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

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Cobra magnet status
COBRA Magnet Status

  • W.Ootani

  • ICEPP, University of Tokyo

  • MEG experiment review meeting

  • Feb.11 2004, PSI

First excitation test in june 03
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.

First excitation test in june 031
First Excitation Test in June’03

Graded field profile measured at 200A

Design field

Measured field

Problems in first test
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.

Second excitation test
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.

Quench tests
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.

Sqd reaction in quench test



Heater at central coil is fired


SQD reacted at central coil


Protection heaters ON




SQDs reacted at the other coils

SQD Reaction in Quench Test

Voltage change in quench test
Voltage Change in Quench Test

  • Maximum voltage across the central coil of 1200V was observed ~500msec after the quench in the central coil.

Temperature rise in quench test
Temperature Rise in Quench Test

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

Mechanical strength



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

Cobra arrived at psi
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

Excitation test at psi
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

Excitation test at psi1
Excitation Test at PSI

5.6% over excitation was successfully done at PSI.

COBRA seems to survive long journey from Japan.

Excitation test at psi2
Excitation Test at PSI

Good performance was confirmed in quench test up to 360A.

Mechanical strength



Influence of fringing field





< 20mG



< 20mG



< 1-2G


πM3 beam line




Influence of Fringing Field

COBRA fringing field would affect neighboring facilities.

What can we do
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

Effect of iron wall
Effect of Iron Wall

  • 3cm-thick and 5m height soft iron wall

  • Finite element calculation with 3D model

Iron wall


Effect of iron wall1


Without wall

With wall


1.9 Gauss

1.1 Gauss


0.8 Gauss

0.6 Gauss


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.

Active shielding
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

Possible solution
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.


  • 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.