Compton Polarimeter Status Report

1. Compton Polarimeter StatusReport Alexandre Camsonne Hall A Collaboration Meeting January 4th 2007

2. Status in 2006 • Not very big year for Compton since most running was low energy • Data taken at 2 GeV • Gen • He4 polarisation ( test data ) • Current status • Problem with electron detector seems to have been damaged • Upgrade developments on going to allow low energy running and better systematics

3. The Hall A Compton Polarimeter Upgrade Motivation: Improve accuracy of experiments by providing 1% beam polarimetery down to 1 GeV. High precision Parity violating experiments are feasible with this upgrade Scope: • New Electron Detector • High resolution silicon microstrips to improve tracking resolution Integrating Photon Detector Improve systematic uncertainties experienced in the counting method 1.5kW Green Fabry-Perot Cavity Twice the Analyzing power of present IR cavity Four-fold increase in Figure-of-Merit Participating Institutions:Jefferson Lab, Saclay, Syracuse, Clermont-Ferrand, Uva, Duke

4. Expected Performance Simulation by David Lhuillier < 1% error @0.85 GeV obtained in about 4 hrs with 50 uA beam

5. Optical Setup • Laser frequency is continuosly tuned to lock to Fabry-Perot Cavity

6. Prometheus Green Laser • Manufacturer: Innolight GmbH, Germany • 708 nm Pump Diodes (2) • 1064 nm Nd:YAG • 532 nm SHG via PPKTP 1064 nm @ 2 W 532 nm @ 0.1 W

7. Green Fabry-Perot Cavity • Intra-cavity power 1.5 kW • The Mirrors (source Research Electo Optics) • Dielectric HR @532 super mirrors Reflectivity > 99.9937 % • Finesse > 50,000 • Loss < 20 ppm • ROC 50 cm Design Goal: Improve upon Saclay IR cavity non-adjustable mirror design, while keeping mirror distance fixed. • Fixed mirror distance • 2 Axis adjustable mirror mount • 2 Axis adjustable stands • UHV compatible

8. Assembled Cavity Photograh: Alan Gavalya

9. Cavity Mirror Mount

10. Optics Summary • Laser • The Prometheus laser lost power dramatically and developed large intensity noise within a few moths of operation • Sent back to Innolight, Germany for repairs. 2 month down time and 9K$ • After repairs (replacing the pump diodes and realignment) the power is back to over 100 mW exceeding specification. • However, Botao’s profiling finds that the beam is still 70% elliptic. We will try to solve it with optical circularization. • Intensity noise suppression electronics now installed on the laser. 4k$ • Polarization transfer function has been fully characterized (Xiaochao) • Mode matching solution developed (Alex Jousse and Botao Jia) • Alignment • Cavity mirror alignment with red HeNe laser works nicely • High resolution motorized mirror alignment and beam steering implemented (David Jacobs) EPICS interface under way (Sue Witherspoon) • Position sensitive photo-detectors installed • Cavity • The cavity is mechanically stable and responsive to mirror alignment ( Joyce and Ravi) • In 10-8 Torr vacuum (Greg Marble and Ed Folts) • The cavity demonstrates strong TEM00 resonance • However, laser-to-cavity coupling is poor (30%) -> improve mode matching

11. New Electron Detector • Electron Detector (Bernard Michel) • MOU with LPC Clermont-Ferrand • LPC takes responsibility for a new high resolution electron detector • Contribute 1 physicist, 1 engineer, 2 designer/techs • The new detector will be compatible with “12 GeV” machine upgrade • Specification • 768 ch 240 mm pitch silicon mstrips • 4 Planes, 192 strips/plane, 1 cm spacing between planes • 120 mm Vertical motion to allow full coverage of Compton Edge from 0.8-11 GeV • New custom front-end, FPGA trigger module (ETROC) • New DAQ and Analysis Software

12. Electron Detector Status • Silicon Microstrips (Michel Brossard) • Silicon microsrtip final design completed • Four planes have been ordered from Canberra-Eurisys system • Delivery of first plane expected late December • Mechanical (Francois Daudon, Alan Gavalya, Joyce Miller, Chris Curtis) • Vacuum chamber final design completed, fabrication to begin soon • Survey and alignment scheme finalized • Vertical motion system design in progress • Installation plan in development with AES (Tim Whitlatch) • DAQ/Analysis (Alex Camsonne) • Working on CODA interface for new ETROC • Need new electron event analyzer • Control System (Sue Witherspoon) • Motion Control EPICS interface • Beam shut-down safety interlock

13. Silicon Microstrip Plane

14. Electron Detector Assembly Drawing

15. Silicon Microstrip Mount

16. Compton photon integrating method • A couple of shifts dedicated to the Compton tune during E03-104 ( Special thanks to S. Strauch and R. Ent ) • Test Flash ADC integrating method • Beam tuning to get a reasonable signal over background ratio • First test with DC coupled base for the PMT of the central crystal • Commissionning of the Flash ADC • Production data ( invasive because of bad injector tune)Low current running First Compton events in FADC

17. Setup FADC Amplifier Upstairs counting house Experimental hall New base Fan in/out Amplifier PMT

18. Signal from Flash ADC

19. Signal from Flash ADC

20. Compton events

21. Compton signal

22. Compton signal

23. Measurement principle • Compton asymmetry ?

24. Concluding Remarks The Hall A Green Compton Polarimeter Upgrade Project is on track to be operational in 2008 - Issues and concerns: • The Prometheus laser’s short lifetime is a concern • Alternative single pass PPLN based green laser is important backup to be studied • Coupling efficiency of the laser to the cavity is rather low at 20%. • More work is needed to improve coupling efficiency. • Locking servo electronics needs more effort and interaction with the manufacturer. An electronics engineer/student is needed. - On going developments for 2007 : • Electron detector • Cavity • Photon detector integrating method