COBRA Magnet

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COBRA Magnet

• Wataru OOTANI
• MEG Experiment Review Meeting, Feb. 2004

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Topics

• Fringing field problem
• Positioning accuracy of mapping machine
• Hall probe calibration

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Fringing Field Problem

• Problems are solved or are about to be solved in
  all places.
• pM3 and µE1(µSR facilities)
• Existing compensation coils can cancel the
  fringing field.
• New MSCB power supply is installed in their
  compensation coil system.
• ECRIT
• They figured out the problem that the detector
  doesnt work under COBRA fringing field.
• pE3
• Either passive or active shielding seems to work.
• Passive shield will be installed if they conclude
  it is really necessary with their measurements
  during the next COBRA excitation in March.

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Shielding in pE3

• Passive shielding is more realistic than active
  one.
• Magnet group keeps lots of shielding material.
• 37 ARMCO plates (1m x 2m x t10mm, high µ and low
  remnant field)
• Can be used for pE3 shielding.
• The design of the shielding house in pE3 is being
  optimized with the help of the magnet group.

Calculated by V. Vrankovic from magnet group
lt1Gauss
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A New Idea to Solve Fringe Field Problem

• A new idea to solve the fringing field problem is
  proposed by R. Eichler.
• Large compensating coils surrounding the COBRA
  magnet to cancel the fringe field outside pE5
  area.
• Simulation
• A pair of large compensation coils facing each
  other along the beam line (Helmholtz
  configuration)
• 5mx5m square-shape coils
• 10m apart from each other
• The idea is attractive, but not feasible compared
  with the other methods.
• B field around COBRA and LXe detector gt30Gauss
• B field on the beam line 100-1KGauss
• Need a huge power supply (the current of 10K AT)
• Difficult to completely cancel the fringe field.

COBRA center
Compensation coils
Beam
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Positioning Accuracy of Mapping Machine

• Maximum gradient of the COBRA field is 3/cm
• 1mm ? ?B/B0.3 (30Gauss for 1T)
• We need positioning accuracy lt 1mm for the
  mapping machine.
• Specification from the manufacturer lt 0.5mm
• 3-D motion of the mapping machine
• R-motion
• ?-motion (rotation)
• Z-motion
• Position of the moving stage will be monitored
  with three optical position sensors. They should
  be calibrated.

Uniformity (calculation)
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Positioning Accuracy of Mapping Machine

• Positioning accuracy of the mapping machine was
  measured with the help of the survey group
• Laser tracker
• Highly precise measurement (10µm) based on laser
  interferometry
• 3 coordinates (x, y, z) are directly measured at
  a time.

Reflector
R
?
Moving stage
Z
Laser tracker
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Positioning Accuracy of Mapping Machine
R-motion
y mm
? measured at radius30cm
R mm
Z-motion
?-motion
x mm
Z mm
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Positioning Accuracy of Mapping Machine

• Positioning accuracy was measured to be lt 0.5mm
  as expected except for Z-motion.
• Poor accuracy in ?-direction of Z-motion is not a
  serious problem because the COBRA field is
  axi-symmetric.
• The problem in Z-motion is almost figured out.
• Z-mover was not fixed on tight to the shaft.
• The accuracy shall be improved hopefully.

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Hall Probe Calibration

• Hall probe
• 3-axis probe (LakeShore MMY-1802-UH)
• Corrected accuracy 0.25 to 20kG
• Temperature coefficient 0.015/?
• The probe was calibrated in a highly uniform
  field up to 1.5T with the help of the magnet
  group (Many thanks to V. Vrankovic).
• Absolute calibration of Hall probe readings using
  NMR
• Measurement of planar Hall effect

Magnet with a pole gap
Hall probe
NMR
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Absolute Calibration

• Two probes (X Y) were successfully calibrated,
  while the probe Z was not able to be calibrated
  because the gap distance is limited in the
  calibration magnet.
• The probe Z will be cross-calibrated against the
  probe X and Y in the COBRA field later.

Difference in readings
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Planar Hall Effect

• Hall voltage can be shifted by the planar Hall
  effect caused by the transverse component of the
  applied magnetic field.
• Planar Hall voltage VPHE ? B//2sin(2?)
• The planar hall effect was measured only for the
  probe Z because of the limited gap distance.
• The effect was measured to be 0.44 at 1.3T.

Transverse field 1.3T
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Planar Hall Effect
VPlanar Hall / VNormal Hall

• Only one probe can suffer from the planar Hall
  effect in the COBRA field measurement because of
  the layout of the probes and the cylindrical
  motion in the mapping machine.
• The planar Hall effect expected in the field
  measurement is calculated based on the
  measurement on the Z-probe.
• The effect is expected to be less than 0.15.
• Not serious. It can be corrected anyway.

Zm
Rm
Zm
Rm
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Schedule

• Shielding house will be installed in pE3 if they
  conclude it is really necessary with their
  measurements during the next COBRA excitation in
  March.
• Test measurement of the COBRA filed with the
  mapping machine will be made in March.
• Final measurement will be carried out in the next
  shutdown period.

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End of Slides
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