Status of COBRA Magnet

1. Status of COBRA Magnet Wataru OOTANI MEG review meeting July 11th, 2003 PSI Switzerland

2. Construction finished! • Construction was finished! • Excitation test was done between Jun.21th and Jul.7th. Compensation coils Superconducting coil in cryostat Potentiometer Support frame

3. Construction finished!, cont’d Power supply for SC Instrument panel Refrigerator Power supply for NC NC current adjustor

4. 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.

5. 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.

6. 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.

7. DV (central coil) Coil current DV (end coil w/o heater) 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.

8. Quench Test Results, cont’d • 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.

9. 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, DT=110K) • For the NC, I=400A(111% of normal operating current) reached.

10. 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.

11. 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

12. 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

13. Field Measurement Device, cont’d • 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

14. On the axis Radial direction at z=0 Field Measurement Device, cont’d 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!

15. Fringe Field • Fringe field around the photon detector region was roughly measured. • ISC=180A, INC=180A (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.

16. 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. • I=300A (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. • I=400A (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.

17. Summary, cont’d • 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, DT=110K). • 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.

18. Schedule