The helium refrigeration system (HRS) of the Korea Superconducting Tokamak Advanced Research (KSTAR), which was designed to provide an energetic equivalent cooling power of 9 kW at 4.5 K, has been operated successfully during the last three years since 2008. For the operation of KSTAR, all of the cold components, such as the superconducting (SC) magnets, magnet structure, thermal shields, SC buslines, and current leads, were cooled down to the aimed cryogenic temperature by generated helium in the HRS. The supercritical helium (4.5 K and 5.5 bar) of 300 g/s was delivered for cooling of the SC magnet (poloidal field (PF) magnet: 14; toroidal field (TF) magnet: 16) andmagnet structure and was circulated by the PF and the TF circulators. Thermal loads due to magnet operation are removed at the thermal damper installed in the distribution box of the HRS. Moreover, the pressure change in the magnet will directly affect the circulators. In particular, the pressure drop of the PF circulator drastically changed due to the PF magnet operation depending on the plasma generation scenario. In the 2010 campaign, the operating range of the PF magnet expanded from +/- 4 to +/- 10 kA, and the plasma scenario was improved. Accordingly, the mass flow rate of the PF circuit was increased from 350 to 369 g/s in order to achieve cryogenic stability. However, the operating range of the PF circulator exceeded the allowable range during shot #2526 and shot #3878. Nevertheless, the circulator successfully operated without any significant fault. The operating plan of the PF magnet that will be further expanded in the operating range may cause damages to the circulator continuously. Consequently, the influence of PF operating scenarios on the PF circulator was analyzed to investigate the safety margin of the HRS.
