Thermal Performance Analysis Of Cryogenic Systems For Cooling Of Superconducting Magnets At 4 K Temperature Level

Abstract

Cryogenics is the science and technology of temperatures below 123 K, involving the design and development of systems and components which produce, maintain and utilize low temperatures. Present day applications of cryogenic technology are widely varied, both in scope and magnitude. One of the areas which involve cryogenic application is in fusion devices. Cryogenic system in fusion research tokamak integrates many components, i.e., heat exchangers, valves, cold circulating pumps, cold compressor etc., in various configurations for the cooling of superconducting (SC) magnets like Toroidal Field (TF), Poloidal Field (PF) and Central Solenoid (CS). Helium refrigerator/liquefier (R/L) serves as a source of cold power for the cryogenic cooling of magnets at 4 K temperature level. However, normally the cryogenic cooling to the SC magnets are accomplished indirectly using the secondary circuit by the use of cold circulating pump, which circulates the super-critical helium in closed circuit and rejects the heat from SC magnets to the Liquid Helium (LHe) bath which is maintained at ~4 K temperature level by the helium (R/L). This arrangement provides flexibility for the operation of SC magnets, which operates in pulsed manner, and still establishes stable operation for the helium (R/L). The LHe bath temperature is maintained using the cold compressor by achieving desired pressure in the LHe bath. There are various configurations that are possible for LHe bath and cold compressor arrangements, i.e., there is a common LHe bath for all SC magnets or individual bath for each SC magnet with either individual cold compressor or common cold compressor for each bath. The objective of the optimum configuration is to maintain the SC magnets below the critical temperature despite pulse heat load nature of fusion research tokamak. Thermal system modeling and analysis of the different cryogenic cooling configuration reveals the optimum configuration satisfying the main function of cryogenic cooling of SC magnets with required thermal performance. The modeling of the cryogenic system is done using ASPEN HYSYS 7.1 software, and is a very useful tool for checking various configurations and observing the effects of the changes on the whole system. The modeling of the cryogenic system is based on helium which is first done in steady state simulation and then in dynamic simulation. The steady state simulation result is verified with that of the analytical values calculated from HEPAK. The dynamic simulation for various system configurations was conducted using the software.