Polarized Electron Beam at CEBAF Matt Poelker 13
Polarized Electron Beam at CEBAF Matt Poelker 13 June, 2006 Polarized Source Group: M. Poelker, P. Adderley, J. Brittian, J. Clark, J. Grames, J. Hansknecht, James Mc. Carter, M. Stutzman, K. Surles-Law (3 scientists, 4 technical staff, 2 graduate students) Science and Technology Review Jefferson Lab June 12 -13, 2006 Thomas Jefferson National Accelerator Facility Operated by Jefferson Science Associates, LLC for the U. S. Department of Energy
Highlights Since the Last S&T Review Ø Beam Polarization 85% typical, 80% guaranteed Ø New Fiber-Based Drive Laser: high power, reliable Ø Parity Violation Experiments: becoming more routine Ø Load-Locked Gun developments for high current future experiments Ø Low Voltage Mott polarimeter for photocathode studies Thomas Jefferson National Accelerator Facility Operated by Jefferson Science Associates, LLC for the U. S. Department of Energy 2
Continuous Electron Beam Accelerator Facility 0. 6 Ge. V linac (20 cryomodules) 1497 MHz RF-pulsed drive lasers 67 Me. V injector (2 1/4 cryomodules) 1497 MHz 499 MHz, Df = 120 A B RF separators 499 MHz C B A C A Pockels cell B C Chopper Wien filter Double sided septum Gun Thomas Jefferson National Accelerator Facility Operated by Jefferson Science Associates, LLC for the U. S. Department of Energy 3
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. 5 mm spot on one photocathode inside 100 k. V Ga. As 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 (10 u. A) – Hall B: GDH (3 n. A) – Hall C: G 0 Backward Angle (60 u. A) Thomas Jefferson National Accelerator Facility Operated by Jefferson Science Associates, LLC for the U. S. Department of Energy 4
Photocathode Material High QE ~ 10% Pol ~ 35% 14 pairs 100 nm Bulk Ga. As Superlattice Ga. As: Layers of Ga. As on Ga. As. P 100 nm Strained Ga. As: Ga. As on Ga. As. P “conventional” material QE ~ 0. 15% Pol ~ 75% @ 850 nm No strain relaxation QE ~ 0. 8% Pol ~ 85% @ 780 nm Both are results of successful SBIR Programs Thomas Jefferson National Accelerator Facility Operated by Jefferson Science Associates, LLC for the U. S. Department of Energy 5
Beam Polarization at CEBAF Experiment Figure of Merit 2 P I sup. = 1. 38 2 P I str. Reasonable to request >80% polarization in PAC proposals Thomas Jefferson National Accelerator Facility Operated by Jefferson Science Associates, LLC for the U. S. Department of Energy 6
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) M. Baylac et al. , “Effects of atomic hydrogen and deuterium exposure on high polarization Ga. As photocathodes” PRST-AB 8, 123501 (2005) Anodized edge: a critical step Polarization No depolarization over time! Oct 13 QE dropped by factor of 2 Thomas Jefferson National Accelerator Facility Operated by Jefferson Science Associates, LLC for the U. S. Department of Energy Nov 9 7
Synchronous Photoinjection Only electrons within 110 ps window can be accelerated. Electrons outside window are dumped in the chopper. Chopper viewer DC drive laser, Most beam thrown away B C A Three independent RF-Pulsed lasers Efficient beam extraction prolongs operating lifetime of photogun Lasers with GHz pulse repetition rates have been hard to come by Thomas Jefferson National Accelerator Facility Operated by Jefferson Science Associates, LLC for the U. S. Department of Energy 8
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 Thomas Jefferson National Accelerator Facility Operated by Jefferson Science Associates, LLC for the U. S. Department of Energy 9
System Availability FY 05 Q 4 – FY 06 Q 3 Realign Ti-Sapphire lasers Thomas Jefferson National Accelerator Facility Operated by Jefferson Science Associates, LLC for the U. S. Department of Energy 10
New Fiber-Based Drive Laser Ti-Sap power J. Hansknecht and M. Poelker, Phys. Rev. ST Accel. Beams 9, 063501 (2006) Ø CEBAF’s last laser! Ø Gain-switching better than modelocking; no phase lock problems Ø Very high power Ø Telecom industry spurs growth, ensures availability Ø Useful because of superlattice photocathode (requires 780 nm) Thomas Jefferson National Accelerator Facility Operated by Jefferson Science Associates, LLC for the U. S. Department of Energy 11
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: Normal Ops; Three beams at 499 MHz Beat Frequency Technique; One laser at 467. 8125 MHz Every 15 th pulse delivered to hall: 31 MHz beam B C A Thomas Jefferson National Accelerator Facility Operated by Jefferson Science Associates, LLC for the U. S. Department of Energy 12
What is “Parity Quality”? Helicity-correlated asymmetry specifications Experiment 1999 Physics Asymmetry Max run-average helicity correlated Position Asymmetry Max run-average helicity correlated Current Asymmetry Spec Achieved HAPPEx-I 13 ppm 10 nm 1 ppm 0. 4 ppm G 0 Forward 2 to 50 ppm 20 nm (4 ± 4) nm 1 ppm (0. 14 ± 0. 3) ppm HAPPEx-He [2004] HAPPEx-He [2005] 8 ppm 3 nm 20* nm 0. 6 ppm 0. 08 ppm 0. 1 ppm HAPPEx-II-H [2004] HAPPEx-II-H [2005] 1. 3 ppm 2 nm 8** nm 1 nm 0. 6 ppm 2. 6** ppm 0. 1 ppm Lead 0. 5 ppm 1 nm - 0. 1 ppm - Qweak 0. 3 ppm 20 nm - 0. 1 ppm - 2008 * Results affected by electronic crosstalk at injector. ** Results at Hall A affected by Hall C operation. Spec was met in 2005 run. Thomas Jefferson National Accelerator Facility Operated by Jefferson Science Associates, LLC for the U. S. Department of Energy 13
Routine Parity Violation Experiments? We need: Ø Long lifetime photogun (i. e. , slow QE decay) Ø Stable injector (especially RF phases) Ø Properly aligned laser table, pockels cell (HAPPEx method) Ø Proper beam-envelope matching throughout machine for optimum adiabatic damping Ø Set the phase advance of the machine to minimize position asymmetry at target Ø Eliminate electronic ground loops: isolate electronics Ø Feedback loops; charge and position asymmetry Ø Specific requirements for each experiment; e. g. , 31 MHz pulse repetition rate, 300 Hz helicity flipping, beam halo < , etc. , Thomas Jefferson National Accelerator Facility Operated by Jefferson Science Associates, LLC for the U. S. Department of Energy 14
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 E 158, 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, PAVI 04 June 11, 2004 Thomas Jefferson National Accelerator Facility Operated by Jefferson Science Associates, LLC for the U. S. Department of Energy 15
Origins of Helicity Correlated Beam Asymmetries Pockels Cell = active lens. Laser beam needs to pass through center of cell. X position diff. (um) Red, IHWP Out Blue, IHWP IN No HV Y position diff. (um) HV + HV - Translation (inches) From G. Cates presentation, PAVI 04 June 11, 2004 Thomas Jefferson National Accelerator Facility Operated by Jefferson Science Associates, LLC for the U. S. Department of Energy 16
New Developments High Current at High Polarization; Qweak to test standard model, 2008 180 u. A at 85% polarization Higher Current, High Polarization; ~ > 1 m. A Proposed new facilities ELIC, e. RHIC High Current, No Polarization: ~ 100 m. A JLab FEL, electron cooling Solution: Fiber-based laser + Load locked gun Thomas Jefferson National Accelerator Facility Operated by Jefferson Science Associates, LLC for the U. S. Department of Energy 17
Load Locked Gun for Qweak Load locked gun: replace photocathodes quickly without bakeout. 8 hours versus 4 days. 100 k. V load locked gun Spot size diagnostic 1 W green laser, DC, 532 nm Bulk Ga. As Faraday Cup Baked to 450 C NEG-coated large aperture beam pipe Insertable mirror Differential Pumps w/ NEG’s Focusing lens on x/y stage Thomas Jefferson National Accelerator Facility Operated by Jefferson Science Associates, LLC for the U. S. Department of Energy 18
Lifetime versus Laser Spot Size Ø Imperfect vacuum limits photocathode lifetime - damage from ion backbombardment Ø Can we increase operating lifetime by merely increasing the laser spot size? Same number electrons, same number ions, but distributed over larger area. • • • Exceptionally high charge lifetime, >1000 C at beam current to 10 m. A! Lifetime scales with laser spot size but simple scaling not valid. Factor 10 instead of factor 20. Repeat measurements with high polarization photocathode material Thomas Jefferson National Accelerator Facility Operated by Jefferson Science Associates, LLC for the U. S. Department of Energy 19
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 Thomas Jefferson National Accelerator Facility Operated by Jefferson Science Associates, LLC for the U. S. Department of Energy 20
R&D program to obtain polarization > 90% Low voltage gun and mini-Mott polarimeter • “Spintronics”, with graduate student James Mc. Carter 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) Thomas Jefferson National Accelerator Facility Operated by Jefferson Science Associates, LLC for the U. S. Department of Energy 21
Conclusions and Future Plans The Polarized Source Group will: Ø Continue to deliver high polarization beam from long lifetime photoguns, using superlattice photocathodes and reliable fiberbased lasers Ø Install our new load-locked gun, to improve operating lifetime and support Qweak and other high current experiments Ø Support parity violation experiments that have tighter and tighter beam specifications Ø Continue working on exciting R&D projects: • Lifetime studies at current > 1 m. A using load locked gun and high polarization photocathode material • Mini-Mott commissioning + photocathode studies to provide polarization >90% Thomas Jefferson National Accelerator Facility Operated by Jefferson Science Associates, LLC for the U. S. Department of Energy 22
Based on Optimistic 07 Budget We will: Ø Purchase more superlattice photocathode material, to keep us happy for many years, just in case vendor loses interest. Ø Purchase two more fiber-based laser systems (that would give us one for each hall plus green version for unpolarized beam experiments) Ø One more staff scientist to manage R&D program at Test Cave, to replace Maud Baylac who returned to France Thomas Jefferson National Accelerator Facility Operated by Jefferson Science Associates, LLC for the U. S. Department of Energy 23
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 Er. Yb-Doped Fiber Amplifier, ” J. Hansknecht and M. Poelker, Phys. Rev. ST Accel. Beams 9, 063501 (2006) “The Effects of Atomic Hydrogen and Deuterium Exposure on High Polarization Ga. As 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, 123501 (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 2006. “Operation of CEBAF photoguns at average beam current > 1 m. A, ” M. Poelker, J. Grames, P. Adderley, J. Brittian, J. Clark, J. Hansknecht, M. Stutzman, Polarized Sources and Targets Workshop, Nov. 14 -17, 2005, Tokyo, JAPAN. “Polarized Photoguns and Prospects for Higher Current, ” M. Poelker, Workshop on Energy Recovered Linacs, Jefferson Lab, March 19 -22, 2005. Thomas Jefferson National Accelerator Facility Operated by Jefferson Science Associates, LLC for the U. S. Department of Energy 24
Origins of Helicity Correlated Beam Asymmetries Photocathode QE Anisotropy, aka Analyzing Power Different QE for different orientation of linear polarization Beam Charge Asymmetry minimum analyzing power maximum analyzing power Ga. As photocathode Rotating Halfwaveplate Angle Thomas Jefferson National Accelerator Facility Operated by Jefferson Science Associates, LLC for the U. S. Department of Energy 25 From G. Cates presentation, PAVI 04 June 11, 2004
Origins of Helicity Correlated Beam Asymmetries Pockels cell aperture Gradient in phase shift leads to gradient in charge asymmetry which leads to beam profiles whose centroids shift position with helicity. Non-uniform polarization across laser beam + QE anisotropy… Thomas Jefferson National Accelerator Facility Operated by Jefferson Science Associates, LLC for the U. S. Department of Energy 26 From G. Cates presentation, PAVI 04 June 11, 2004
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