Operated by Jefferson Science Associates, LLC for the U.S. Department of Energy Thomas Jefferson National Accelerator Facility Polarized Electron Beam at CEBAF Matt Poelker 13 June, 2006 Science and Technology Review Jefferson Lab June 12-13, 2006 Polarized Source Group: M. Poelker, P. Adderley, J. Brittian, J. Clark, J. Grames, J. Hansknecht, James McCarter, M. Stutzman, K. Surles-Law (3 scientists, 4 technical staff, 2 graduate students)

Operated by Jefferson Science Associates, LLC for the U.S. Department of Energy Thomas Jefferson National Accelerator Facility 2 Highlights Since the Last S&T Review  Beam Polarization 85% typical, 80% guaranteed  New Fiber-Based Drive Laser: high power, reliable  Parity Violation Experiments: becoming more routine  Load-Locked Gun developments for high current future experiments  Low Voltage Mott polarimeter for photocathode studies

Operated by Jefferson Science Associates, LLC for the U.S. Department of Energy Thomas Jefferson National Accelerator Facility 3 Continuous Electron Beam Accelerator Facility

Operated by Jefferson Science Associates, LLC for the U.S. Department of Energy Thomas Jefferson National Accelerator Facility 4 Everyone Gets Beam from Pol. Electron Gun! CEBAF’s first polarized e-beam experiment 1997 Now polarized beam experiments comprise ~ 80% of our physics program All beam originates from the same 0.5mm spot on one photocathode inside 100kV GaAs photogun (the thermionic gun was removed in 2000) For example, during April 2006 there were three high profile polarized beam experiments on the floor simultaneously; –Hall A: Gen (10uA) –Hall B: GDH (3nA) –Hall C: G0 Backward Angle (60uA)

Operated by Jefferson Science Associates, LLC for the U.S. Department of Energy Thomas Jefferson National Accelerator Facility 5 Photocathode Material High QE ~ 10% Pol ~ 35% Bulk GaAs “conventional” material QE ~ 0.15% Pol ~ 850 nm Strained GaAs: GaAs on GaAsP 100 nm Superlattice GaAs: Layers of GaAs on GaAsP No strain relaxation QE ~ 0.8% Pol ~ 780 nm 100 nm 14 pairs Both are results of successful SBIR Programs

Operated by Jefferson Science Associates, LLC for the U.S. Department of Energy Thomas Jefferson National Accelerator Facility 6 Beam Polarization at CEBAF P I 2 2 sup. str. = 1.38 Experiment Figure of Merit Reasonable to request >80% polarization in PAC proposals

Operated by Jefferson Science Associates, LLC for the U.S. Department of Energy Thomas Jefferson National Accelerator Facility 7 Superlattice Photocathodes Success required ~ 1 year of effort Cannot be hydrogen cleaned (M. Baylac) Arsenic capped (worked with vendor SVT) No solvents during preparation! (M. Stutzman) Anodized edge: a critical step No depolarization over time! Oct 13Nov 9QE dropped by factor of 2 Polarization M. Baylac et al., “Effects of atomic hydrogen and deuterium exposure on high polarization GaAs photocathodes” PRST-AB 8, (2005)

Operated by Jefferson Science Associates, LLC for the U.S. Department of Energy Thomas Jefferson National Accelerator Facility 8 Synchronous Photoinjection Only electrons within 110 ps window can be accelerated. Electrons outside window are dumped in the chopper. Efficient beam extraction prolongs operating lifetime of photogun Lasers with GHz pulse repetition rates have been hard to come by Three independent RF-Pulsed lasers DC drive laser, Most beam thrown away Chopper viewer A B C

Operated by Jefferson Science Associates, LLC for the U.S. Department of Energy Thomas Jefferson National Accelerator Facility 9 Commercial Ti-Sapphire Laser 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

Operated by Jefferson Science Associates, LLC for the U.S. Department of Energy Thomas Jefferson National Accelerator Facility 10 System Availability FY05Q4 – FY06Q3 Realign Ti-Sapphire lasers

Operated by Jefferson Science Associates, LLC for the U.S. Department of Energy Thomas Jefferson National Accelerator Facility 11 New Fiber-Based Drive Laser  CEBAF’s last laser!  Gain-switching better than modelocking; no phase lock problems  Very high power  Telecom industry spurs growth, ensures availability  Useful because of superlattice photocathode (requires 780nm) J. Hansknecht and M. Poelker, Phys. Rev. ST Accel. Beams 9, (2006) Ti-Sap power

Operated by Jefferson Science Associates, LLC for the U.S. Department of Energy Thomas Jefferson National Accelerator Facility 12 Other Benefits of Fiber Drive Laser Maybe replace some lossy optics components with telecom stuff? Green version for RF-pulsed Compton Polarimetry, FEL Drive Laser “Beat Frequency Technique” to create Low Rep Rate Beam for Particle Identification at Halls: Beat Frequency Technique; One laser at MHz Normal Ops; Three beams at 499 MHz A B C Every 15 th pulse delivered to hall: 31 MHz beam

Operated by Jefferson Science Associates, LLC for the U.S. Department of Energy Thomas Jefferson National Accelerator Facility 13 What is “Parity Quality”? Helicity-correlated asymmetry specifications Experiment Physics Asymmetry Max run-average helicity correlated Position Asymmetry Max run-average helicity correlated Current Asymmetry SpecAchievedSpecAchieved HAPPEx-I 13 ppm10 nm 1 ppm0.4 ppm G 0 Forward2 to 50 ppm20 nm(4 ± 4) nm1 ppm(0.14 ± 0.3) ppm HAPPEx-He [2004] HAPPEx-He [2005] 8 ppm3 nm 20* nm 0.6 ppm 0.08 ppm 0.1 ppm HAPPEx-II-H [2004] HAPPEx-II-H [2005] 1.3 ppm2 nm 8** nm 1 nm 0.6 ppm 2.6** ppm 0.1 ppm Lead0.5 ppm1 nm-0.1 ppm- Q weak 0.3 ppm20 nm-0.1 ppm * Results affected by electronic crosstalk at injector. ** Results at Hall A affected by Hall C operation. Spec was met in 2005 run.

Operated by Jefferson Science Associates, LLC for the U.S. Department of Energy Thomas Jefferson National Accelerator Facility 14 Routine Parity Violation Experiments? We need:  Long lifetime photogun (i.e., slow QE decay)  Stable injector (especially RF phases)  Properly aligned laser table, pockels cell (HAPPEx method)  Proper beam-envelope matching throughout machine for optimum adiabatic damping  Set the phase advance of the machine to minimize position asymmetry at target  Eliminate electronic ground loops: isolate electronics  Feedback loops; charge and position asymmetry  Specific requirements for each experiment; e.g., 31 MHz pulse repetition rate, 300 Hz helicity flipping, beam halo <, etc.,

Operated by Jefferson Science Associates, LLC for the U.S. Department of Energy Thomas Jefferson National Accelerator Facility 15 What is HAPPEx Method? Developed jointly with Source Group Identify Pockels cells with desirable properties: –Minimal birefringence gradients –Minimal steering –Must be verified through testing! Install Pockels cell using good diagnostics: –Center to minimize steering –Rotationally align to minimize unwanted birefringence Adjust axes to get small (but not too small) analyzing power. Adjust voltage to get maximum circular polarization! Use feedback to reduce charge asymmetry. –Pockels cell voltage feedback maximizes circular polarization. –“Intensity Asymmetry” Pockels cell provides most rapid feedback. –During SLAC E158, both were used. If necessary, use position feedback, keeping in mind you may just be pushing your problem to the next highest order. From G. Cates presentation, PAVI04 June 11, 2004

Operated by Jefferson Science Associates, LLC for the U.S. Department of Energy Thomas Jefferson National Accelerator Facility 16 Origins of Helicity Correlated Beam Asymmetries Pockels Cell = active lens. Laser beam needs to pass through center of cell. From G. Cates presentation, PAVI04 June 11, 2004 Translation (inches) X position diff. (um) Y position diff. (um) Red, IHWP Out Blue, IHWP IN No HV HV + HV -

Operated by Jefferson Science Associates, LLC for the U.S. Department of Energy Thomas Jefferson National Accelerator Facility 17 New Developments High Current at High Polarization; Qweak to test standard model, uA at 85% polarization Higher Current, High Polarization; ~ > 1 mA Proposed new facilities ELIC, eRHIC High Current, No Polarization: ~ 100mA JLab FEL, electron cooling Solution: Fiber-based laser + Load locked gun

Operated by Jefferson Science Associates, LLC for the U.S. Department of Energy Thomas Jefferson National Accelerator Facility 18 Load Locked Gun for Qweak Bulk GaAs 100 kV load locked gun Faraday Cup Baked to 450C NEG-coated large aperture beam pipe Differential Pumps w/ NEG’s 1W green laser, DC, 532 nm Focusing lens on x/y stage Spot size diagnostic Insertable mirror Load locked gun: replace photocathodes quickly without bakeout. 8 hours versus 4 days.

Operated by Jefferson Science Associates, LLC for the U.S. Department of Energy Thomas Jefferson National Accelerator Facility 19 Lifetime versus Laser Spot Size Exceptionally high charge lifetime, >1000C at beam current to 10mA! Lifetime scales with laser spot size but simple scaling not valid. Factor 10 instead of factor 20. Repeat measurements with high polarization photocathode material  Imperfect vacuum limits photocathode lifetime - damage from ion backbombardment  Can we increase operating lifetime by merely increasing the laser spot size? Same number electrons, same number ions, but distributed over larger area.

Operated by Jefferson Science Associates, LLC for the U.S. Department of Energy Thomas Jefferson National Accelerator Facility 20 New Load-Locked Gun Better than first load locked gun design No more edge-anodizing Multiple samples Better vacuum in high voltage chamber –No more venting –Less surface area –NEG coated Longer photocathode lifetime? Commissioning now, Ready for installation Fall 2006

Operated by Jefferson Science Associates, LLC for the U.S. Department of Energy Thomas Jefferson National Accelerator Facility 21 R&D program to obtain polarization > 90% “Spintronics”, with graduate student James McCarter and Dr. Stuart Wolf of University of Virginia. New photocathode material. Collaborating with Dr. Tim Gay of University of Nebraska, polarimeter expert. (We have borrowed his polarimeter) Low voltage gun and mini-Mott polarimeter

Operated by Jefferson Science Associates, LLC for the U.S. Department of Energy Thomas Jefferson National Accelerator Facility 22 Conclusions and Future Plans The Polarized Source Group will:  Continue to deliver high polarization beam from long lifetime photoguns, using superlattice photocathodes and reliable fiber- based lasers  Install our new load-locked gun, to improve operating lifetime and support Qweak and other high current experiments  Support parity violation experiments that have tighter and tighter beam specifications  Continue working on exciting R&D projects: Lifetime studies at current > 1 mA using load locked gun and high polarization photocathode material Mini-Mott commissioning + photocathode studies to provide polarization >90%

Operated by Jefferson Science Associates, LLC for the U.S. Department of Energy Thomas Jefferson National Accelerator Facility 23 Based on Optimistic 07 Budget We will:  Purchase more superlattice photocathode material, to keep us happy for many years, just in case vendor loses interest.  Purchase two more fiber-based laser systems (that would give us one for each hall plus green version for unpolarized beam experiments)  One more staff scientist to manage R&D program at Test Cave, to replace Maud Baylac who returned to France

Operated by Jefferson Science Associates, LLC for the U.S. Department of Energy Thomas Jefferson National Accelerator Facility 24 Source Group Recent Publications Papers: “A High Average Current Polarized Electron Source with Long Cathode Operational Lifetime,” C. K. Sinclair, M. Poelker, P. A. Adderley, B. M. Dunham, J. C. Hansknecht, P. Hartmann, J. S. Price, P. M. Rutt, W. J. Schneider, and M. Steigerwald, in press. M. Poelker, J. Grames, J. Hansknecht, R. Kazimi, J. Musson, “Generation of Electron Microbunches at Low Repetition Rates Using Beat Frequency Technique”, in press. “Synchronous Photoinjection Using a Frequency-Doubled Gain-Switched Fiber-Coupled Seed Laser and ErYb-Doped Fiber Amplifier,” J. Hansknecht and M. Poelker, Phys. Rev. ST Accel. Beams 9, (2006) “The Effects of Atomic Hydrogen and Deuterium Exposure on High Polarization GaAs Photocathodes,” M. Baylac, P. Adderley, J. Brittian, J. Clark, T. Day, J. Grames, J. Hansknecht, M. Poelker, M. Stutzman, A.S. Terekhov and A.T. Wu, Phys. Rev. ST Accel. Beams 8, (2005). Conferences: “Probing Hadron Structure at CEBAF Using Polarized Electron Scattering,” M. Poelker, presented at the annual meeting of the American Physical Society, Dallas, TX, April “Operation of CEBAF photoguns at average beam current > 1 mA,” M. Poelker, J. Grames, P. Adderley, J. Brittian, J. Clark, J. Hansknecht, M. Stutzman, Polarized Sources and Targets Workshop, Nov , 2005, Tokyo, JAPAN. “Polarized Photoguns and Prospects for Higher Current,” M. Poelker, Workshop on Energy Recovered Linacs, Jefferson Lab, March 19-22, 2005.

Operated by Jefferson Science Associates, LLC for the U.S. Department of Energy Thomas Jefferson National Accelerator Facility 25 Origins of Helicity Correlated Beam Asymmetries maximum analyzing power minimum analyzing power Beam Charge Asymmetry Rotating Halfwaveplate Angle Photocathode QE Anisotropy, aka Analyzing Power Different QE for different orientation of linear polarization GaAs photocathode From G. Cates presentation, PAVI04 June 11, 2004

Operated by Jefferson Science Associates, LLC for the U.S. Department of Energy Thomas Jefferson National Accelerator Facility 26 Origins of Helicity Correlated Beam Asymmetries Gradient in phase shift leads to gradient in charge asymmetry which leads to beam profiles whose centroids shift position with helicity. From G. Cates presentation, PAVI04 June 11, 2004 Non-uniform polarization across laser beam + QE anisotropy… Pockels cell aperture