Developing Control of Cryo-Pump Test Cold-Box System: Some Investigations

Tokamak, a leading contender of practical fusion reactor, is a popular system using powerful superconducting magnets to confine plasma in a toroidal space to conduct controlled thermo-nuclear reactions required for fusion power research. Tokamak uses state-of-the-art superconducting magnet system requiring specialized cryogenic cooling, which is achieved and maintained by a forced-flow of helium at supercritical state. Supercritical helium mass flow rate as high as 2.21 kg/s at the cryogenic temperature of ~ 4.6 K is necessary not only to obtain cryogenic conditions required for these toroidal superconducting magnet system, but also to regulate cryo-conditions against severe thermal shocks and disturbances due to exothermic plasma reactions and other operational variations. In prior work, the largest cryogenic centrifugal pump was developed to meet and obtain such special cryogenic conditions with various normal and off-normal operating modes. However, such special cryo-pump needs to be tested first rigorously in a special, custom-designed cryogenic test set-up before using them in actual Tokamak systems. A dedicated test-system, called Test Cold-Box, is proposed and developed to test such large cryogenic centrifugal pump. The test-system consists of a liquid helium sub-cooler bath, heat-exchangers, control valves, heaters, etc. In our work, a suitable multi-loop control scheme for the Test Cold-Box is also proposed. Classic multi-loop PID control scheme with classic Ziegler-Nichols and modern IMC tuning have been evaluated using ASPEN HYSYS® simulation environment. The paper describes this Test Cold-Box system including cryo-pump along with suitable controls, and its operational analysis in ASPEN HYSYS® simulation environment.