Polarized Electron Beams in the MEIC at JLab

1. Polarized Electron Beams in the MEIC at JLab Fanglei Lin for MEIC Study Group EIC Users Meeting, Stony Brook University, June 27, 2014

2. Outline • MEIC layout and electron polarization requirements • MEIC electron polarization design • Overview of strategies • Initial Injection bunch pattern • Polarization collision pattern and measurement • Optimization of average polarization • Continuous injection option • Conclusions & Outlook

3. MEIC & e- Polarization Requirements Warm large booster (up to 25 GeV/c) Three Figure-8 rings stacked vertically Ion source SRF linac Pre-booster Cold 25-100 GeV/c proton collider ring Electron cooling Warm 3-12 GeV electron collider ring Medium-energy IPs with horizontal beam crossing Injector 12 GeV CEBAF Pre-booster Ion Source • Electron polarization of 70% or above • Longitudinal electron polarization at collision points • Spin flipping Linac • Three compact rings: • 3 to 12 GeV electron • Up to 25 GeV/c proton (warm) • Up to 100 GeV/c proton (cold) Cross sections of tunnels for MEIC 12 GeV Electron cooling 11 GeV IP IP Full Energy EIC Collider rings MEIC collider rings 12 GeV CEBAF

4. Overview of e− Polarization Strategies • Highly vertically polarized electron beams are injected from CEBAF • avoid spin decoherence, simplify spin transport from CEBAF to MEIC, alleviate the detector background • Polarization is designed to be vertical in the arc to avoid spin diffusion and longitudinal at collision points using spin rotators • Universal spin rotator rotates the electron polarization from 3 to 12GeV • Desired spin flipping is implemented by changing the source polarization • Compton polarimeter is considered to measure the electron polarization • Two long opposite polarized bunch trains (instead of alternate polarization between bunches) simplify the Compton polarimetry • Polarization configuration with figure-8 geometry removes electron spin tune energy dependence • Continuous injection of electron bunch trains from the CEBAF is considered to • preserve and/or replenish the electron polarization, especially at higher energies, and • maintain a constant beam current in the collider ring • Spin matching in some key regions is considered if it is necessary … bunch train & polarization pattern 1.33ns 748.5MHz Empty buckets Empty buckets … … … Polarization (Down) Polarization (Up)

5. Initial Injection Bunch Pattern half ring injection half ring injection Macro bunch train 2.3μs, ~1700 bunches …… 2.4 pC 2.4 pC 1.33 ns, 748.5 MHz …… …… 2.3μs, ~1700 bunches (Iave= 1.8 mA) 100 ms (~21740 turns) 1.33 ns, 748.5 MHz • Such injection bunch pattern needs no upgrade of CEBAF beyond 12 GeV upgrade 200 us Iave = 82.6 nA

6. Polarization Collision Pattern • HERA: • Each ion bunch only sees the same electron bunch • MEIC: • Each ion bunch sees all electron bunches • No non-colliding bunches ion ion electron electron • Therefore, in the MEIC • Bunch-to-bunch variation does not contribute to the uncertainty • One can measure average polarization of each macro bunch train

7. Polarization Measurement Laser + Fabry Perot cavity Photon calorimeter Quasi real photon tagger c Quasi real photon tagger Electron tracking detector e- beam • Compton polarimetry: (next talk given by Alexandre Camsonne) • same polarization at laser as at IP due to zero net bend • Spin dancing (using spin rotators): • Experimentally optimize (calibrate) longitudinal polarization at IP Arc IP IP forward ion detection ions forward e- detection Schematic drawing of USR e- spin rotator e- final focusing elements Illustration of spin rotation by a USR v-step and50 mrad crossing

8. Optimization of Average Polarization : Initial polarization : Injection time : Measurement time : Depolarization time Average Pol. * Includes duty factor

9. Continuous Injection Option • Continuous injection principle • Low injected current preserves high polarization • One possible injection bunch pattern • Damping time at energy > 5 GeV << 250ms • No beam dump needed 2.3μs, ~1700 bunches …… 1.33 ns, 748.5 MHz …… 2.4 pC 2.4 pC …… 2.3μs, ~1700 bunches 1.33 ns, 748.5 MHz Lost or Extracted Iave = 33 nA 200 us P0 (>Pt) 250 ms Pave /P0= 92% Pt

10. Conclusion and Outlook • Electron polarization schemes have been developed • Initial injection bunch pattern • Polarization collision pattern and measurement • Optimization of average polarization • Continuous injection option • Outlook • Scheme or technique optimization • Spin matching to improve polarization lifetime • Spin tracking with realistic errors • Acknowledgements • Y.S. Derbenev, J. Grames, J. Guo, L. Harwood, V.S. Morozov, P. Nadel-Turonski, F. Pilat, R. Rimmer, M. Poelker, R. Suleiman, H. Wang, S. Wang, Y. Zhang, – JLab • D. P. Barber – DESY/Liverpool/Cockcroft • M. Sullivan  SLAC

11. Thank You for Your Attention

12. Injection Bunch Pattern Macro bunch train 2.3μs, ~1700 bunches …… 2.4 pC 2.4 pC 1.33 ns, 748.5 MHz duty factor 4.3e-4 …… …… 2.3μs, ~1700 bunches (Iave= 1.8 mA) 100 ms (~21740 turns) 1.33 ns, 748.5 MHz 200 us Iave = 82.6 nA