Probing Hadron Structure at CEBAF Using Polarized Electron Scattering

1. Probing Hadron Structure at CEBAF Using Polarized Electron Scattering M. Poelker, Jefferson Lab APS Meeting, Dallas, TX, April 2006 Structure Functions, Form Factors, Parity Violation, DVCS, GPD, more? • Outline; • CEBAF Overview • What Can You Expect at CEBAF? • Parity Violation Experiments (becoming routine?) • New Developments for New Experiments

2. RF-pulsed drive lasers A B C 499 MHz, Df = 120 Continuous Electron Beam Accelerator Facility 0.6 GeV linac (20 cryomodules) 1497 MHz 67 MeV injector (2 1/4 cryomodules) 1497 MHz RF separators 499 MHz B A C B A C Pockels cell Chopper Wien filter Double sided septum Gun

3. CEBAF Overview • CEBAF Benefits; • Recirculating LINACs • Superconducting Cavities • Three Halls; 3x the physics • CEBAF Headaches? • … • … • … What I’m going to talk about

4. CEBAF Headaches? • Many shared components link experimental programs at neighboring halls • Ambitious schedule with frequent energy changes: demands precise knowledge of magnet field maps • All beams originate from the same polarized photogun: more complicated compared to thermionic gun • Experiments grow more complicated, Beam specifications grow more demanding. Commissioning at one hall inconvenient to other halls • Beamtime oversubscribed: rush to complete 6GeV program

5. Everyone Gets Beam from Polarized Electron Gun! • CEBAF’s first polarized e-beam experiment 1997 • Now polarized e-beam experiments comprise ~80% of our physics program • All beams originate from the same 0.5mm spot on one photocathode inside 100kV GaAs photogun (we removed the thermionic gun in 2000) • At the moment, there are three polarized e-beam experiments on the floor; Hall A: GEn (10uA) Hall B: GDH (3nA) Hall C: G0 Backward Angle (60uA)

6. Spin precession angle: Shared Spin Manipulator, Shared LINAC Spin precession at arcs and transport lines Wien filter spin manipulator at injector, used to properly orient spin at Hall

7. Hall B Hall C At 5-pass, precession angle >10,000 degrees! No depolarization through machine Hall A Wien Angle Shared Spin Manipulator, Shared LINAC • Pure longitudinal pol for one hall at any beam energy • Many energy and pass configurations provide simultaneous longitudinal polarization at two halls • Simultaneous longitudinal polarization at three halls limited to ~ 2 and 4 GeV • In practice however, many settings provide nearly longitudinal polarization to all three halls J. Grames, et al. PRST-AB 7, 042802 (2004)

8. CEBAF Photoinjector 1998 1997 NOW • Long photocathode lifetime: • Good vacuum with NEGs • Spare-gun • NEG-coated beampipe • No short focal length elements • Wien filter • Photocathodes with anodized edge • Synchronous photoinjection

9. Synchronous Photoinjection Shared Injector Chopper Efficient beam extraction prolongs operating lifetime of photogun. DC Light, Most beam thrown away Lasers with GHz pulse repetition rates have been hard to come by Three independent RF-Pulsed lasers Lasers don’t turn completely OFF between pulses: Leakage (aka crosstalk, bleedthrough) B Now add prebuncher C A

10. Harmonic-modelocked Ti-Sapphire CEBAF Lasers Diode-seed + diode-amp 1996 M. Poelker, Appl. Phys. Lett. 67, 2762 (1995). 2000 C. Hovater and M. Poelker, Nucl. Instrum. Meth. A 418, 280 (1998);

11. Commercial Ti-Sapphire • 1st commerical 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

12. Complicated Laser Table • Many lossy optical components; tune mode generators, IAs, isolators • Time consuming alignment to ensure coincident, colinear beams • No “clean-up” polarizer for parity Users • Fussy Ti-Sapphire lasers; lose phase lock, require weekly maintenance

13. New Fiber-Based Drive Laser • CEBAFs last laser! • Gain-switching better than modelocking; no phase lock problems • Very high power • Telecom industry spurs growth • Useful only because of superlattice photocathode… J. Hansknecht and M. Poelker, submitted PRST-AB

14. Other Benefits of Fiber-Based Laser? • Replace lossy laser-table components with telecom stuff? • Tune mode generator (fast phase shifter and injector chopper) • IA and laser attenuator: fiber amplitude modulator • Fiber optic beam combiners? Extremly good mode quality, good for parity Users? Low repetition rate beam for particle ID and background studies, using beat frequncy method. Polarized beam without Pockels cell? Green version good for RF-pulsed Compton Polarimeter?

15. Superlattice GaAs: Layers of GaAs on GaAsP Strained GaAs: GaAs on GaAsP Bulk GaAs 100 nm 100 nm 14 pairs “conventional” material QE ~ 0.15% Pol ~ 75% @ 850 nm No strain relaxation QE ~ 0.8% Pol ~ 85% @ 780 nm High QE ~ 10% Pol ~ 35% Both are results of successful SBIR Programs Photocathode Material Superlattice reference; T. Maruyama et al, Appl. Phys. Lett. 85, 2640 (2004)

16. 2 2 P I P I sup. = 1.38 str. Beam Polarization at CEBAF Reasonable to request >80% polarization in PAC proposals

17. Oct 13 QE dropped by factor of 2 Nov 9 Anodized edge: a critical step Superlattice Photocathodes • No depolarization over time • Cannot be hydrogen cleaned • Arsenic-capped • No solvents during preparation!

18. Availability

19. What Can a User Expect at CEBAF? • Beam current from 100pA to 120 uA • Polarization > 80% • Photogun Lifetime ~ 100C (weeks of uninterrupted operation of gun) • Availability ~ 70% • Leakage from neighboring beams, < 3% • Energy Spread 1E-4 (can be made smaller) • Charge asymmetry 500ppm routine • Parity-Quality…

20. 1999 2007 HAPPEx notes: * Part 1 completed 2004, Part 2 during 2005, awaiting final numbers $Results at Hall A affected by Hall C operation. Expect specs were met in part2 What is Parity Quality? Helicity-correlated asymmetry specifications(achieved)

21. Routine Parity Violation Experiments? We need: • Long lifetime photogun (i.e., slow QE decay) • Stable injector • Properly aligned laser table (HAPPEx method) • Eliminate electronic ground loops • Proper beam-envelope matching throughout machine for optimum adiabatic damping: need to develop tools • Set the phase advance of the machine to minimize position asymmetry at target • Feedback loops; charge and position asymmetry • Specific requirements for each experiment; e.g., 31 MHz pulse repeitition rate, 300 Hz helicity flipping, beam halo < , etc.,

22. What is HAPPEx Method? • Identify Pockels cells with desirable properites: • 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 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

23. Photocathode QE Anisotropy, aka Analyzing Power Different QE for different orientation of linear polarization minimum analyzing power maximum analyzing power Beam Charge Asymmetry Rotating Halfwaveplate Angle Origins of HC Beam Asymmetries GaAs photocathode From G. Cates presentation, PAVI04 June 11, 2004

24. Origins of HC beam asymmetries cont. 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… From G. Cates presentation, PAVI04 June 11, 2004

25. Pockels Cell acts as active lens X position diff. (um) Y position diff. (um) Translation (inches) Red, IHWP Out Blue, IHWP IN Origins of HC Asymmetries cont. Use quad photodiode to minimize position differences From G. Cates presentation, PAVI04 June 11, 2004

26. High Current at High Polarization; Qweak to test standard model 180uA at 85% polarization CEBAF and ELIC New Developments Higher Current and High Polarization; > 1 mA Proposed new facilities ELIC, eRHIC Solution: Fiber-based laser + Load locked gun

27. 100 kV load locked gun Spot size diagnostic 1W green laser, DC, 532 nm Bulk GaAs Faraday Cup Baked to 450C Differential Pumps w/ NEG’s Insertable mirror NEG-coated large aperture beam pipe Focusing lens on x/y stage Test Cave LL-Gun and 100 kV Beamline Side-view

28. Ion Backbombardment Limits Photocathode Lifetime Bigger laser spot – same # electrons, same # ions laser light IN anode residual gas cathode But QE at (x ,y ) degrades more slowly because ion damage distributed over larger area (?) ionized residual gas hits photocathode i i (Best Solution – Improve Vacuum, but this is not easy) Can increasing the laser spot size improve charge lifetime? electron beam OUT Reality more complicated, Ions focused to electrostatic center

29. 342 um and 1538 um laser spots High Current Lifetime Experiments • Exceptionally high charge lifetime, >1000C at beam current to 10mA! • Lifetime scales with laser spot size but simple scaling not valid • Repeat measurements with high polarization photocathode material

30. Plus: • Multiple samples, • No more anodizing, • Better gun vacuum • Less surface area • No more venting Installation at CEBAF September, 2006 Load Locked Gun Development No more gun bakeouts! Photocathode replaced in 8 hours versus 4 days. Longer photocathode lifetime?

31. Beat Frequency Technique Normal Ops; Three beams at 499 MHz Beat Frequency Technique; One laser at 467.8125 MHz B C A Halls receives Low Rep Rate Beam at Beat Frequency between Laser and Chopper RF, in this case, 31.1875 MHz Why? Particle identification, background studies

32. s-polarized l /4 s and p polarized p-polarized l /2 Polarized beam without PC 60 degree optical delay line steering mirror atten Fiber-based laser l /2 atten Fast RF phase shifter Fast phase shifter moves beam IN/OUT of slit; Downside: extract 2x required beam current

33. CEBAF Headaches not so bad • Healthy polarized beam program at CEBAF with (mostly) happy Users. • Easy to satisfy ~60uA experiments. 100uA beam experiments at high polarization still keep us on our toes (i.e., we have to provide photocathode maintenence 1/mo.). • Ongoing gun and laser development to support high current Ops. • Parity violation experiments are not yet “routine” but we are getting there. Experience helps, new tools are being developed, better hardware • Fiber laser and load locked gun will help a great deal • We’ve enjoyed a great relationship with our Users, hopefully Users feel simialrly about CEBAF accelerator staff.

34. X-BPM (mm) Y-BPM (mm) without X-PZT (Source) 1C-Line 1C-Line with X-BPM (mm) Y-BPM (mm) Y-PZT (Source) 1C-Line 1C-Line Routine Parity Violation Experiments • HC position differences are generated at the source. • “Matching” the beam emittance to the accelerator acceptance realizes damping, • Well matched beam => position differences reduced. • Poorly matched beam => reduced damping (or even growth). • Accelerator matching (linacs & arcs) routinely demonstrated. • Injector matching has been arduous, long (~2 year) process.