Thomas Jefferson National Accelerator FacilityS. Nanda, June 22, 2007 1

The Hall A Compton Polarimeter

Sirish Nanda Jefferson Laboratory

Hall A Collaboration Meeting June 22, 2007

Thomas Jefferson National Accelerator FacilityS. Nanda, June 22, 2007 2

Compton Polarimetry

e-

e-

<Ath>=2 %

><××== −+

−+

+−

thennnn APPA γexp

kmax=340 MeV

E,E’ k,k’

• Figure-of-Merit x A2 ~ k2 x E2

Thomas Jefferson National Accelerator FacilityS. Nanda, June 22, 2007 3

The Hall A Compton Polarimeter Upgrade

Motivation:

High Power Green Fabry-Perot Cavity Twice the Analyzing power of present IR cavityFour-fold increase in Figure-of-Merit

Improve accuracy of polarization experiments by providing 1% beam polarimetery down to 1 GeV. High precision Parity violating experiments are feasible with this upgrade

New Electron Detector High resolution silicon microstrips to improve tracking resolution

New Photon Detector Improve systematic uncertainties experienced in the counting methodWhile preserving counting abilities

Participating Institutions: Jefferson Lab, Saclay, Syracuse, Clermont-Ferrand, Uva,

Duke,Carnegie-Mellon

Scope:

Thomas Jefferson National Accelerator FacilityS. Nanda, June 22, 2007 4

Expected Performance

Simulation byDavid Lhuillier

< 1% error @0.85 GeVobtained in about 4 hrs with 50 uA beam

Thomas Jefferson National Accelerator FacilityS. Nanda, June 22, 2007 5

Electron Detector LPC Clermont-Ferrand (contact: B. Michel)

• Specification• 768 ch 240 m pitch silicon strips• 4 Planes, 192 strips/plane, 1 cm spacing between planes• 120 mm Vertical motion to allow coverage of Compton Edge from 0.8-11 GeV• New custom front-end, FPGA trigger module (ETROC)• New DAQ and Analysis Software

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Electron Detector Assembly

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Electron Detector Status

• Detector & Electronics (Michel Brossard)

• Four planes of Silicon microsrtip delivered by Canberra Systems • Front-end analog electronics completed• ETROC Trigger system in fabrication• Test scheduled for July/Aug 07• Fifth spare microstrip ordered from Canberra

• Mechanical (Francois Daudon)

• Detector chamber design completed and approved• Parts in fabrication• Vertical motion controller to be done

• DAQ (Alex Camsonne)

• CODA readout for new ETROC• New electron event decoder• The new detector will be compatible with old DAQ and Analysis with only 48 strips active Just

in case…

• Support Structure and Installation (Alan Gavalya, Tim Whitlach, Chris Curtis, Ed Folts…)

• Survey alignment scheme finalized• Support structure in design• Preliminary installation plan developed

On track for Jan-Feb 08 installation

Thomas Jefferson National Accelerator FacilityS. Nanda, June 22, 2007 8

Photon DetectorCarnegie-Mellon University (Contact: Gregg Franklin)

• Calorimeter

• Simulation and Conceptual design Completed• Single large crystal GSO/Single PMT• High light output, fast decay time (less than 60 ns) • Preserve triggered counting capabilities. • Small sample test at CMU this summer with loaner from the detector group• Crystal procurement negotiations with Hitachi in Progress

• Integrating DAQ

• Flash ADC prototype has been beam tested (R. Michaels)• Improvements needed• New ADCs in procurement• Further beam tests planned for 2007/8

Thomas Jefferson National Accelerator FacilityS. Nanda, June 22, 2007 9

Green Fabry-Perot CavityJefferson Lab and Duke University (contact: SN)

• Specification

• Intracavity power 1.5 kW • Wavelength 532 nm• Mode CW, TEM00

• CIP Spot size () 65 m• Locking PDH

• Solutions• Primary: a) Tunable “smart” single pass Green Laser -> “Passive” High Finesses cavity,

Feedback to laser PZT to lock (Botao Jia)

b) Tunable “smart” IR Laser + single pass PPLN SHG -> “Passive” High Finesse

Cavity, Feedback to laser PZT (Diana Parno)

• Alternate: non-tunable “dumb” Green Laser + Electro-optic frequency modulator -> “Active” low Finesse cavity. Feedback to cavity exit mirror PZT (Could use help)

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Optical Setup

Thomas Jefferson National Accelerator FacilityS. Nanda, June 22, 2007 11

Setup in Compton Lab

Thomas Jefferson National Accelerator FacilityS. Nanda, June 22, 2007 12

Assembled Cavity

Photograh: Alan Gavalya

Thomas Jefferson National Accelerator FacilityS. Nanda, June 22, 2007 13

Pound-Drever-Hall Locking Scheme

Photo detector

Beam Splitter Cavity

Oscillator

Phase Shifter

MixerLow Pass Filter

0

Tunable Laser

PID-Regulator

Error signal

Detect phase of the resonance from reflected light

Feedback to tunable element to stay “locked” to resonance

Thomas Jefferson National Accelerator FacilityS. Nanda, June 22, 2007 14

Locking ExperienceProblems:

Salcay PDH Electronics: Did not work with Prometheus green laser

Commercial Electronics: Required substantial modifications. Produced unstable intermittent locks of short duration

Mechanical: Cavity Input mirror showed mechanical instability

Picomotor mount re-design solved mechanical instability (Ravi)

For locking electronics We pursued a home brew…

Thomas Jefferson National Accelerator FacilityS. Nanda, June 22, 2007 15

Digital CavLock Prototype home-brewed approach

• ARM7 RISC 32-bit Processor (Analog Devices ADuc7026)

• 12-bit ADCs and DACs• Fast interrupt handler• Flash memory

Host PC C++ Control Program via JTAG Port

• Feed-Back Servo

• Resosnace Phase detector• PID Regulator• Fast and Slow Error

Feedback to Laser4”

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Locking with CavLock

Transmission

Reflection

Fast Scan

Slow Search

Fast Ramp + Error Signal

Slow Ramp

Transmission

Reflection

Minutes

20 minutes

Lock Command

Unlock Command

10-sec Scope Trace

40-min Strip Chart

• On-demand lock within seconds• Stable and reliable Lock for hours• Production version to be implemented soon (Fernado Barbosa)

Thomas Jefferson National Accelerator FacilityS. Nanda, June 22, 2007 17

Cavity Power

• We have succeeded in locking, Next is optimization of Cavity Power

• So, How much power do we really need?

• Must have 0.5 - 1 kW we can be in business with this if the beam tune is clean

• Should have 1 - 2 kW Our design goal for routine operation

• Would like to have 2 - 3 kW gives us better operating margin when the beam tune is poor

• Too Much > 3 kW Present DAQ will choke at the counting rate

• Unlikely > 5 kW Limit of present optics damage threshold

Thomas Jefferson National Accelerator FacilityS. Nanda, June 22, 2007 18

PPLN SHG

• The Prometheus Green laser has some problems

• The laser may not be able provide the injection power we need to achieve 1.5 kW we should have in the cavity

• The pump diodes have short lifetime requiring frequent offsite service

• We are developing our own ~ kHz linewidth tunable green laser by frequency doubling a Lightwave 1064 nm IR Laser to 532 nm (Diana)

• With a PPLN:MgO crystal from HC Photonics 10-15 mW 532 nm output has been achieved with 700 mW IR input.

Temperature Scan at Full Power

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

66 67 68 69 70 71 72

Temperature (deg C, as reported by controller)

Green Power (mW)

Pursuing power amplification to several hundred milliwats with a fiber amplifier

Thomas Jefferson National Accelerator FacilityS. Nanda, June 22, 2007 19

12-GeV ComptonScope• Reduce Chicane deflection from 30 to 22 cm to use existing dipole magnets• Raise FP cavity and Photon detector by 8 cm• Vertical motion for e-detector to cover higher energy

Green FP Cavity

e-detector

-detector

Raised Dipoles and Optics

Line between 6 and 12 GeV Compton Polarimeter design and implementation effort is very fuzzy!

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Compton Upgrade Schedule

• Electron Detector:• Installation: Jan-Feb 08• Commissioning Mar-Apr 08

• Green FP Cavity • Installation: Apr-May 08• Commissioning: Jun-Jul 08

• Photon Detector • Installation: Sep 08• Commissioning: Oct 08

Production Operation: 2009

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Summary

• Electron detector progress is consistent with Jan 08 Installation

• Progress with Green FP cavity development

• Robust locking has been achieved• Power optimization in progress

• PPLN based backup green laser solution is in sight

• Photon detector design proceeding well

Project progress is limited by manpowerParticipation welcome!

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Schedule