Practical study of optimizing the cool-down scenario for the KSTAR helium refrigeration system

The cooling of superconducting (SC) magnet system for fusion device is the first procedure of the experiment. The Korea Superconducting Tokamak Advanced Research (KSTAR) device is fully SC tokamak that is made of NbTi and Nb3Sn. The KSTAR SC magnets are composed of 16 Toroidal Field (TF) coils and 14 Poloidal Field (PF) coils with 300 tons of cold mass. A Helium Refrigeration System (HRS) with 9kW@4.5 K was installed and has been operated since 2008 to cool the SC magnets and keep all magnets at appropriate temperature conditions. This paper describes and analyzes the operational result of HRS during the KSTAR cool-down in 2008 and 2013. According to the cool-down analysis, the larger cryogenic helium flow from HRS made more efficient cooling of SC magnets. Furthermore, the study of cool-down optimization based on practical approach is presented and the build-up of control logic by using of cool-down optimization is also discussed.