Polarimetry for Qweak Overview Status Plans Qweak Polarimetry

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

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

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

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 40 m. A, 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 -2 m. A) 4

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

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 m. A § Rather than moving continuously, beam will dwell at certain point on target for a few ms 6

Status: tests with 1 mm “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 40 m. A – mabe due to distorted edge of foil – runs at 40 m. A frequently interrupted by BLM trips 7

Status: kicker + half-foil test summary Ø Ø 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. Ø The next step is to make 1% polarization measurements at 80 m. A during G 0 backward angle run. 8

Plans: kicker + half-foil Moller R&D Configuration Nominal Kick width - Precision <1% Max. Current 2 m. A Prototype I 20 ms few % 20 m. A Prototype II 10 ms few % 40 m. A G 0 Bkwd. (2006) 3. 5 -4 ms 80 m. A QWeak 2 ms Required: 2% Goal: 1% Required: 1% Goal: 1% 180 m. A 9

Plans: operation during Qweak phase I q 1 mm foil with kicker should work fine at 1 m. A average current (instantaneous current 180 m. A) q 1% measurement will take ~30 minutes q Conservative heating calculations indicate foil depolarization will be less than 1% in the worst case under these conditions – can be checked q Compton being shaken down during this phase 10

Plans: operation during Qweak phase II q To reach 1% combined systematic and statistical error, plans are to operate both Compton and Moller polarimeters during phase II. q Duration and frequency of Moller runs can be adjusted to reach the highest precision in average P 1 q Can we estimate the systematic error associated with drifts of polarization between Moller samplings? Is there a worst-case model for polarization sampling errors? 11

Moller performance during G 0 (2004) 12

Plans: estimation of Moller sampling systematics Worst-case scenario for sampling instantaneous jumps at unpredictable times Ø model completely specified by just two parameters Ø 1. average rate of jumps 2. r. m. s. systematic fluctuations in P maximum effective jump rate is set by duration of a sampling measurement (higher frequencies filtered out) Ø unpredictability of jumps uniquely specifies the model Ø y sampling 13

Plans: estimation of Moller sampling systematics § Inputs: Pave = 0. 70 d. Prms = 0. 15 fjump = 1/10 min T = 2000 hr fsamp = variable § Rule of thumb: sampling systematics only model calculation Monte Carlo simulation Adjust the sample frequency until the statistical errors per sample match d. P. 14

Plans: time line for Hall C beamline Ø Short term plans (2006) – Improve beamline for Moller and Moller kicker operation Ø Long term plans (2008) – Install Compton polarimeter Ø Longer term plans (12 Ge. V) Jlab view: these are not independent – Upgrade Moller for 12 Ge. V operation 15

Overview: Compton design criteria q measure luminosity-weighted average polarization over period of ~1 hour with statistical error of 1% under Qweak running conditions q control systematic errors at 1% level q coexist with Moller on Hall C beamline q be capable of operation at energies 1 -11 Ge. V fomstat ~ E 2 (for same laser and current) 16

Overview: the Compton chicane q q q 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 D 4 D 1 D 2 D 3 Compton detector 17

Overview: the Compton chicane q 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 q Focus with bx = by = 8 m near center of chicane 18

Overview: the Compton chicane 19

Overview: the Compton chicane 20

Overview: the Compton chicane q 3 configurations support energies up to 11 Ge. V Beam energy (Ge. V) 1. 165 2. 0 2. 5 3. 0 6. 0 4. 0 11. 0 qbend (deg) 10 4. 3 2. 3 B (T) 0. 67 1. 16 1. 45 0. 625 0. 75 1. 50 0. 54 1. 47 D (cm) Dxe (l=520 nm) (cm) 57 2. 4 4. 1 5. 0 2. 2 2. 6 4. 9 1. 8 4. 5 25 13 21

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 / 4 p) § scales like 1/qcross down to scale of beam divergence 22

Overview: Compton detectors Ø Detect both gamma and recoil electron – two independent detectors – different systematics – consistency checks Ø Gamma – electron coincidence – necessary for calibrating the response of gamma detector – marginally compatible with full-intensity running Ø Pulsed laser operation – backgrounds suppressed by duty factor of laser ( few 103 ) – insensitive to essentially all types of beam background, 23 eg. bremsstrahlung in the chicane

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

Plans: “counting” in pulsed mode q cannot count individual gammas because pulses overlap within a single shot Q. How is the polarization extracted? A. By measuring the energy-weighted asymmetry. q Consider the general weighted yield: For a given polarization, the asymmetry in Y depends in general on the weights wi used. 25

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 26

Plans: “counting” in pulsed mode • Define a figure-of-merit for a weighting scheme l f (ideal) f (wi=1)> f (wi=k) 514 nm 2260 9070 3160 248 nm 550 2210 770 193 nm 340 1370 480 27

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 28

Status: Monte Carlo simulations q Needed to study systematics from – detector misalignment – detector nonlinearities – beam-related backgrounds q Processes generated – – Compton scattering from laser synchrotron radiation in dipoles (with secondaries) bremsstrahlung from beam gas (with secondaries) standard Geant list of physical interactions 29

Monte Carlo simulations • Compton-geant: based on original Geant 3 program by Pat Welch dipole chicane backscatter exit port gamma detector 30

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

Monte Carlo simulations 32

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) – 65 W unit now in tabletop size 3. High-power doubled solid-state laser (pulsed) – 90 W commercial units available 33

Status: laser options laser l option (nm) P (W) Hall A 1064 1500 23. 7 480 1. 03 5 UV Ar. F 193 32 119. 8 0. 8 5. 42 100 UV Kr. F 248 65 95. 4 2. 2 4. 27 58 Ar-Ion (IC) 514 100 48. 1 10. 4 2. 10 51 DPSS 100 46. 5 10. 8 532 Emax rate <A> (Me. V) (KHz) (%) 2. 03 t (1%) (min) 54 34

Status: laser configuration monitor electron beam laser q 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 35

Detector options q Photon detector – – – q Lead tungstate Lead glass BGO Electron detector – Silicon microstrip – Quartz fibers 36

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. 37

extra slides (do not show) 38

Addendum: recent progress 39

Addendum: recent progress 40

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 (? ) 41

Addendum: laser choices • High-power UV laser (50 W @ 248 nm) – – • sold by several firms industrial applications: micromachining and lithography excimer laser (Kr. F) 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 42

Addendum: laser choices • Properties of LPX 220 i – – • 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% 43

Addendum: laser choices • purchase cost for UV laser system – – • LPX-220 i (list): LPX-220 amplifier (list): control electronics: mirrors, ¼ wave plates, lenses: 175 k$ 142 k$ 15 k$ 10 k$ cost of operation (includes gas, maintenance) – per hour @ full power: $35 (single) $50 (with amplifier) – continuous operation @ full power: 2000 hours 44

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 45