High Power Polarized Positron Source A Mikhailichenko Cornell

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 -200 Me. V Not discussed in this talk ILC source has ~48 μA ; plans to have 200 μA at CEBAF (DULY R Inc) ● High power Target ● Beam collection system ● Protection and shielding Not discussed in this talk 2

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~150 m, λu =1 cm 3

(POLARIZED) POSITRON PRODUCTION +cross diagram Longitudinal polarization as function of particle’s + fractional energy E /(Eγ-2 mc 2) 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

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

Size of shower (cascade) Definition of radiation length Elements C W Cu Al Ti Fe Z 6 74 29 13 22 26 A 12 183. 8 63. 5 27 47. 9 55. 8 Ec, Me. V 84. 2 8. 1 20. 2 42. 8 26. 2 22. 4 X 0 g/cm 2 43. 3 6. 8 13 24. 3 16. 1 13. 84 , cm 19. 2 0. 35 1. 45 9 3. 58 1. 75 RM/X 0 (=Es/Ec) 0. 25 2. 57 1. 05 0. 49 0. 7 0. 95 l. M, cm 4. 8 0. 9 1. 5 4. 4 2. 5 6 1. 65

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

EFFICIENCY OF POSITRON PRODUCTION E. G. Bessonov, A. A. Mikhailichenko, “A Method of Polarized Positron Beam Production”. Jun 1996. 3 pp. , Published in EPAC 96, 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 mc 2 g within energy interval ΔEg around energy Eg in a solid angle do is ~ g 2 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. 8

Differential cross section of pair production (H. Bethe, W. Heitter, Proc. Roy. Sot. A 146 (1934) 83. ) (2)-(3) Conversion Energy at creation The numbers at the top of each curve is an energy of incoming gammas/mc 2. The curves for Eg=6, 10 are valid for any material. Hatched are corresponds to collected particles. The probability, Wd. E+ 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 9

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 10

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 micro. Ampere, the electron current must be Ielectron= 0. 5 m. A Target has a thickness~ 0. 3 X 0 which corresponds to ~0. 12 cm=1. 2 mm for Tungsten or Lead; Titanium will be extremely non effective here Energy deposited in a target is 2 Me. V/(gr/cm 2) x 0. 3 x 6. 8 g/cm 2 =4. 1 Me. V This yields the power deposition in a target ~0. 5 m. Ax 4 Me. V~2 k. W So the target must be designed for ~5 k. W For positron current 0. 1 micro. Ampere the power comes to moderate 0. 5 k. W 11

Threshold energy for neutron photo-production R. Montalbetti, L. Katz, J. Goldemberg, “Photoneutron Cross Sections”, Phys. Rev. 91, 659 (1953). Elements Me. V [1] C W Cu Al Fe Pb U 18. 72[1] 6. 19 9. 91 13. 03 11. 21 6. 73 6. 04 Natural Graphite contains 1. 1% of C 18 which has a threshold of 4. 9 Me. V 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

NEUTRON FLUX Neutron dose for electron beam carrying power P[k. W] at distance R [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 Me. V conversion efficiency 10 -3 requires primary electron current ~1 m. A this brings power deposition to 4 k. W For 100 Me. V efficiency ~10 -2 , so again, power deposition in target ~ 4 k. W again –not in surroundings, where it rises proportionally With polarization, efficiency goes down ~5 times coming to 20 k. W 13

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

EXAMPLE OF ELECTRON BEAM DUMP Dump for 15 -Me. V electron beam at ERL Conical shape allows easy expansion 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

Two examples for positron conversion systems 16

E-166 experiment at SLAC Target Lens Pb wall Magnetized Iron 3 x 3 Cs. I array Trajectories calculated in 3 D field 7 Me. V e+ 9 Me. V e+ 17

FOCUSING WITH DC SOLENOIDAL LENS Under-focusing 8 x 12 turns with up to 350 A; dimensions -inches Over-focusing Field distribution ~Right focusing 18

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 Me. V Conversion efficiency~2. 5%, DC power consumption ~2. 5 k. W J. Barley, V. Medjidzade, A. Mikhailichenko , “New Positron Source for CESR”, CBN-01 -19, Oct 2001. 16 pp. 19

63% particles inside, incoming beam with ~200 Me. V E+ = 5 Me. V E+ = 10 Me. V E+ = 20 Me. V 0. 043 0. 05 0. 032 0. 35 0. 22 0. 12 0. 025 0. 052 0. 038 0. 084 0. 085 0. 034 =0. 25, 0. 83, 1. 1 mm for 5, 10 and 20 Me. V respectively These parameters used for generation of ensemble of positrons for further usage with PARMELA 20

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

HIGH POWER TARGES 22

LIQUID METAL TARGET CONCEPT High Z metals could be used here such as Bi-Pb alloy (83 Bi, 82 Pb), Mercury (80 Hg). Bi. Pb has melting temperature 154 o. C. Hg has boiling temperature 354 o. C Gaskets-OK for this temperature Gear pump. Hg Jet velocity~10 m/s Calculations show absolute feasibility of this approach for ILC: 5 k. W DC power. A. Mikhailichenko, “ Liquid Metal Target for ILC”, EPAC 2006, MOPLS 108. 23

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 Li lens is shown in this Figure Lens Primary Beam Spinning disk W (+Ti) Target disc Coolant Size ~30 cm in diameter Appropriate for <10 k. W as the volume limited by topological cycling with limited perimeter (radius) 24

COLLECTION OPTICS DESIGN Efficiency= N e+ /Ngammas, % Angle shown~0. 3 rad Target – Tungsten (W) Thickness – 1. 5 mm 20 Me. V photons Angle of capture, rad Particles from 10 to 19 Me. V only Efficiency as function of capturing angle; within this angle the particles are captured 25 by collection optics

SOLENOIDAL LENS Focal length of the solenoidal lens where (HR) =pc/300 stands for magnetic rigidity ~33 k. Gxcm for 10 Me. V particles. For f~2 cm Integral comes to Maximal field comes to 66 k. G 2 xcm ; For contingency ~ 4 cm the field value comes to 4 k. G 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

Solenoidal lens could be designed with compact dimensions De-ionized water in For the number of turns =10, current in one turn goes to I 1~1. 3 k. A Conductor cross-section~ 5 x 10 mm 2; Coolant-oil Current density ~26 A/mm 2, which is ordinary, maximal ~100 A/mm 2 Spacers Coolant out Shown here is 20 -turn lens Solenoidal lens located here 27

Some estimation for the liquid metal target. Velocity of jet=10 m/sec Pb/Bi : Liquid at Tl=129. 5 o. C, boiling at Tb=~1500 o. C, Latent heat =860 k. J/kg, r ~10 g/cm 3 Mercury: Liquid at Tl=-38. 87 o. C, boiling at Tb=~357 o. C, Latent heat =294 k. J/kg, r ~13. 54 g/cm 3 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. 2 mm, so for 1 second the volume of material flowing through the nozzle comes to V≈2. 4 x 104 mm 3=2. 4 x 10 -2 Dm 3 For average power deposition 5 k. W ( Q=5 k. J per 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 take ΔQ ≈860 x 0. 024 x 10=206 k. J for Bi-Pb and ΔQ ≈ 95 k. J for Hg So Bi-Pb will remain at ~300 o. C and Hg at 160 o. C By increase the jet speed up to 20 m/sec the temperature could drop to 80 o. C for Bi. Pb 28 To avoid melting and damage we suggested (see above) that jet hits the free liquid metal surface

Bi-Pb alloy composed with 55. 51 Mass% of Bi and 44. 49 Mass% of Pb has liquid phase at 125. 9 o. C. 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 @1668 o. C ) or Niobium (melt @ 2464 o. C) As the energy deposited in the windows could be small, theirs cooling could be carried by metal jet itself. Material for windows: 4 Be; 22 Ti; Boron Nitride- BN (5 B 7 N, sublimates @2700 o. C) Window could be omitted, but this will require differential pumping and cooled traps. Careful design required in this case. 29

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)

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

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 k. W (2. 5 times of theoretical minimum) for positron current ~1 μA; proportionally lower for lower positron current desirable. ● Usage of rotated target possible below 10 k. W 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 Me. V. ● With differential pumping the windows could be eliminated ● We recommend design for 20 k. W of absorption power as a safe margin, the cost will not rise much. 32