1. /SC-PAC2001-6.19.01 Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy Joe Grames CEBAF Operations 8-31-05 1 Thomas Jefferson National Accelerator Facility Higher Polarization - Superlattice Photocathodes Powerful Ti-Sapphire Lasers Parity Violation Experiments Load locked gun development Polarized Source Advances Introduction, Motivation and History of Polarized Beams at CEBAF Matt Poelker (Group Leader) & Joe Grames (Deputy)

2. Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy Thomas Jefferson National Accelerator Facility Joe Grames CEBAF Operations 8-31-05 Motivation – Why Polarized Electrons?  Approximately half of CEBAF experiments require polarized electrons.  High profile nuclear physics research at all halls structure functions nucleon form factors parity violation (10 -> 1 -> 0.1 part-per-million)  Past and present success due in part to reliable, high polarization and high ave. current beam delivery from GaAs photoguns  New experiments are even more demanding: more current, higher polarization, greater availability, tighter beam specifications. Future success requires continued R&D focus

3. Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy Thomas Jefferson National Accelerator Facility Joe Grames CEBAF Operations 8-31-05 Polarized Electron Tutorial P = 3-1 3+1 = 50% Electron energy levels of bulk GaAs Step 1: Put electrons into conduction band Bare GaAs; No electrons Cs reduces work function. Some electrons Cs + NF3 = “NEA” Many electrons Step 2: Put electrons into the accelerator

4. Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy Thomas Jefferson National Accelerator Facility Joe Grames CEBAF Operations 8-31-05 Continuous Electron Beam Accelerator Facility 499 MHz,  = 120  Pockels cell Gun 0.6 GeV linac (20 cryomodules) 1497 MHz 67 MeV injector (2 1/4 cryomodules) 1497 MHz RF separators 499 MHz Double sided septum “Synchronous Photoinjection” to prolong gun lifetime A B C

5. Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy Thomas Jefferson National Accelerator Facility Joe Grames CEBAF Operations 8-31-05 CEBAF Photogun Challenges  Highest average current polarized beam in the world  Long periods of uninterrupted operation: 24/7 for more than 30 weeks/yr. This places tough demands on gun vacuum and lasers  Simultaneous beam delivery to three halls with large beam current dynamic range (100 pA to 120  A)  Demanding Beam Specifications: small energy spread, tight beam profiles, no cross talk between halls, small helicity correlated beam asymmetries, etc.  Today’s achievements = Tomorrow’s Standard

6. Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy Thomas Jefferson National Accelerator Facility Joe Grames CEBAF Operations 8-31-05 History of Polarized Electrons at JLab  Z-style spin manipulator  Bulk GaAs with 35% polarization  Relatively poor vacuum limits gun lifetime  Vacuum greatly improved with NEGs  Photocathode edge anodizing  Wien spin manipulator  Shorter beamline  Two gun design (one spare).  Eliminate 90 bend.  Large bore vacuum tube with NEG coating. First Gun (1997)Second Gun (1998) 1999 to Present

7. Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy Thomas Jefferson National Accelerator Facility Joe Grames CEBAF Operations 8-31-05 History of Polarized Electrons at JLab  Gain switched seed laser and diode amplifier (MOPA)  One laser provides beam to all halls First Laser (1996)  Three diode MOPAs, One laser for each hall for more flexibility.  Technology transferred to Mainz Microtron Second Laser (1998) Invented new technique to modelock Ti-Sapphire laser at 499 MHz. More powerful than diode lasers. Wavelength tunable Third Laser (2000)

8. Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy Thomas Jefferson National Accelerator Facility Joe Grames CEBAF Operations 8-31-05 Noteworthy Achievements  Highest average current polarized beam in world  Longest demonstrated operational charge lifetime  First to demonstrate synchronous photoinjection  Results of first parity violation experiment published within 6 months of run.  Many new techniques and technologies have been transferred to other labs: hydrogen cleaning, edge anodizing, NEG pumped guns, gain switched diode laser and amplifier.

9. Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy Thomas Jefferson National Accelerator Facility Joe Grames CEBAF Operations 8-31-05 History of Polarized Electrons at JLab 0 10 20 30 40 50 60 70 80 FY97FY98FY99FY00FY01FY02FY03FY04FY05FY06 Fiscal Year P 2 I One diode laser at 1497 MHz and bulk GaAs Three diode lasers at 499 MHz Strained GaAs and Ti-Sapp lasers Superlattice GaAs and Ti-Sapp lasers P 2 I = Figure of Merit (P is polarization and I is beam current) We can do more experiments and more difficult experiments when P 2 I is high.

10. Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy Thomas Jefferson National Accelerator Facility Joe Grames CEBAF Operations 8-31-05 Photocathode Material High QE ~ 10% Pol ~ 35% Bulk GaAs Both are results of successful SBIR Programs “conventional” material QE ~ 0.15% Pol ~ 75% Strained GaAs: GaAs on GaAsP 100 nm Superlattice GaAs: Layers of GaAs on GaAsP No strain relaxation QE ~ 0.8% Pol ~ 85% 100 nm 14 pairs

11. Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy Thomas Jefferson National Accelerator Facility Joe Grames CEBAF Operations 8-31-05 Superlattice Photocathodes photon electron From Hall A Compton Polarimeter Beam polarization ~ 85% - the highest polarization ever measured at JLab High QE, 0.8% versus 0.15% for “conventional” strained GaAs Small analyzing power should help parity violation experiments Polarization (%) Wavelength (nm) Typ. Polarization from conventional strained GaAs 100 kV Mott scattering at Test Stand

12. Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy Thomas Jefferson National Accelerator Facility Joe Grames CEBAF Operations 8-31-05 Superlattice Photocathodes Learning to use this material has been challenging  In particular, it cannot be hydrogen cleaned, an essential follow up step of the original edge anodizing procedure  Protective Arsenic cap layer developed with vendor  Anodization procedure modified to minimize contamination of inner portion of photocathode sample  Problems solved. High polarization, high QE and long lifetime for HAPPEx parity violation experiment  Ready to switch to superlattice material as new CEBAF standard. Users can expect 85% instead of 75% polarization. Superlattice photocathodes in both guns by October 2005

13. Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy Thomas Jefferson National Accelerator Facility Joe Grames CEBAF Operations 8-31-05 Powerful Mode-locked Ti-Sapphire Lasers 1 st commercial laser w/ 499 MHz rep rate Higher power compared to diode lasers Wavelength tunable for highest polarization Feedback electronics to lock optical pulse train to accelerator RF

14. Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy Thomas Jefferson National Accelerator Facility Joe Grames CEBAF Operations 8-31-05 Powerful Mode-locked Ti-Sapphire Lasers  Over the course of ~ two years we have purchased 4 TBWP lasers  One Ti-Sapphire laser with 31 MHz pulse repetition rate for g0 experiment  Three Ti-Sapphire lasers with 499 MHz repetition rate for conventional experiments at high current halls A and C  Two lasers in the tunnel and one hot spare Drawbacks (compared to diode lasers):  Complicated lasers need maintenance every ~ 2 weeks.  Complicated laser table (more optical elements required).  Laser jock required.

15. Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy Thomas Jefferson National Accelerator Facility Joe Grames CEBAF Operations 8-31-05 New Laser Clean Room Clean laser environment, Temperature Stable, Safe for other injector occupants

16. Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy Thomas Jefferson National Accelerator Facility Joe Grames CEBAF Operations 8-31-05 Parity Violation Experiments at CEBAF Experiment Beam Properties (µA, %) Physics Asymmetry Position Asymmetry Current Asymmetry HAPPEx-I(100, 35) 13 ppm10 (10) nm1.0 (0.4) ppm HAPPEx-II(40, 85)1.3 ppm2 (8) nm0.6 (2.6) ppm HAPPEx-He(40, 85)8 ppm3 (3) nm0.6 (0.08) ppm G0G0 (40, 70)2 to 50 ppm20 (4) nm1 (0.14) ppm P-REX(100, 85)0.5 ppm1 nm0.1 ppm Q weak (180, 85)0.3 ppm40 nm0.1 ppm 1999 2010? Helicity-correlated asymmetry specifications (achieved) averaged over the experiment

17. Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy Thomas Jefferson National Accelerator Facility Joe Grames CEBAF Operations 8-31-05 Preliminary Results from HAPPEx-He  Steady beam delivery since July.  40 uA and beam polarization ~ 85%. HAPPEx receives ~ 3 C/day  Run appears to be successful but position asymmetry could be smaller  Helicity magnets installed Gun Current (uA) Time (hr) Charge (C) Daily Gun Current and Charge Plot

18. Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy Thomas Jefferson National Accelerator Facility Joe Grames CEBAF Operations 8-31-05 Load Locked Gun Development Installation at CEBAF January, 2006 No more gun bakeouts! Photocathode replaced in 8 hours versus 4 days. Multiple samples, No more photocathode edge anodizing, Smaller surface area and no more venting means: Better gun vacuum, Longer photocathode lifetime

19. Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy Thomas Jefferson National Accelerator Facility Joe Grames CEBAF Operations 8-31-05Conclusions  Extremely good beam quality and knowledgeable staff at CEBAF  Much of CEBAF success can be attributed to Polarized Source Program.  Many Photogun Achievements and “World’s Firsts” : highest ave current polarized beam, longest charge lifetime, novel lasers and techniques, etc.  CEBAF – on our way to becoming a production parity violation lab

20. Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy Thomas Jefferson National Accelerator Facility Joe Grames CEBAF Operations 8-31-05 Future Challenges  Beam specifications typically grow more demanding. For example, consider Qweak: polarization 85%, current 180 uA, run average helicity-correlated charge asymmetry 0.1 ppm.  Today’s achievements = Tomorrow’s standard  Infrequent maintenance periods: Lasers should be more reliable. Laser table is very complicated Gun lifetime (and therefore vacuum) – always room for improvement.  Apply CEBAF photogun technology to future NP machines