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Compact High-Intensity CW Polarized Positron Source for New Frontier Research

This research project aims to develop a compact high-intensity continuous wave (CW) polarized positron source for frontier research in physics. The source will allow for the study of quantum interference, search for new physics beyond the standard model, and facilitate the development of spintronic devices.

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Compact High-Intensity CW Polarized Positron Source for New Frontier Research

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  1. Compact High-Intensity CW Polarized Positron Source for New Frontier Research Joe Grames, Jiquan Guo, Fanglei Lin, Vasiliy Morozov, Yuhong Zhang February 14, 2017

  2. Research Impact and Benefits • Physics interest in the CEBAF (E. Voutier) • allows to uniquely distinguish the quantum interference when more than one Quantum Electro-Dynamics based mechanism contributes to a reaction process • isolate the interference contributions betwen the known Bethe-Heitler and the unknown virtual Compton amplitudes • Provide the measurement of the effective electron-quark coupling qualifying charge-conjugation violation • search a U-boson or heavy photon, candidate of Standard Model Dark Matter interaction mediator • Physics interest in the EIC (Y. Furletova, W. Melnitchouk, R. Ent) • Study the flavor separation of the pion and kaon structure • Study flavor decomposition of the nucleon’s quark and antiquark distribution • Study the gluon structure of nucleons and the diffraction mechanism in QCD • Search for right-handed W-boson exchange • Study other physics beyond the standard model • Physics interest in the applied physics (F.A. Selim) • facilitate the development of spintronic devices • study ferromagnetism at surfaces and interfaces and provide an effective method for measuring spin densities

  3. Polarized Positrons at JLEIC A group (J. Grames, J. Guo, F. Lin, V. Morozov, E. Voutier, Y. Zhang) is exploring a polarized positron injector suitable for Jefferson Lab Electron Ion Collider (JLEIC). L ≥1033cm-2s-1 Pe+≥40% CEBAF 17 MHz bunch train, 1.75 us 30 bunches (I= 3.4 uA) Bunch structure from the injector: 0.2 pC ×30 … A pulsed beam with low average current (~10 nA) is required for injection into JLEIC with a reasonably short injection time and reasonably high equilibrium polarization (>40%). 37 s 19 ms 1.1 ms ~ 50 Hz, Iave= 9 nA

  4. Positron Production Scheme Accumulator Ring (18m) 500-turn phase-space painting Positron Conversion/Collection Efficiency ~ 10-4 Bunch Management Polarized Electron 10 MeV Injector to CEBAF Harmonic kicker Extraction 10 MeV polarized e- 4 pC @ 748.5 MHz 10 MeV polarized e- 2 nC bunches @ 748.5 MHz 5-7 MeV Polarized e+ 0.2 pC@ 17 MHz 10 MeV polarized e- 2 nC @ 17 MHz This scheme is based on the Polarized Electrons for Polarized Positrons (PEPPo) technique.

  5. JLEIC Positron Parameters Primary Conservative Version

  6. JLEIC Positron Parameters Primary Aggressive Version

  7. Key Challenges • High bunch charge and high repetition rate polarized electron source • QE decays quickly at high mA beam current • Operation of spin polarized GaAs beams at high current and/or high bunch charge • Polarized electron accumulator ring • Beam dynamics during the multi-turn injection • Spin dynamics • Harmonic stripline RF kicker for the extraction of the electron ring • Polarized positron source and collection (from 10-6 to 10-4) • Improve collection efficiency of positrons • Optimize IP^2 figure of merit • Increase electron beam energy at the target (at the expense of radiation/activation)

  8. Anticipated Outcomes • Polarized electron source (P > 80%) operating at high voltage (> 300 kV) with various repetition rates, high bunch charge (> 1 pC) with good operating lifetime (> 200 C) at high average current (> 1 mA) • Electron accumulation ring with high gain (> 100), preservation of polarization in accumulator ring using Siberian snake (P > 80%), and suitable injection/extraction (< 100 ns) • PEPPo pair-creation target and a liquid metal target with collection optimized for final yield, polarization, emittance, momentum spread, bunch length and bunch train structure

  9. Timetable of Activities

  10. Back Up

  11. Positron Production Scheme Accumulator Ring (18m) 500-turn phase-space painting Positron Conversion/Collection Efficiency ~ 10-4 Bunch Management Polarized Electron 10 MeV Injector to CEBAF Harmonic kicker Extraction 10 MeV polarized e- 4 pC @ 748.5 MHz 10 MeV polarized e- 2 nC bunches @ 748.5 MHz 5-7 MeV Polarized e+ 0.2 pC@ 17 MHz 10 MeV polarized e- 2 nC @ 17 MHz 20ns gap allows more reasonable kicker rise time 17MHz allows reducing collider ring bunch reprate by factor of 4, may help increase luminosity for low beam current/high energy collision cases, when beam-beam is not a bottleneck.

  12. Multi-Turn Injection x > Septum thickness +bunch width Accumulator ring x Injected beam • Concept: an orbit bump created near a septum and then slowly reduced as beam being injected (phase-space painting) • A number of painting schemes have been developed • Process can also be simultaneously occurring in vertical and longitudinal dimensions • CERN’s LEIR has a design for 75-turn injection of Pb54+, we plan to push this number to a few hundred to a thousand using low electron emittance • Main injection system components • Magnetic or electrostatic septum • Four bumper magnets with ~1 s rise time, reasonably fast fall-off time and ~10 mrad maximum deflection

  13. Harmonic Kicker Extraction Y. Huang. Phys. Rev. Accel and Beams, 19, 084201 (2016) Empty ring buckets already kicked out to linac Ring buckets still occupied by bunches Empty linac buckets Linac buckets occupied by extracted bunches Harmonic kicker that kicks every 3rd bunch

  14. High Power Target • Liquid Metal Target – lead-bismuth eutectic (LBE) • High Z = 82, 83 • Low melting point: 124°C • High boiling point: 1670°C • Multiple LBE targets tested on various accelerators • Natural Circulation • Mechanical Pumping • Electromagnetic Pumping • Approaching 10 kW power level, CW Stainless Steel Windows (0.25mm) LBE (2mm) Output (e-,e+,γ) Input (e-)

  15. High Polarization Photocathode R&D • 6.4% QE & 84% polarization at 776 nm from strained GaAs/GaAsP superlattice photocathode with GaAsP/AlAsP Distributed Bragg Reflector(DBR) • The highest QE & FOM of any reported high polarization photocathode • Possible to improve both QE & Pol • SBIR partnership Photo-cathode DBR Substrate Paper for SPIN and APL submission: W. Liu, S. Zhang, M. Stutzman, M. Poelker, Y. Chen, W. Lu, and A. Moy

  16. Higher Voltage (300 kV) Inverted Photo Gun 11 MV/m CsK2Sb 1mA 8 MV/m First insulator commissioned to 325 kV New insulator, getting installed now, supports higher voltage operation

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