Contact resistance between two REBCO tapes: the effects of cyclic loading and surface coating

Abstract

No-insulation (NI) superconducting REBCO magnets have advantages of self-quench-protection, a very high engineering current density and high mechanical strength, and the potential to reach very high magnetic fields. However, NI REBCO magnets have drawbacks of a long magnet charging time and high field ramp losses. These can be mitigated by controlling the turn-to-turn contact resistivity (Rc). In an effort to control Rc, we consider two approaches. One is coating a REBCO conductor with various resistive thin films, and the other is to use a stainless steel (SS) tape as an interlayer which is also coated with different metallic films. We present experimental results of Rc of an as-received sample under cyclic contact pressure of 2.5–25 MPa up to 30 000 cycles. After an initial increase in Rc for the first 10–20 cycles, Rc decreases to about one tenth of its initial value after a few hundred cycles. A warm-up and cool-down thermal cycle does not significantly change the low Rc resulting from a previously high number of load cycles. We also studied Rc of REBCO tapes that are coated with different resistive layers and interlayers. In order to increase Rc, we experimented with electro- and electroless plating of Ni, Cr, and Ni–P. We also measured Rc with a thin metallic interlayer as a coil co-winding material which included Cu, SS, and SS plated with Ni and Cu. A SS interlayer increases Rc by about three orders of magnitude; while the Cu plated SS interlayer only increases Rc by one order of magnitude. Finally, we treated the as-received REBCO surface by oxidation using an Ebonol® C solution. This controlled oxidation allowed the Rc to be controlled over a wide range.