COBRA Magnet Status

1. COBRA Magnet Status • W.Ootani • ICEPP, University of Tokyo • MEG experiment review meeting • Feb.11 2004, PSI

2. First Excitation Test in June’03 • Summary • First excitation test was carried out in Japan last June. • The test was not completed because of some problems. • Some of the quench protection heaters were burned. • Cold spots on the cryostat inner wall. • Superconducting magnet was tested up to 83% excitation. • Compensation coils were successfully tested up to 110% excitation. • Field measurement device was tested. • Field profile in the bore and fringing field around photon detector region were roughly measured.

3. First Excitation Test in June’03 Graded field profile measured at 200A Design field Measured field

4. Problems in First Test • Protection heaters were burned. • All heaters were replaced with larger heaters and back-up heaters were added. • Cold spots on the cryostat wall • Too thick super Insulation layers in narrow gap of the cryostat. • Radiation shield cylinder was slightly displaced. The problems were quickly fixed and the second excitation test was carried out last August.

5. Second Excitation Test • The second test was done in Japan last August. • The magnet was successfully tested up to 380A • (5.6% higher than the operating current, 360A) • No cold spot • No protection heater was broken.

6. Quench Tests • Quench propagation observed by voltage taps, temperature sensors and superconducting quench detectors (SQDs). • Severest test: heater quench test at central coil at 360A. • Quench induced by firing a heater in the central coil, which is the farthest coil from the refrigerator. • DC OFF and quench protection heater ON after the quench is detected. Quench was propagated in the magnet fast enough to keep ΔT and ΔV below the acceptable level.

7. Time[msec] -130 Heater at central coil is fired 0 SQD reacted at central coil +54 Protection heaters ON +74 DC OFF +(150-200) SQDs reacted at the other coils SQD Reaction in Quench Test

8. Voltage Change in Quench Test • Maximum voltage across the central coil of 1200V was observed ~500msec after the quench in the central coil.

9. Temperature Rise in Quench Test • Temperature was peaked at 110K in the central coil 16sec after the quench occurred.

10. 360A 380A Mechanical Strength • Strains in the central coil and support cylinder measured up to coil current of 380A. • Fairly linear relation between strain and I2 Sufficient mechanical strength

11. COBRA arrived at PSI • COBRA arrived at PSI Nov.12&13, 2003. • Placed in SLS hall for the initial test before the installation in πE5, which is planned this April. • System check after the transportation was carried out last December and no serious problem was found. Power supply, compressor, mapping machine, etc Main body

12. Excitation Test at PSI • COBRA is placed at Axis34-36 in SLS. • Excitation test was done between Jan.17-27,2004. • Full excitation for SC and 8% excitation for NC because of limited utility at SLS. COBRA in SLS hall MEG magnet team

13. Excitation Test at PSI 5.6% over excitation was successfully done at PSI. COBRA seems to survive long journey from Japan.

14. Excitation Test at PSI Good performance was confirmed in quench test up to 360A. Mechanical strength Voltage Temperature

15. Position Brequirement BCOBRA GPS(πM32) < 20mG 2~3G LTF(πM32) < 20mG ~1G μLAN(πE3) < 1-2G 4~5G πM3 beam line ? 5~10G COBRA Influence of Fringing Field COBRA fringing field would affect neighboring facilities.

16. What Can We Do? • COBRA is placed inside shielding box • Strong EM interaction bw/ shield and COBRA • Destroy field suppression around photon detector • Beam time sharing • The beam lines are supposed to be used all the time. • Iron walls between πE5 and neighboring beam line (passive shielding). • Active shielding for each device in neighboring beam line Not possible Not possible

17. Effect of Iron Wall • 3cm-thick and 5m height soft iron wall • Finite element calculation with 3D model Iron wall πM3

18. Position Without wall With wall GPS(πM32) 1.9 Gauss 1.1 Gauss LTF(πM32) 0.8 Gauss 0.6 Gauss μLAN(πE3) 4.8 Gauss 2.6 Gauss Effect of Iron Wall • Some effect (25-45% reduction), • Not sufficient especially for GPS and LTF • B field at this level can be distorted easily by surrounding materials not only in direction but also in strength Difficult to predict what actually happens.

19. Active Shielding • Cube shape active shielding composed of six compensating coils. • B field in any direction can be canceled. • Two settings of coil current are necessary corresponding to two states of COBRA magnet (ON & OFF). • More efficient and much lighter. • This type of active shielding is already working in GPS and LTF in πM3 to compensate earth field. • Details of effect are being investigated. Active shielding in LTF of πM3

20. Possible Solution • COBRA Magnet has only two states (ON and OFF). • COBRA field will be highly stabilized within 0.1%. • Active shielding is already working in GPS and LTF of πM3 to compensate earth field. • What is reasonable solution? • Active shielding with two settings of compensating coil current corresponding to COBRA ON and OFF. Passive shielding can be added if necessary. • Possibility that existing compensating coils in πM3 can cancel COBRA fringing field. It has to be tested after the installation of COBRA in πE5. • We plan to measure the fringing field around πE5 after the installation.

21. Summary • The second excitation test of the COBRA magnet was successfully performed last August in Japan after fixing the problems in the first test. • The magnet was tested up to 380A(5.6% higher than normal operating current). • Good quench propagation and mechanical performance were observed. • The magnet was transported to the PSI. • The excitation test was carried out in SLS hall between Jan.17 and 27 and the magnet was successfully tested up to 380A. • Field mapping study is starting in SLS hall and COBRA is planned to move to πE5 this April. • COBRA is going to be used in various tests (LXe, timing counter,...) this year and final field measurement will be done after arrival of BTS around at the end of this year. • Reasonable solution to fringing field problem might be a combination of active and passive shielding. Further investigation needed.