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Abstract-TheNo-Insulation (NI) winding technique for NI REBCO pancake coil is expected to improve the dynamic and thermal stability and to enhance the current density. The investigations of the electromagnetic and thermal behaviors are important for the development of NI REBCO coils. Many stability investigations of the NI REBCO coil itself have been carried out by experiments and simulations. However, the detailed behavior of the bypass current between turns has been not shown yet. Although the contact resistivity was obtained through the priori experiment, it included the resistivity of not only contact surface but also the components, i.e., the copper stabilizer and the Hastelloy substrate, of REBCO tape. To investigate the detailed bypass current behavior in this paper, the true contact surface resistivity is taken into account in the simulation. The bypass current on the cross section of NI REBCO tape is simulated using the 2-D FEM. From the simulation results, the influence of the coil-radial resistivity between turns on the turn-to-turn contact surface resistivity is clarified. In addition, the heat loss is also reported, and a simple equivalent circuit of the turn-to-turn contact is proposed.

Abstract-It is important to investigate the stability and behavior of an epoxy-resin-impregnated no-insulation (NI) REBCO pancake coil in order to implement high-field applications, such as ultra-high-field magnetic resonance imaging. We have performed sudden discharging and overcurrent tests for the impregnated NI REBCO pancake coil. From the discharging test, the contact resistivity is estimated, and it changes depending on the initial current. From the overcurrent test, the high thermal stability of the impregnated NI REBCO pancake coil is confirmed. The REBCO pancake coil is charged up to 67 A though the critical current is only 46 A, and no degradation has been found.To investigate in detail the electromagnetic behavior of an impregnated NI REBCO pancake coil, the simulation is performed by means of the partial element equivalent circuit (PEEC) model. In the sudden discharging test, the simulation results for the case of lower initial current are in good agreement with the experimental data. As can be inferred from the simulation results, the current drastically decreases from the inside of the impregnated NI REBCO pancake coil. The result of the overcurrent simulation is almost identical to the experimental one. However, since the contact resistivity is presumed to be constant in the simulation, the difference is observed in the high current region.