Transcript Document

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
Chuck Davis, incoming
Hall C Polarimetry Workshop
Newport News, June 9-10, 2003
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
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)
ALR =
Note:
sP
s P -1
-1 = P
P
2
s+ +
r+ - rr+ + r-
(1+P)
2
s-
1
(P)
dP
(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 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)
4
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)
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 mA
 Rather than moving
continuously, beam will
dwell at certain point on
target for a few ms
6
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
7
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.
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Plans: kicker + half-foil Moller R&D
Configuration
Kick width
Precision
Max. Current
Nominal
-
<1%
2 mA
Prototype I
20 ms
few %
20 mA
Prototype II
10 ms
few %
40 mA
G0 Bkwd.
(2006)
3.5-4 ms
80 mA
QWeak
2 ms
Required: 2%
Goal: 1%
Required: 1%
Goal: 1%
180 mA
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Plans: operation during Qweak phase I

1mm foil with kicker should work fine at 1mA
average current (instantaneous current 180mA)

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
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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?
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Moller performance during G0 (2004)
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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
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)
 unpredictability of jumps uniquely specifies the model

y
sampling
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Plans: estimation of Moller sampling systematics
 Inputs:
Pave = 0.70
dPrms = 0.15
fjump = 1/10min
T = 2000hr
fsamp = variable
 Rule of thumb:
sampling systematics only
model calculation
Monte Carlo simulation
Adjust the sample
frequency until the
statistical errors per
sample match dP.
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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)
Jlab view:
these are
not
independent
– Upgrade Moller for 12 GeV operation
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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



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
2m
D
D4
D1
D2
D3
Compton
detector
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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
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Overview: the Compton chicane
19
Overview: the Compton chicane
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Overview: the Compton chicane

3 configurations support energies up to 11 GeV
Beam energy
(GeV)
1.165
2.0
2.5
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=520nm)
(cm)
57
2.4
4.1
5.0
2.2
2.6
4.9
1.8
4.5
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13
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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/qcross down
to scale of beam
divergence
22
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
23
Plans: example of pulsed-mode operation
laser
output
detector
signal
signal gate
background gate
* pulsed design used by Hermes, SLD
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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 the energy-weighted asymmetry.

Consider the general weighted yield: Y =
w
i
i
For a given polarization, the asymmetry in Y
depends in general on the weights wi used.
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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
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Plans: “counting” in pulsed mode
Define a figure-of-merit for a weighting scheme
f
V( pˆ ) =
N
l
f (ideal)
f (wi=1)>
f (wi=k)
514 nm
2260
9070
3160
248 nm
550
2210
770
193 nm
340
1370
480
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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
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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
29
Monte Carlo simulations
Compton-geant: based on original Geant3 program by Pat Welch
dipole chicane
backscatter exit port
gamma detector
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Monte Carlo simulations
Example events (several events superimposed)
electron beam
Compton backscatter (and bremsstrahlung)
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Monte Carlo simulations
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Status: laser options
1.
External locked cavity (cw)
–
2.
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
3.
High-power doubled solid-state laser (pulsed)
–
90W commercial units available
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Status: laser options
laser
option
l
(nm)
P
(W)
Hall A
1064 1500
Emax
(MeV)
rate
(KHz)
<A>
(%)
t (1%)
(min)
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
34
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
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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.
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 (?)
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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
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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 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-220i (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
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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
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