Polarized Electron Positron RD at CEBAF Joe Grames
Polarized Electron & Positron R&D at CEBAF Joe Grames APEX, BNL November 19, 2012
The Talk • Jefferson Lab Introduction • “APEX @ JLAB” – Vying for time at CEBAF • PEPPo – Polarized Positrons • Critical Polarized Electron R&D • Conclusions Page 2
CEBAF Overview First large high-power CW recirculating e-linac based on SRF technology Operations since 1995 served ~1400 nuclear physics users, 178 experiments Capabilities: 5 passes, multiple energies, beam characteristics, polarization 3 Halls running simultaneously Upgrade to 12 Ge. V: proposal late 1990’s approved and funded in 2004 Page 3
Parity Violation Experiments at CEBAF Experiment Energy (Ge. V) I (µA) Target Apv (ppb) Maximum Charge Asym (ppb) Maximum Position Diff (nm) Maximum Angle Diff (nrad) Maximum Size Diff (δσ/σ) HAPPEx-II (Achieved) 3. 0 55 1 H 1400 1 0. 2 Was not specified HAPPEx-III (Achieved) 3. 484 16900 200± 100 3± 3 0. 5± 0. 1 10 -3 PREx 1. 063 500 100± 10 2± 1 0. 3± 0. 1 10 -4 234 100± 10 2± 1 30± 3 10 -4 35. 6 10± 10 0. 5± 0. 5 0. 05± 0. 05 10 -4 (20 cm) 100 1 H (25 cm) 70 208 Pb (0. 5 mm) QWeak 1. 162 180 1 H (35 cm) Møller 11. 0 75 1 H (150 cm) PV experiments motivate polarized e-source R&D Page 4
Beam Current Electron Gun Requirements • Ultrahigh vacuum • No field emission • Maintenance-free Charge from photogun CEBAF Polarized Electron Source 24 Hours Record Performance (2012): 180 m. A at 89% polarization Page 5
High Current Polarized Electron Source R. Suleiman et al. , PAC 2011 Parameter Laser Rep Rate 1500 MHz Laser Pulselength 50 ps Laser Wavelength 780 nm Laser Spot Size High-Pol Photocathode Gun Voltage Page 6 Value 350 µm FWHM SSL Ga. As/Ga. As. P 200 k. V DC CW Beam Current 4 m. A Run Duration 1. 4 hr Extracted Charge 20 C 1/e Charge Lifetime 85 C
Ave. Beam Current (m. A) Spin Polarization from Ga. As at Jefferson Lab ? R&D for the happy user First polarized beam from Ga. As photogun @ SLAC Physics Program @ CEBAF • Low P : Bulk Ga. As • High P: Superlattice Ga. As/Ga. As. P Page 7
How do Accelerator Physics Experiments Fit In? Experiment Year Scheduling Spin-Orbit Sensitivity 1999 4 Facility Days Ph. D – J. Grames Spin Dance Experiment 2001 3 Facility Days PRST-AB Energy Recovery Exp’t 2003 14 PAC Days Ph. D – C. Tennet CW Positron Source 2010 Beam Studies Ph. D – S. Golge C 100 Beam Breakup Exp’t 2010 4 Facility Days Ph. D expected – I. Shin Beam Optics Optimization 2011 Beam Studies Ph. D – M. Spata Polarized Positrons Exp’t 2012 20 PAC Days Ph. D – J. Dumas Ph. D expected – A. Adeyemi Publication expected Beam Optics Optimization Ongoing Beam Studies Ph. D expected – R. Bodenstein 2013 Beam Studies Ph. D expected – M. Mc. Hugh Publication expected Mott 1% Experiment Output q Facility Days were included in early days of operations q Parity Violation experiments necessitated “beam studies” q RF Improvements (push to 6 Ge. V, preparing 12 Ge. V) too Page 8
Beam Studies Paradigm Shift Agreement between Physics and Accelerator Divisions provides for typically 12 to 16 hours each week of program for “beam studies” q Most often time is allocated for operational development q Savvy Users also lined up in this same queue q Sometimes PI’s (or even plans) weren’t present for scheduling q Lively discussion often ensued (imagine stock exchange floor) Beginning in January 2012 we shifted to a Campaign philosophy q Any request entertained but Head of Operations set priorities Ø C 100 – Testing 12 Ge. V cryomodules Ø PEPPo Ø CEBAF Modeling Ø 1% Mott Upgrade Ø Miscellaneous q PI’s required to defend requests submitted in advance Not surprisingly the number of User requests dropped dramatically Page 9
Polarized Brem & Pair Production e- → g → e+ Suggested by E. G. Bessonov, A. A. Mikhailichenko, EPAC (1996) and A. P. Potylitsin, NIM A 398 (1997) 395 Bremsstrahlung Pair Creation Rexplored at low energy for PEPPo by E. A. Kuraev, Y. M. Bystritskiy, M. Shatnev, E. Tomasi-Gustafsson, PRC 81 (2010) 055208 Page 10
PEPPo Concept @ JLab Viewers Beam position monitors Solenoid ELEGANT beam optics Viewers Collimator Polarized Analyzing Solenoid Target Dipoles (7. 5 cm Fe) Corrector magnets PEPPo branch jonction Quadrupoles Faraday Cup G 4 PEPPo experiment model Production Target T 1 (0. 1 -1 mm W) Annihilation detector Positron Selection Device Viewer Corrector magnets Reconversion Target Beam Profiler T 2 (2 mm W) Compton Xmission Polarimeter Page 11
PEPPo Lobbying PAC 35’s enthusiastic endorsement of LOI-10 -010 “Any accelerator facility, like JLab, using polarized electrons for its physics program would like an intense beam of polarized positrons. ” The PEPPo experiment (ER 12 -11 -105) proposes to measure the polarization transfer from longitudinally polarized electrons to longitudinally polarized positrons as a proof-of-principle for a new technique for polarized positron source. Page 12
Installation Complete ~ November 2011 Page 13
By July 2012 Preliminary PEPPo Results Data Set 1 Preliminary momentum scan Characterized backgrounds Learned rate & equipment limitations Data Set 2 • Shielded polarized background • Improved spectrometer PS stability • Improved coincidence timing Consistent with polarization well above 50% pe- ~ 8. 25 Me. V/c Pe- ~ 85% Page 14
Positron Yield Consideration CEBAF Pre-Injector CEBAF 6 Ge. V Injector CEBAF 12 Ge. V Injector Conversion efficiencies are low, but 100 n. A – 10 m. A sufficient for user requirements Corresponding milliamp electron beams are challenging, yet possible (more later). Page 15
Positron Source Concept at CEBAF A. Freyberger, JLAB Town Hall Mtg (2011) S. Golge, Ph. D Thesis, ODU/JLAB (2010) Requires ~ 1 m. A I ~ 300 n. A dp/p ~ 10 -2 ex, y ~ 1. 6 mm. mrad Page 16
Polarized Electron Source “Musts” Good Electron Gun Good Laser Ø Ultrahigh vacuum Ø No field emission Ø Maintenance-free Ø “Headroom” Ø Suitable pulse structure Ø Low jitter Page 17 Good Photocathode Ø High Polarization Ø Many electrons/photon Ø Fast response time Ø Long lifetime
Imperfect Vacuum = Finite Lifetime ~2. 5 E-11 Torr ~5. 0 E-11 Torr >15. 0 E-11 Torr Page 18
Trenching & Laser Management laser light IN electron beam OUT Ions create QE trough to electrostatic center Active area = 5. 0 mm Laser spot = 0. 5 mm Characteristic “trench” from laser to electrostatic center of photocathode
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Improve Lifetime with Large Laser Spot? (Best Solution – Improve Vacuum, but not easy) Bigger laser spot, same # electrons, same # ions Ionized residual gas strikes photocathode Ion damage distributed over larger area Page 21
CEBAF High-P Lifetime vs. Laser Spot Size Page 22
Always Tweaking the Gun Design 1 4 Endless (? ) quest for perfection 2 3 Page 23
Load-lock Photogun • Best vacuum inside HV Chamber, which is never vented • Photocathodes - Heated and Activated inside Preparation Chamber • Use “Suitcase” to replace photocathodes through a Loading Chamber Activation Laser Preparation Chamber Loading Chamber HV Chamber Storage Manipulators “Load-locked dc high voltage Ga. As photogun with an inverted-geometry ceramic insulator, ” Phys. Rev. ST Accel. Beams 13, 010101 (2010) Page 24
Inverted Gun Design Ø Higher voltage = ü better beam quality ü potentially better lifetime Ø Inverted design = ü Less metal at HV (compact) ü Not directly exposed to FE Old Design New Inverted Design Page 25
Inverted Gun Page 26
Higher Voltage Gun Building a 350 k. V Gun Built a 200 k. V Gun a. Longer insulator b. Spherical electrode c. Thin NEG sheet further away (a) (b) (c) Page 27
HV Issues: inside and outside the gun 600 k. V supply at FEL gun test stand, with SF 6 tank Inverted insulator 4. 5” long CEBAF insulator reached 470 k. V before arcing to ground Learn to apply high voltage without breakdown, dielectric plug inside insulator, Then address the field emission problems inside the gun Page 28
Conclusions • Accelerator Physics & Operations : a fruitful chicken & egg scenario • Good ideas & the squeaky wheel “get the grease” • Polarized Positrons : PEPPo concept successful => publish in 2013 • Polarized Electrons : Critical Source R&D Ø Powerful lasers & their management (thermal, reflections) Ø Higher quantum efficiency & robust photocathodes Ø Higher voltage & lower vacuum electron guns Page 29
Supplemental Slides Follow Page 30
Jefferson Lab at a Glance • Created to build and Operate the Continuous Electron Beam Accelerator Facility (CEBAF), worldunique user facility for Nuclear Physics: – – – Mission is to gain a deeper understanding of the structure of matter • Through advances in fundamental research in nuclear physics • Through advances in accelerator science and technology In operation since 1995 ~1, 400 Active Users 178 Completed Experiments to-date Produces ~1/3 of US Ph. Ds in Nuclear Physics (406 Ph. Ds granted, 180 more in progress) • Managed for DOE by Jefferson Science Associates, LLC (JSA) • Human Capital: – – ~800 FTEs 22 Joint faculty; 27 Post docs; 14 Undergraduate, 33 Graduate students • K-12 Science Education program serves as national model • Site is 169 Acres, and includes: – – 83 SC Buildings & Trailers; 749 K SF Replacement Plant Value: $331 M Biological & Environ. Research, 0. 78 Basic Energy Sciences; 1. 17 High Energy Physics; 2. 80 Advanced Scientific Computing Research, 0. 05 Energy Efficiency and Renewable Energy, 0. 02 Other DOE; 0. 03 WFO, 13. 3 Other Office of Science, 33. 8 Nuclear Physics, 133. 4 FY 2011: Total Lab Operating Costs: $185 M Non-DOE Costs: $13 M Page 31
12 Ge. V Upgrade Project Upgrade is designed to build on existing facility: vast majority of accelerator and experimental equipment have continued use Upgrade arc magnets and supplies Add 5 cryomodules CHL upgrade 20 cryomodules New Hall Add arc Maintain capability to deliver lower pass beam energies: 2. 2, 4. 4, 6. 6…. 20 cryomodules The completion of the 12 Ge. V Upgrade of CEBAF was ranked the highest priority in the 2007 NSAC Long Range Plan. Add 5 cryomodules Scope of the project includes: • Doubling the accelerator beam energy • New experimental Hall and beamline • Upgrades to existing Experimental Halls Enhanced capabilities in existing Halls Page 32
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