Transcript ASIPP Operation of cryostat vacuum vessel of HT

ASIPP
Operation of cryostat vacuum vessel of
HT-7 superconducting tokamak
Y. YANG, M. SU and HT-7 vacuum group
Institute of Plasma Physics, Chinese Academy of Sciences
December 2005
(For the 15th International Toki Conference, Toki, Japan)
Operation of cryostat vacuum vessel of HT-7 superconducting tokamak
Outline
1. Introduction
2. Inner structure of the HT-7 CVV
3. Vacuum requirements on the HT-7 CVV
4. Pressure evolution of CVV
5. Anomalous pressure disturbance
6. Solutions
7. Summary
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Operation of cryostat vacuum vessel of HT-7 superconducting tokamak
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Introduction
HT-7 vacuum system is divided into two groups, the ‘inner vacuum’ and the
Cryostat Vacuum Vessel (CVV). Focuses of vacuum operation have been put
primarily on the former. During its over 10 year’s operation since 1994, operation
of the device has been pushed towards its limits, while aging effects of the
materials emerge gradually.
A safe and stable operation of this CVV is essential for the experimental runs
because the superconducting toroidal field coil cryostat is contained in this vessel.
Anomalous pressure rises and the corresponding solutions are reviewed to
provide guideline for HT-7 vacuum operation. It’s valuable for the operation of
other superconducting devices, e.g. a whole superconducting tokamak under
construction, EAST.
Operation of cryostat vacuum vessel of HT-7 superconducting tokamak
Inner structure of the HT-7 CVV (I)
Adapted from the HT-7 website. (W.W. XIAO)
Layer
Temperature
CVV shell
Room
temperature
Outer heat
shield
<80K
Cryostat
<5K
Inner heat
shield
<80K
Inner VV shell
<500K
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Operation of cryostat vacuum vessel of HT-7 superconducting tokamak
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Inner structure of the HT-7 CVV (II)
Liquid Helium circuit:
Continuous liquid Helium is pressurized to
flow through the copper tubes holding the
superconducting coils. The utilized
superconducting material, NbTi requires
the coil temperature be lower than 9K for
superconductive operation.
Symposium of HT-7 Physics Programme,
S.H. WANG (1994)
Liquid Nitrogen circuit:
Flowing liquid Nitrogen in the embedded tubes in both
shields keeps the shields at a temperature around 80K.
Small epoxy resin blocks keep these 5 layers of different
temperature from contacting each other for better heat
isolation. High vacuum is kept in the CVV.
Operation of cryostat vacuum vessel of HT-7 superconducting tokamak
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Vacuum requirements on the HT-7 CVV (I)
Heat isolation:
The heat load power (in the molecular state),
=cPAT
in which,
c is the co-efficient; P is the pressure; A is the total area;
T is the temperature difference between the liquid nitrogen shield and the
cryostat.
Heat load is at the level of 1W, at 10-4Pa, while exceeds 100W at 10-2Pa,
which is not acceptable for HT-7 cryogenic system*.
(* Symposium of HT-7 Physics Programme, Y.N. PAN, 1994)
Operation of cryostat vacuum vessel of HT-7 superconducting tokamak
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Vacuum requirements on the HT-7 CVV (II)
Electrical insulation:
Another important requirement of the
cryostat magnet is to prevent the
breakdown through the residual gases.
There are two cryogenic circuits inside the
CVV, liquid nitrogen and liquid helium.
The electrode spacing being 1cm,
Townsend breakdown threshold voltage
for N2 is more than 6kV at 20Pa; while for
He, more than 10kV at 60Pa. Different
references give similar experimental data.
It’s required that the CVV pressure be at
the level of 10-4Pa for heat isolation;
while be lower than 10 Pa for long
enough to avoid Townsend breakdown.
J. Gerhold, Cryogenic 38, 1063-1081, 1998
Operation of cryostat vacuum vessel of HT-7 superconducting tokamak
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Vacuum operation of the HT-7 CVV
Cryostat starts cooling down when
the pressure drops to 10-2Pa, and
the ultimate pressure during the
plasma operation is at the
magnitude of 10-4 Pa.
• HT-7 CVV pump
stations (top)
• Current lead unit
(left)
Operation of cryostat vacuum vessel of HT-7 superconducting tokamak
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Anomalous pressure rise during operation (I)
Reason
Air leak
LN leak
LHe leak
In-vessel
leak
Outgas
Power
loss
Criticality
High
High
Depends
Low
Low
Low
Pmax
Could be
as high as
100 Pa
Depends Commonly <10-2Pa
less than 2E3Pa,
accidentally
could be as
high as 1000
Pa
<10-3Pa
<10-3Pa
Possibility
Very
Less
Less
Likely
Less
Differentiability
Difficult
Difficult Difficult
Difficult
Easy
Easy
Likely
Operation of cryostat vacuum vessel of HT-7 superconducting tokamak
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Anomalous pressure rise during operation (II)
Air leak:
Rubber sealing, bellows, etc., show aging
effect gradually. Vibration during discharges
could lead to air leak in the low T sealing.
Heaters have to be used to keep sealing from
frosting.
Due to the big pumping capacity of the low T
cryostat, the rise of the pressure is slow
commonly. However, it’s not easy to find
small leak owing to the high ratio of He in the
residual gas caused by the Helium leak.
Air leak
Air leak (Apr06,2005)
Finding and solving the leak on time would prevent the vicious circle:
leak-> higher pressure-> poorer heat isolation-> lower T near leak-> bigger leak.
Operation of cryostat vacuum vessel of HT-7 superconducting tokamak
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Anomalous pressure rise during operation (III)
LHe circuit leakage:
There are more than 4.5km superconducting
coils, and hundreds of welding points in the
LHe circuit. The leakage at room temperature
is similar to that of the air leak. Consequently,
the CVV pressure rises corresponding to the
He pressure.
Under present situation, the pressure
disturbance is observed to be at the level of
10-3Pa as long as there is no further damage
caused by arching on the circuit.
Sudden leak of the LHe circuit during the plasma operation is serious problem. It
not only might cause air leak due to the T drop of the sealing, but also might
cause breakdown between the electrodes, which is extremely dangerous.
LN circuit has more robust structure, and effect on the pressure should be slower.
Operation of cryostat vacuum vessel of HT-7 superconducting tokamak
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Anomalous pressure rise during operation (IV)
In-vessel leak:
Outgas:
It happened that there was a leak
between the inner VV shell and
the CVV. Despite the huge leak as
high as 1Pam3/s, effect on the
PCVV was observed to be lower
than 1E-3Pa and only
corresponding to the short
pressure pulses inside inner VV.
Increased gradually, 1E-4Pa pulses.
Loss of power:
The pumping capacity of the cryostat is 2 magnitudes higher than the CVV
pump stations. It’s observed that temporary power loss wouldn’t lead to
pressure rise bigger than 1E-2Pa.
Operation of cryostat vacuum vessel of HT-7 superconducting tokamak
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Solution and scheme
It’s important to discover the anomalous pressure rise and distinguish the
source in time. Therefore, there should be:
• Continuous monitoring; (set points of total and partial pressure)
• In-time leak detecting (differential and local RGA)
It’s crucial to keep the CVV
pressure lower than requirements
for long enough time for solving the
problems.
• Wide-range backup pump
• Interlock with quench system
Characteristic of a pump to be used on HT-7,
by J.G. CHU, accepted by Vacuum Science
and Technology (2005)
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Summary
• After more than 10 year’s operation, the importance of the vacuum operation of
the HT-7 CVV should be recognized.
• The primary risk of HT-7 CVV is the air leak due to the material aging. However, if
it could be handled properly and in time, plasma operation will not be influenced.
• Electrical insulation hasn’t been a problem for HT-7 for over 10 years. But it might
be an important issue for EAST, in which several kVs of voltage might exist inside
the CVV.
•It’s essential for a CVV to have enough and stable pumping capacity before the
pressure threshold for breakdown or heat isolation is reached.
• It’s valuable to have in-time monitoring and set points based on the total and
partial pressure measurements. Interlock with quench protection system should
be useful for superconducting device like EAST.