BaselHall C Mller Upgrade

1
Basel/Hall C Møller Upgrade

• Existing Hall C Møller can do 1 measurements
  (stat) in a few minutes

• Limitations
• -Maximum current 10 mA. At higher currents the
  Fe
• target depolarizes due to target heating
• -Measurement is destructive
• Goals for and upgraded Møller
• Measure beam polarization at 100 mA or higher
• Make measurement quasi-continuously

2
Kicker Magnet for High Current Møller Polarimetry

• Chen Yan originally presented idea to use a fast
  kicker magnet to scan the electron beam across
  some iron wire or strip target
• Kicker would move quickly and at low duty cycle
  to minimize time on iron target and beam heating
• First generation kicker was installed in Fall 2003

3
Kicker and 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

4
Modified Kicker with Half-Target

• The only way to keep random coincidences at an
  acceptable level is to reduce the instantaneous
  rate
• 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

5
Kicker and Half-Target

• With 4 mm target and 2 mA, a run with better than
  1 statistical precision takes about 5 minutes
• The 1 mm foil with kicker will take 1 mA average
  current (Iavg I fkick Dtkick) measurement will
  take about 8 times longer 40 minutes
• Detailed and conservative heating calculations
  indicate foil depolarization will be less than 1
  in the worst case
• Depolarization will depend on beam size -gt can be
  measured and depolarization calculated
• Deliberately running with larger frequency will
  allow us to map out depolarization and check
  predictions

6
Preliminary Tests of 2nd Generation Kicker and
Half -Target

• Chen Yan improved existing kicker to have 10 ms
  kick interval - still uses triangular wave-form
• Dave Meekins mounted 1 mm strip on Møller target
  ladder

7
Kicker Half Target Test Run

• Attempted high current test Dec. 16 bad beam
  tune caused too many BLM trips had to back out
• Tried again with more success on Dec. 21
• Accomplishments
• Found the target!
• Verified kick direction and determined rough
  calibration
• Took polarization measurements up to 40 mA
  (previous best was 20 mA with wire target)

8
Beam Position Scan

• Scanned beam position at BPM 3C17 (BPM just
  upstream of Møller target)
• Found lower edge of target from Møller
  coincidence rate
• Some indications that the target was deformed
  somehow
• Rates at edge were too high
• Rates decreased as beam moved further on to
  target
• Visual inspection of target later confirmed that
  bottom edge of target was bent

Rate expected based on 2 mA running on 4 mm target
9
Kicker Measurements

• Run conditions
• 2 mA on 4mm foil (nominal Møller run conditions),
  kicker on and off
• Kicker runs at 10, 20, and 40 mA
• Beam trips prevented us from running at higher
  currents
• Required average current on target less than 1mA
  to minimize target heating
• Used beam from Hall A laser -gt maximize
  polarization
• Measured polarization was reasonably consistent
  for all configurations but
• Charge asymmetries were quite large, sometimes
  1!
• Some instability, even for nominal Møller
  configurations (no kicker)

10
Preliminary Kicker Results

• Measurements consistent
• at the 2 level

11
Kicker Test Summary

• Preliminary results look promising
• 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
  distortion
• Dave Meekins already thinking about improved foil
  mounting design
• Future tests would be much more easily done when
  Møller already tuned and has been used for some
  period of time so that we are confident we
  understand the polarimeter and polarized source
  properties
• Thanks to Howard Fenker and Bill Vulcan for
  Møller tuneup help and power supply switcheroo
  and trouble shooting