Observation of REBCO delamination in the resistive insulation nested coils

TL;DR

This paper reports the observation of delamination in REBCO coated conductors used in high-field magnet coils, highlighting a key mechanical challenge and its implications for future superconducting magnet designs.

Contribution

It provides detailed postmortem analysis of REBCO delamination in resistive-insulation nested coils, a novel investigation into a critical failure mode.

Findings

Delamination caused resistive transitions at lower currents.

Chemical etching and microscopy revealed delaminated sections with low critical current.

Delamination poses a significant challenge for high-field REBCO magnet reliability.

Abstract

The REBCO coated conductor has the potential to be widely used in ultrahigh field magnets. It is well known, however, that it is not mechanically strong against delamination in the direction normal to its surface due to its intrinsic layered structure. Therefore, conductor delamination is one of the major design challenges for REBCO magnet coils. As a part of the development of the 40 T all-superconducting magnet at the National High Magnetic Field Laboratory, USA (NHMFL), a dry-wound resistive-insulation-nested-coils (RINC) was designed to reach 25.8 T. It used surface-treated stainless-steel tape as a co-wind to control the turn-to-turn contact resistance, and was fabricated and tested in a liquid helium bath. During the test, two of the double pancake modules exhibited resistive transitions at a current significantly lower than the designed value. The postmortem inspection of the REBCO conductor of these modules by reel-to-reel magnetization at 77 K found sections of very low critical current. Further investigations of one section by chemical etching, visual inspection, and electron microscopy revealed that conductor of this section was delaminated. We present the detailed findings of these postmortem characterizations. The implication of this type of delamination for future magnet designs will be discussed.