Compton Polarimeter for Qweak Evaluation of a Fiber Laser reference laser high-power fiber laser comparison 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 Jeff Martin, Regina hopefully more…
Summary of reviewed options: laser l option (nm) P (W) Hall A 1064 1500 23. 7 480 1. 03 5 193 32 119. 8 0. 8 5. 42 65 95. 4 2. 2 4. 27 58 Ar-Ion (IC) 514 100 48. 1 10. 4 2. 10 DPSS 100 46. 5 10. 8 UV Ar. F UV Kr. F 248 532 Emax rate <A> (Me. V) (KHz) (%) 2. 03 t (1%) (min) 100 51 54 2
refererence design: 100 W green pulsed n High-power green laser (100 W @ 532 nm) q q sold by Talis Laser industrial applications frequency-doubled solid state laser pulsed design, MW peak power D. Gaskell: news as of 10/2005 n q q q product no longer being advertised Google search: “talis laser” finds “laser tails” mispelled Coherent has a device with similar properties 3
New option: fiber laser with SHG Original suggestion by Matt Poelker (4/6/2006) n q q source group has good experience with fiber laser capable of very short pulses (40 ps), high rate (500 MHz) current design delivers 2 W average power might be pushed up to 60 W, duty factor around 50 Published result: Optics Letters v. 30 no. 1 (2005) 67. n q q high average power: 60 W average power (520 nm). demonstrated high peak power: 2. 4 KW (d. f. = 30) almost ideal optical properties: M 2 = 1. 33 polarization extinction ratio better than 95% 4
Optics Letters v. 30 no. 1 (2005) 67. fiber laser (grating mirrors) pulse starts here polarizer modulator (chopper) pumped fiber preamplifier laser diode source: cw, broadband pulse comes out here non-linear doubling crystal coupling to LMA amplifier laser main amplifier pump laser (976 nm) main pulse amplifier (1080 nm) 5
Optics Letters v. 30 no. 1 (2005) 67. n q q q 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. 6
Optics Letters v. 30 no. 1 (2005) 67. n Performance: pictures tell the story! 7
Comparison n 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) n Can one gain something by matching the laser pulse structure to the machine? 1. answer depends on crossing angle 2. quantitative estimate follows… 8
Comparison reference laser option fiber laser option average power 100 W 60 W minimum pulse width 100 ns < 40 ps pulse repetition rate 300 – 1000 Hz 10 – 500 MHz (3 - 10) 10 -5 (0. 05 – 2. 5) 10 -2 1 -3 MW 2. 4 - ? KW ~30 1. 33 3° 0. 5° duty factor range instantaneous power M 2 factor (emittance/HUP) minimum crossing angle 9
Comparison n q q How is “minimum crossing angle” derived? crossing angle is important for stable alignment. Raleigh range + crossing angle → eff. target length larger M 2 => shorter RR might allow conversion of raw power into an “effective power factor” expected range 10
Comparison n Near-ideal emittance feature of this device is impossible to beat with diode-pumped SHG lasers. n 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 n Advantages of fiber laser design: q q q in-house expertise at Jefferson Lab potential x 10 effective power increase for same average power more flexible pulsing scheme (large range in duty factor) 11
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 12
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 13
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. § § Ø 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 m. A during G 0 backward angle run. 14
Plans: kicker + half-foil Moller R&D Configuration Kick width Precision Max. Current Nominal - <1% 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 Required: 2% Goal: 1% 80 m. A QWeak 2 ms Required: 1% Goal: 1% 180 m. A 15
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 16
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? 17
Moller performance during G 0 (2004) 18
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 19
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. 20
Plans: time line for Hall C beamline Ø Short term plans (2006) q Ø Long term plans (2008) q Ø Improve beamline for Moller and Moller kicker operation Install Compton polarimeter Longer term plans (12 Ge. V) q Upgrade Moller for 12 Ge. V operation Jlab view: these are not independent 21
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) 22
Overview: the Compton chicane q q q 4 -dipole design accommodates both gamma and recoil electron detection nonzero beam-laser crossing angle (~1 degree) q q q 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 23
Overview: the Compton chicane q Alex Bogacz (CASA) has found a way to fit the chicane into the existing Hall C beamline. q independent focusing at Compton and target q last quad triplet moved 7. 4 m downstream q two new quads added, one upstream of Moller and one between Moller arms q q q fast raster moves closer to target, distance 12 m. beamline diagnostic elements also have to move Focus with bx = by = 8 m near center of chicane 24
Overview: the Compton chicane 25
Overview: the Compton chicane 26
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 27
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 28
Overview: Compton detectors Ø Detect both gamma and recoil electron q q Ø two independent detectors different systematics – consistency checks Gamma – electron coincidence – – Ø 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 29
Plans: example of pulsed-mode operation laser output detector signal gate background gate * pulsed design used by Hermes, SLD 30
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. 31
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 32
Plans: “counting” in pulsed mode n 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 33
Plans: “counting” in pulsed mode n Systematics of energy-weighted counting q q n 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 q q rate per segment must be < 1/shot weighting used when combining results from different segments 34
Status: Monte Carlo simulations q Needed to study systematics from q q detector misalignment detector nonlinearities beam-related backgrounds Processes generated q q Compton scattering from laser synchrotron radiation in dipoles (with secondaries) bremsstrahlung from beam gas (with secondaries) standard Geant list of physical interactions 35
Monte Carlo simulations n Compton-geant: based on original Geant 3 program by Pat Welch dipole chicane backscatter exit port gamma detector 36
Monte Carlo simulations Example events (several events superimposed) electron beam Compton backscatter (and bremsstrahlung) 37
Monte Carlo simulations 38
Status: laser options External locked cavity (cw) 1. q Hall A used as reference High-power UV laser (pulsed) 2. q large analyzing power (10% at 180°) q technology driven by industry (lithography) q 65 W unit now in tabletop size High-power doubled solid-state laser (pulsed) 3. q 90 W commercial units available 39
Status: laser configuration monitor electron beam laser q two passes make up for losses in elements q small crossing angle: 1° q q q effective power from 2 passes: 100 W mirror reflectivity: >99% length of figure-8: 100 cm 40
Detector options Photon detector q q Lead tungstate Lead glass BGO Electron detector q q q Silicon microstrip Quartz fibers 41
Summary n n n n 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. 42
extra slides (do not show) 43
Addendum: recent progress 44
Addendum: recent progress 45
Addendum: laser choices Properties of LPX 220 i n q q 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” n q q maximum power: 50 W repetition rate unchanged focal spot size: 100 x 150 mm polarization above 90% 46
Addendum: laser choices purchase cost for UV laser system n q q 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) n q q per hour @ full power: $35 (single) $50 (with amplifier) continuous operation @ full power: 2000 hours 47
Status: tests with iron wire target § Initial tests with kicker and an iron wire target performed in Dec. 2003 § Many useful lessons learned q q q 25 mm wires too thick Large instantaneous rate gave large rate of random coincidences Duty factor too low – measurements would take too long 48
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