Status of COBRA Magnet

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Status of COBRA Magnet

• Wataru OOTANI

MEG review meeting July 11th, 2003 PSI
Switzerland
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Construction finished!

• Construction was finished!
• Excitation test was done between Jun.21th and
  Jul.7th.

Compensation coils
Superconducting coil in cryostat
Potentiometer
Support frame
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Construction finished!, contd
Power supply for SC
Instrument panel
Refrigerator
Power supply for NC
NC current adjustor
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Excitation test

• Purposes
• Excitations with up to 5 and 10 higher coil
  current for the SC and NC, respectively.
• Intentional quench tests.
• Quench propagation in the SC.
• Temperature rise.
• Voltage rise.
• Optimization of the protection heater
  parameters.
• Stress distribution in the superconducting
  magnet.
• Interaction between SC and NC.
• Safety circuit of the magnet control system.
• Stability and accuracy of power supply.
• Test of the device for the magnetic field
  measurement.
• Installation procedure.
• Driving system of Hall probe.
• Test measurement of the magnetic field.
• Fringe field in the photon detector region.

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Two Troubles in Excitation Test

• Cold spots on the inner wall of the cryostat.
• Dew condensation around the spots.
• Radiation shield is touching the cryostat wall.
• No problem in the magnet operation.
• But, it might be problem for the detectors
  installed into the magnet or filed measurement
  (Hall probe stability).
• Protection heaters in the downstream end-coil
  were broken.
• Protection heater When a quench occurs somewhere
  in the magnet, all coils are intentionally
  quenched using protection heaters to avoid local
  energy dump.
• Quench without the protection heater might damage
  the coil due to voltage and temperature rise in
  the coil.

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Cold Spots

• Radiation shield is thermally connected to the
  cryostat wall due to too much superinsulation
  layers in narrow gap.
• Dew condensation was observed around the cold
  spots.
• The magnet will be disassembled to fix this
  problem.

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Quench Test Results

• Voltage rise in the quench triggered by the
  protection heaters in all the coils except the
  end coil with broken heater.
• Coil current 300A

Voltage across the end coil with broken heater
rose up to 900V.
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Quench Test Results, contd

• Temperature rise in the quench triggered by the
  protection heaters in all the coils except the
  end coil with broken heater.
• Coil current 300A

DT (central coil)
Temperature of the central coil rose up to 110K.
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Excitation w/o protection heater

• We were increasing coil current step by step and
  carefully checking the temperature and voltage
  rise in the intentional quench tests, but we had
  to give up the excitation with full operating
  current (360A) for the SC only for safety reason.
• We reached 300A (83 of normal operating current)
  for the SC and successfully done the quench test
  at the coil current.
• We found the SC magnet worked well under much
  more severe conditions. (DV-900V, DT110K)
• For the NC, I400A(111 of normal operating
  current) reached.

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Strain

• No strange behavior up to coil current of 300A.
• Mechanical strength was already confirmed in the
  previous excitation test of the central part of
  the magnet last November.

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External Control System of Magnet

• External operation of the magnet using MIDAS
  slow control system (MSCB) was tested in this
  excitation test.
• Control software based on LabVIEW.

MSCB modules
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Field Measurement Device

• Point-by-point precision of the field map
  10Gauss
• 3-axis Hall probe.
• Position accuracy a few hundred mm
• Ultrasonic motor with a rotary encoder
  (4000pulses/rotation)
• for R- and Z-motion and high-torque AC
  servo-motor for q-motion
• Timing belt will be used for the motion in all
  directions
• Probe position is measured by optical scale
  sensors

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Field Measurement Device, contd

• Construction completed.
• Installed into the COBRA cryostat and tested
  during the excitation test.
• Test measurement of the COBRA field was done
  with the Hall probe used in the Belle magnet.

Hall probe
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Field Measurement Device, contd

• Test measurement of the magnetic field was done
  in the excitation test.
• SC Coil only.
• Coil current 200A (Normal operating current
  360A).
• No calibration of Hall probe.
• Hall probe and driving system were roughly
  aligned to the magnet cryostat.

Really graded!!
Good agreement with calculation!
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Fringe Field

• Fringe field around the photon detector region
  was roughly measured.
• ISC180A, INC180A (50 of full excitation)
• The fringe field in the full excitation can be
  estimated by simply scaling.

• The compensation of the fringe field works very
  well!
• Fringe field is suppressed below 50 Gauss all
  over the photon detector region as designed.

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Summary

• Construction of the COBRA magnet was finished.
• Excitation test was done between Jun.21th and
  Jul.7th.
• Two problems.
• Cold spots in the inner wall of the cryostat.
• Protection heaters in the downstream end-coil are
  broken.
• I300A (83 of normal operating current) reached
  for the SC, but full excitation had to be given
  up because the full excitation w/o protection
  heater is dangerous.
• I400A (111 of normal operating current) reached
  for the NC.
• Field measurement device was completed.
• Test measurement was done in the excitation test.
• Measured magnetic field shows good agreement with
  calculation.
• Precise field measurement will be done at PSI.
• Fringe field around the photon detector region
  was measured.
• Good suppression of the fringe field.

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Summary, contd

• We are now strongly confident in the performance
  of the COBRA magnet.
• Graded magnetic field and small fringe field as
  designed.
• Survived under much more severe conditions
  (DV-900V, DT110K).
• Good mechanical performance up to 300A (no
  strange behavior in strain plots).
• Two problems can be easily fixed after the test.
• More reliable and powerful protection heater
• Reduce the superinsulation layers.

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Schedule