Electron Polarization In MEIC Pavel Chevtsov

Outline MEIC Electron Polarization Requirements MEIC Electron Polarization Design Universal Spin Rotator Spin dynamics in MEIC Electron Ring Summary

Electron Polarization Requirements High (>80%) polarization of stored high-current electron beam Sufficiently long polarization lifetime (>10 min) in 3 to 11 GeV energy range Longitudinal polarization available at all interaction points Alternating bunch-to-bunch polarization Comparably high polarization of a stored positron beam through S-T self-polarization effect Sufficiently fast self-polarization at low positron beam energies (~3 GeV)

Electron Polarization Design Choices High electron polarization through full-energy injection from CEBAF Over 80% electron beam polarization from CEBAF’s state-of-the-art polarized source/injector High polarization well preserved during acceleration in the multi-pass recirculated SRF Linac of CEBAF Polarization direction coming out of CEBAF easily controlled by two Wien filters in the CEBAF injector Longitudinally-polarized electron beam injected from CEBAF into a long straight section of the MEIC electron collider ring After polarization degradation, beam replaced by new bunches from CEBAF. Possibility of continuous top-off injection from CEBAF

Electron Polarization Design Choices Maintaining high electron beam polarization S-T self-polarization by making the spin anti-parallel to bending magnetic field in the arcs Spin-tune superconducting solenoid(s) in straight(s) to adjust the spin tune for polarization stability Longitudinal electron polarization at IP’s using spin rotators Four 90º spin rotators at the ends of the arcs Upstream spin rotator rotates spin from vertical to longitudinal Downstream spin rotator rotates spin back to vertical Spin rotators work over entire energy range

Electron Polarization In Figure-8 Collider Ring

Impact of Figure-8 Shape Figure-8 shape chosen exclusively due to its unique advantages for ion polarization Electron collider ring matches ion figure-8 footprint to share same tunnel and accommodate interaction points No complications due to figure-8 shape for electrons, potentially advantageous due to energy-independent zero spin tune SC solenoids in the straights (where spin is longitudinal) to improve polarization stability s x

Self-Polarization Time Positron Beam Positrons can be accelerated in CEBAF as effective as electrons with similar beam quality Both polarized and unpolarized positron sources being developed for fixed target program Only unpolarized positron source can provide sufficiently high beam current Positron beam polarized by S-T self-polarization Polarization time too long at low energies (<6 GeV). Shortening polarization time by Using small damping ring to enhance S-T effect Accelerating to high (~7 GeV) energy for quick self-polarization and subsequent deceleration Self-Polarization Time GeV Hours 3 14.6 4 3.5 5 1.1 7 0.21 9 0.06

Spin Flip and Polarization Lifetime From CEBAF and at IP’s spin In arcs Polarization depolarization Bunch trains with alternating polarization from CEBAF by changing polarization of photo-injector’s driving laser S-T effect could cause significant depolarization of wrong-direction polarization Depolarized beam can be replaced periodically by injection from CEBAF, provided polarization lifetime is not too short (>10 min) Possibility of multiple spin flips by spin resonance crossing

Universal Spin Rotator Spin rotator rotates spin from vertical to longitudinal works in whole 3 to 11 GeV energy range same orbit geometry for different energies Universal spin rotator (USR) composed of two solenoid interspersed with arc bending dipoles Fixed dipole bending angles  no orbit change, solenoid strengths adjusted for different energies x-y coupling compensated for each solenoid individually E (GeV) 1 BL1 (Tm) 1 2 BL2 (Tm) 2 3 /2 15.7 /3 /6 4.5 /4 11.8 23.6 6 0.62 12.3 2/3 1.91 38.2 9  62.8 12 24.6 4/3 76.4

Solenoid Coupling Compensation Transport matrix of a solenoid X-Y decoupling scheme (V. Livinenko & A. Zholents, 1980) (H. Sayed) BdL = 28.7 T m e- v Decoupling insertion L L/2 solenoid 4.16 m solenoid 4.16 m decoupling quad insert M = T - T

Spin Dynamics Analytic estimate of equilibrium electron polarization gives ~85% Preliminary tracking results (D. Barber) consistent with ~85% More detailed depolarization/polarization studies including spin matching underway (in collaboration with Profs. D. Barber and A. Kondratenko).

Summary Electron polarization requirements High polarization (>80%) Sufficient polarization lifetime Longitudinal polarization at IP’s Alternating bunch-to-bunch polarization / spin-flipping High electron polarization by full-energy injection from CEBAF High degree of polarization maintained by S-T effect and by spin tune control with spin-tune solenoids Longitudinal polarization at IP’s provided by four spin rotators in the entire 3 to 11 GeV/c energy range without affecting orbit No complications due to figure-8 shape, potential advantage Possibility of polarized positron beam with unpolarized positron source and S-T self-polarization Alternating polarization by flipping polarization at the source Detailed spin tracking and simulations underway in collaboration with Profs. D. Barber and A. Kondratenko