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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…. Qweak Polarimetry Working Group:.

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compton polarimeter for qweak evaluation of a fiber laser

Compton Polarimeter for QweakEvaluation of a Fiber Laser

reference laser

high-power fiber laser


S. Kowalski, M.I.T. (chair)

D. Gaskell, Jefferson Lab

R.T. Jones, U. Connecticut

Jeff Martin, Regina

hopefully more…

Qweak Polarimetry Working Group:

Hall C Polarimetry Workshop

Newport News, June 9-10, 2003

summary of reviewed options
Summary of reviewed 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

refererence design 100w green pulsed
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
    • Google search: “talis laser” finds “laser tails” mispelled
    • Coherent has a device with similar properties
new option fiber laser with shg
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: M2 = 1.33
    • polarization extinction ratio better than 95%
optics letters v 30 no 1 2005 67
Optics Letters v.30 no. 1 (2005) 67.

fiber laser

(grating mirrors)




pumped fiber


pulse starts


laser diode

source: cw,


pulse comes

out here




coupling to LMA

amplifier laser

main amplifier

pump laser

(976 nm)

main pulse


(1080 nm)


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.

Optics Letters v.30 no. 1 (2005) 67.

  • Performance: pictures tell the story!


  • Relevant features for a Compton laser:
    • high average power (in one polarization state)
    • high instantaneous power (low duty factor)
    • diffraction-limited optics (M2 of order unity)
  • Can one gain something by matching the laser pulse structure to the machine?
    • answer depends on crossing angle
    • quantitative estimate follows…


reference laser option

100 W

100 ns

300 – 1000 Hz

(3 - 10) 10-5

1-3 MW


fiber laser option

60 W

< 40 ps

10 – 500 MHz

(0.05 – 2.5) 10-2

2.4 - ? KW



average power

minimum pulse width

pulse repetition rate

duty factor range

instantaneous power

M2 factor (emittance/HUP)

minimum crossing angle



  • How is “minimum crossing angle” derived?
  • crossing angle is important for stable alignment.
  • Raleigh range + crossing angle → eff. target length.
  • larger M2 => shorter RR
  • might allow conversion of raw power into an “effective power factor”

expected range



  • Near-ideal emittance feature of this device is impossible to beat with diode-pumped SHG lasers.
  • 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:
    • in-house expertise at Jefferson Lab
    • potential x10 effective power increase for same average power
    • more flexible pulsing scheme (large range in duty factor)

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

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
status kicker half foil test summary
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.

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

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?


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




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


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



overview compton design criteria
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)

overview the compton chicane
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




10 m

2 m








overview the compton chicane1
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
overview the compton chicane4
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

plans use of a crossing angle
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
overview compton detectors
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
plans example of pulsed mode operation
Plans: example of pulsed-mode operation





signal gate

background gate

  • pulsed design used by Hermes, SLD
plans counting in pulsed mode
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 weights wi used.

plans counting in pulsed mode1
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
plans counting in pulsed mode2
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

plans counting in pulsed mode3
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
status monte carlo simulations
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
monte carlo simulations
Monte Carlo simulations
  • Compton-geant: based on original Geant3 program by Pat Welch

dipole chicane

backscatter exit port

gamma detector

monte carlo simulations1
Monte Carlo simulations
  • Example events (several events superimposed)

electron beam

Compton backscatter (and bremsstrahlung)

status laser options
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
status laser configuration
Status: laser configuration


electron beam


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

extra slides

(do not show)

addendum laser choices
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%
addendum laser choices1
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
status tests with iron wire target
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