1. Compton Polarimeter for Qweak Evaluation of a Fiber Laser reference laser high-power fiber laser comparison S. Kowalski, M.I.T. (chair) D. Gaskell, Jefferson Lab R.T. Jones, U. Connecticut Jeff Martin, Regina hopefully more… Hall C Polarimetry Workshop Newport News, June 9-10, 2003 Qweak Polarimetry Working Group:

2. 2 laser l P E max rate t (1%) option(nm) (W)(MeV)(KHz) (%)(min) Hall A10641500 23.7 4801.03 5 UV ArF 193 32119.8 0.85.42100 UV KrF 248 65 95.4 2.24.27 58 Ar-Ion (IC) 514 100 48.110.42.10 51 DPSS 532 100 46.510.82.03 54 Summary of reviewed options:

3. 3 refererence design: 100W green pulsed High-power green laser (100 W @ 532 nm)  sold by Talis Laser  industrial applications  frequency-doubled solid state laser  pulsed design, MW peak power D. Gaskell: news as of 10/2005  product no longer being advertised “talis laser” finds “laser tails” mispelled  Google search: “talis laser” finds “laser tails” mispelled  Coherent  Coherent has a device with similar properties

4. 4 New option: fiber laser with SHG Original suggestion by Matt Poelker (4/6/2006)  source group has good experience with fiber laser  capable of very short pulses (40ps), high rate (500MHz)  current design delivers 2W average power  might be pushed up to 60W, duty factor around 50 Published result: Optics Letters v.30 no. 1 (2005) 67.  high average power: 60W average power (520 nm).  demonstrated high peak power: 2.4KW (d.f. = 30)  almost ideal optical properties: M 2 = 1.33  polarization extinction ratio better than 95%

5. 5 laser diode source: cw, broadband Optics Letters v.30 no. 1 (2005) 67. pulse starts here polarizermodulator (chopper) pumped fiber preamplifier fiber laser (grating mirrors) coupling to LMA amplifier laser main pulse amplifier (1080 nm) main amplifier pump laser (976 nm) non-linear doubling crystal pulse comes out here

6. 6 Optics Letters v.30 no. 1 (2005) 67. Is there anything exotic in this design?  all optics elements are coated for 1080 nm.  FOPA pump coupling mirror has dual coating.  minimum pulse peak power for efficienct SGH in non-linear crystal  minimum pulse width to avoid SRS in fiber.  LBO crystal has a narrow temperature range.

7. 7 Optics Letters v.30 no. 1 (2005) 67. pictures tell the story! Performance: pictures tell the story!

8. 8 Comparison Relevant features for a Compton laser: 1.high average power (in one polarization state) 2.high instantaneous power (low duty factor) 3.diffraction-limited optics (M 2 of order unity) Can one gain something by matching the laser pulse structure to the machine? 1.answer depends on crossing angle 2.quantitative estimate follows…

9. 9 Comparison average power minimum pulse width pulse repetition rate duty factor range instantaneous power M 2 factor (emittance/HUP) minimum crossing angle reference laser option 100 W 100 ns 300 – 1000 Hz (3 - 10) 10 -5 1-3 MW ~30 3° fiber laser option 60 W < 40 ps 10 – 500 MHz (0.05 – 2.5) 10 -2 2.4 - ? KW 1.33 0.5°

10. 10 Comparison How is “minimum crossing angle” derived?  crossing angle is important for stable alignment.  Raleigh range + crossing angle → eff. target length  Raleigh range + crossing angle → eff. target length.  larger M 2 => shorter RR “effective power factor”  might allow conversion of raw power into an “effective power factor” expected range

11. 11 Comparison Near-ideal emittance feature of this device is impossible to beat with diode-pumped SHG lasers. either or or some combination To exploit this requires either going to very small crossing angles (~ 1 mr) or matching the laser pulse train to the electron pulse train, or some combination. Advantages of fiber laser design: Advantages of fiber laser design:  in-house expertise at Jefferson Lab  potential x10 effective power increase for same average power  more flexible pulsing scheme (large range in duty factor)

12. 12  Target heating limits maximum pulse duration and duty factor  Instantaneous rate limits maximum foil thickness  This can be achieved with a 1  m foil N real /N random ≈10 at 200  A  Rather than moving continuously, beam will dwell at certain point on target for a few  s Status: tests with “half-target” foil

13. 13  tests by Hall C team during December 2004  measurements consistent at the ~2% level  random coincidence rates were larger than expected – reals/randoms 10:1 at 40  A – mabe due to distorted edge of foil – runs at 40  A frequently interrupted by BLM trips Status: tests with 1  m “half-target” foil

14. 14 Status: kicker + half-foil test summary  Preliminary results look promising.  Source polarization jumps under nominal run conditions make it impossible to confirm ~1% stability.  Running at very high currents may be difficult – problem may have been exacerbated by foil edge distortion.  Development is ongoing.  Dave Meekins is thinking about improved foil mounting design.  Future tests should be done when Moller already tuned and has been used for some period of time so that we are confident we understand the polarimeter and polarized source properties.  The next step is to make 1% polarization measurements at 80  A during G0 backward angle run.

15. 15 ConfigurationKick widthPrecisionMax. Current Nominal-<1% 2  A Prototype I 20  s few % 20  A Prototype II 10  s few % 40  A G0 Bkwd. (2006) 3.5-4  s Required: 2% Goal: 1% 80  A Q Weak 2  s Required: 1% Goal: 1% 180  A Plans: kicker + half-foil Moller R&D

16. 16  1  m foil with kicker should work fine at 1  A average current (instantaneous current 180  A)  1% measurement will take ~30 minutes  Conservative heating calculations indicate foil depolarization will be less than 1% in the worst case under these conditions – can be checked  Compton being shaken down during this phase Plans: operation during Qweak phase I

17. 17  To reach 1% combined systematic and statistical error, plans are to operate both Compton and Moller polarimeters during phase II.  Duration and frequency of Moller runs can be adjusted to reach the highest precision in average P -1  Can we estimate the systematic error associated with drifts of polarization between Moller samplings? Plans: operation during Qweak phase II Is there a worst-case model for polarization sampling errors?

18. 18 Moller performance during G0 (2004)

19. 19 Plans: estimation of Moller sampling systematics Worst-case scenario for sampling  instantaneous jumps at unpredictable times  model completely specified by just two parameters  maximum effective jump rate is set by duration of a sampling measurement (higher frequencies filtered out)  unpredictability of jumps uniquely specifies the model 1.average rate of jumps 2.r.m.s. systematic fluctuations in P y sampling

20. 20 Plans: estimation of Moller sampling systematics model calculation Monte Carlo simulation  Inputs: P ave = 0.70   P rms = 0.15 f jump = 1/10min T = 2000hr f samp = variable  Rule of thumb:  Rule of thumb: Adjust the sample frequency until the statistical errors per sample match  P. sampling systematics only

21. 21  Short term plans (2006)  Improve beamline for Moller and Moller kicker operation  Long term plans (2008)  Install Compton polarimeter  Longer term plans (12 GeV)  Upgrade Moller for 12 GeV operation Plans: time line for Hall C beamline Jlab view: these are not independent

22. 22 Overview: Compton design criteria  measure luminosity-weighted average polarization over period of ~1 hour with statistical error of 1% under Qweak running conditions  control systematic errors at 1% level  coexist with Moller on Hall C beamline  be capable of operation at energies 1-11 GeV fom stat ~ E 2 (for same laser and current)

23. 23 Overview: the Compton chicane 10 m 2 m D1 D2D3 D4 Compton detector Compton recoil detector D  4-dipole design  accommodates both gamma and recoil electron detection  nonzero beam-laser crossing angle (~1 degree)  important for controlling alignment  protects mirrors from direct synchrotron radiation  implies some cost in luminosity

24. 24  Alex Bogacz (CASA) has found a way to fit the chicane into the existing Hall C beamline.  independent focusing at Compton and target  last quad triplet moved 7.4 m downstream  two new quads added, one upstream of Moller and one between Moller arms  fast raster moves closer to target, distance 12 m.  beamline diagnostic elements also have to move  Focus with  x  y  = 8m near center of chicane Overview: the Compton chicane

25. 25 Overview: the Compton chicane

26. 26 Overview: the Compton chicane

27. 27  3 configurations support energies up to 11 GeV Beam energy  bend B D  x e ( =520nm) (GeV)(deg)(T)(cm)(cm) 1.165 100.67 57 2.4 2.0 1.16 4.1 2.51.45 5.0 2.5 4.30.625 25 2.2 3.00.75 2.6 6.01.50 4.9 4.0 2.30.54 13 1.8 11.01.47 4.5 Overview: the Compton chicane

28. 28 Plans: use of a crossing angle  assume a green laser = 514 nm  fix electron and laser foci at the same point  = 100  m  emittance of laser scaled by diffraction limit  = M (  / 4   scales like 1/  cross down to scale of beam divergence

29. 29 Overview: Compton detectors  Detect both gamma and recoil electron  two independent detectors  different systematics – consistency checks  Gamma – electron coincidence – necessary for calibrating the response of gamma detector – marginally compatible with full-intensity running  Pulsed laser operation – backgrounds suppressed by duty factor of laser ( few 10 3 ) – insensitive to essentially all types of beam background, eg. bremsstrahlung in the chicane

30. 30 Plans: example of pulsed-mode operation detector signal signal gate background gate laser output * pulsed design used by Hermes, SLD

31. 31  cannot count individual gammas because pulses overlap within a single shot Q. How is the polarization extracted? A.By measuring the energy-weighted asymmetry.  Consider the general weighted yield: For a given polarization, the asymmetry in Y depends in general on the weights w i used. Plans: “counting” in pulsed mode

32. 32  Problem can be solved analytically w i = A(k)  Solution is statistically optimal, maybe not for systematics.  Standard counting is far from optimal w i = 1  Energy weight is better! w i = k Plans: “counting” in pulsed mode

33. 33 Define a figure-of-merit for a weighting scheme  f (ideal) f ( w i =1)> f ( w i = k ) 514  nm226090703160 248 nm 5502210 770 193 nm 3401370 480 Plans: “counting” in pulsed mode

34. 34 Systematics of energy-weighted counting  measurement independent of gamma detector gain  no need for absolute calibration of gamma detector  no threshold  method is now adopted by Hall-A Compton team Electron counter can use the same technique  rate per segment must be < 1/shot  weighting used when combining results from different segments Plans: “counting” in pulsed mode

35. 35 Status: Monte Carlo simulations  Needed to study systematics from  detector misalignment  detector nonlinearities  beam-related backgrounds  Processes generated  Compton scattering from laser  synchrotron radiation in dipoles (with secondaries)  bremsstrahlung from beam gas (with secondaries)  standard Geant list of physical interactions

36. 36 Monte Carlo simulations Compton-geant : based on original Geant3 program by Pat Welch dipole chicane backscatter exit port gamma detector

37. 37 Monte Carlo simulations Example events (several events superimposed) electron beam Compton backscatter (and bremsstrahlung)

38. 38 Monte Carlo simulations

39. 39 Status: laser options 1. External locked cavity (cw)  Hall A used as reference 2. High-power UV laser (pulsed)  large analyzing power (10% at 180°)  technology driven by industry (lithography)  65W unit now in tabletop size 3. High-power doubled solid-state laser (pulsed)  90W commercial units available

40. 40 Status: laser configuration  two passes make up for losses in elements  small crossing angle: 1 °  effective power from 2 passes: 100 W  mirror reflectivity: >99%  length of figure-8: 100 cm laser electron beam monitor

41. 41 Detector options  Photon detector  Lead tungstate  Lead glass  BGO  Electron detector  Silicon microstrip  Quartz fibers

42. 42 Summary Qweak collaboration should have two independent methods to measure beam polarization. A Compton polarimeter would complement the Moller and continuously monitor the average polarization. Using a pulsed laser system is feasible, and offers advantages in terms of background rejection. Options now exist that satisfy to Qweak requirements with a green pulsed laser, that use a simple two-pass setup. Monte Carlo studies are underway to determine tolerances on detector performance and alignment required for 1% accuracy. Space obtained at Jlab for a laser test area, together with Hall A. Specs of high-power laser to be submitted by 12/2005.

43. 43 extra slides (do not show)

44. 44 Addendum: recent progress

45. 45 Addendum: recent progress

46. 46 Addendum: laser choices Properties of LPX 220i  maximum power: 40 W (unstable resonator)  maximum repetition rate: 200 Hz  focal spot size: 100 x 300 mm (unstable resonator)  polarization: should be able to achieve ~90% with a second stage “inverted unstable resonator”  maximum power: 50 W  repetition rate unchanged  focal spot size: 100 x 150 mm  polarization above 90%

47. 47 Addendum: laser choices purchase cost for UV laser system  LPX-220i (list):175 k$  LPX-220 amplifier (list):142 k$  control electronics: 15 k$  mirrors, ¼ wave plates, lenses: 10 k$ cost of operation (includes gas, maintenance)  per hour @ full power:$35 (single) $50 (with amplifier)  continuous operation @ full power:2000 hours

48. 48  Initial tests with kicker and an iron wire target performed in Dec. 2003  Many useful lessons learned  25 mm wires too thick  Large instantaneous rate gave large rate of random coincidences  Duty factor too low – measurements would take too long Status: tests with iron wire target