Control of turn-to-turn contact resistivity in resistively insulated REBCO coils
Resistively insulated (RI) REBCO magnets feature short ramp times and low ramp losses while maintaining the advantages of no-insulation coils with high engineering current density and tolerance for defects in the REBCO conductor. Control of the turn-to-turn contact resistivity Rc is key to RI technology. Rc must be sufficiently high to prevent a large transient current, which could result in high mechanical stress during magnet quenches. Meanwhile it must be lower than the quench propagation limit to avoid conductor burn-out during a quench. Therefore, it is critical to control Rc within a suitable range of values which is usually coil specific. Previously, we discovered that Rc between two REBCO tapes with a stainless steel interlayer decreases dramatically with contact pressure cycling by up to three orders of magnitude. This drastic change made it impossible to design a suitable Rc value for a stainless steel co-wound RI magnet. In this work, we first present methods for mitigating Rc pressure cycling sensitivity. We found that by adding conductive fillers, such as conductive paste or epoxy, the Rc load cycling sensitivity is largely mitigated. For dry-wound coils, Rc load cycling sensitivity is mitigated by coating REBCO tape with a layer of 2- 3 um of PbSn solder. In addition, Rc can be controlled by oxidizing the stainless steel co-wind tape by heating stainless steel tapes at different temperatures in air. Using above methods, short sample tests showed that Rc was controlled to prescribed values of 1000 and 5000 uOhm-cm2 and was not sensitive to contact pressure cycling up to 30,000 cycles at 4.2 K. The new Rc control method was applied to a 6 double-pancake test coil which was tested at 4.2 K. The Rc in this test coil was comparable with the short sample results. This demonstrated the ability of this new method to control Rc in large coils.