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In high temperature superconducting (HTS) transformer with double pancake windings, the perpendicular component of leakage magnetic flux density (B/sub r/) applied to HTS tapes of double pancake windings is larger than that of solenoid windings, which acts as a cause to decrease the critical current in HTS tape and increase AC loss. In this paper, in order to reduce the B/sub r/ applied to HTS windings, the use of ferrite flux diverters is presented. Flux diverters are located in between the primary windings and the secondary windings, and must be made of low-loss materials to minimize the heat. And electromagnetic analysis of a single-phase 1 MVA 22.9 kV/6.6 kV HTS transformer with flux diverters in HTS windings was accomplished. The variation of B/sub r/ applied to the secondary windings and magnetic flux density in flux diverters were analyzed by changing their relative permeability of flux diverters respectively. Suitable relative permeability of flux diverters to reduce B/sub r/ applied to HTS windings was selected, considering flux diverter losses by increment of B in flux diverters.

The central solenoid (CS) magnet system of the Korea Superconducting Tokamak Advanced Research (KSTAR) device consists of four pairs of segmented CS coils and a CS coil structure. The maximum repulsive force between CS coils is about 12 MN. The functions of the CS structure are to apply preload on the CS coils and to support the repulsive force between CS coils during operation. The designed axial compression of 15 MN at 4.5 K will be applied partly by assembling the preload structure at room temperature with preload of about 13.4 MN and partly by the thermal contraction difference between the CS coils and the structure during cool down. Additional preload will be given by minute adjustment of wedges. The structural analysis of the CS magnet system has been performed to verify the CS structure design reliability.

Several types of Lorenz forces and Joule heating generated in the magnet system of the Korea Superconducting Tokamak Advanced Research (KSTAR) device have been studied numerically and analytically. We have examined 225 plasma equilibrium states to calculate the maximum magnetic forces and to determine which equilibrium state(s) generate such forces. The obtained results are used as input for structural analysis. A precompression should be applied to the eight-segmented central solenoid (CS) coil assembly to prevent free motion of each coil due to attractive and repulsive forces during operation. We have also evaluated the maximum values of the vertical and lateral forces for each of the CS coils and poloidal field coils and also for the entire CS coil stack. The in-plane force due to toroidal field (TF) coil charging and the out-of-plane force due to interaction of the TF coil current with the poloidal field have been computed. The Joule heating on the TF structure due to plasma disruption has also been calculated.

The toroidal field (TF) magnet system of Korea Superconducting Tokamak Advanced Research (KSTAR) device consists of 16 superconducting coils enclosed in steel cases. The TF cases are wedged along the inboard straight legs to sustain in-plane centering forces. The inter-coil structures contain adjustable shear keys and conical bolts to provide pre-loading in toroidal direction and to resist in-plane and out-of-plane forces, which are the most critical loads on the TF magnet system. For effective cooling of the TF case, a pad-type cooling channel has been designed to ensure structural and thermal stability. In order to investigate the structural integrity and to increase the structural reliability of the KSTAR magnet system, structural analyses have been conducted. From the analysis results, it has been found that the TF magnet structure can safely withstand the reference scenario operations.

The Korea Superconducting Tokamak Advanced Research (KSTAR) device is a steady-state-capable experimental fusion device with a fully superconducting magnet system, including toroidal field (TF) coils, central solenoid (CS) coils, and poloidal field (PF) coils. The major design consideration of the magnet system is to meet the KSTAR mission with plasma current of 2 MA and toroidal field of 3.5 T at the major radius 1.8 m and z=0. The preliminary analyses show that the magnet structure design has mechanical, electrical, and thermal stability during operation. The TF magnets have a wedged structure, including coil cases, inter-coil structures, and inter-octant joints. The CS and PF structures are designed to support the electromagnetic forces. To support the coil system against gravity and lateral loads, gravity support and lateral load structures are designed.