of 16 microns isavailable for tuning the SRF cavity . Based on the knowncharacteristics of the Jefferson Laboratory SRF cavities,this corresponds to a tuning bandwidth of 6400Hz. This isgreater than the design goal by a factor of three.

It is clear that there is some hysteresis in thesemeasurements. The source of this hysteresis is theanisotropic magnetic properties of the material and can bereduced by properly adjusting the chemistry of thematerial.

At Jefferson Laboratory, the tuner was installed on aSRF cavity as shown in Figure 5. In operation, acombination superconducting and mu-metal shield wouldbe used to prevent the magnetic field generated by themagnetostrictive tuner from affecting the performance ofthe cavity.

The cavity was inserted vertically in a dewar andcooled in liquid Helium to approximately 2 Kelvin. A

network analyzer (Hewlett Packard 8753) was used todrive the cavity and measure the cavity's resonantfrequency (fo). An RF frequency preamp was use to boostthe signal from the cavity field probe before returning tothe analyzer. A Sorenson programmable power supplycapable of 45 A was used as the current source for thetuner. A 50 Amp shunt was installed in the return leg ofthe circuit between tuner and power supply. The shuntwas calibrated for 1 mV/A. A digital multimeter wasused to measure the shunt voltage. The apparatus wascontrolled with a laptop computer running Labview usinga GPIB card to talk to the network analyzer and themultimeter. A National Instruments analog box wasattached to the parallel port. This provided an analogoutput with which we could control the current source.The resolution of the current drive under this setup wasabout 0.5 A. The resolution of the network analyzer wasabout 10 Hz.

Figure 5 - The magnetostrictive tuner was installed inplace of the dead leg of the mechanical tuner.

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Four different current sweep tests were done. In thefirst two tests, the maximum current for the tuner was 30A. While the pressure in the helium vessel was constantduring each test, there is a slight variation over a longerperiod of time. This pressure variation affects the tuningof the cavities. During the third test, when the current wasincreased to 50 A, a large change in pressure from 21 torrto 12 torr occurred during the test. There was likely a shiftin frequency because of the pressure change exacerbatingthe hysteresis that is evident in the data. In the final test,the cryostat pressure was maintained at 12 torr and theresultant tuning capability measured shows a frequencyrange of 2 kHz.

Figure 6 Ð Measurement of the cavity tuningcapability of the fine tuning mechanism

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Figure 7 - A compact linear stepper motor built byEnergen, Inc.

Figure 6 - Stepper motor operating sequence.

LINEAR STEPPER MOTOR

Energen has taken the basic concept of themagnetostrictive actuator and develop a linear motorcapable of longer range motion than the linear actuator.

The stepper motor actuator consists of a shaft held bya pair of magnetostrictive clamps. The shaft consists of alinear actuator with a connecting rod extending from eachend. Clamps that hold each of the connecting rods, are

opened by energizing the magnetostrictive actuatorcontained therein thereby enabling the connecting rod tomove along its axis. A photograph of a stepper motor isshown in Figure 7.

Figure 8 Ð Stepper motor operating sequence.

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OPERATION

The stepper motor can be operated in one of twomodes Ð in stepper mode or fine tune mode. In steppermode, the translating rod is moved in discrete steps alongits axis by operating the clamp and translating actuators ina predefined sequence. The sequence is as follows and isillustrated in Figure 8.

1) Starting from the power off position,

2) The forward clamp is energized causing it to releaseits hold on the connecting rod.

3) The translating actuator is energized causing themagnetostrictor rod to elongate pushing the frontconnecting rod forward.

4) The forward clamp is then closed to grab onto rod.

5) The rear clamp is energized to release.

6) The translating actuator is de-energized causing therear half of the rod to be pulled forward.

7) The rear clamp is then closed to provide maximumholding.

The rod has moved a step forward. Reversing thesequence of operations above causes the rod to move inthe opposite direction.

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To achieve high positioning resolution Energenprovides a fine tuning mode of operation. In this mode,the forward actuator is energized to release and thecurrent in the translating actuator is modulated therebymoving the forward connecting rod proportionally.

Thus this linear motor is capable of providing a longstroke with high positioning resolution. It is capable ofholding position with zero power dissipation since theclamps hold at zero current.

DRIVE ELECTRONICS

The drive electronics for the position control actuatorenables a wide range of actuator operational flexibilitythrough a digital state control circuit. In addition tosetting the operating mode (stepper or fine tune), thedirection of motion, the step size and stepping frequencycan be adjusted. Figure 9 shows a block diagram of thedrive electronics.

Figure 9 -A digital state control circuit allowscomplete flexibility in positioning control with thestepper motor.

CurrentRegulator

AC/DCConverter

Low Voltage DC Bus

Displacement

Clamp

Stepping/ Fine TuneDirection

SpeedLockout

Step size

Clamp

StateController

STEPPER TUNER DESIGN

Energen has developed a SRF cavity tuner based onits stepper motor. The basic principal of operation for thetuner is to use a pair of stepper motors to expand orcompress the cavity along its axis relative to the cryostat.For this application, a large force capacity is required ofthe stepper motor. The force capability is directly relatedto the cross section of the magnetostrictive element. Thegeometry of the stepper motor and its interface to thecavity is shown in Figure 10. The stepper motor canmove forward or backward with a total stroke of 12.5 mmwith a force capability of 3100 N. As it moves forward, itpulls on the cavity with respect to the helium vessel.Reversing the direction of motion causes the cavity to becompressed. When the desired resonant frequency is

reached, power to the stepper motor can be turned off andthe stepper motor will remain locked in position.

Figure 10 - Conceptual design of stepper motor andits interface to the SRF cavity.

5.00 inch [127 mm]

3.8 inch[97 mm]

1.25 inch[32 mm]

Stepper Motor

SRF Cavity

Helium Vessel

Bellows

Stepper Motor

ADVANTAGES

This tuning technique has several advantages overexisting systems.

q Accurate tuning Ð positioning resolution of 0.01micron gives accurate frequency tuning

q Compact design Ð since only wires re needed toconnect to this tuner from outside the cryostat,the tuner is more compact that a moreconventional tuner system

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q Low hysteresis Ð since the actuators are turnedon and off, the stepping motion is digital innature and minimizes the effect of the hysteresisof the magnetostrictor material.

q Elimination of mechanical penetrations throughcryostat reduces the complexity of the cryostatsystem and increases reliability since mechanicalpenetrations through the vacuum space areeliminated.

q ÒSet and forgetÓ tuning Ð the tuner will holdposition when the power is turned off

q Multiplexing of cavity tuners Ð one set ofelectronics can be used to tune multiple cavities

q Low cost Ð it is anticipated that as the productionquantities increase, the cost of themagnetostrictive material will be reduced by afactor of 5 to 20 from the present costs.

CONCLUSION

Energen, Inc. has designed, built and demonstrated afine tuning mechanism for superconducting radiofrequency (SRF) cavities used in particle accelerators.The tuner is designed specifically for the JeffersonLaboratory and operates in parallel with an existingmechanical tuning system that uses a wormwheel and ballscrew assembly. The magnetostrictive fine tuner hasdemonstrated the desired 2000 Hz of tuning bandwidthand can be actively controlled during the operation of theSRF cavity.

A new SRF cavity tuner design based on EnergenÕslinear stepper motor promises to provide reliable, accuratetuning capability and will eliminate the complexity andexpense of mechanical tuning systems. This new tuningsystem is being considered for use with the upgrade of theelectron beam accelerator at Jefferson Laboratory.

1 Magnetostrictive Materials, K. B. Hathaway and A. E.Clark, MRS Bulletin, Vol. XVIII, No. 4, April 1993.2 A. E. Clark, High Power Magnetostrictive Materialsfrom Cryogenic Temperatures to 250 C, MaterialsResearch Society Fall Meeting, Boston, MA, November28-30, 1994.

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