Re­cent­ly re­port­ed re­sults on the tung­sten life­time/fa­tigue tests under con­di­tions ex­pect­ed in the Neu­tri­no Fac­to­ry tar­get have strength­ened the case of solid tar­get op­tion for a Neu­tri­no Fac­to­ry. This paper gives de­scrip­tion of the de­tailed mea­sure­ments of the tung­sten prop­er­ties at high tem­per­a­ture and high stress. We have per­formed ex­ten­sive set of mea­sure­ments of the sur­face dis­place­ment and ve­loc­i­ty of the tung­sten wires that were stressed by pass­ing a fast, high cur­rent pulse through a thin sam­ple. Ra­di­al and lon­gi­tu­di­nal os­cil­la­tions of the wire were mea­sured by a Laser Doppler Vi­brom­e­ter. The wire was op­er­at­ed at tem­per­a­tures of 300-2500 K by ad­just­ing the pulse rep­e­ti­tion rate. In doing so we have tried to sim­u­late the con­di­tions (high stress and tem­per­a­ture) ex­pect­ed at the Neu­tri­no Fac­to­ry. Most im­por­tant re­sult of this study is an ex­per­i­men­tal con­fir­ma­tion that strength of tung­sten re­mains high at high tem­per­a­ture and high stress. The ex­per­i­men­tal re­sults have been found to agree very well with LS-DY­NA mod­elling re­sults

Back, J.J.

Bennett, J.R.J.

Booth, C.N.

Edgecock, R.

Gray, S.A.

McFarland, A.J.

Škoro, G.P.

English

University of Huddersfield Repository

University of Huddersfield RepositoryŠkoro, G.P., Back, J.J., Bennett, J.R.J., Edgecock, R., Booth, C.N., Gray, S.A. and McFarland, A.J.Tungsten Behavior at High Temperature and High StressOriginal CitationŠkoro, G.P., Back, J.J., Bennett, J.R.J., Edgecock, R., Booth, C.N., Gray, S.A. and McFarland, A.J. (2010) Tungsten Behavior at High Temperature and High Stress. In: Proceedings of the 1st International Particle Accelerator Conference. IPAC 2010 . JACoW, Kyoto, Japan, pp. 4269-4271. ISBN 978-92-9083-352-9 This version is available at http://eprints.hud.ac.uk/21171/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/TUNGSTEN BEHAVIOUR AT HIGH TEMPERATURE AND HIGHSTRESS∗G. P. Sˇkoro† , C. N. Booth, Department of Physics and Astronomy, University of Shefﬁeld, UKJ. R. J. Bennett, T. R. Edgecock, S. Gray, A. McFarland, STFC, Rutherford Appleton Laboratory, UKJ. J. Back, Department of Physics, University of Warwick, UKAbstractRecently reported results on the behaviour of the tung-sten under conditions expected in the Neutrino Factory tar-get have strengthened the case for a solid target option forthe Neutrino Factory. This paper gives a description ofthe measurements of the surface motion of tungsten wires,stressed by a pulsed current, using a Laser Doppler Vi-brometer in order to measure tungsten properties at hightemperature and high stress. The experimental results havebeen compared with modelling results, and the results ofprevious lifetime tests have been conﬁrmed.INTRODUCTIONThe magnitude of thermal stress on the target is the mainissue for solid target option for the Neutrino Factory. In or-der to make thermal shock measurements on tantalum andtungsten samples it would be best to do a lifetime test on areal size target in a proton beam over several years. How-ever, beams of the required power are not readily availablefor any length of time. Hence, it was decided to pass afast, high current pulse through a thin wire made of thecandidate materials [1]. This paper concentrates on mea-surements of wire surface motion during the pulsing usinga laser interferometry technique. The main goal of the ex-periment is to measure tungsten properties as a functionof applied stress and temperature, as well as to check themodelling results and, hopefully, to conﬁrm the results ofprevious lifetime tests [2].EXPERIMENTAL SETUPA 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 tungsten, the wire cannot begreater than ∼ 0.75 mm in diameter. A power supply forthe ISIS [3] kicker magnets is being used, supplying a max-imum of 60 kV and 9200A at up to 50 Hz in a pulse whichrises in 100 ns and is ∼ 800 ns long. The wires, of 3 − 5cm length, are supported in a vacuum chamber to avoidoxidation. One end of the wire is ﬁrmly clamped, whilethe other end is allowed to expand freely through a pair ofgraphite (or copper) conductors that lightly clamp the wire.The wire is operated at temperatures from 300 to 2500 Kby adjusting the pulse repetition rate. The wire tempera-ture is measured by a manually operated optical pyrometer.∗Work supported by Science and Technology Facilities Council (UK).† g.skoro@shefﬁeld.ac.ukThe shape of the current pulse is shown in Fig. 1 (dottedline). After the ﬁrst pulse (∼ 1µs long) there is a series ofreﬂections that last up to 5µs.Figure 1: Measured and calculated (see text) waveform ofthe current pulse used in the experiment.In order to calculate energy density and temperature riseacross the wire an analytic current penetration formula wasused [4]. The idea of this procedure was to obtain a goodﬁt of the measured current pulse shape. That formula andthe results of measurements of the voltage waveform wereused to obtain the current density j(r, t) distribution acrossthe wire (and the current waveform) as a function of time.It is seen in Fig. 1 that the calculated current (full line) isvery close to the measured current waveform (dotted line).After conﬁrming that analytic current penetration formuladescribes nicely the experimental results, we have used itto produce the input needed for LS-DYNA [5] calculations:the temperature rise across the wire (proportional to the in-tegral of the j2) and the Lorenz force from the magneticﬁeld produced by the current passing through wire. In theLS-DYNA calculations, the geometry of cylindrical wirewas meshed with a 3D element type 164 (LS-DYNA Ex-plicit Solid) or corresponding 2D element type. The em-pirical ’Temperature Dependent Bilinear Isotropic Model’was used as the material model. This is a model that usestwo temperature-dependent slopes to represent the exper-imental data on the material stress-strain curve. A singlepoint Laser Doppler Vibrometer (LDV) from Polytec [6]has been used for measurements of the wire’s longitudinaland radial surface oscillations. The measurement systemProceedings of IPAC’10, Kyoto, Japan THPEC09104 Hadron AcceleratorsT19 Collimation and Targetry 4269consists of the optical sensor head OFV-534 which is con-nected with the OFV-5000 vibrometer controller. The sen-sor head consists of a laser source, an interferometer and aphoto-detector. The most important parts of the vibrometercontroller are the decoder units. There is a wide range ofavailable decoders and we have used three different unitsto cover the complete range of amplitudes and frequenciesof the wire vibrations.RESULTSWe have performed an extensive set of measurements oflongitudinal wire oscillations but only the measurements ofradial oscillations have been used for the material charac-terization at high temperatures. This is because the longi-tudinal vibrations are the results of the non-homogeneoustemperature distribution along the wire so it is difﬁcult tosay what the relevant temperature is. It is important to notethat we have used two LDV decoder units for this set ofmeasurements, and that they utilise completely differentmeasurements techniques.Figure 2: (Top) Radial displacement of the 0.5 mm wireas a function of applied current. (Bottom) Correspondingfrequency spectra and the result of LS-DYNA prediction.First data were taken with a DD-300 displacement de-coder. Fig. 2 (top) shows radial displacements of the 0.5mm diameter tungsten wire as a function of time and ap-plied current, as well as the fact that displacement increaseswith increasing current (as expected), one can see that highfrequency oscillations are much more prominent for highercurrent values. This can be seen in Fig. 2 (bottom), togetherwith almost perfect agreement between the measured fre-quency (6.85 MHz) and LS-DYNA prediction. It must benoted here that the frequency spectra lie on top of eachother but they have been separated in Fig. 2 by scaling inorder to show the rise of the 6.85 MHz line with increasingcurrent. The measured radial frequency f can be used toextract the value of Young’s modulus of tungsten using theformula [7]:E =(2πf)2r2ρζ2·(1 + ν)(1 − 2ν)(1− ν), (1)where r is the wire radius, ρ is the density, ν is Poisson’sratio for tungsten and ζ is the root of the following Besselfunction [7]:ζJ0(ζ)−(1 − 2ν)(1− ν)J1(ζ) = 0. (2)Unfortunately, the amplitude range of the DD-300 de-coder unit (±75 nm) was a limiting factor for high temper-ature measurements.Tungsten  Properties  at  High  Temperature  and  High StressThe VD-05 velocity decoder unit has been found to bemuch better suited for the high temperature measurements.High temperature measurements are the most importantgoal of this study because this places the material sam-ple under high thermal stress conditions as expected at theNeutrino Factory. We have tested wires of different sizes,and the radial velocity of the 0.38 mm diameter wire isshown in Fig. 3 (bottom, dotted line). The wire in thiscase was stressed well above the Neutrino Factory level.The equivalent stress here was calculated to be 560 MPawhile at the Neutrino Factory we expect ∼ 300 MPa. TheLS-DYNA result (full line) nicely follows the experimentaldata.Figure 3: (Top) The current pulse shape for 6.5 kA peakcurrent; (Bottom) Radial velocity of the 0.38 mm diametertungsten wire at 1200 0C.This is also illustrated in Fig. 4 where the compari-son is shown between the measured and calculated radialfrequency spectra (only the region of interest is plotted).The difference between experiment and simulation is at thelevel of a fraction of a percent, which is additional proof ofTHPEC091 Proceedings of IPAC’10, Kyoto, Japan427004 Hadron AcceleratorsT19 Collimation and TargetryFigure 4: Measured and simulated characteristic frequencyof radial vibrations of the 0.38mm diameter wire measuredat 1200 0C.the quality of both the experimental method and simulationprocedure.Fig. 5 shows the comparison between our experimentalresults (ﬁlled circles) and existing data [8]. The line inFig. 5 represents the result of a ﬁt to the results obtained inprevious so-called dynamicmeasurements [8]. One can seethat our measurements show that Young’smodulus of tung-sten remains high at high temperatures and high stresses.Figure 5: Comparison between our experimental resultsand previous results [8] on tungsten Young’s modulus.When discussing measured tungsten properties as afunction of temperature, it should be noted that results de-pend on a set of parameters that deﬁne the material be-haviour (electrical conductivity, coefﬁcient of thermal ex-pansion, etc ...). All of these are temperature dependent.However, if we choose the characteristic frequency as ameasured observable we reduce this multi-parameter phasespace to only three degrees of freedom: modulus of elas-ticity, density and Poisson’s ratio. The Poisson’s ratio andtungsten density depend on temperature relatively weaklyand deﬁnitely more weakly than the Young’s modulus.As a result, we have chosen the Young’s modulus to bethe variable which represents the change of tungsten be-haviour with temperature. Also, Young’s modulus is di-rectly related to applied stress which is additional impor-tant information needed for the interpretation of our re-sults. Our measurements of tungsten material propertieswere dynamic and with relatively high strain-rates. Thesigniﬁcant difference compared to the static measurements(see Fig. 5) should be noted. From the point of view of theNeutrino Factory solid target concept, it is important thatwe have tested our tungsten samples under conditions asclose as possible to those expected at the Neutrino Factory.We have induced the same level of shock and operated thewire in the same temperature region. Also, the wire vi-brations in our experiment lie in the frequency region fromtens of kHz up to tens of MHz. Dimensions of a singleNeutrino Factory tungsten target are planed to be ∼ 2 cmin diameter and 20 cm in length, and it will vibrate domi-nantly in the frequency region around 10 kHz, so here toowe have quite a good match. As a result, we also expectto see that tungsten’s modulus of elasticity remains highunder the Neutrino Factory conditions.CONCLUSIONSWe have performed an extensive set of measurements ofthe surface displacement and velocity of tungsten wires athigh temperature and high stress. Stress was induced bypassing a fast, high current pulse through a thin wire andradial and longitudinal motion of the sample were mea-sured by a Laser Doppler Vibrometer. The wire was op-erated at temperatures of 300 − 2500 K by adjusting thepulse repetition rate. In doing so we have tried to simulatethe conditions (high stress and temperature) expected at theNeutrino Factory. The most important result of this studyis that Young’s modulus of tungsten remains high at hightemperature and high stress. The experimental results havebeen compared with modelling results and we have foundthat they agree very well. This means that the results ofprevious lifetime tests have been conﬁrmed.REFERENCES[1] J.R.J. Bennett et al., Journal of Nuclear Materials 377(1), p.285-289, 2008.[2] G.P. Skoro et al., PAC09 Proceedings, 2009 (in press).[3] The ISIS web site: http://www.isis.rl.ac.uk/.[4] The Mathematics of Diffusion, J. Crank, Oxford UniversityPress, 1967.[5] Livermore Soft. Techn. Corp., http://www.lstc.com/.[6] Polytec web site: http://www.polytec.com.[7] J. R. Airey: Arch. d. Math. u. Phys. (3) 20 (1913), 289-294.[8] J.W. Davis, ITER Material Properties Handbook, 1997, Vol-ume AM01-2111.Proceedings of IPAC’10, Kyoto, Japan THPEC09104 Hadron AcceleratorsT19 Collimation and Targetry 4271

ITER Material Properties Handbook,

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

Tungsten Behavior at High Temperature and High Stress

Tungsten Behavior at High Temperature and High Stress

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