W.Ootani

1
COBRA Magnet Status

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

2
First Excitation Test in June03

• 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 June03
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 ?T and ?V below the acceptable level.
7
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
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.1213, 2003.
• Placed in SLS hall for the initial test before
  the installation in pE5, 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
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 pE5 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
pM3
18
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 pM3 to compensate earth field.
• Details of effect are being investigated.

Active shielding in LTF of pM3
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 pM3 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
  pM3 can cancel COBRA fringing field. It has to be
  tested after the installation of COBRA in pE5.
• We plan to measure the fringing field around pE5
  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 pE5 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.