Polarimetry for Qweak

1
Polarimetry for Qweak

• Overview
• Status
• Plans

S. Kowalski, M.I.T. (chair) D. Gaskell, Jefferson
Lab R.T. Jones, U. Connecticut Chuck Davis,
incoming
Qweak Polarimetry Working Group
Hall C Polarimetry Workshop Newport News, June
9-10, 2003
2
Overview

• Phase I 8 measurement of ALR
• 2 combined systematicstatistical error on
  polarization
• sampling measurements with Moller polarimeter
• Phase II 4 measurement of ALR
• 1 systematicstatistical error on polarization
• continuous running with Compton polarimeter,
  combined with periodic Moller samplings

3
Overview polarimetry goals for Qweak

• What statistic is relevant for quoting precision?

but in terms of measured rates r
1
(
)
P
the relevant quantity
sP
dP
sP -1
(
)
Note
1 2


P
P
P -1
4
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)

5
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)

6
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/Nrandom10 at 200 mA
• Rather than moving continuously, beam will dwell
  at certain point on target for a few ms

7
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 101 at 40mA
• mabe due to distorted edge of foil
• runs at 40mA frequently interrupted by BLM trips

8
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.

9
Plans kicker half-foil Moller RD
10
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

11
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?
12
Moller performance during G0 (2004)
13
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

y
sampling
14
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
15
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 not independent
16
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)

17
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
2 m
D
D4
D1
Compton detector
D2
D3
18
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

19
Overview the Compton chicane
20
Overview the Compton chicane
21
Overview the Compton chicane

• 3 configurations support energies up to 11 GeV

Beam energy qbend B D Dxe (l520nm)
(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
22
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

23
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

24
Plans example of pulsed-mode operation
laser output
detector signal
signal gate
background gate

• pulsed design used by Hermes, SLD

25
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
• For a given polarization, the asymmetry in Y
  depends in general on the weights wi used.

26
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

27
Plans counting in pulsed mode

• Define a figure-of-merit for a weighting scheme

l f (ideal) f (wi1)gt f (wik) 514
nm 2260 9070 3160 248 nm 550 2210 770 193 nm
340 1370 480
28
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 lt 1/shot
• weighting used when combining results from
  different segments

29
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

30
Monte Carlo simulations

• Compton-geant based on original Geant3 program
  by Pat Welch

dipole chicane
backscatter exit port
gamma detector
31
Monte Carlo simulations

• Example events (several events superimposed)

electron beam
Compton backscatter (and bremsstrahlung)
32
Monte Carlo simulations
33
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

34
Status laser options

• laser l P Emax rate ltAgt 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

35
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 gt99
• length of figure-8 100 cm

36
Detector options

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

37
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.

38
extra slides (do not show)
39
Addendum recent progress
40
Addendum recent progress
41
Addendum laser choices

• High-power green laser (100 W _at_ 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 (?)

42
Addendum laser choices

• High-power UV laser (50 W _at_ 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

43
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

44
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 _at_ full power 35 (single)
• 50 (with amplifier)
• continuous operation _at_ full power 2000 hours

45
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