Mechanical behavior of the KSTAR central solenoid preloading structure according to the coil current

The central solenoid (CS) magnet system of the KSTAR consists of four pairs of superconducting coils which are stacked with up-down symmetry and supporting structures which can give pre-compression in a vertical direction on the coil stack to sustain the contact between coils. The CS system of the KSTAR was mechanically preloaded with about 7.3 MN at room temperature by preloading structure in order to preserve the structural stability from thermal and electromagnetic loads which can make coil stack misaligned. It has been shown that the CS system stably operated in terms of mechanical behavior during seven thermal cycles from 300 K to 4.5 K and its peak coil current of 9 kA per turn for plasma operation until 2014. However, the performance test of the CS coils at a higher coil current was required for a long pulse operation. In 2014, the CS coils were successfully charged up to its current of 15 kA per turn step by step during the comprehensive commissioning of the KSTAR Motor Generator (MG) system whose peak power is 200 MVA compared with 100 MVA of available grid power. The adverse effect on the structural stability of the CS system was not found although the axial compressive force preloaded on the CS system was gradually released due to the attractive force between the coils which can make coil stack contracted according to the increase in coil current. This paper describes the effect of the CS coil current on the mechanical behavior of the KSTAR CS system as well as the results of the variation of compressive force according to the coil current.