Transcript Document

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
Jeff Martin, Regina
Hall C Polarimetry Workshop
Newport News, June 9-10, 2003
hopefully more…
Summary of reviewed 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
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refererence design: 100W green pulsed
High-power green laser (100 W @ 532 nm)
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sold by Talis Laser
industrial applications
frequency-doubled solid state laser
pulsed design, MW peak power
D. Gaskell: news as of 10/2005
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product no longer being advertised
Google search: “talis laser” finds “laser tails” mispelled
Coherent has a device with similar properties
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New option: fiber laser with SHG
Original suggestion by Matt Poelker (4/6/2006)
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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.
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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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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)
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Optics Letters v.30 no. 1 (2005) 67.
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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.
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Optics Letters v.30 no. 1 (2005) 67.
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Performance: pictures tell the story!
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Comparison
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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)
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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
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
M2 factor (emittance/HUP)
minimum crossing angle
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Comparison
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How is “minimum crossing angle” derived?
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crossing angle is important
for stable alignment.
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Raleigh range + crossing
angle → eff. target length.
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larger M2 => shorter RR
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might allow conversion of
raw power into an
“effective power factor”
expected range
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Comparison
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Near-ideal emittance feature of this device is
impossible to beat with diode-pumped SHG lasers.
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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.
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Advantages of fiber laser design:
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in-house expertise at Jefferson Lab
potential x10 effective power increase for same average power
more flexible pulsing scheme (large range in duty factor)
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Status: tests with “half-target” foil
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Target heating limits
maximum pulse duration
and duty factor
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Instantaneous rate limits
maximum foil thickness
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This can be achieved with
a 1 mm foil
Nreal/Nrandom≈10 at 200 mA
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Rather than moving
continuously, beam will
dwell at certain point on
target for a few ms
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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
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Status: kicker + half-foil test summary
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Preliminary results look promising.
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Source polarization jumps under nominal run conditions make it
impossible to confirm ~1% stability.
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Running at very high currents may be difficult – problem may
have been exacerbated by foil edge distortion.
Development is ongoing.
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Dave Meekins is thinking about improved foil mounting design.
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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
Required: 2%
Goal: 1%
80 mA
QWeak
2 ms
Required: 1%
Goal: 1%
180 mA
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Plans: operation during Qweak phase I
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1mm foil with kicker should work fine at 1mA
average current (instantaneous current 180mA)
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1% measurement will take ~30 minutes
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Conservative heating calculations indicate foil
depolarization will be less than 1% in the worst
case under these conditions – can be checked
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Compton being shaken down during this phase
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Plans: operation during Qweak phase II
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To reach 1% combined systematic and statistical
error, plans are to operate both Compton and
Moller polarimeters during phase II.
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Duration and frequency of Moller runs can be
adjusted to reach the highest precision in average
P-1
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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
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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
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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
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Short term plans (2006)
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Long term plans (2008)
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Improve beamline for Moller and Moller kicker
operation
Install Compton polarimeter
Longer term plans (12 GeV)
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Upgrade Moller for 12 GeV operation
Jlab view:
these are
not
independent
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Overview: Compton design criteria
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measure luminosity-weighted average
polarization over period of ~1 hour with
statistical error of 1% under Qweak running
conditions
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control systematic errors at 1% level
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coexist with Moller on Hall C beamline
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be capable of operation at energies 1-11 GeV
fomstat ~ E2
(for same laser and current)
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Overview: the Compton chicane
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4-dipole design
accommodates both gamma and recoil electron
detection
nonzero beam-laser crossing angle (~1 degree)
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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
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Alex Bogacz (CASA) has found a way to fit the
chicane into the existing Hall C beamline.
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independent focusing at Compton and target
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last quad triplet moved 7.4 m downstream
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two new quads added, one upstream of Moller and one
between Moller arms
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fast raster moves closer to target, distance 12 m.
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beamline diagnostic elements also have to move
Focus with bx = by = 8m near center of chicane
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Overview: the Compton chicane
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Overview: the Compton chicane
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Overview: the Compton chicane
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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)
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2.4
4.1
5.0
2.2
2.6
4.9
1.8
4.5
25
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Plans: use of a crossing angle
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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
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Overview: Compton detectors
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Detect both gamma and recoil electron
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two independent detectors
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different systematics – consistency checks
Gamma – electron coincidence
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necessary for calibrating the response of gamma
detector
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marginally compatible with full-intensity running
Pulsed laser operation
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backgrounds suppressed by duty factor of laser ( few
103 )
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insensitive to essentially all types of beam background,
eg. bremsstrahlung in the chicane
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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
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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.
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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
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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
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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
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Systematics of energy-weighted counting
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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
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rate per segment must be < 1/shot
weighting used when combining results from
different segments
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Status: Monte Carlo simulations
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Needed to study systematics from
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detector misalignment
detector nonlinearities
beam-related backgrounds
Processes generated
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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 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
External locked cavity (cw)
1.
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Hall A used as reference
High-power UV laser (pulsed)
2.
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large analyzing power (10% at 180°)
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technology driven by industry (lithography)
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65W unit now in tabletop size
High-power doubled solid-state laser (pulsed)
3.
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90W commercial units available
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Status: laser configuration
monitor
electron beam
laser
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two passes make up for losses in elements
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small crossing angle: 1°
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effective power from 2 passes: 100 W
mirror reflectivity: >99%
length of figure-8: 100 cm
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Detector options
Photon detector
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Lead tungstate
Lead glass
BGO
Electron detector
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Silicon microstrip
Quartz fibers
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Summary
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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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extra slides
(do not show)
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Addendum: recent progress
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Addendum: recent progress
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Addendum: laser choices
Properties of LPX 220i
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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”
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maximum power: 50 W
repetition rate unchanged
focal spot size: 100 x 150 mm
polarization above 90%
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Addendum: laser choices
purchase cost for UV laser system
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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)
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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
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Initial tests with kicker and
an iron wire target
performed in Dec. 2003
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Many useful lessons
learned
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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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