There is a layer of steel on the bottom that provides the structure for the tape and a layer of silver on top of the REBCO ceramic layer for electrical connecti

There is a layer of steel on the bottom that provides the structure for the tape and a layer of silver on top of the REBCO ceramic layer for electrical connection, but the whole tape is coated in copper which is the secondary conductor. When superconducting, the tapes, which are wound in a coil, carry the current and the copper acts as an insulator between tapes, but if it stops superconducting, the copper can become a resistive conductor which helps prevent quenching.