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Polarimetry for Qweak
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  1. Polarimetry for Qweak Overview Status Plans S. Kowalski, M.I.T. (chair) D. Gaskell, Jefferson Lab R.T. Jones, U. Connecticut Chuck Davis, incoming Qweak Polarimetry Working Group: Hall C Polarimetry Workshop Newport News, June 9-10, 2003

  2. Overview • Phase I: 8% measurement of ALR • 2% combined systematic+statistical error on polarization • sampling measurements with Moller polarimeter • Phase II: 4% measurement of ALR • 1% systematic+statistical error on polarization • continuous running with Compton polarimeter, combined with periodic Moller samplings

  3. s+-s- ALR = s++s- s- ( 1 ± P ) ( 1 + P ) s+ + r± = 2 2 r+-r- ALR = r++r- Overview: polarimetry goals for Qweak • What statistic is relevant for quoting precision? but in terms of measured rates r± 1 ( ) P the relevant quantity dP sP sP-1 ( ) + … Note: 1 + 2 = P P P-1

  4. Overview: Polarimetry methods for Qweak • Moller polarimeter for Qweak • uses existing Hall C Moller spectrometer • incorporates fast kicker to enable operation at high beam currents – pulsed Moller operation • early tests demonstrate operation at 40mA, development is ongoing [following slides] • impact on beam and hall backgrounds probably prevents simultaneous running with Qweak • statistics at 1% level obtained in ~40 min. • sub-percent systematic errors (based on experience with standard cw Moller operation at 1-2mA)

  5. Status: the Hall C Moller upgrade • Existing Hall C Moller can do 1% measurements in a few minutes. • Limitations: • - maximum current ~10mA • - at higher currents the Fe target depolarizes due to target heating • - measurement is destructive • Goals for the upgrade: • measure beam polarization up to 200mA • make measurement quasi-continuously (not for Qweak)

  6. Status: tests with “half-target” foil • Target heating limits maximum pulse duration and duty factor • Instantaneous rate limits maximum foil thickness • This can be achieved with a 1 mm foil Nreal/Nrandom≈10 at 200 mA • Rather than moving continuously, beam will dwell at certain point on target for a few ms

  7. Status: tests with 1mm “half-target” foil • 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 40mA • mabe due to distorted edge of foil • runs at 40mA frequently interrupted by BLM trips

  8. 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 80mA during G0 backward angle run.

  9. Plans: kicker + half-foil Moller R&D

  10. Plans: operation during Qweak phase I • 1mm foil with kicker should work fine at 1mA average current (instantaneous current 180mA) • 1% measurement will take ~30minutes • 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

  11. Plans: operation during Qweak phase II • 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? Is there a worst-case model for polarization sampling errors?

  12. Moller performance during G0 (2004)

  13. 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 • average rate of jumps • r.m.s. systematic fluctuations in P y sampling

  14. Plans: estimation of Moller sampling systematics • Inputs: Pave = 0.70 • dPrms = 0.15 • fjump = 1/10min • T = 2000hr • fsamp= variable • Rule of thumb: Adjust the sample frequency until the statistical errors per sample match dP. sampling systematics only model calculation Monte Carlo simulation

  15. Plans: time line for Hall C beamline • 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 Jlab view: these are not independent

  16. 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 fomstat~ E2(for same laser and current)

  17. Overview: the Compton chicane • 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 Compton recoil detector 10 m 2 m D D4 D1 Compton detector D2 D3

  18. Overview: the Compton chicane • 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 bx = by= 8m near center of chicane

  19. Overview: the Compton chicane

  20. Overview: the Compton chicane

  21. Overview: the Compton chicane • 3 configurations support energies up to 11 GeV Beam energy qbend B D Dxe (l=520nm) (GeV) (deg) (T) (cm) (cm) 1.165 10 0.67 57 2.4 2.0 1.16 4.1 2.5 1.45 5.0 2.5 4.3 0.625 25 2.2 3.0 0.75 2.6 6.0 1.50 4.9 4.0 2.3 0.54 13 1.8 11.0 1.47 4.5

  22. Plans: use of a crossing angle • assume a green laser l = 514 nm • fix electron and laser foci at the same point s = 100 mm • emittance of laser scaled by diffraction limit e = M (l / 4p) • scales like 1/qcrossdown to scale of beam divergence

  23. 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 103 ) • insensitive to essentially all types of beam background, eg. bremsstrahlung in the chicane

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

  25. Plans: “counting” in pulsed mode • cannot count individual gammas because pulses overlap within a single shot Q. How is the polarization extracted? A. By measuring theenergy-weightedasymmetry. • Consider the general weighted yield: For a given polarization, the asymmetry in Ydepends in general on the weightswiused.

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

  27. Plans: “counting” in pulsed mode • Define a figure-of-merit for a weighting scheme l f (ideal) f(wi=1)> f (wi=k) 514nm2260 9070 3160 248 nm550 2210 770 193 nm340 1370 480

  28. Plans: “counting” in pulsed mode • 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

  29. 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

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

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

  32. Monte Carlo simulations

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

  34. Status: laser options laser l P Emax rate <A> t (1%) option (nm) (W) (MeV) (KHz) (%) (min) Hall A 1064 1500 23.7 480 1.03 5 UV ArF 193 32 119.8 0.8 5.42 100 UV KrF 248 65 95.4 2.2 4.27 58 Ar-Ion (IC) 514 100 48.1 10.4 2.10 51 DPSS 532 100 46.5 10.8 2.03 54

  35. Status: laser configuration monitor electron beam laser • 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

  36. Detector options • Photon detector • Lead tungstate • Lead glass • BGO • Electron detector • Silicon microstrip • Quartz fibers

  37. 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.

  38. extra slides (do not show)

  39. Addendum: recent progress

  40. Addendum: recent progress

  41. Addendum: laser choices • High-power green laser (100 W @ 532 nm) • sold by Talis Laser • industrial applications • frequency-doubled solid state laser • pulsed design • D. Gaskell: visit from Talis Laser reps June 2003 • not confident that they could deliver • product no longer being advertised (?)

  42. Addendum: laser choices • High-power UV laser (50 W @ 248 nm) • sold by several firms • industrial applications: micromachining and lithography • excimer laser (KrF) • pulsed design • R. Jones: visit from Lambda Physik reps • sales team has good technical support • plenty of experience with excimer lasers • strong interest in our application

  43. 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%

  44. 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

  45. Status: tests with iron wire target • 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