The UK pro­gramme of high power tar­get de­vel­op­ments for a Neu­tri­no Fac­to­ry is cen­tred on the study of high-Z ma­te­ri­als (tung­sten, tan­ta­lum). A de­scrip­tion of life­time shock tests on can­di­date ma­te­ri­als is given as part of the re­search into a solid tar­get so­lu­tion. A fast high cur­rent pulse is ap­plied to a thin wire of the sam­ple ma­te­ri­al and the life­time mea­sured from the num­ber of puls­es be­fore fail­ure. These mea­sure­ments are made at tem­per­a­tures up to ~2000 K. The stress on the wire is cal­cu­lat­ed using the LS-DY­NA code and com­pared to the stress ex­pect­ed in the real Neu­tri­no Fac­to­ry tar­get. It has been found that tan­ta­lum is too weak to sus­tain pro­longed stress at these tem­per­a­tures but a tung­sten wire has reached over 26 mil­lion puls­es (equiv­a­lent to more than ten years of op­er­a­tion at the Neu­tri­no Fac­to­ry). An ac­count is given of the op­ti­mi­sa­tion of sec­ondary pion pro­duc­tion from the tar­get and the is­sues re­lat­ed to mount­ing the tar­get in the muon cap­ture solenoid and tar­get sta­tion are dis­cussed

Back, J. J.

Bennett, J.R.J.

Booth, C.N.

Brooks, S.J.

Edgecock, R.

Škoro, G.P.

English

University of Huddersfield Repository

University of Huddersfield RepositoryEdgecock, R., Back, J. J., Bennett, J.R.J., Booth, C.N., Brooks, S.J. and Škoro, G.P.Overview of Solid Target Studies for a Neutrino FactoryOriginal CitationEdgecock, R., Back, J. J., Bennett, J.R.J., Booth, C.N., Brooks, S.J. and Škoro, G.P. (2010) Overview of Solid Target Studies for a Neutrino Factory. In: Proceedings of the 1st International Particle Accelerator Conference. IPAC 2010 . JACoW, Kyoto, Japan, pp. 4263-4265. ISBN 978-92-9083-352-9 This version is available at http://eprints.hud.ac.uk/21168/The University Repository is a digital collection of the research output of theUniversity, available on Open Access. Copyright and Moral Rights for the itemson this site are retained by the individual author and/or other copyright owners.Users may access full items free of charge; copies of full text items generallycan be reproduced, displayed or performed and given to third parties in anyformat or medium for personal research or study, educational or not-for-profitpurposes without prior permission or charge, provided:• The authors, title and full bibliographic details is credited in any copy;• A hyperlink and/or URL is included for the original metadata page; and• The content is not changed in any way.For more information, including our policy and submission procedure, pleasecontact the Repository Team at: E.mailbox@hud.ac.uk.http://eprints.hud.ac.uk/OVERVIEW OF SOLID TARGET STUDIES FOR A NEUTRINOFACTORY∗T. R. Edgecock† , J. R. J. Bennett, S. Brooks, STFC, Rutherford Appleton Laboratory, UKG. P. ˇSkoro, C. N. Booth, Department of Physics and Astronomy, University of Sheffield, UKJ. J. Back, Department of Physics, University of Warwick, UKAbstractThe UK programme of high power target developmentsfor a Neutrino Factory is centred on the study of high-Zmaterials (tungsten, tantalum). A description of lifetimeshock tests on candidate materials is given as part of the re-search into a solid target solution. A fast high current pulseis applied to a thin wire of the sample material and the life-time measured from the number of pulses before failure.These measurements are made at temperatures up to 2000K. The stress on the wire is calculated using the LS-DYNAcode and compared to the stress expected in the real Neu-trino Factory target. It has been found that tantalum is tooweak at these temperatures but a tungsten wire has reachedover 26 million pulses (equivalent to more than ten yearsof operation at the Neutrino Factory). Measurements ofthe surface velocity of the wire using a single point LaserDoppler Vibrometer are in progress, which, combined withLS-DYNA modelling, will allow the evaluation of the con-stitutive equations of the material. An account is given ofthe optimisation of secondary pion production and capturein a Neutrino Factory and of the latest solid target engineer-ing ideas.INTRODUCTIONThere are proposals [1] to build a Neutrino Factory inthe US, Europe or Japan, in order to understand some ofthe basic properties of neutrinos. The Neutrino Factorywill consist of a proton driver accelerator delivering shortpulses of beam to a heavy metal target at GeV energies atup to ∼ 50 Hz, with a mean power of ∼ 4 MW. As a re-sult of the beam interactions with the target, copious num-bers of pions will be produced, as well as other secondaryproducts. The pions decay to muons which are cooled andaccelerated to tens of GeV. The muons then circulate in alarge storage/decay ring with long straight sections wherethey decay to neutrinos. The neutrinos come off in a nar-row cone along the axis of the muon beam and the armsof the decay ring are directed at suitable long baseline neu-trino detectors thousands of kilometres away. ExtensiveNeutrino Factory R&D is underway around the world ona number of fronts. Target R&D is particularly importantbecause it is a potential showstopper. The UK programmeof high power target developments for a Neutrino Factoryis centred on the study of high-Z solid materials (tungsten,∗Work supported by Science and Technology Facilities Council (UK).† t.r.edgecock@rl.ac.uktantalum).THE NEUTRINO FACTORY TARGETThe Neutrino Factory target is not a stopping target. Itdissipates a mean power of about 700 kW in a 2 − 3 cmdiameter bar of tungsten about 20 cm long. The remain-ing 3.3 MW of beam is absorbed in a beam dump and sur-rounding shielding. The energy density averaged over thetarget volume is ∼ 300 J/cm3 per pulse (assuming a 2 cmdiameter target). Tungsten and tantalum are candidate ma-terials since they have high Z values, are refractory and arerelatively strong. Also, these metals have been shown to beextremely resistant to radiation damage effects in the ISIStarget [2] (up to 12 dpa), similar to the Neutrino Factorytarget after 10 years of operation.Figure 1: Variation of the peak stress versus macro-pulselength in 2 and 3 cm diameter targets, with 3 and 5 equallyspaced bunches (each 2 ns long).The magnitude of the thermal stress on the target, whichis the main issue for solid targets, is governed by the mag-nitude and rate of change of the energy density (or tempera-ture rise). With the Neutrino Factory there is a requirementfor short micro-pulses of 1 − 2 ns length within a macro-pulse of a few micro-seconds. MARS [3] calculations havebeen made of the beam hitting tantalum and tungsten tar-gets to assess the distribution of energy deposition, andto calculate the corresponding temperature rise. The peaktemperature rise is about 200 K, in which a parabolic trans-verse beam current distribution is assumed, where the beamradius and the target radius are equal.The temperature rises are then used in the commercialpackage LS-DYNA [4] to calculate the dynamic stresses inthe target. As can be seen in Fig. 1, the equivalent VonMises stress can reach several hundred MPa, depending onProceedings of IPAC’10, Kyoto, Japan THPEC08904 Hadron AcceleratorsT19 Collimation and Targetry 4263the target diameter, the number of bunches (an odd numberof bunches is preferred for the muon accelerator) and thepulse length. Also, we can see that by a clever choice ofpulse length (assuming equally spaced bunches) we will beable to reduce the stress on the target by a factor of two ormore. When discussing a solid tungsten (or tantalum) baras a choice for a Neutrino Factory target we do not meanto have a single bar; this would be impossible if we keepin mind the energy density, repetition rate and temperaturerise per pulse. In our case, the individual bar concept as-sumes that we have hundreds of bars, and that a ’new’ barwould be presented for each beam pulse and then cooledby radiation until the next turn. We have studied differentpossible designs [5] but the most interesting one is a spoke-less wheel concept [6] in which targets are mounted on awheel which will rotate perpendicularly to the beam direc-tion. In this case, around 150 bars (spaced apart by 100mm) will form an outer rim that is connected to an inner’guide/driver’ rim of the wheel. The outer diameter will be5 m, giving a rim speed of 5 m/s. In this concept, the mag-netic solenoid magnet used for pion capture has to be splitinto two halves (Helmholtz coil). This could affect the cap-tured pion yield, but calculations [7] have shown that thecapture rate is very similar to the corresponding results foran alternative mercury jet target. These simulations havebeen done by calculating the pion production rate in thetarget using MARS and then transporting the captured pi-ons and muons to the end of the front-end using ICOOLcode [8]. A lot of work is still required to develop this newengineering idea into a full concept, for example to exam-ine the effect of eddy currents on the wheel, to estimatethe forces that the support structure will need to withstand,etc. However, the first step in this direction is to determinethe lifetime of tungsten and tantalum as potential NeutrinoFactory targets.LIFETIME SHOCK TESTSIn order to make thermal shock measurements on tanta-lum and tungsten samples it would be best to do a lifetimetest on a real size target in a proton beam over several years.However, beams of this power are not readily available forany length of time. Hence, it was decided to pass a fast,high current pulse through a thin wire made of the can-didate materials. Coupled with the use of codes (such asLS-DYNA) to model the thermal shock in the wire it willbe possible to find the constitutive equations of state of thematerial under dynamic conditions. Using the correct con-stitutive equations for tungsten at high temperatures willallow a better computation of the thermal stresses in a realtarget of various geometries.A thin wire is necessary to allow the current to diffuseinto the centre of the wire in a sufficiently short time forthe shock to be effective. For tantalum and tungsten, thewire cannot be greater than ∼ 0.75 mm in diameter. Apower supply for the ISIS [9] kicker magnets is being used,supplying a maximum of 60 kV and 9200 A at up to 50Figure 2: Photograph of a 0.75 mm diameter tungsten wireduring current pulse tests (a) and illustration of wire clamp-ing (b)Hz in a pulse which rises in 100 ns and is ∼ 800 ns long.The wires, of 3 − 5 cm length, are supported in a vacuumchamber to avoid oxidation. One end of the wire is firmlyclamped, while the other end is allowed to expand freelythrough a pair of graphite conductors that lightly clamp thewire (see Fig. 2). The wire is operated at temperatures from300 to 2500 K by adjusting the pulse repetition rate . Thewire temperature is measured by a manually operated opti-cal pyrometer.Figure 3: Beam power into a target of 2 and 3 cm diameterversus wire current for the same peak stress (at the initialtemperature of 2000 K). Using a proton beam requires thatwe have 3 bunches at optimum spacing to minimise thestress in the target.The stress in the wire is calculated including both tem-perature and the Lorenz force from the magnetic field pro-duced by the current passing through it. It was shown thatVon Mises stress in the wire reaches similar values to thebeam-target case, hence it is possible to relate the currentin the wire that produces the same peak stress as the beamin the full sized target (see Fig. 3). The illustration of theresults of a number of tests with 0.5 mm diameter tungstenwires is shown in Table 1. The tantalum was too weak attemperatures of 1400 K or more. The tungsten was muchmore robust and the wire only failed when operated at tem-peratures well over 2000 K. As can be seen in Table 1, insome cases the wire(s) survived tens of millions of pulses,for example 26.4 million pulses in the high temperatureregime at the equivalent beam power of 4 MW in a 2 cmdiameter target. This corresponds to the target lifetime atTHPEC089 Proceedings of IPAC’10, Kyoto, Japan426404 Hadron AcceleratorsT19 Collimation and Targetrythe Neutrino Factory of more than 10 years (more than 20years for a 3 cm diameter target). The situation is evenbetter at lower temperature (see the last row in Table 1).Table 1: Results of tungsten wire tests. The ’Beam Power’column shows the equivalent beam power for a full sizetarget of 2(3) cm diameter for the same stress in the testwire. These results assume a parabolic beam distribution,3 bunches per macro-pulse of 20µs, and a beam diameterequal to the target diameter.Current Tempe- No. of Beam Targetrature pulses Power Diameter(A) (K) (*106) (MW) (cm)5560 1900 4.2 2.7 (5.0) 2 (3)5840 2050 9.0 3.0 (5.4) 2 (3)6200 2000 10.1 3.3 (6.1) 2 (3)6520 1940 26.4 4.1 (8.7) 2 (3)4720 1840 54.4 2.1 (4.5) 2 (3)6480 600 80.8 4.0 (8.6) 2 (3)LDV MEASUREMENTSA single point Laser Doppler Vibrometer (LDV) fromPolytec [10] has been used for measurements of the wire’slongitudinal and radial surface oscillations. The main goalof these measurements is to measure tungsten properties asa function of applied stress and temperature, as well as tocheck the modelling results and, hopefully, confirm the re-sults of lifetime tests. The measurement system consists ofthe optical sensor head OFV-534 which is connected to theOFV-5000 vibrometer controller. The sensor head consistsof a laser source, an interferometer and a photo-detector.We have performed an extensive set of measurements ofthe surface displacement and velocity of tungsten wires athigh temperature and high stress [11]. A characteristic setof the results is shown in Fig. 4. The top plot in Fig. 4shows the radial velocity of the 0.5 mm diameter tungstenwire as a function of time for 50 kV peak voltage while thebottom plot shows the same but for 60 kV peak voltage.The full lines represent the results of LS-DYNA simula-tions. The time scale starts at t < 0 in order to illustrate thesignal to noise ratio. At t = 0 the current pulse arrives andthe wire starts first to contract (because the magnetic pres-sure effect is dominant at the beginning of the pulse) andthen to expand. As can be seen, the LS-DYNA result (fullline) nicely follows the experimental data. All the other ex-perimental data agree very well with modelling results [11]which means that the results of the lifetime tests have beenconfirmed.CONCLUSIONSOur lifetime shock tests demonstrated that our originalmaterial candidate, tantalum, is not strong enough at highFigure 4: (Top) Radial velocity of the 0.5 mm diametertungsten wire as a function of time for 50 kV peak voltage;(Bottom) Radial velocity of the 0.5 mm diameter tungstenwire as a function of time for 60 kV peak voltage. The fulllines represent the results of LS-DYNA simulations.temperatures, but a tungsten wire has reached over 26 mil-lion pulses, equivalent to more than ten years of operationat the Neutrino Factory. Measurements of the surface os-cillations of the wire using a single point Laser DopplerVibrometer are in progress, and the first results fully con-firm our modelling results. These measurements, combinedwith additional LS-DYNA modelling, will allow the eval-uation of the constitutive equations of the material underconditions expected at the Neutrino Factory.REFERENCES[1] NuFact proceedings 1999-2006, e.g. NuFact05; Nucl.Physics B (Proc. Suppl.) 155 (2006).[2] J. Chen et al., J. Nucl. Mater., 318 (2003) 56.[3] N. Mokhov, http://www-ap.fnal.gov/MARS.[4] Livermore Soft. Techn. Corp., http://www.lstc.com/.[5] J.R.J. Bennett et al., J. Nucl. Mater., 377 (2008) 285.[6] http://hepunx.rl.ac.uk/uknf/wp3/targeteng/.[7] J. Back, PoS NuFact08:084 (2008); arXiv: 0812.3520.[8] R.Fernow et al., pubweb.bnl.gov/people/fernow/icool/.[9] The ISIS web site: http://www.isis.rl.ac.uk/.[10] Polytec web site: http://www.polytec.com.[11] G.P. Skoro et al., This Proceedings (THPEC091).Proceedings of IPAC’10, Kyoto, Japan THPEC08904 Hadron AcceleratorsT19 Collimation and Targetry 4265

NuFact proceedings 1999-2006, e.g.

Overview of Solid Target Studies for a Neutrino Factory Original Citation Edgecock,

The ISIS web site: http://www.isis.rl.ac.uk/.

Overview of Solid Target Studies for a Neutrino Factory

Overview of Solid Target Studies for a Neutrino Factory

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