Bailey IR

Clarke D

Clarke JA

Davies K

Gallagher A

Gronberg J

Hagler Li

Hesselbach S

Jenner LJ

Nelson C

Piggott WT

Rochford James

English

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A STUDY OF MECHANICAL AND MAGNETIC ISSUES FOR APROTOTYPE POSITRON SOURCE TARGETI.R. Bailey  , D.G. Clarke, J.A. Clarke  , K.P. Davies, A.J. Gallagher, J. Gronberg,L. Hagler, S. Hesselbach, L.J. Jenner   , C.J. Nelson  , T. Piggott, J.H. Rochford  Cockcroft Institute, Daresbury Campus, Warrington, Cheshire, WA4 4AD, UK Department of Physics, University of Liverpool  , Liverpool, L69 7ZE, UK STFC Daresbury Laboratory, Warrington, Cheshire, WA4 4AD, UK Lawrence Livermore National Lab  , Livermore, CA 94551, USA Institute of Particle Physics Phenomenology, University of Durham, UK STFC Rutherford Appleton Laboratory, Didcot, OX11 0QX, UKAbstractIn order to construct a high yield, positron source thatcan meet the intensity requirements of future facilities, arobust conversion target is needed. One solution is to use arotating titanium alloy wheel upon which a beam of pho-tons is incident. The efficiency of capturing the result-ing positrons can be optimised by immersing this systemin a magnetic field. As described elsewhere [1], a pro-totype of such a target has been built at Daresbury Labo-ratory, to investigate the mechanical challenges associatedwith its construction and to study the magnetic effects thatthe wheel will experience. In this paper, predictions of theeddy current power losses in the target prototype obtainedfrom magnetic modelling simulations are presented, and aninitial analysis of the thermal shocks that the ILC targetwheel is expected to experience due to beam loading dur-ing operation is described.INTRODUCTIONIn the baseline design for the ILC positron source thehigh-energy (  GeV) electron beam from the electronlinac is diverted through a beamline containing a helicalundulator to generate gamma rays with an average energyclose to   MeV.The average integrated power of the photon beam gener-ated by the ILC undulator will be approximately    kW,with each bunch of photons carrying a total energy of ap-proximately   J and consisting of order    photons. Thebeam will pass through a collimator before being incidenton the rim of a Ti Al V target wheel   radiationlengths thick. The proposed target wheel comprises a cir-cular rim   m in diameter connected to a central drive shaftby five equally-spaced radial struts. The wheel will be ori-ented with the photon beam parallel to the drive shaft, suchthat photons will strike the rim, which will have a radial  l.j.jenner@dl.ac.uk This work is supported in part by the Commission of the EuropeanCommunities under the th 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.width of  mm. The target will be positioned downstreamof the undulator such that the photon beam spot will havean rms radius of at least  mm.Particle tracking simulations predict that approximately	 of the power of the photon beam will be dissipated inthe target. Despite the fact that the wheel rotates to miti-gate the effects of the intense beam incident upon it, simu-lations must be carried out to ensure the target is capable ofsurviving in this environment. The possibility of damageor fracture due to shock waves in the real target wheel isconsidered later in this paper. Results from one model arepresented in the following section.The efficiency of the positron source design can be sub-stantially improved if the conversion target is partially im-mersed in the magnetic field of the positron capturing op-tics, the next element in the beamline after the target itself.However, the effect of eddy currents generated in the targetwheel rim due to the interaction between the rotating metalsurface and the magnetic field of the adjacent capture opticsmust be taken into account.The construction of a first prototype of the target wheelhas nearly been completed with the aim of investigating theeddy currents and benchmarking the associated numericalsimulations. The wheel will be partially immersed in thefield of a GMW 3474-140 250 mm dipole electromagnet tosimulate the effect of the capture optics. At a rim velocityof   ms   and a magnetic field strength of   T simula-tions predict eddy current power loads in the prototype ofapproximately   kW but estimates differ widely depend-ing on the assumptions made in the computer models.MAGNETIC MODELLINGThe eddy currents in the target wheel are calculated inVector Fields Opera [2] using the ELEKTRA rotationalsolver. ELEKTRA uses a combination of vector and re-duced potentials to model time varying electromagneticfield problems. The rotational solver also includes the ef-fects of motion induced eddy currents about a given rota-tional axis. In reality the target wheel model is not fullyaxially symmetric due to the spokes, however under the as-sumption that the interaction of the disk with the magneticProceedings of EPAC08, Genoa, Italy MOPP07203 Linear Colliders, Lepton Accelerators and New Acceleration Techniques T02 Lepton Sources715field is localised to the rim of the wheel the geometry canbe represented as being axially symmetric, see Fig 1.Figure 1: Left: Showing an engineering drawing of the pro-totype 5-spoked wheel (green). Right: The simplified, ax-ially symmetric model used to allow modelling in ELEK-TRA. The solenoidal coils (red) are approximately equiva-lent to the pole caps of the dipole magnet being used in theprototype tests.Figure 2: Showing the calculated values for the torqueexpected at incremental rotation rates and benchmarkedagainst real copper disk measurements.The rim of the titanium alloy wheel material is mod-elled to have a thickness of 14 mm, an outer radius of0.5 m, an inner radious of 0.47 m and a conductivity of9.26x  S/m. Since the code models the field penetrationinto a moving material it is essential to have a mesh den-sity smaller then the characteristic skin depth of the com-ponents being modelled. The nominal rotational speed ofthe final target wheel will be  2000 rpm, and the proto-type is designed match this. If we assume that one rotationis comparable to one cycle of an oscillating electromag-netic field, then the frequency is  40 Hz, yielding an es-timate of  6 mm for the skin depth for this regime. TheELEKTRA modeller has been benchmarked against someexperimental data produced by measuring the torque of aspinning copper disk in a magnetic field, carried at outat LLNL/SLAC [3]. The model was found to be in goodagreement with the experiment giving confidence in its pre-dictions, as seen in Fig 2. A typical output of the ELEK-TRA simulation is shown in Fig 3.Figure 3: Showing the Opera model of the titanium wheel(grey) immersed in the field region between the coils. Thefield strength along the z-axis is shown as a coloured con-tour plot superimposed on the wheel. The peak fieldstrength is 1.6 T.The simulation is carried out for various speeds and fieldstrengths, which will be mimicked in the prototype experi-mental program. The distance into the field that the wheelrim is moved, or the immersion depth, is also varied in sim-ulation and in experiment. The immersion depth affects thetotal area of the wheel that is within the field at any one timeand also the amount to which it experiences the strongestand most homogenous field that the magnet produces.Fig 4 shows the gradually rising eddy currents as thespeed of the wheel increases. As expected, the simula-tion also shows that a lesser immersion into the field causesthe wheel to experience a lower peak field and hence lesspower loss. The nominal working point of the real targetwheel will be 2000 rpm in a 1.16 T magnetic field. A sim-ulation tailored to these parameters yields an expected eddycurrent power loss of 3 kW at an immersion depth of 248mm. These eddy currents create a magnetic force whichopposes the motion of the wheel in the dipole field. Thesimulation calculates this force to be 30 N, correspondingto a torque of 17 Nm.MOPP072 Proceedings of EPAC08, Genoa, Italy03 Linear Colliders, Lepton Accelerators and New Acceleration Techniques716T02 Lepton SourcesFigure 4: Showing how Eddy Current Power Loss varieswith Speed at four different immersion depths at 1.6T.ANALYSIS OF THERMAL SHOCKS INTHE TARGETDuring the operation of the target at the ILC, the rapidenergy deposition from the photon beam will cause a pres-sure shock wave which may damage the target. The shockwave can be described by a hydrodynamical model for thetemperature      and pressure     [5].Initial simulations carried out at LLNL [4] showed thatthe stresses on the target would be within acceptable limits,but more recent studies at Cornell [6] have shown greaternegative pressures associated with the shock wave on thedownstream face of the target, which could lead to a failureof the target. In [6] the distribution of the energy depositionhas been modelled by a Gaussian approximation, 	 	   			  		(1)with the energy deposited by one bunch   , the bunch dimensions  and the targetthickness .In order to check these assumptions, we have comparedthe resulting energy distribution of Eq. (1) with a FLUKAsimulation of the energy deposition in the target, wherehigher harmonics of the undulator radiation are includedand hence the photon beam intensity extends to larger than for a Gaussian distribution [7]. In Fig. 5 the contoursof the volume are shown where 90% of the bunch energy isdeposited for both the FLUKA simulation and the Gaussianapproximation according to Eq. (1). For the more realisticFLUKA simulation the energy is deposited in a consider-ably larger volume than for the Gaussian approximation.Although this result casts some doubt on the applicabilityof the Cornell model in this case, further studies of the hy-drodynamic models will be needed to investigate the sur-vivability of the target.Figure 5: Contours of the volume where 90% of the bunchenergy is deposited for a FLUKA simulation and the Gaus-sian approximation of Eq. (1).SUMMARY AND OUTLOOKA design for a titanium target wheel that satisfies the re-quirements of the ILC baseline positron source has beendeveloped, and benchmarking of magnetic simulationsagainst real data is underway.In our simplified model, at 2000 rpm in a realistic field of1.16 T, calculations show that the wheel will be susceptibleto maximum eddy current power losses of 3 kW. The asso-ciated opposing force is 30 N, corresponding to a torque of17 Nm.Current results appear to indicate that a gaussian distri-bution is insufficient to describe the particle flow throughthe target, which could have a positive bearing on the sur-vivability of the titanium wheel. However, before firm con-clusions can be drawn the heat flow and the pressure wavehave to be simulated with the full hydrodynamical model.REFERENCES[1] I.R. Bailey et al., “A Prototype Target Wheel for the ILCPositron Source”, EPAC 2008 Conf. Proc.[2] http://www.vectorfields.com[3] T. Piggott et al., THPMN087, PAC 2007 Conf. Proc.[4] SLAC-TN-03-044, LCC-0088, UCRL-ID-148936[5] A.A. Mikhailichenko, CBN06-1, 2006.[6] A.A. Mikhailichenko, talk at ILC Positron Source Collabora-tion Meeting, Argonne National Laboratory, September 17–19, 2007.[7] A. Ushakov, talk at ILC Positron Source Collaboration Meet-ing, DESY Zeuthen, April 7–9, 2008.Proceedings of EPAC08, Genoa, Italy MOPP07203 Linear Colliders, Lepton Accelerators and New Acceleration Techniques T02 Lepton Sources717

A Prototype Target Wheel for the ILC Positron Source”, EPAC

talk at ILC Positron Source Collaboration Meeting,

A Study of  Mechanical and Magnetic Issues for a Prototype Positron Source Target

A Study of Mechanical and Magnetic Issues for a Prototype Positron Source Target

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