Transcript COBRA Magnet Status W.Ootani MEG experiment review meeting

COBRA Magnet
Status
W.Ootani
ICEPP, University of Tokyo
MEG experiment review meeting
Feb.11 2004, PSI
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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.
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First Excitation Test in June’03
Graded field profile measured at 200A
Design field
Measured field
3
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.
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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.
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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.
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SQD Reaction in Quench Test
Time[msec]
-130
0
+54
+74
+(150-200)
Heater at central coil is fired
SQD reacted at central coil
Protection heaters ON
DC OFF
SQDs reacted at the other coils
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Voltage Change in Quench
Test
Maximum voltage across the central coil of
1200V was observed ~500msec after the quench
in the central coil.
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Temperature Rise in Quench
Test
Temperature was peaked at 110K in the central
coil 16sec after the quench occurred.
9
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
360A
380A
10
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.
Main body
Power supply, compressor,
mapping machine, etc
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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
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Excitation Test at PSI
5.6% over excitation was successfully done at PSI.
COBRA seems to survive long journey from
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Excitation Test at PSI
Good performance was confirmed in quench test up to 360A.
Mechanical strength
Voltage
Temperature
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Influence of Fringing Field
COBRA
Position
Brequirement
GPS(πM32)
< 20mG
LTF(πM32)
< 20mG
μLAN(πE3)
< 1-2G
πM3 beam line
?
BCOBRA
2~3G
~1G
4~5G
5~10G
COBRA fringing field would affect neighboring facilities.
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What Can We Do?
COBRA is placed inside shielding box
Not possible
Strong EM interaction bw/ shield and COBRA
Destroy field suppression around photon detector
Beam time sharing
Not possible
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
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Effect of Iron Wall
3cm-thick and 5m height soft iron wall
Finite element calculation with 3D model
Iron wall
πM3
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Effect of Iron Wall
Position
Without wall
GPS(πM32) 1.9 Gauss
LTF(πM32) 0.8 Gauss
μLAN(πE3) 4.8 Gauss
With wall
1.1 Gauss
0.6 Gauss
2.6 Gauss
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.
18
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
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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.
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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.
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