Compton Polarimeter for Qweak
Evaluation of a Fiber Laserreference laserhigh-power fiber lasercomparison
S. Kowalski, M.I.T. (chair)
D. Gaskell, Jefferson Lab
R.T. Jones, U. Connecticut
Jeff Martin, Regina
hopefully more…Hall C Polarimetry WorkshopNewport News, June 9-10, 2003
Qweak Polarimetry Working Group:
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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
Summary of reviewed options:
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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” “talis laser” findsfinds “laser tails” “laser tails” mispelledmispelled CoherentCoherent has a device with similar properties
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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%
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laser diodesource: cw,broadband
Optics Letters v.30 no. 1 (2005) 67.
pulse startspulse startsherehere
polarizer modulator(chopper) pumped fiber
preamplifier
fiber laser(grating mirrors)
coupling to LMAamplifier laser
main pulseamplifier
(1080 nm)
main amplifierpump laser(976 nm)
non-lineardoublingcrystal
pulse comespulse comesout hereout here
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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 LBO crystal has a narrow temperature range.temperature range.
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Optics Letters v.30 no. 1 (2005) 67.
Performance: pictures tell the story!pictures tell the story!
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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 (M2 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…
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Comparison
average power
minimum pulse width
pulse repetition rate
duty factor range
instantaneous power
M2 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°
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Comparison How is “minimum crossing angle” derived?
crossing angle is important for stable alignment.
Raleigh range + crossing Raleigh range + crossing angle angle → eff. target length→ eff. target length.
larger M2 => shorter RR
might allow conversion of raw power into an “effective power factor”“effective power factor”
expected range
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Comparison Near-ideal emittance feature of this device is
impossible to beat with diode-pumped SHG lasers.
To exploit this requires eithereither going to very small crossing angles (~ 1 mr) oror matching the laser pulse train to the electron pulse train, or some combinationor some combination.
Advantages of fiber laser design:Advantages of fiber laser design: in-house expertise at Jefferson Labin-house expertise at Jefferson Lab potential x10 effective power increase for same average powerpotential x10 effective power increase for same average power more flexible pulsing scheme (large range in duty factor)more flexible pulsing scheme (large range in duty factor)
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Target heating limits maximum pulse duration and duty factor
Instantaneous rate limits maximum foil thickness
This can be achieved with a 1 m foil
Nreal/Nrandom≈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
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tests by Hall C team during December 2004
measurements measurements consistent at the consistent at the ~2% level~2% level
random coincidence rates were larger than expected
– reals/randoms 10:1 at reals/randoms 10:1 at 4040AA
– mabe due to distorted mabe due to distorted edge of foiledge of foil
– runs at 40A frequently interrupted by BLM trips
Status: tests with 1m “half-target” foil
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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 80A during G0 backward angle run.
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Configuration Kick width Precision Max. 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 sRequired: 2% Goal: 1%
80 A
QWeak 2 sRequired: 1% Goal: 1%
180 A
Plans: kicker + half-foil Moller R&D
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1m foil with kicker should work fine at 1A average current (instantaneous current 180A)
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
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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?
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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 jumps2. r.m.s. systematic fluctuations in P
y
sampling
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Plans: estimation of Moller sampling systematics
model calculation
Monte Carlo simulation
Inputs:Inputs:
Pave = 0.70
Prms = 0.15
fjump =
1/10min T = 2000hr fsamp =
variable
Rule of Rule of thumb: thumb: Adjust the sample frequency until the statistical errors per sample match P.
sampling systematics only
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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 arenotindependent
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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)
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Overview: the Compton chicane
10 m
2 m
D1
D2 D3
D4
Comptondetector
Comptonrecoildetector
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
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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
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3 configurations support energies up to 11 GeV
Beam energy bend B D xe (=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 1.811.0 1.47 4.5
Overview: the Compton chicane
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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
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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
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Plans: example of pulsed-mode operation
detectorsignal
signal gate
background gate
laseroutput
* pulsed design used by Hermes, SLD
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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 wi used.
i
iw Y
Plans: “counting” in pulsed mode
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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 mode
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Define a figure-of-merit for a weighting scheme
f (ideal) f (wi=1)> f (wi=k)
514nm 2260 9070 3160
248 nm 550 2210 770
193 nm 340 1370 480
N
fp )ˆ(V
Plans: “counting” in pulsed mode
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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
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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
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Monte Carlo simulations
Compton-geant: based on original Geant3 program by Pat Welch
dipole chicane
backscatter exit portgamma detector
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Monte Carlo simulations
Example events (several events superimposed)
electron beam
Compton backscatter (and bremsstrahlung)
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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
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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
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Detector options
Photon detector Lead tungstate Lead glass BGO
Electron detector Silicon microstrip Quartz fibers
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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.
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Addendum: laser choices
Properties of LPX 220i maximum power: 40 W (unstable resonator) maximum repetition rate: 200 Hz focal spot size: 100 x 300 m (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 m polarization above 90%
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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