Bailey IR

Clarke DG

Clarke James

Davies KP

Gallagher AG

Gronberg J

Hagler Li

Jenner LJ

Nelson CJ

Piggott WT

English

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A PROTOTYPE TARGET WHEEL FOR THE ILC POSITRON SOURCEI.R. Bailey1,2∗, D.G. Clarke3, J.A. Clarke1,3, K.P. Davies3, A.G. Gallagher3,J. Gronberg5, L. Hagler5, L.J. Jenner1,2, C.J. Nelson4, T. Piggott51 Cockcroft Institute, Daresbury Campus, Warrington, Cheshire, WA4 4AD, UK2 Department of Physics, University of Liverpool † , Liverpool, L69 7ZE, UK3 STFC Daresbury Laboratory, Warrington, Cheshire, WA4 4AD, UK4 STFC Rutherford Appleton Laboratory, Didcot, OX11 0QX, UK5 Lawrence Livermore National Lab ‡ , Livermore, CA 94551, USAAbstractIn this paper we describe the design, construction andcommissioning of a prototype based on the positron pro-duction target wheel planned for the International LinearCollider (ILC) positron source. The efficiency of the cur-rent baseline positron source design for the ILC can be im-proved if the conversion target is partially immersed in themagnetic field of the capture optics, thereby increasing theoverall capture efficiency for positrons by a factor of twoor more. However, immersion of the rotating target wheelgenerates strong eddy currents leading to additional heat-ing and stresses on the wheel. The primary purpose of ourprototype, which has been assembled at Daresbury Labo-ratory, is to investigate the effects of eddy currents inducedin a titanium alloy wheel moving with rim speeds up to100 ms−1 in magnetic fields of the order of 1 T.INTRODUCTIONIn the baseline design for the ILC positron source [1]the high-energy (150 GeV) electron beam from the elec-tron linac is diverted through a beamline containing a heli-cal undulator insertion device 147 m in length with a periodof 1.15 cm, generating gamma rays with an average energyclose to 10 MeV. A prototype undulator module with pa-rameters close to those specified for the ILC has recentlybeen constructed by the HeLiCal collaboration [2].The average integrated power of the photon beam gener-ated by the ILC undulator will be approximately 131 kW,with each bunch of photons carrying a total energy of ap-proximately 10 J and consisting of order 1013 photons. Thebeam will pass through a collimator [3] before being inci-dent on the rim of a Ti 6%Al 4%V target wheel 0.4 radia-tion lengths thick.The proposed target wheel comprises a circular rim1 m in diameter connected to a central drive shaft by fiveequally-spaced radial struts. The wheel will be orientedwith the photon beam parallel to the drive shaft, such that∗ i.r.bailey@cockcroft.ac.uk† This work is supported in part by the Commission of the EuropeanCommunities under the 6th Framework Programme “Structuring the Eu-ropean Research Area”, contract number RIDS-011899.‡ This work was performed under the auspices of the U.S. Departmentof Energy by the University of California, Lawrence Livermore NationalLaboratory under Contract No. W-7405-Eng-48.photons will strike the rim, which will have a radial widthof 30 mm. The target will be positioned at least 500 mdownstream of the centre of the undulator, giving a photonbeam spot with a rms radius of at least 2 mm. As the targetwheel will be housed in a vacuum vessel at a pressure of10−7 Torr or less, the wheel will be water-cooled and thewater will flow through the hollow drive shaft via a rotatingunion and through an internal channel contained inside thetarget rim.Particle tracking simulations [4] predict that approxi-mately 8% of the power of the photon beam will be dis-sipated in the target. The average energy density in the re-gion of the target rim struck by a photon macropulse (2625bunches in 1 ms) is therefore expected to be approximately900 J/g for a static target, which reduces to 40 J/g whenthe rim is rotated at the design speed of 100 ms−1. At thisspeed, the target will rotate approximately 12◦ over the du-ration of a macropulse and undergo a rotation of more thansix full revolutions between consecutive macropulses.Positrons emitted from the target wheel are to be fo-cused in a pulsed flux concentrator device before beingaccelerated first to 400 MeV by normally-conducting r.f.cavities, and subsequently to 5 GeV by a superconductingbooster linac before finally being injected into a dampingring. The capture efficiency of the positron source can besubstantially improved if the conversion target is partiallyimmersed in the magnetic field of the capture optics [5].However, the effect of eddy currents generated in the targetwheel rim due to the interaction between the rotating metalsurface and the magnetic field must be taken into account.We are currently finishing construction of a first proto-type of the target wheel with the aim of investigating theeddy currents and benchmarking the associated numericalsimulations. At a rim velocity of 100 ms−1 and a mag-netic field strength of 1 T simulations predict eddy currentpower loads in the prototype of several kW, but estimatesdiffer widely depending on the assumptions made in thecomputer models [6, 7].The mechanical design and data acquisition system forthe prototype are discussed below followed by a descriptionof the proposed experimental programme.MOPP069 Proceedings of EPAC08, Genoa, Italy03 Linear Colliders, Lepton Accelerators and New Acceleration Techniques706T02 Lepton SourcesMECHANICAL DESIGNThe prototype wheel closely resembles the baseline tar-get but does not have internal water cooling channels, andis designed to be operated in air rather than in a vacuum. Adrawing of the wheel and the target assembly is shown inFig. 1.The wheel has been manufactured from the same tita-nium alloy as specified for the baseline target and has thesame outer diameter, radial wheel width and number ofspokes. The thickness of the prototype wheel rim has beenmeasured to be 15.6±0.1 mm, where the uncertainty corre-sponds to the rms variation in the thickness of the material.The wheel has been balanced to a balancing quality gradeof G2.5, and has a small residual unbalance measured tobe approximately 0.8 g. A central drive shaft connects thewheel via a torsionally-stiff flexible coupling and torquetransducer assembly to a 15 kW drive motor. The shaft issupported by two Plummer block bearing units mountedon a custom-made frame. The motor stands on a separateframe, and the motor and the wheel have been aligned usinga laser tracker to a radial alignment tolerance of±0.42 mmand an angular alignment tolerance of ±0.75◦ as requiredby the torque transducer assembly, which is itself balancedto balancing quality grade G2.5.Figure 1: A 3d model of the target wheel prototype. Themotor (light grey) is shown on the left, and the dipole mag-net (light grey) is shown on the right behind the wheel (yel-low). The torque transducer (yellow) is shown on the driveshaft (pink).A (constant) magnetic field will be generated by a water-cooled dipole electromagnet with cylindrical pole caps250 mm in diameter. The magnet support structure (shownin red in Fig. 1) will allow the fraction of the wheel rimimmersed in the high-field region to be adjusted by varyingthe relative distance between the centre of the wheel and thecentre of the pole caps. The minimum pole gap envisagedduring operation of the rotating wheel is 50 mm, whichcorresponds to a maximum peak field of 1.5 T. The mag-net geometry limits the maximum length of the immersedarc of the wheel rim to be 190 mm as illustrated in Fig. 2.Figure 2: Diagram showing a transverse slice of the proto-type assembly. The immersed arc of the target wheel rim isshown cross-hatched.OPERATIONAL CONSIDERATIONSTwo independent finite element analyses of the stresseson the wheel during operation have been carried out andcross-checked against each other. The models are in goodagreement and show the maximum deformation of thewheel rim due to inertial loads at an angular velocity of2000 rpm to be 0.29 mm, whilst the regions of maximumstress are predicted to be located either side of each spoke,with a peak value of 126 MPa. The maximum stress pre-dicted is well below the minimum tensile strength of thetitanium alloy (960 MPa). Notwithstanding these results, itis intended to house the whole target assembly inside localguarding to contain the wheel in the event of a catastrophicfailure during operation. The total kinetic energy of thewheel at 2000 rpm is approximately 50 kJ. Preliminarycalculations suggest steel plates with a thickness between5 mm and 20 mm will be needed, where the variation de-pends on the plastic deformation properties of the gradeof steel chosen. Additional protection for personnel andequipment will be provided by sandbags where appropri-ate. When the guarding is not in place, the wheel is re-stricted to run at angular velocities of 60 rpm or less.As the prototype does not have an internal cooling chan-nel, this heat must be dissipated by another mechanism.Initial calculations show that convective cooling of the ro-tating wheel in air at room temperature will lead to anequilibrium temperature of the wheel rim of approximately200◦ C. During operation the temperature is monitored asdescribed in the data acquisition section below, and airfrom a centrifugal fan fitted with a dust filter is to be di-rected through the shielding by a duct in order to protect themagnet and transducers from overheating. In addition, themagnet is fitted with temperature interlocks which isolatethe magnet in the event of the coils exceeding a tempera-ture of 50◦ C. The resistances of the magnet coils are alsomonitored to ensure that they do not exceed their maximumrated value of 0.54 Ω.Excess vibrations of the system due to bearing wear orother failure of the prototype are detected by uniaxial ac-celerometers mounted vertically on each of the two bear-Proceedings of EPAC08, Genoa, Italy MOPP06903 Linear Colliders, Lepton Accelerators and New Acceleration Techniques T02 Lepton Sources707ing units. The accelerometers are rated to have a sensi-tivity of one thousandth of the acceleration due to gravityat sea-level at a readout rate of 1 kHz. One accelerom-eter is connected to a motor interlock which isolates themotor when the signal from the accelerometer exceeds anadjustable threshold. The second accelerometer is used bythe data acquisition system. Additional machine protectionand personnel safety is provided by motor interlocks fittedto all removable panels on the local guarding and to thedoor of the caged area in which the prototype is situated.DATA ACQUISITIONThe data acquisition system consists of two PC’s inter-faced to the power supplies (for the drive motor and themagnet) and to a suite of transducers via USB and serialports. A rotary torque sensor comprised of strain gauges ismounted on the drive shaft to measure the axial torque act-ing on the wheel at rates of up to 3 kHz. Infra-red sensors(single-pixel thermal cameras) and thermocouples are usedto monitor the temperature of the magnet pole caps, targetrim, drive shaft and support structures. As the responseof the infra-red sensors depends on the emissivity of thematerial whose temperature is being studied, calibration ofthe sensors is currently underway using a thermostatically-controlled heating element and off-cuts of the target wheelmaterial that were produced during the wheel’s manufac-ture. The angular velocity of the drive shaft is monitoredby optical encoders both in the torque transducer unit and inthe motor controller. As described above, the vibrations ofthe bearings are monitored using a uniaxial accelerometer.Finally, a Hall probe monitors the magnetic field betweenthe pole caps.EXPERIMENTAL PROGRAMMEThe first phase of the experiment will aim to measurevibrations of the wheel as a function of angular velocity inthe absence of a magnetic field. Data from the torque trans-ducer and optical encoder will be logged and compared toprovide a cross-check of the angular velocity measurement.Safe operating thresholds for the vibration interlock will beestablished and the characteristic Fourier spectra of the ac-celerometer signals will be recorded over a range of angularvelocities up to 2000 rpm, and compared to the predictionsof a rotordynamic model. Out of plane flex modes of thewheel are predicted to be excited at angular velocities closeto 200 rpm and 1100 rpm. Special care will be taken nearthese critical speeds.During the second (subsequent) phase of the experiment,the magnetic field strength and wheel speed will be system-atically incremented in steps of 0.2 T and 50 rpm respec-tively, and the resulting torque and temperature readingswill be compared with the predictions of the eddy currentcomputer simulations. Data will be taken over a range ofimmersion depths of the rim in the magnetic field. Usinginformation on the azimuthal angle of the wheel providedby the optical encoders, data taken whilst the spokes of thewheel are travelling through the magnetic field will be con-sidered separately from data taken during the rest of thewheel’s rotation so that the transient eddy currents in thespokes can be studied. Models of the effect of the transienteddy currents are currently under development. Simple an-alytical calculations based on the relative area of rim andspoke immersed in the field suggest that the transient in-creases in torque associated with the spokes will be lessthan 2% of the torque associated with the rim, where thedistance between the centre of the wheel and the centre ofthe pole caps is taken as 0.57 m.During the third phase of the experiment, the wheel willbe operated at 2000 rpm for prolonged periods and thecomponents will be monitored for wear. Additional studieswith a replacement aluminium wheel are also under consid-eration in order to investigate the dependence of the eddycurrents on the conductivity of the material.SUMMARY AND OUTLOOKA design for a titanium target wheel that satisfies the re-quirements of the ILC baseline positron source has beendeveloped, and construction of a target wheel prototype toinvestigate eddy currents effects is almost complete. Ini-tial commissioning of the wheel transducers at speeds of60 rpm is ongoing, and faster operation of the wheel willcommence when sufficient guarding is in place. It is antic-ipated that the experiment will be complete at the end of2008. Results from the experiment will be useful in opti-mising the choice of target and capture optics designs forthe ILC.REFERENCES[1] G. Aarons et al., “ILC Reference Design Report”,http://www.linearcollider.org/.[2] J.A. Clarke et al., “Construction of a Full Scale Supercon-ducting Undulator Module for the International Linear Col-lider Positron Source”, these proceedings.[3] L. Zang et al., “Studies on the Role of a Photon Collimatorfor the ILC Positron Source”, these proceedings.[4] A. Ushakov et al., EPAC 2006 Conf. Proc. 2478 (2006).[5] W. Liu et al., PAC 2007 Conf. Proc. 2918 (2007).[6] S.P. Antipov et al., PAC 2007 Conf. Proc. 2909 (2007).[7] I.R. Bailey et al., “A Study of the Mechanical and MagneticIssues for a Prototype Positron Source”, these proceedings.MOPP069 Proceedings of EPAC08, Genoa, Italy03 Linear Colliders, Lepton Accelerators and New Acceleration Techniques708T02 Lepton Sources

A Study of the Mechanical and Magnetic Issues for a Prototype Positron Source”, these proceedings.

Construction of a Full Scale Superconducting Undulator Module for the International Linear Collider Positron Source”, these proceedings.

Studies on the Role of a Photon Collimator for the ILC Positron Source”, these proceedings.

A Prototype target wheel for the ILC Positron Source

A Prototype target wheel for the ILC Positron Source

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