Magnetic stress is of particular concern for ultra high field magnets, especially, peak hoop stress of those magnets may be locally concentrated by the screening current stress (SCS). For instance, the Little Big Coil (LBC), which was designed, constructed, and operated by a collborate team led by NHMFL, generated the world record magnetic field of 45.5 T, but the conductor in LBC experienced plastic deformation due to excessive SCS. In this paper, to further analyze details of the mechanical stress behavior of LBC magnet, the effect of friction coefficient and overband radial build on stress alleviation is investigated by the parametric study. First, total current density with screening current is calculated by using the H-formulation together with domain homogenization method. Later, the entire magnet structure is modeled by the 2D finite element method to represent preload and axial interactions between each adjacent single pancake (SP), and 3 turns are assumed to be one engineering turn with frictionless radial turn-to-turn contact. While, friction acts between the coil and spacer in a direction that limits the radial displacement. Finally, we discuss the effects of friction coefficient and overband thickness by sweeping those parameters. Our parametric study implies that both friction and overband radial build can reduce peak hoop stress levels of LBC, however, effect of friction is dominant when overbanding is thicker than 0.5 mm.

The screening current and its relaxation cause the variation of the self- and mutual inductances of REBCO coils—REBCO is one of the high-temperature superconductors. However, most studies of coil voltage analysis on a REBCO magnet, a stack of coils, have reported simulation results assuming invariant self- and mutual inductances so far. Although the conventional assumption of invariant inductances is still acceptable for fundamental coil voltage analyses, it can cause misleading conclusions due to inductive voltage errors when a precise coil voltage analysis is demanded. Hence, here we report a numerical method to calculate screening-current-dependent self- and mutual inductances of REBCO coils for advanced studies based on a lumped-circuit analysis model. In this work, we aim to investigate the inductance variation due to the screening current with a case study and discuss its effects on the coil voltage. We assume that there is a stack of 12 REBCO single-pancake coils. No transverse current in each coil is considered for simplicity. A numerical simulation of the current density in the magnet is performed, and then the inductances are calculated by considering the spatially non-uniform current density due to the screening current. From this case study, we confirm that the self- and mutual inductances are changed by up to 110% and 30% each. It is also confirmed that the discrepancy is notable at the beginning of the charge while marginal at the end. Finally, we discuss the effect of inductance variation on the quench voltage analysis.

No-insulation (NI) high temperature superconductor (HTS) toroidal field (TF) coil or its variations have been regarded as potential options for a compact, high magnetic field tokamak due to compactness in size and robustness in operational reliability. However, its real application still faces multiple challenges, e.g., limited experience in the design, construction, and operation of “D-shaped” NI HTS coils for toroid magnets. Here we report, as the first attempt in Korea, fabrication, test, and analysis of a reduced size D-shaped NI HTS single pancake coil to explore the feasibility of NI HTS technology in TF coil. The D-shaped NI HTS coil is designed to be 1/20 of the Korea Superconducting Tokamak Advanced Research (KSTAR) TF coil in height. One of the three test coils is wound with single HTS tape, and the other two test coils are co-wound with double and triple tapes, respectively. Current charging/discharging tests and sudden discharge tests were conducted for each coil, and the results are analyzed by the well-known lumped circuit model of a typical NI HTS coil. The results show possible variations in the contact resistance during charging and sudden discharge within a coil.

This paper presents a system for real-time monitoring spatial and temporal electromagnetic behaviors of a high temperature superconductor (HTS) coil with the no-insulation (NI) winding technique. We implemented a real-time monitoring system that consists of Hall sensors and voltage taps to measure spatial and temporal magnetic fields, and local voltages. An NI HTS test coil was wound, and the real-time monitoring system was combined with the test coil. An experimental study was performed in a liquid nitrogen bath at 77 K, where the test coil was charged with a constant ramp rate of 0.05 As ^$-1$ up to its operating current of 100 A. To describe measurement results, including local voltages and magnetic fields, we propose a set of simulations for coil voltages and currents, current density including screening current , and field gradient including screening current induced field . Using this set, we successfully simulated spatial and temporal electromagnetic behaviors generated by the test coil to a reasonable extent. The discussion section will provide comparison results between calculations and measurements.

This paper presents numerical studies on the mechanical behavior of dry-wound high-temperature superconducting (HTS) racetrack coil. After calculating the Lorentz force induced from the self-field of the racetrack coil based on the 3D finite element method (FEM), the 2D mechanical stress analysis is performed based on the plane-stress assumptions to mitigate computational loads, and the mechanical behavior including displacement and stress are investigated. Based on analysis model, two following case studies are conducted: 1) analysis of mechanical behavior on various-shaped of racetrack coils having a different ratio of straight section length and semicircular section radius, and 2) analysis of stress reduction effect according to the degree of curvature when slight curvature is included in the straight section. In each case study, the factors affecting the mechanical behavior of racetrack coils are numerically analyzed and their trends are presented. The results obtained in this paper would be useful for the design of dry-wound racetrack coil, including no-insulation coil, applied in a large-scale and high-field application.

This paper presents characteristics analysis results of no-insulation (NI) high-temperature superconducting (HTS) motor with saliency according to motor control schemes. In case of a salient motor, for efficient operation, the commonly-known maximum torque per ampere (MTPA) control method is applied to generate the maximum torque at the given absolute value of stator current by using optimal dq-axes current pair ( $i^{r}_{ds}$ , $i^{r}_{qs}$ ). To figure out whether MTPA control is applicable for NI HTS motor, we applied three control schemes referred from conventional motor control method: 1) MTPA with constant parameters; 2) MTPA with variable parameters; and 3) $i^{r}_{ds}=0$ control. In addition, we proposed MTPA control with exponential function for suppressing radial leak current. The following key transient profiles of 500 kW NI HTS motor according to motor control schemes are presented: 1) dq-axes current; 2) field coil radial leak current; and 3) torque per ampere value.