Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
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 ASIPP Operation of cryostat vacuum vessel of HT-7 superconducting tokamak ASIPP 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 ASIPP Operation of cryostat vacuum vessel of HT-7 superconducting tokamak ASIPP 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 ASIPP Vacuum requirements on the HT-7 CVV (I) Heat isolation: The heat load power (in the molecular state), =cPAT 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 ASIPP 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 ASIPP 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 ASIPP 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 ASIPP 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 ASIPP 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 ASIPP 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 1Pam3/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 ASIPP 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) Operation of cryostat vacuum vessel of HT-7 superconducting tokamak ASIPP 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.