Developing a Vacuum Pressure Impregnation Procedure for CORC Wires

Abstract

Superconducting magnets designed for high energy physics and nuclear fusion require mechanical and electrical integrity to perform at high currents and magnetic fields. Vacuum Pressure Impregnation (VPI), a process of curing epoxy in and around the superconducting wires, is often used to support and consolidate a magnet. However, the heat and mechanical stresses associated with the process can degrade the wires, significantly lowering their critical current. This study explores different methods of potting and curing CORC ® wire with the aim of reducing wire performance degradation to less than 3% measured at 77 K, self-field. The wires were 2.9 mm in diameter consisting of a total of six REBCO tapes (three layers of two tapes). Two bending radii (20 mm and 50 mm) were tested to mimic the winding shape of a magnet. Mix 61 epoxy was used in preliminary tests for potting. For each test, two wires were used, and their critical currents were measured simultaneously in liquid nitrogen at 77 Kin their straight form, then bent, followed by the heat treatment used for Mix 61 but without epoxy and finishing with the full epoxy impregnation test. Later tests were performed using CTD-528 to explore a room temperature cure, limiting possibility of degradation from thermal expansion and prolonged exposure of the REBCO tapes to elevated temperature. Here we report the experimental results with multiple CORC ® wires and different curing schedules. The results obtained are the first steps toward identifying the VPI process with minimum degradation in critical current to be implemented in high-field magnets using CORC ® wires.

An initial magnet experiment using high-temperature superconducting STAR® wires

“…Although the star® wires can be resilient to the mechanical deformation and the resulting strain during the test (figures 7 and 17), we should avoid conductor deformation during magnet operation. Built on the significant experience that the community has gained [10,[64][65][66][67][68][69][70][71][72], a vacuum-pressure impregnation technique should be developed to support individual rebco tapes in a star® wire.…”

Distributed Fiber Optic Sensing to Identify Locations of Resistive Transitions in REBCO Conductors and Magnets

“…The CORC ® wire used in the experiments is 0.178 m long. The details on the CORC ® wire used here can be seen in [18][19] [22]. The wire contains six REBCO tapes commercially available from SuperPower Inc.…”

“…The wire was first bent with a 2 cm radius and an optic fiber was cowound around the wire afterwards. Then the wire and fiber were impregnated with an epoxy (NHMFL Mix 61) and cured for 16 hours at 60 ˚C and 24 hours at 100˚C [22].…”

“…GHz criterion of 20 µV, is 224 A [22]. The raw voltage included a linear voltage component (resistive foot) that is due to the termination joint resistance.…”

Uni-layer magnets: a new concept for LTS and HTS based superconducting magnets

“…In addition to the very high forces imposed in the conductor, when using HTS such as REBCO, and derived wires such as corc ® [21,22], one of the main difficulties is their susceptibility to degradation when wound around relatively small apertures [23,24]. As the use of HTS is normally more advantageous in the high field region of the magnet, the minimum radius of curvature required by the winding is relatively low.…”