Contact Resistance Change Due to Air Gap Generation Between Turns of NI REBCO Pancake Coils

Rare-earth barium copper oxide (REBCO) magnets with excellent current characteristics under high temperature and high magnetic field has been developed for various applications worldwide. No-insulation (NI) REBCO coils allow currents to avoid flowing into normal-state transition regions due to turn-to-turn direct contacts, improving the thermal stability on the NI REBCO coils. The turn-to-turn contact resistances on NI REBCO coils are very important for thermal stability; but they change according to the turn-to-turn contact surface conditions. The contact condition is severely deteriorated when REBCO tape surfaces are not in contact. Our previous research pointed out that deformations on small-bore NI insert coils derived from strong stresses make some air gaps under high external magnetic field and high operating current, resulting in increase in the turn-to-turn contact resistances. In this paper, to investigate the air gap behaviors, an electromagnetic and stress simulation considering individual turn movements is conducted. A contact area ratio and a whole contact resistance of an NI REBCO pancake coil is computed. As the result, under the operating current of 50 A, the turn-to-turn contact conditions deteriorate by air gap generation between turns with the increase of the background fields. However, under 10 T, the turn-to-turn contact conditions improve as the operating current increases beyond 50 A. The air gap related with the distribution pattern of
BJR
stress, but not the strength of
BJR
stress.