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Vacuum 84 (2010) 699–704

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Vacuum

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Development of the radiation-hardened Magnetically SuspendedCompound Molecular Pump

Kaoru Wada*, Takashi Inohara, Motoo Yoshida, Yukio Yamato, Tatsuo Nakayasu,Masashi Iguchi, Yusuke Hikichi, Norio Ogiwara, Keigo MioOsaka Vacuum, Ltd., Nabari Technological Dopt., 1221 Kunugida-cho, Hachioji, Tokyo 193-0942, Japan

a r t i c l e i n f o

Article history:Received 24 October 2008Received in revised form12 February 2009Accepted 11 March 2009

Keywords:Magnetically Suspended CompoundMolecular PumpGamma-raySynchrotron

* Corresponding author. Tel.: þ81 42 664 5302; faxE-mail address: wada@osakavacuum.co.jp (K. Wad

0042-207X/$ – see front matter � 2009 Elsevier Ltd.doi:10.1016/j.vacuum.2009.03.036

a b s t r a c t

The Magnetically Suspended Compound Molecule Pump is used for the vacuum pumping system in the3 GeV Rapid Cycle Synchrotron (3 GeV-RCS) at Japan Proton Accelerator Research Complex (J-PARC). Dueto the radiation environment it is used under, the pump must be resistant to at least 10 MGy of radiation.The standard Magnetically Suspended Compound Molecular Pump is only capable of withstanding up to3.5 MGy under the radiation environment. For this reason, the radiation-hardened Magnetically Sus-pended Compound Molecular Pump was developed based on the gamma-ray irradiation examinationresults for the standard Magnetically Suspended Compound Molecular Pump, and an irradiation test wasperformed. The radiation-hardened Magnetically Suspended Compound Molecular Pump got a sensortuning error when the accumulative radiation dose reached to 73.8 MGy. As a result of a disassemblycheck, cause of the failure was determined to be a decreased movement of the shaft due to the defor-mation of epoxy resin for the mandrel. However, all other parts such as the position sensor, the elec-tromagnet and the motor, were no problem.

� 2009 Elsevier Ltd. All rights reserved.

1. Introduction

A sputter ion pump is used for the vacuum pumping system ina conventional accelerator, because the pump is exposed to radia-tion while the accelerator is operating. The sputter ion pump isa type of entrapment vacuum pump. Therefore, it is unable to pumpdown efficiently the vacuum chamber with large amount ofoutgassing.

For this reason, the vacuum pumping system in the 3 GeVRapid Cycle Synchrotron (3 GeV-RCS), operated by Japan AtomicEnergy Research Institute in Proton Accelerator Complex (J-PARC),uses both a sputter ion pump and a Magnetically SuspendedCompound Molecule Pump. The Magnetically SuspendedCompound Molecular Pump can evacuate high throughputbecause it is a type of kinetic vacuum pump. In particular, theMagnetically Suspended Compound Molecular Pump is installedin the place where there is a large amount of outgassing from thevacuum chamber.

The pump requires to resist radiation up to at least 10 MGy.At first the standard Magnetically Suspended Compound

Molecular Pump (TG1300 M) was tested under a gamma-ray

: þ81 42 664 6420.a).

All rights reserved.

irradiation examination, because there has been no experience touse it under a radiation environment.

The examination results were used for the development of theradiation-hardened Magnetically Suspended Compound MolecularPump.

2. Standard Magnetically Suspended Compound MolecularPump (TG1300 M)

In general, semiconductor and resin are used for parts (ex.positon sensor) inside the conventional Magnetically SuspendedCompound Molecular Pump. These parts cannot be used under theradiation environment, as they are affected by radiation.

Therefore, the conventional Magnetically Suspended CompoundMolecular Pump must be removed while the accelerator isoperating.

The standard TG1300 M has been improved to withstand theradiation dose from a conventional pump.

1. The position sensor with low carrier frequency has beenadopted to eliminate the influence by the length of thecable.

2. The oscillation circuit composed of semiconductor parts hadbeen removed from inside the pump by retrofitting an auto-matic sensor adjustment mechanism.

Table 1Main parts of TG1300 M.

Parts name Compositions

Casing Aluminum alloyRotor Aluminum alloyShaft Carbon steel

Magnetic steel sheet for coreHousing Aluminum alloyElectromagnet Hetro polar type

Enameled wire/Epoxy resinMagnetic steel sheet for core

Sensor Inductance methodEnameled wire/Epoxy resinMagnetic steel sheet for core

Motor Induction motorEnameled wire/Epoxy resinMagnetic steel sheet for core

Connector Epoxy resin hermeticO-ring Nitrile rubber

Fluorine rubberWire Teflon sheath wire

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3. The pump has been designed without using nonmetal mate-rials for the structure.

[The compositions of main parts of TG1300 M are listed inTable 1].

A 5-axis active magnetic bearing control has been adopted. Itallows for highly-reliable control with five position sensors and fiveelectromagnets. The magnetic suspension system inside thecontroller (TC010 M) uses a DSP (Digital Signal Processor) as mainprocessor. TC010 M can be controlled and confirmed operatingcondition remotely through serial communication.

Fig. 1 shows the sectional drawing of TG1300 M.

Fig. 1. Sectional drawing of TG

3. Gamma-ray irradiation examination of TG1300 M

The gamma-rays irradiation examination was done in Co-60Gamma-ray Irradiation Facility No. 1 of Takasaki Advanced Radia-tion Research Institute.

The measurements were performed for the following things:output of the sensor, the transfer function for the control system ofthe magnetic suspension, start/stop time, resistance of the cable,and insulation resistance of the sheath of cable for the drivingsystem was measured.

TC010 M and the backing vacuum pump were set up outside thegamma-ray irradiation room.

The radiation dose was 5.47 kGy/h on the surface of TG1300 Mfor the gamma-ray source side, and 0.37 kGy/h on the other side.The mean value between gamma-ray source side and the other sidewas 2.92 kGy/h, which was assumed as the radiation dose for thecenter of TG1300 M. Fig. 2 shows the measurement of radiationdose.

TG1300 M got a leakage from the O-ring (nitrile rubber) on thecasing for vacuum seal when the accumulative radiation dosereached 3.47 MGy.

It was found that O-ring of the gamma-ray source side washardened and deformed plastically, as shown in Fig. 3. The otherside of the O-ring also hardened slightly, but retained elasticity andwas not deformed. The outer surface of the O-ring (atmosphericside) became sticky and appeared melted, but the inner surface ofthe O-ring (vacuum side) did not become sticky. The molecularbonds on the atmospheric side of the O-ring were broken by thegamma-ray, causing molecules to react with the oxygen in theatmosphere. On the other side, the vacuum side of the O-ring wasunable to react with oxygen because there is no oxygen present invacuum. The accumulative radiation dose was 6.49 MGy for thegamma-ray source side, and 0.44 MGy for the other side.

1300 M (standard model).

Fig. 2. The measurement of radiation dose of gamma-ray to TG1300 M.

Fig. 3. O-ring (nitrile rubber) hardened and deformed after irradiation examination.

Fig. 4. Resistance of position and rotation sensors with th

K. Wada et al. / Vacuum 84 (2010) 699–704 701

The Teflon sheath of the electric wire inside the pump hadhardened. The Teflon sheath got crack by stress change when thepump was disassembled. Teflon is vulnerable to radiation. There-fore, the pump connector was positioned on the opposite side ofthe gamma-ray source in order to reduce the irradiation to it.

The sensor output, the transfer function, the start/stop time, andthe output cable insulation resistance were not changed. Thismeans its parts were not affected by the gamma-ray.

The irradiation examination was restarted after replacing to theO-ring made by fluorine rubber. The fluorine rubber has betterresistance to radiation than nitrile rubber.

In the restarted examination, TG1300 M got a leakage from theO-ring (fluorine rubber) on the casing for vacuum seal when theaccumulative radiation dose reached to 3.62 MGy. The accumula-tive radiation dose was 6.84 MGy for the gamma-ray source side,and 0.41 MGy for the other side. The total accumulative radiationdose on TG1300 M was 7.09 MGy. The degree of the influence ofgamma-ray to the fluorine rubber O-ring was almost same as to thenitrile rubber O-ring. The fluorine rubber theoretically has higher

e accumulation in radiation dose (standard model).

Table 2Parts improvement for TG1300MR.

Parts name TG1300 M TG1300MR

O-ring Fluorine rubber MetalElectric wire Teflon wire PEEK wireOutput cable PVC cable PEEK cableNumber of connectors 1 2 (Magnetic Suspension

and Motor)Connector Epoxy resin hermetic Ceramic hermeticConnector pin Iron alloy with tin plating Cu with gold plating

PVC: polyvinylchloride; PEEK: polyetheretherketone.

K. Wada et al. / Vacuum 84 (2010) 699–704702

resistance to radiation than nitrile rubber, but no difference couldbe observed on this examination. The Teflon sheath of the electricwire used inside the pump line had hardened further. The Teflonsheath of the electric wire peeled due to stress change of it, whenthe pump was disassembled. And the electric wire became unableto keep sufficient insulation.

Therefore, TG1300 M was not able to operate due to the insu-lation failure.

The sensor output, the transfer function, the start/stop time, andthe cable insulation resistance were not affected by the gamma-rayirradiation.

TG1300 M would likely have continued to operate if the Teflonsheath of the wire did not peel off.

Fig. 4 shows how the resistance of the position sensor and therotation sensor changed with the accumulation in radiation dose.

The resistance of the position sensor and the rotation sensordecreased, because the room temperature was changed over thewinter and summer seasons.

The insulation resistance of the lower radial electromagnet andthe axial electromagnet decreased to 1500 MU from 2000 MU. Thedecreased value of insulation resistance is no problem because thestandard value of specification is 100 MU.

The start/stop time of the pump did not change.

Fig. 5. Sectional drawing of TG1300M

4. Improving resistance to radiation

The material of the O-ring was changed from rubber to metal,because TG1300 M was unable to withstand a radiation of 10 MGydue to the deterioration of the O-ring. And also, parts easily affectedby radiation have been improved. Table 2 lists the improved parts.

The model name of TG1300MR has been assigned.Fig. 5 shows the sectional drawing of TG1300MR.A lead block was used to protect the PVC (polyvinylchloride)

output cable during the gamma-ray irradiation test of TG1300 M,because PVC is vulnerable to radiation. The lead block was removedfor the gamma-ray irradiation test of TG1300MR because theoutput cable was changed to PEEK (polyetheretherketone) cable forthe radiation resistance. The controller and the backing vacuumpump were set up outside the gamma-ray irradiation room.

The PEEK cable had undergone gamma-ray irradiation testbefore being used for the electric wire inside the pump and theoutput cable.

At the accumulative radiation dose was 10 MGy, though thesheath changed color, there was no change in the performance.

At the accumulative radiation dose was 30 MGy, the elongationratio and the tensile strength were deteriorated, but the insulationresistance was retained.

In conclusion, the permissible radiation dose of the PEEK cable isup to 30 MGy.

5. Gamma-ray irradiation examination of TG1300MR

The method for irradiation examination for TG1300MR was thesame as for TG1300 M.

TG1300MR got a matching error when the accumulative radia-tion dose reached to 34.3 MGy. The matching error is indicated toavoid an incorrect combination of a pump and a controller. Thecause of the failure was a malfunction due to the decreased

R (radiation-hardened model).

Fig. 7. The amount of deformation of the epoxy resin inside the magnetic suspensionunit.

K. Wada et al. / Vacuum 84 (2010) 699–704 703

insulation resistance between pins of the magnetic suspensionconnector. The insulation resistance had dropped to 0.1 MU from100 MU. The model identification circuit was operating normally.The electrical discharge marks on the atmospheric side of themagnetic suspension connector were found on most pins such asthe pins for model distinction circuit and the electromagnet. Thecause of decreasing of the insulation resistance was the electricaldischarge mark between these pins. Fig. 6 shows the electricaldischarge mark on the magnetic suspension connector. There wasno electrical discharge mark on the opposite connector on theoutput cable side, and no change was observed.

The electrical discharge mark could be removed easily withalcohol. The magnetic suspension connector is made of ceramichermetic, and the pins are gold-plated copper installed by silverbrazing. The insulation resistance came back to normal value whenelectrical discharge marks were removed. Therefore, the deterio-ration of insulation resistance was caused by the electricaldischarge mark on the surface of a ceramic. This electrical dischargemark was marked when 500 VDC was charged to measure theinsulation resistance of the motor, the position sensor, the rotationsensor and the electromagnet. The operation voltage of eachnormal operation is 120 VAC for the motor, 15 VDC for the positionsensor and the rotation sensor, and 24 VDC for the electromagnet.The electrical discharge doses not occur because the impressedvoltage is low at normal operation.

The PEEK sheath of the wire inside the pump changed colorfrom gray to yellow ocher, and had hardened slightly. However, itkept sufficient elasticity and did not crack with bending, and itsinsulation resistance did not change. The heat shrinkage tubewhich is insulation of the wire on the vacuum side of the magneticsuspension connector had hardened and cracked, and peeled offwhen touching the wire. But the insulation resistance between thewire had not decreased. Such a change was not observed in the lastgamma-ray irradiation test for TG1300 M under 7 MGy. Theconnector of output cable had formed rust in some parts, butshowed no change in the insulation resistance or resistance.

The calibration data, the transfer function and the start/stoptime of the pump did not change, even though a change in colorwas observed on the epoxy resin for molding material to the mainparts such as the position sensor, the electromagnet and the motorstator.

The gamma-ray irradiation test was restarted after replacing themagnetic suspension connector and the heat shrink tube which isinsulation of the wire.

Fig. 6. Electrical discharge marks on the atmospheric side of the magnetic suspensionconnector.

TG1300MR got the output cable error when the accumulativeradiation dose reached to 39.3 MGy. The cause of the failure was thedeterioration of the insulation resistance for the pump sideconnector of the output cable. The cause of the insulation failure ofthe output cable was peeled off the PEEK sheath. The output cablewas replaced, and the gamma-ray irradiation examination wasrestarted. TG1300MR became incapable of operating with a sensortuning error when the accumulative radiation dose reached to73.8 MGy.

The sensor tuning error occurs when automatic tuning whichadjusts the sensor for the magnetic suspension control cannot beperformed.

The epoxy resin molded for the position sensor, the motor stator,and the electromagnet inside the magnetic suspension unit hadcracked. The sensitivity of the sensor could not be adjusted due todecreasing movable amount of the rotor because of epoxy resincrack. That’s condition become a failure. Fig. 7 shows the amount ofdeformation of the epoxy resin inside the magnetic suspension unit.

TG1300MR could operate normally again after correcting thedeformation of the epoxy resin. Fig. 8 shows the change in thesensor value. The sensor value decreases as the radiation dose isincreased from the accumulative radiation dose is 30 MGy–70 MGy.

Fig. 8. Sensor value of the position sensor.

Fig. 9. Resistance of position and rotation sensor.

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The epoxy resin surface flaked off like powder and entered intothe gap between the shaft and the dry touching down bearing. Thusthe movable amount of the shaft was reduced.

The sensor value is changed greatly at the accumulative radiationdose reached to 73.8 MGy because of decreasing the movable amountof the shaft due to the deformation of the epoxy resinwith crack. Fig. 9shows the resistance of the position sensor and the rotation sensor.

The resistance of each sensor has varied. This is due to thetemperature change in the irradiation room.

The performance of the motor, the position sensor, the rotationsensor and the electromagnets were not deteriorated. The sensorvalues were restored to normal values after correcting the defor-mation of the epoxy resin.

6. Conclusion

The permissible accumulative gamma-ray radiation dose for thestandard Magnetically Suspended Compound Molecular Pump

(TG1300 M) is 7.09 MGy because of a leakage from the O-ring due toelasticity deteriorating.

The permissible accumulative gamma-ray radiation dose for theradiation-hardened Magnetically Suspended Compound MolecularPump (TG1300MR) is 73.8 MGy because of deformation of theepoxy resin of mandrel for the magnetic suspension system.

TG1300MR could operate normally again after the deformationof the epoxy resin was corrected.

The permissible accumulative gamma-ray radiation dose fora PEEK output cable is 39.3 MGy because PEEK sheath could notkeep the insulation resistance.

We confirmed that TG1300MR can operate without troubleunder the permissible gamma-ray irradiation dose of 10 MGyrequired by 3 GeV-RCS of J-PARC.

The main parts of the magnetic suspension system such as themotor, the position sensor, the rotation sensor and electromagnetare not influenced by gamma-ray irradiation until the accumulativeradiation dose is 73.8 MGy.