In recent years, rare-earth barium copper oxide (REBCO) pancake coils have shown excellent performances in high magnetic field applications. The no-insulation (NI) winding technique enables to enhance the thermal stability and current density of REBCO pancake coils. Still, further developments of REBCO coils are expected; e.g., the mechanical deteriorations of REBCO tapes caused by electromagnetic forces have been reported. The screening currents are considered as one of the causes to damage the REBCO tapes because of a nonuniform current distribution. Meanwhile, the electromagnetic forces on non-impregnant NI REBCO coils, whose windings can be deformed separately with turns and moved easily, are complicated, and the detailed mechanism has not yet been clarified. Commonly, the circumferential movement/deformation of the winding is considered as a rigid body. In this study, we investigate the stresses and displacements due to the electromagnetic forces on NI REBCO pancake coils in high magnetic field using a current simulation and a 2D elastic finite element analysis in the radial and circumferential directions; i.e., each turn can individually move in the radial and circumferential directions considering the spiral winding structure. The simulation results show the circumferential movement/deformation of windings and the nonuniform hoop stress due to the electromagnetic force.

No-insulation (NI) Rare-Earth Barium Copper Oxide (REBCO) pancake coils can generate high magnetic fields and have the high thermal stability against normal-state transition due to allowing currents to radially bypass a local normal-state region through the turn-to-turn contacts. Despite these merits, no turn-toturn insulation causes a charging delay. To solve this charging delay problem, an NI REBCO coil wound with multi-bundled (MB) conductors was proposed. As an experimental result, a charging delay was improved because the inductance per tape of MB coil is smaller than that of single-tape NI coil. Meanwhile, the current behaviors of MB coils are complicated and has not been clarified. We have investigated the current and thermal behaviors of MB REB-CO coils during charging and against normal-state transition, comparing with and without turn-to-turn insulation. As the result, it is shown that the MB coil can be operated stably, and no pessimistic temperature rise is observed. However, the operating current of MB coils with turn-to-turn insulation may concentrate on one tape when the normal-state transition occurs. Therefore, MB coils without turn-to-turn insulation seems to be desirable.  Index Terms-Charging delay, high magnetic field generation, multi-bundled REBCO tape, no-insulation winding technique.

The no-insulation (NI) winding technique proposed by Hahn on 2011 drastically improves the thermal stability of REBCO pancake coils. Thereafter, several different kinds of NI winding techniques have been proposed, such as the partialinsulation, the metal-as-insulation (MI), the conductive-epoxy-resin-covering (CERC), and the intra-layer NI winding techniques.Recently, we have reported the thermal stability comparison of REBCO coils which these NI winding techniques are applied to.The reduction of contact resistance improves the thermal stability even more.In addition, the multiple REBCO tape bundled (MB) technique was recently proposed. REBCO tapes may have a few defects before winding. When the MB technique applies to REBCO coils, the current can avoid the defect region to transfer into the neighboring REBCO tapes. That is why the technique can improve the charging delay. The MB winding technique is getting attention. We have also evaluated REBCO pancake coils with the MB winding technique comparing them with a conventional NI REBCO pancake coil.