Polarized Positrons in JLEIC Fanglei Lin, Yuhong Zhang, Jiquan Guo, Vasiliy S. Morozov, JLEIC R&D Meeting, April 21, 2016 F. Lin

Positron Beams The question always comes up “what about positron beams at an EIC?” We can dream about polarized positron beams of similar luminosity and polarization as the electron beams foreseen for EIC, and some science would drive/require that, but… We should give our accelerator colleagues some ROUGH idea about a reasonable lower threshold for Luminosity Polarization requirements for valuable science at an EIC to give them a target. Caveat: we do NOT want our accelerator colleagues to make this a direct priority, but more have them to keep such options in the back of their minds. So, a discussion on what thresholds are in the ballpark will be useful for them. Here my bold proposal for valuable science with polarized positron beams: Luminosity > 1033 Polarization > 40% from Rolf Ent

JLEIC Primary Positron Parameters For a full acceptance detector Achieved with a single pass ERL cooler CM energy GeV 33.5 40 52.9 p e+ Beam energy 70 4 100 7 Collision frequency MHz 476/3=159 Particles per bunch 1010 1.8 0.59 2.0 Beam current A 0.46 0.15 0.5 Polarization % >70% ~40% Bunch length, RMS cm 2 1.2 Norm. emitt., vert./horz. μm 0.5/0.25 36/18 190/95 Horizontal & vertical β* 4/2 5.8/2.9 2/4 4.1/2.0 7.1/3.55 2.4/1.2 Vert. beam-beam 0.002 0.03 Laslett tune-shift 0.056 small 0.028 Det. space, up/down m 3.6/7 3/3.2 Hour-glass reduction 0.89 0.87 0.82 Lumi./IP, w/HG, 1033 cm-2s-1 0.9 0.7 For a high(er) luminosity detector Horizontal & vertical β* cm 1.6/0.8 2.3/1.2 1.6/0.82 4.8/2.4 Vert. beam-beam 0.002 0.15 0.03 Det. space, up/down m ±4.5 3 Hour-glass reduction 0.67 0.63 0.71 Lumi./IP, w/HG, 1033 cm-2s-1 1.7 2.3 0.9

Luminosity of JLEIC w/ Positrons Assume that the positron beam current is 0.15 A

Formation of Positron Beam collider CEBAF Beam current mA 150 0.05 Bunch freq. MHz 158.8 22.68 Bunch spacing ns 6.3 44.1 m 1.89 13.2 Bunch number 1138 Bunch charge pC 945 2.2 Bunch intensity 109 5.9 0.014 Energy GeV 3 5 6.95 9.3 10 Electron current A 1.4 0.95 0.71 SR power MW 0.16 2.65 Damping time ms 376 81 26 14 Polarization lifetime hour 66 8 2.2 0.9 0.3 Beam lifetime Positron current mA 150 SR power w/o wigglers 0.017 0.13 0.5 1.6 2.1 Initial injection time w/o wigglers min 43.6 9.4 3.5 1.5 1.2 Damping time w/ wigglers 20 Damping time reduction 19.6 4.2 2.6 SR power w/ wigglers 0.32 0.54 0.75 4.0 4.3 Initial injection time w/wigglers 2.3 0.6 Initial injection times are still reasonable short comparing to the beam and polarization lifetimes without wigglers. Dipole wigglers in the straight should not affect the polarization direction at IPs, but may affect the polarization lifetime.

Injected Polarized Positron Structure τd=20ms 44 ns 22.7 MHz bunch train, 3.23µs, 73 bunches (Ipulse = 50 A) injector pulse train up polarization from CEBAF …… injector pulse train down polarization from CEBAF 2.2 pC bunch 73 bunches (Ipulse = 50 A) τd=20ms 25 Hz, 40 ms, Iave=8.1 nA Positron Polarization % > 40 Pulse current Ipulse A 50 Bunch charge pC 2.2 Bunch rep. rate in pulses MHz 22.7 Bunch spacing in pulses ns 44 Pulse rep. rate Hz 25 Average current Iave nA 8.1

Continuous Injection Polarization w/ continuous injection Equilibrium polarization A relatively low average injected beam current of tens-of-nA level can maintain a high equilibrium polarization in the whole energy range.

Positron Production Scheme I It seems easier to stack “cold” electrons than “hot” positrons 1000 turn phase space painting sounds aggressive but it is “only” a factor of ~10 increase of 2D emittance (1  ~10 m) in case of 6D phase space painting or a factor of ~32 increase of 2D emittance (1  ~32 m) in case of 4D phase space painting Option with optimistic assumption about electron stacking 26.5 m accumulator ring with 1000 turn phase-space painting bunch focusing and compression if needed e+ collection system x 10-2 1 mm W target x 10-3 to CEBAF 5-7 MeV Polarized e+ 2 x 0.44 pC @ 11.3 kHz = 10 nA 10 MeV polarized e- 44.1 pC @ 22.7 MHz = 1 mA 10 MeV pol e- 2 x 44.1 nC bunches @ 11.3 MHz = 1 A 10 MeV polarized e- 2 x 44.1 nC @ 11.3 kHz = 1 mA 0-9.5 MeV polarized e+ 2 x 44.1 pC @ 11.3 kHz = 1 A

Positron Production Scheme II Option with less optimistic assumption about electron stacking 26.5 m accumulator ring with 100 turn phase-space painting bunch focusing and compression if needed e+ collection system x 10-2 1 mm W target x 10-2 to CEBAF ~60 MeV polarized e+ 2 x 0.44 pC @ 11.3 kHz = 10 nA 10 MeV polarized e- 44.1 pC @ 22.7 MHz with 10% duty factor = 0.1 mA acceleration to 100 MeV 10 MeV pol e- 2 x 4.41 nC bunches @ 11.3 MHz = 0.1 A 10 MeV polarized e- 2 x 4.41 nC @ 11.3 kHz = 0.1 mA 0-99.5 MeV polarized e+ 2 x 44.1 pC @ 11.3 kHz = 1 A

Experimental Test Minimum experimental setup e+ collection system 1 mm W target x 10-3 to diagnostics 5-7 MeV Polarized e+ 0.0007 fC @ 1497 MHz = 1 nA 10 MeV polarized e- 70 fC @ 1497 MHz = 100 A 0-9.5 MeV polarized e+ 0.07 fC @ 1497 MHz = 0.1 A