Positron Sources for Linear Colliders*

1. Positron Sources for Linear Colliders* Wei Gai JPOS 2009, Jefferson Lab, March 26, 2009 * Acknowledgement of contributions from the ILC and CLIC e+ collaborations

2. Content • Overview • Undulator Based Positron Source • Conventional Positron Source • Compton based Positron Source

3. Overview

4. positron production Gamma generation Conversion target Capturing optics Acceleration • Planar/Helical Wiggler/Undulator • Bremsstrahlung /Channeling radiation • Laser Compton scattering e- beam (multi MeV – hundreds of GeV) Gamma ray Positrons Gamma generation schemes

5. Helical undulator Based Scheme: requires very high energy drive beam (~100 GeV) Undulator technology is straightforward. (SC or PM) i -i Supper conducting helix Can produce circularly polarized photon, good for polarized e+ source. Drive beam energy: 150GeV Proposed: A. Mikhailichenko K. Flottmann, et al

6. E1 hw=E2-E1 e + E2 e Conventional (Bremsstrahlung) Drive beam energy can be as low as ~ 100 MeV. AMD e- 6GeV e- RF LINAC e+ 4 X0 tungsten target Here, bremsstrahlung refers to radiation from electrons stopping in matter. If the incident electron is polarized, the photon produced will be circularly polarized. And this can give us a possible polarized e+ source using conventional scheme.

7. Channeling radiation-- Coherent bremsstrahlung (separateγand e+ production) Enhancement can be as high as 40 comparing with incoherent bresstrahlung (R. Chehab et al.) Schematic illustration of channeling An example of positron source using channeling radiation

8. Laser Compton scattering Circularly polarized YAG Laser or CO2 Laer Circularly polarized g Multi GeV e- Mr. Omori-San’s favorite drawing

9. Undulator based sources for ILC and CLIC

10. ILC (500 GeV CM) Positron Source Layout (undulator based scheme)

11. Beam parameters for different machines

12. Photon Spectrum and Polarization of ILC baseline undulator • Photon energy spectrum and polarization from a ILC “baseline” undulator (K=1, lu=1cm and Edrive =150GeV) up to the 9th harmonics. • Note photons close to critical energy (also near axis) for each harmonic have higher polarization. Collimating incoming photons will result polarized e+.

13. Target Energy Deposition Profile: Energy deposition profile showing here is calculated per drive e- bunch • Energy deposition in target is about 0.5255J per bunch • Energy deposition : about 1482J per pulse • Power deposition 1482(J)/0.874e-3(s) ~= 1.696MW per pulse • Average power deposition: 1482*5=7.4KW Target has to be rotating at high speed to survive Rotating the 2m diameter target wheel at 1000rpm was estimated for safe operation of the target. Ti target

14. Energy and polarization distribution e+ source at the target Large energy spread

15. Transverse phase space distribution at the target Large divergence, high emittance beam

16. Positron collection and acceleration:Adiabatic Matching Device (target immersed in a solenoid B field) L-band Standing Wave Accelerator. AMD field:5T-0.25T in 50cm Accelerating gradient in pre-accelerator: 12 MV/m for first 6 m, 10 MV/m for next 6 m and 8.9 MV/m for the rest.

17. Comparison of positron yield from different undulators Target: 1.42cm thick Titanium

18. Proposed ILC target geometry and simulation of the target rotating in magnetic fields. 1m Solenoid positioned at 0.95m 1.4cm The model is checked against known experiments.

19. Power vs RPMs for the ILC Target

20. Cockroft institute prototype experiment simulation Technical drawing provided by I.Bailey z0 Simulation, Induced field, z-component, 2000RPM D – 1m, rim width – 30mm, rim thickness – 14mm, distance between magnet poles is 5cm, field – 1.5Tesla

22. Another proposed solution: A pulsed flux concentrator • Pulsing the exterior coil enhances the magnetic field in the center. • Needs ~ 1ms pulse width flattop • Similar device built 40 years ago. Cryogenic nitrogen cooling of the concentrator plates. • ANL and LLNL did initial rough electromagnetic simulations. Not impossible but an engineering challenge. • No real engineering done so far.

23. Advanced Solution: Lithium lens • Lithium Lens • Will lithium cavitate under pulsed heating? • window erosion • Will lithium flow adequately cool the windows? • Increased heating and radiation load in the capture section • Needs R&D to demonstrate the technology. A. Mikhailichenko A. Mikhailichenko et al. P.G. Hurh & Z. Tang

24. What if every capturing magnet technology fails, a safe solution: ¼ wave solenoid • Low field, 1 Tesla on axis, tapers down to ¼ T. • Capture efficiency is only 25% less than flux concentrator • Low field at the target reduces eddy currents • This is probably easier to engineer than flux concentrator • SC, NC or pulsed NC? ANL ¼ wave solenoid simulations W. Liu

25. Summary of Capture Efficiency for Different AMD

26. Undulator based e+ for CLIC (3 TeV) J. Sheppard L. Rinolfi, W. Gai

27. To the IP e- beam Cleaning chicane Ti alloy e+ e+ 250 GeV 2.2 GeV NC Linac 450 m A possible CLIC scheme for polarized e+ Pre-Injector Linac G = 12 MV/m E = 200 MeV fRF = 1.5 GHz B = 0.5 T Injector Linac G = 17 MV/m E = 2.424 GeV f RF = 1.5 GHz f rep= 50 Hz Undulator K = 0.75 λu = 1.5 cm L = 100 m

28. Booster linac Following the tunnel back to e+ injector e+ e- e- main linac e+ main linac undulator e+ capturing optics and preaccelerator Bending assemblies, 20 of them, each one bends the electron beam by 1/20 of the angle between axis of undulator and the axis of the rest of electron main linac undulator >2m e+ capturing etc target A possible CLIC complex layout with undulator based e+ source

29. Numerical Simulation on the effect of undulator parameter and accelerating gradient • Drive e- beam energy: 250GeV • Undulator parameters: K = 0.5 - 0.75, λ= 1.3 - 1.5cm, L= 100 m • Drift to target: 450m • Accelerator L-band Linac, AMD: 7T - 0.5T in 20cm; • Target material: 0.4 rl Titanium, • Positron capture is calculated by numerical cut using damping ring acceptance window: +/-7.5 degrees of RF(1.3GHz), ex+ey<0.09p.m.rad,1% energy spread with beam energy ~2.4GeV

30. Yield and polarization for the CLIC undulator based source Yield is calculated as Ne+ captured/Ne- in drive beam Bottom line: It works

31. Conventional e+ for LCs

32. Superconducting linacs With quadropole focusing Target AMD PPA 5 GeV e+ e- ~ 120 MeV The original ILC conventional source schematic layout To damping ring • Target • Material W23Re • Length 4.5 RL • Electron Beam • Energy 0.25 - 6 GeV • Transverse size, σx = σy 2 mm • Longitudinal size, σt 1.5 ps • Polarized electron →polarized positron (?) After sweeping through the parameter space, this original scheme seems to be not viable for ILC due to the excessive energy deposition in target.

33. Courtesy of M.Kuriki

34. Courtesy of M.Kuriki

35. Liquid metal target (BINP design)

36. Liquid metal target development Lead flow

37. Cog-wheel pump test bench (BINP)

38. x x z z Temperature distribution using ILC beam time structure: 600MeV drive beam, 1mm spot size, AMD immersed target (130 and 260 bunches) 260 bunches 130 bunches Too hot to handle!!!!!!!!! Ways to improve: higher energy, larger spot size and increasing flow rate

39. Need 30m/s pumping speed to keep the liquid from boiling.

41. Courtesy of T.Omori Time structure of 300Hz conventional source Output timing structure from DR per ILC specs Advantage: Only deal with 132 pulse each time Low speed target

42. Temperature in target after 2 tripletsTarget is moving at 10m/s

45. Compton Based Scheme

47. Photon spectrums a CO2 laser compton scattering with 3 different drive beam energy Photon number is high but the interaction time is short. Total number of photon produced is small. Stacking is needed.

50. F. Zimmerman et al.