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High Power Polarized Positron Source

High Power Polarized Positron Source. A.Mikhailichenko Cornell LEPP CEBAF/ JLab March 26, 2009. 1. Components of Positron Source suitable for CEBAF: ● Source of (Polarized) electrons 10-200MeV  Not discussed in this talk

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High Power Polarized Positron Source

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  1. High Power Polarized Positron Source A.Mikhailichenko Cornell LEPP CEBAF/ JLab March 26, 2009 1

  2. Components of Positron Source suitable for CEBAF: ● Source of (Polarized) electrons 10-200MeV  Not discussed in this talk ILC source has ~48 μA ; plans to have 200μA at CEBAF (DULY R Inc) ● High power Target ● Beam collection system ● Protection and shielding  Not discussed in this talk 2

  3. Positron source serving for generation of (polarized) positrons Positrons could be obtained by two ways 1 Beta-decay 2 From gammas In its turn- gammas could be obtained from ● Undulator radiation ● Compton scattering ● Breamsstrahlung Low efficiency results to high power deposition in a target –extensive way For ILC polarized positrons created by gammas generated by main beam in helical undulator K<1, L~150m, λu =1cm 3

  4. (POLARIZED) POSITRONPRODUCTION +cross diagram Longitudinal polarization as function of particle’s fractional energy E+/(Eγ-2mc2) Only gamma quanta can create positron (with electron) H.Olsen, L.Maximon, 1959 Photon polarization Polarization could be enhanced by selection positrons by theirs energy This procedure stays in line with limiting energy acceptance of CEBAF SRF structure 4

  5. The ways to create (circularly polarized) gammas in practical amounts Well known processes reviewed for practical utilization in positron source Polarized electron E.Bessonov, A.Mikhailichenko,1996 V.Balakin, A. Mikhailichenko, 1979 E.Bessonov, 1992 This we recommend for CEBAF 5

  6. Size of shower (cascade) Definition of radiation length 6

  7. One example: For energy deposited, say Temperature rise per pulse (Cp –heat capacity) Temperature rise in Graphite Temperature rise in Tungsten Fatigue destruction even for occasional hits. For positron production business of our interest, the thickness is much less, than Xo 7

  8. EFFICIENCY OF POSITRON PRODUCTION E.G. Bessonov, A.A. Mikhailichenko, “A Method of Polarized Positron Beam Production”. Jun 1996. 3pp., Published in EPAC96, Barcelona, June 9-14, 1996, Proceedings, p.1516-1518. (1) (polarized) electron  (2) (circularly polarized) photon  (3) (polarized) positron (1)-(2) Gamma production The number of photons emitted by one electron with energy mc2g within energy interval ΔEg around energyEg in a solid angledo is ~ g2 where -thickness of the target measured in fractions of radiation length - angle between initial electron velocity and direction of photon Degree of circular polarization of photon for Olsen, L.C. Maximon, Phys. Rev. 114 (3) (1959 ) 887.

  9. (2)-(3) Conversion Differential cross section of pair production (H. Bethe, W. Heitter, Proc. Roy. Sot. A 146 (1934) 83.) Energy at creation The numbers at the top of each curve is an energy of incoming gammas/mc2. The curves for Eg=6,10 are valid for any material. Hatched are corresponds to collected particles. The probability, WdE+ that positron created at depth t with energy E+ will have the energy between B. Rossi, “High Energy Particles”, N/Y, 1982 Polarization could be approximated

  10. After some mathematics, the total number of positron created by each electron comes to Mean square scattering angle and transverse size Example : for x+max =0.25, i.e. collection arranged for the positrons in 25 energy interval around maximal energy, d=0.3 ,then i.e. efficiency 1.5% Degree of polarization

  11. Finally we have a number for efficiency, ~1% which means, that the electron current must be 100 bigger, than the positron one. Now the efficiency of collection must be taken into account also. For narrow energy interval this could be ~20%, coming to the current ratio Ielectron/Ipositron~500 So for positron current desired Ipositron=1 microAmpere, the electron current must be Ielectron= 0.5 mA Target has a thickness~ 0.3 X0 which correspondsto ~0.12cm=1.2mm for Tungsten or Lead; Titanium will be extremely non effective here Energy deposited in a target is 2MeV/(gr/cm2) x0.3x6.8g/cm2 =4.1MeV This yields the power deposition in a target ~0.5mAx4MeV~2 kW So the target must be designed for ~5kW For positron current 0.1 microAmpere the power comes to moderate 0.5 kW

  12. Threshold energy for neutron photo-production R.Montalbetti, L.Katz, J. Goldemberg, “Photoneutron Cross Sections”, Phys.Rev. 91, 659 (1953). [1] Natural Graphite contains 1.1% of C18 which has a threshold of 4.9 MeV for reaction . Choice of materials is important for the electron-positron conversion system Lower energy-less neutrons, but as efficiency drops drastically, for the same yield one needs to increase intensity of primary beam Optimization of energy is a primary task 12

  13. NEUTRON FLUX Neutron dose for electron beam carrying power P[kW] at distanceR [m]  Above threshold W.P.Swanson, “Calculation of Neutron Yields Released by Electrons Incident on Selected Materials”, Health Physics, Vol.35, pp.353-367, 1978. Rule of thumb for Tungsten Safe level Example: for positron yield 1 μA: For 10 MeV conversion efficiency10-3 requires primary electron current ~1 mA this brings power deposition to 4 kW For 100 MeV efficiency ~10-2 , so again, power deposition in target ~ 4 kW again –not in surroundings, where it rises proportionally With polarization, efficiency goes down ~5 times coming to 20 kW

  14. Example of Protection shield There is a possibility to add the surrounding materials Collimator for ERL, Cornell Protection shield might wary for collimators, depending on its location A.Mikhailichenko,“Physical Foundations for Design of High Energy Beam Absorbers’’ , CBN 08-8, Cornell 2008. 14

  15. EXAMPLE OF ELECTRON BEAM DUMP Conical shape allows easy expansion Dump for 15-MeV electron beam at ERL The concept of an electron dump system with vapor cooling in first stage. Two-phase flow comes out through peripheral tube(s). Coolant enters at the center. Entrance orifice has diameter 4 in. Absence of parasitic cavities protects against theirs excitation by beam hawing 1.3 GHz component; 2 MW of DC power absorption is possible in this compact design 15

  16. Two examples for positron conversion systems

  17. E-166 experiment at SLAC Target Lens Pb wall Magnetized Iron 3x3 CsI array Trajectories calculated in 3D field 7MeV e+ 9 MeV e+ 17

  18. FOCUSING WITH DC SOLENOIDAL LENS Under-focusing 8x12 turns with up to 350A; dimensions -inches Over-focusing ~Right focusing Field distribution

  19. CORNELL POSITRON SOURCE W Target Focusing coil with flux concentrator This short-focusing lens followed by RF structure immersed in solenoid Positron rate ~1011 /sec at 50 Hz operation at ~200 MeV Conversion efficiency~2.5%, DC power consumption ~2.5 kW J. Barley, V. Medjidzade, A. Mikhailichenko, “New Positron Source for CESR”, CBN-01-19, Oct 2001. 16pp.

  20. 63% particles inside, incoming beam with ~200 MeV =0.25, 0.83, 1.1 mm for 5,10 and 20 MeV respectively These parameters used for generation of ensemble of positrons for further usage with PARMELA

  21. Efficiency of capture for three different values of energy as a function of the feeding current in the pulsed lens. Geometry of capturing optics This conversion system doubled the rate of positron productions

  22. HIGH POWER TARGES

  23. LIQUID METAL TARGET CONCEPT High Z metals could be used here such as Bi-Pb alloy (83Bi,82Pb), Mercury (80Hg). BiPb has melting temperature 154 oC. Hg has boiling temperature 354 oC Gaskets-OK for this temperature Gear pump. Hg Jet velocity~10m/s Calculations show absolute feasibility of this approach for ILC: 5kW DC power. 23 A.Mikhailichenko, “ Liquid Metal Target for ILC”, EPAC 2006, MOPLS108.

  24. Conversion unit on a basis of spinning W+Ti It is a good idea to make first section of accelerating structure from Aluminum; Focusing solenoid (not shown here) made with Al conductor also. RF structure; input-far from the target side Lens Li lens is shown in this Figure Primary Beam Size ~30cm in diameter Spinning disk W (+Ti) Target disc Appropriate for <10 kW as the volume limited by topological cycling with limited perimeter (radius) Coolant 24

  25. COLLECTION OPTICS DESIGN Angle shown~0.3 rad Efficiency= N e+ /Ngammas, % Target – Tungsten (W) Thickness – 1.5 mm 20 MeV photons Particles from 10 to 19 MeV only Angle of capture, rad Efficiency as function of capturing angle; within this angle the particles are captured by collection optics 25

  26. SOLENOIDAL LENS Focal length of the solenoidal lens where (HR) =pc/300 stands for magnetic rigidity ~33kGxcm for 10MeV particles. For f~2 cm Integral comes to Maximal field comes to 66kG2xcm ; For contingency ~ 4cm the field value comes to 4 kG For generation of such field the amount of Ampere-turns required goes to be No flux concentrator possible for DC; however the current density is different in accordance with the path length difference, so manipulation with thickness of conductor is possible 26

  27. Solenoidal lens could be designed with compact dimensions For the number of turns =10, current in one turn goes to I1~1.3kA Conductor cross-section~ 5x10mm2; Coolant-oil Current density ~26A/mm2, which is ordinary, maximal ~100A/mm2 De-ionized water in Spacers Shown here is 20-turn lens Coolant out 27 Solenoidal lens located here

  28. Some estimation for the liquid metal target. Velocity of jet=10m/sec Pb/Bi : Liquid at Tl=129.5oC, boiling at Tb=~1500oC, Latent heat =860 kJ/kg, r ~10g/cm3 Mercury: Liquid at Tl=-38.87oC, boiling at Tb=~357oC, Latent heat =294 kJ/kg, r ~13.54g/cm3 One negative property of Mercury, what may strictly influence to the choice – is its toxicity. Hg considered as one of mostly toxic materials; it could be handled properly, however. In some installations the Mercury is in use in turbine circle, instead of water, what give assurance of success of its implementation for our purposes. Let us jet transverse size 2 mm, along the beam -1.2mm, so for 1 second the volume of material flowing through the nozzle comes to V≈2.4x104 mm3=2.4x10-2Dm3 For average power deposition 5kW ( Q=5 kJper second) the temperature rise of this amount of material comes to It begins vaporizing, so the latent heat of vaporization needs to taken into account, and it will takeΔQ≈860x0.024x10=206kJ for Bi-Pb and ΔQ ≈95kJ for Hg So Bi-Pb will remain at ~300 oC and Hg at 160 oC By increase the jet speed up to 20 m/sec the temperature could drop to 80oC for BiPb To avoid melting and damage we suggested (see above) that jet hits the free liquid metal surface

  29. Bi-Pb alloy composed with 55.51Mass% of Bi and 44.49 Mass% of Pb has liquid phase at 125.9oC. Phase diagram of this alloy is rather branchy with different modifications of Pb sub-phases. Bi Pb diagram Pb- Sn diagram So the jet chamber could be made from Ti (Melt @1668oC ) or Niobium (melt @ 2464oC) As the energy deposited in the windows could be small, theirs cooling could be carried by metal jet itself. Material for windows: 4Be; 22Ti; Boron Nitride- BN (5B7N, sublimates @2700oC) Window could be omitted, but this will require differential pumping and cooled traps. Careful design required in this case.

  30. POSSIBLE SCHEME FOR CEBAF Matching triplet Pb-Bi liquid target Compact DC solenoid DC compact solenoid cooled by de-ionized water Triplet serves for matching focusing of compact DC solenoid having azimuthtal symmetry and the rest optics (FODO)

  31. OTHER POSSIBLE ADDITION FOR THE SOURCE Al made Accelerating structure Al made solenoid For DC operation provides significant RF dissipation (~electron DC gun, DULY) Higher efficiency in return

  32. SUMMARY ● Small energy acceptance forces to select positrons by energy. This is in line for selection of positrons by polarization if polarized electron source used as a primary one. ● Polarized electron source of necessary intensity may be borrowed from ILC. One can expect minimal efficiency ~0.1% of electron-positron conversion, maximal-1%. ● Power deposited in a target remains ~5 kW (2.5 times of theoretical minimum) for positron current ~1 μA; proportionally lower for lower positron current desirable. ● Usage of rotated target possible below 10kW but requires careful design. ● Target with liquid metal (Bi/Pb) possible and mostly adequate here. ● Collection optics as a DC solenoid recommended for CEBAF positron source. ● Radioactive isotopes accumulation weakly depends on conversion energy as the efficiency of positron production increases for higher energy as the threshold is ~6 MeV. ● With differential pumping the windows could be eliminated ● We recommend design for 20kW of absorption power as a safe margin, the cost will not rise much. 32

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