Thermodynamic Analysis And Design Of The Helium Refrigeration System For HT-7U Superconducting Tokamak

Large scale superconducting magnets and cryogenic systems have been widely used in the particle accelerators and nuclear fusion experimental devices all over the world. The HT-7U tokamak is an advanced steady-state nuclear fusion experimental device to be built at the Institute of Plasma Physics, Chinese Academy of Sciences. The toroidal field (TF) magnets and poloidal field (PF) magnets of HT-7U are all made of NbTi/Cu cable in conduit conductor (CICC). The total cold mass of the superconducting magnets is about 170 tons, and they operate at 3.8 K and are cooled with forced flow supercritical helium.Superconducting magnets are cooled by a helium refrigerator. According to the heat loads calculation, the basic capacity of the helium refrigerator is 1050W/3.5K + 200W/4.5K + 13 g/s LHe + (13～25) kW/80K,which has a safety factor of 1.5。The refrigeration is mainly used for the cooling of superconducting magnets, current leads and thermal shields. This paper presents the thermodynamic analysis and design of the refrigeration system for HT-7U to get high exergy efficiency, to improve the reliability and to be enough flexible for all the operation modes.A helium refrigerator may have different arrangements for the last cooling stage. Their thermodynamic efficiencies are compared in this thesis. For the Joule-Thomson refrigeration cycle, the optimum state of operation is achieved. Based on the exergy analysis method, this paper discussed the irreversible losses of the helium refrigeration cycle and pointed out how to define the temperature differences of all the cooling stages and how to improve the cycle efficiency.This paper presents the process of thermodynamic analysis with sub-atmospheric stream and multiple expanders helium refrigeration cycle via the founded of a general thermodynamic analysis mode. The objective function with independent variables for the cycle optimization is reasoned out. The corresponding optimum method is presented as well. According to the method, the optimization was performed on the HT-7U helium refrigeration cycle, which gives out the thermodynamic parameters of all of the state points and the design parameters for refrigeration cycle. The exergy efficiency of HT-7U helium refrigerator reaches 21.8%. The thermodynamic analyses for the different operation modes are also discussed. The design of the main equipment and components of cryogenic system, including helium screw compressors, oil removal system and drier, plate-fin heat exchangers, turbines and control valves, are discussed. The design and selection results for all of the components are also presented. Special discussion are given on the design of the oil removal system and plate-fin heat exchangersThe cold components in the HT-7U device consists of TF coil windings, TF coil case and intercoil structure, PF coils, supports, buslines, current leads and thermal shields and need to be cooled by the refrigerator. This paper discussed the design of the cooling loop based on the required cooling temperature, helium pressure and mass flow rates. The friction factor of HT-7U CICC were tested. The test results were described and pressure losses of the HT-7U conductors were calculated. The cooldown and safety protection of the cryogenic system after quench of superconducting magnets are also discussed. Finally, all the operation modes and their transitions between them are presented in this paper.