An assessment of axial pre-compression for the KSTAR central solenoid

The central solenoid (CS) of the KSTAR consists of four pairs of superconducting coils compressed axially by preloading structures. The axial pre-compression is needed to suppress the maximum attractive or repulsive force of the coils based on reference operation scenarios. All CS coils were assembled with eight sets of preloading and supporting structures under precise dimensional control. Only shells of preloading structures were heated to make 6 mm gap between the preloading structure and coil assembly. The gap was, then, filled up by wedge movement so that axial pre-compression be applied after thermal contraction of shells. However measured pre-compression is about 55% of the design value. A detailed investigation has been carried out to find reasons for the preloading reduction by comparing strain measurement and structural analysis results. The minimum residual compression is estimated as 6.5 MN. It is expected to have no serious problem during large plasma current H-mode discharge with consideration of the anti-slip design. A structural analysis model of KSTAR CS magnet has been developed based on the measured strains during operations and will give a guide line for future high performance and long pulse operations of KSTAR.