Probing Hadron Structure at CEBAF Using Polarized Electron

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; Ø CEBAF Overview Ø What Can You Expect at CEBAF? Ø Parity Violation Experiments (becoming routine? ) Ø New Developments for New Experiments

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 C Pockels cell A A B C Chopper Wien filter Double sided septum Gun

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

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

Everyone Gets Beam from Polarized Electron Gun! Ø CEBAF’s first polarized e-beam experiment 1997 Ø Now polarized e-beam experiments comprise ~80% of our physics program Ø All beams originate from the same 0. 5 mm spot on one photocathode inside 100 k. V Ga. As photogun (we removed thermionic gun in 2000) Ø At the moment, there are three polarized e-beam experiments on the floor; Hall A: GEn (10 u. A) Hall B: GDH (3 n. A) Hall C: G 0 Backward Angle (60 u. A)

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

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

CEBAF Photoinjector 1998 1997 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 NOW

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

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

Commercial Ti-Sapphire • 1 st 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

Complicated Laser Table ØMany lossy optical components; tune mode generators, IAs, isolators ØTime consuming alignment to ensure coincident, colinear beams ØNo “clean-up” polarizer for parity Users ØFussy Ti-Sapphire lasers; lose phase lock, require weekly maintenance

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

Other Benefits of Fiber-Based Laser? Replace lossy laser-table components with telecom stuff? ØTune mode generator (fast phase shifter and injector chopper) ØIA and laser attenuator: fiber amplitude modulator ØFiber 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?

Photocathode Material High QE ~ 10% Pol ~ 35% “conventional” material QE ~ 0. 15% Pol ~ 75% @ 850 nm 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 No strain relaxation QE ~ 0. 8% Pol ~ 85% @ 780 nm Both are results of successful SBIR Programs Superlattice reference; T. Maruyama et al, Appl. Phys. Lett. 85, 2640 (2004)

Beam Polarization at CEBAF Psup. 2 I = 1. 38 2 Pstr. I Reasonable to request >80% polarization in PAC proposals

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

Availability

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

What is Parity Quality? Helicity-correlated asymmetry specifications (achieved) 1999 2007 Experiment Physics Asymmetry Max run-average helicity correlated Position Asymmetry Max run-average helicity correlated Current Asymmetry HAPPEX-I 13 ppm 10 (10) nm 1 (0. 4) ppm G 0 Forward 2 to 50 ppm 20 (4 ± 4) nm 1 (0. 14 ± 0. 3) ppm HAPPEX-He* 8 ppm 3 nm (3) nm 0. 6 (0. 08) ppm HAPPEX-II* 1. 3 ppm 2 nm (8)$ nm 0. 6 (2. 6)$ ppm Lead 0. 5 ppm 1 nm 0. 1 ppm Qweak 0. 3 ppm 40 nm 0. 1 ppm 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 part 2

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

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 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

Origins of HC 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 From G. Cates presentation, PAVI 04 June 11, 2004

Origins of HC beam asymmetries cont. Pockels cell aperture Non-uniform polarization across laser beam + QE anisotropy… 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, PAVI 04 June 11, 2004

Use quad photodiode to minimize position differences Red, IHWP Out Blue, IHWP IN Y position diff. (um) Pockels Cell acts as active lens X position diff. (um) Origins of HC Asymmetries cont. Translation (inches) From G. Cates presentation, PAVI 04 June 11, 2004

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

Test Cave LL-Gun and 100 k. V Beamline Side-view 100 k. V load locked gun Spot size diagnostic Bulk Ga. As 1 W green laser, DC, 532 nm 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

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

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

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

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

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

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

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. X-BPM (mm) without X-PZT (Source) Y-BPM (mm) 1 C-Line with X-BPM (mm) Y-PZT (Source) Y-BPM (mm) 1 C-Line