Bennett J R J

Densham C J

Drumm P V

English

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A HIGH POWER RADIATION COOLED TARGET FOR A NEUTRINOFACTORYJ. R. J. Bennett, C. J. Densham and P. V. Drumm, CLRC, Rutherford Appleton Laboratory, Chilton,Didcot, Oxon. OX11 0QX, UK.AbstractA solid target is being considered for the productionof pions by intense high-energy proton beams for aneutrino factory. The target would consist of a rotatingring or a string of discrete bars travelling in a circle. Thetarget would rotate in vacuum and the power would beradiated to the water-cooled walls of the vacuumchamber. Steady powers of up to 100 MW can bedissipated by thermal radiation at temperatures of ~2500K, using a suitable refractory material such as tantalum ortungsten. Under thermal cycling and pulsed beamconditions, the thermal stresses and shock induced in thetoroid are being studied. This is expected to significantlyreduce the power capabilities and shorten the life byfatigue. The present design of the facility requires anaverage power dissipation in the target of up to 1 MW;this appears feasible at present. It is proposed to levitate,guide and rotate the target by electromagnetic forcesgenerated by suitable arrangements of coils. A targetwheel variation is also being considered for practicalreasons.1  INTRODUCTIONA toroid or band, rotating in vacuum and thermallyradiating its power to water-cooled vacuum chamberwalls could provide a simple, clean and reliable highpower target for pion production in a neutrino factory [1].It would not require beam windows between the incomingproton beam and the outgoing pion beam. A similar bandtarget has been proposed by King [2] but is cooled by jetsof water and rotates in atmosphere.2  POWER DISSIPATION IN THEROTATING TOROIDFigure 1 shows schematically a ring or toroid, of radius R,and circular cross-section, radius r (1 cm), rotating atvelocity V, in a vacuum. The proton beam passes througha length L (20 cm) of the toroid at a small angle to itsplane, dissipating energy Q, and heats the target to atemperature Tmax. As the ring rotates it radiates heat to thesurroundings at temperature Te and enters the beam againat temperature T0. The power dissipation and temperatureof the toroid can be calculated by using the relation forthe thermal capacity of the target and Stefan’s Law ofradiation [3].Figure 1: Schematic diagram of the rotating toroid.To collect the pions produced in the target, a largesolenoid [1], producing a central field of 20 T, enclosesthe interaction region. This is the reason that a band isused, as opposed to a wheel with spokes, so that it canthread the axis of the solenoid without cutting it.Assuming a surface thermal emissivity ε = 0.3 and r =2 cm, a power of 1 MW can be dissipated in a toroid of20 m radius rotating at 1m/s [3]. At 10 MW powerdissipation, the toroid could be 40 m radius and rotatingat 20 m/s.Improving the thermal emissivity will increase thepower dissipation. A value of 0.8 can be achieved byeffectively creating a rough surface; for example, byapplying a suitable surface coating, the addition of fins ordrilling small holes in the surface. A larger target sectionincreases the area for thermal emission, but this coulddecrease the pion yield due to self-absorption and reducethe collection efficiency of the solenoid through enlargingthe effective emittance of the pions.3  PULSED EFFECTSIf the proton beam is pulsed, then some of the powerdissipation relations are modified. Assume that the protonbeam has a very short pulse length (~1 ns) and arepetition rate of f (Hz). When the toroid moves slowly,the volumes “illuminated” by consecutive pulses of theproton beam overlap and gradually separate as the toroidrotates faster or the repetition rate falls. The pulses arejust separate when the speed and frequency are given by(this condition is assumed for the remainder of the paper),V = L f                                       (1)For a target length of 20 cm and a repetition rate of100 Hz, this speed is 20 m/s. At higher speeds the pulsesproton beamrotating toroid,velocity VRsolenoidrProceedings of EPAC 2000, Vienna, Austria2228are separate and the toroid is effectively larger thannecessary. The parts of the toroid heated by the beam maycool more before they pass back into the beam, but themaximum temperature is still limited (by Tmax ~2500 Kfor tantalum). Thus the peak power that can be dissipatedis given by,( ) fTTLSrLVQW e−== max2ρπ            (2)where ρ is the density of the target and S the specific heat.For a tantalum target, W = 0.32 f MW, which is 16 MW ata repetition rate of 50 Hz.4  TEMPERATURE RISE, STRESS,SHOCK AND FATIGUEThe temperature of the toroid rises abruptly onentering the proton beam. The temperature change foruniform energy deposition is,SVrWTρπ 2=∆                              (3)At 1 MW the temperature rise is 136 K with the toroid ata velocity of 10 m/s.Large temperature rises are to be avoided because ofthe thermal stress and shock induced in the structure. Thetantalum is weak at high temperatures. Figure 2 shows thetemperature rise as a function of repetition rate (velocity),f = V/L. At 10 MW power dissipation, the velocity needsto be 50 m/s (f = 250 Hz) to keep the temperature risedown to 200 K. The stresses and high velocities are likelyto limit the rotating toroid to ~10 MW.As the toroid starts to radiate the power, a gradient isset up across the section radius, r. In fact the situation ismore complicated than this since the beam, whetheruniform or not, enters the toroid at an angle and producesadditional longitudinal and radial temperature gradients.The temperature gradients and the mechanicalstresses, shock and fatigue are being estimated using afinite element method computer code, ANSYS. Since theenergy is deposited in the target element in such a shorttime (~1 ns) the calculations show considerable shockeffects even at temperature rises of 100-200 K. Thesecalculations may show that the allowed power dissipationis restricted to quite low values – perhaps 1 MW.However, calculations by King [2,4] of the stresses ina water-cooled cupronickel rotating band target indicatethat the problem is not severe at the 1 MW level. Nickelhas a larger specific heat than tantalum and hence willgive smaller temperature rises. Also, the experience of thepulsed targets [5] at ISOLDE (CERN) and the p-bartarget [6] at FNAL indicates that dissipation of 1-10 MWis possible at the power densities that are required.However, fatigue may limit the lifetime of the target atthe relatively high repetition rate.Figure 2: Temperature rise as a function of the repetitionrate at different power dissipations, W (MW).5  LEVITATIONElectromagnetic levitation and guidance of the toroidand rotation by linear motors results in no moving parts(except for the toroid) in the vacuum and no physicalcontact with the toroid.Preliminary calculations of the ac magnetic fieldsrequired to levitate the toroid and to drive it round arefound to be reasonable. The eddy current loses in thetoroid are 50-100 kW. Detailed calculations and the fieldshaping required to provide stable operation have yet tobe made.6  INDUCED FIELDS IN THE SOLENOIDThe rotation of the toroid through the 20 T solenoidproduces eddy currents, which circulate about the axis ofthe target section. They produce a braking effect on therotation of the toroid both on entering and exiting themagnetic field. The braking can be overcome by ~10 kWof additional drive. There is also a radial constrictingpressure on the target section of ~15 bar. Neither effectwould appear to be a major problem.7 SOME PRACTICAL CONSIDERATIONSThe toroid has to be threaded through the high fieldsolenoid. The toroid could be made in parts and thenfastened together to make a rigid structure. However, thismakes maintenance and repair difficult, particularly sincethe whole region will be very radioactive and requireremote handling.An alternative is to have discrete target bars, 20 cmlong, which can be individually levitated and shot throughthe  solenoid. This scheme has the advantage that a failureW10  6  3  10.1f,  Hz110100 1000∆T, K0 50 10 15 200 250 3002229Proceedings of EPAC 2000, Vienna, Austriaof any one target is not catastrophic as in the case of thetoroid. Also, maintenance and replacement of individualtargets is easier.Extending this idea, it would be possible to have alarge number of individual targets, which are shot throughthe solenoid and then stored for a given time to coolbefore being taken up again. This allows the equivalent ofa very large radius toroid and time for the individualtargets to cool to a lower temperature. Then, themaximum temperature of the targets could be reduced andmaterials with lower melting points and higher specificheats than tantalum or tungsten can be chosen. This willreduce the thermal shock and increase the target fatiguelife. Figure 3 shows the system schematically.Figure 3: Levitated target bars are projected through thesolenoid and guided to and from the holding reservoirwhere they are allowed to cool.Cooling by thermal radiation is possible but it willtake a long time at low temperatures. It takes ~4 minutesfor a tantalum bar to cool from 1000 K to 800 K. Hence itmay be necessary to use a flow of helium gas in thereservoir.Another possibility is to construct a wheel consistingof spokes holding a target bar at their end. The assemblywould rotate in a plane perpendicular to the beam. Thesolenoid would have to be split into two halves as in aHelmholtz coil. The target wheel could be virtually self-contained in its own vacuum box. This would greatly helpin containing the radiation and contamination. The targetcould be removed as a unit for maintenance purposes andwould also reduce the local radiation levels when othernearby items required access. Figure 4 shows the targetwheel in the solenoid schematically.Cooling would be by thermal radiation, as in thetoroid. For 1 MW of power dissipated in the target, atTmax ~2500 K, with a 100 K temperature rise, theperipheral speed of the wheel would be 5.5 m/s and thediameter 11 m.Figure 4: Schematic diagram of the rotating target wheel.REFERENCES[1] R.B. Palmer, C. Johnson, E. Keil, “A Cost-EffectiveDesign for a Neutrino Factory”, BNL-66971, CERNSL/99-070 AP, Neutrino Factory Note 09.[2] B.J. King, S.S. Moser, R.J. Weggel, N.V. Mokhov,“A Cupronickel Rotating Band Pion Production Targetfor Muon Colliders”, in Proceedings of the 1999 IEEEParticle Accelerator Conference, New York City, NY,U.S.A., 29 March-2 April, 1999, 3041-3043, IEEE99CH36366.[3] J.R.J. Bennett, “A High Power, Radiation Cooled,Rotating Toroidal Target for Neutrino Production”, to bepublished in Proceedings of the MuMu99 Conference,Berkeley, December 1999.[4] B.J. King, S.S. Moser, R.J. Weggel, N.V. Mokhov,“Rotating Band Pion Production Targets for MuonColliders & Neutrino Factories”, Muon ColliderCollaboration Meeting, LBNL, 12-14 December, 1999,http://pubweb.bnl.gov/people/bking/.[5] H.V. Ravn, private communication.[6] S. O’Day, K. Bieniosek, K. Anderson, “New TargetResults from the Antiproton Source”, Proceedings of the1993 IEEE Particle Accelerator Conference, Washington,DC, 17-20 May, 3096-3098, IEEE, 1993.solenoidcollection andcooling reservoirproton beamproton beamspoketargetsolenoid coilsshaftcontainmentboxProceedings of EPAC 2000, Vienna, Austria2230

A high power radiation cooled target for a neutrino factory

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