Experiences on vacuum conditioning in the cryostat of KSTAR tokamak

Abstract Korea Superconducting Tokamak Advanced Research (KSTAR) device has been successfully operated for the plasma experiment from KSTAR 1st campaign to 4th campaign. The main pumping system for the cryostat has to maintain the target pressure below 1.0 × 10 −4  mbar at room temperature and 1.0 × 10 −5  mbar at extremely low temperature for the plasma experiment against the air leak coming from ports of vessel and/or the helium leak from cooling loops in the cryostat. No leak has been detected at room temperature. Unexpectedly, the cold-leak appeared in the cryostat at temperature around 50 K during the cool-down in the KSTAR 2nd campaign. We carefully analyzed the characteristics of detected cold leak because it can cause the increase of the base pressure in the cryostat. After the cool-down, the leak detection was performed to locate the position and size of the leak by the pressurizing the loops. As a result, it is found that the cold leak was located at cooling loops for PF/CS coils and CS structure. Nevertheless, the vacuum inside the cryostat was well maintained below 6.0 × 10 −8  mbar during the entire operation period. The impact of the He-leak in present status on the plasma operation is negligible. However, we have found that the leak rate increases as a function of time. Therefore careful monitoring on cold-leak is an important technical issue for the operation of superconducting tokamak.