Speaker
Description
The US Magnet Development Program is considering REBCO symmetric tape round (STAR®) wires as candidates for the next generation of high-field hybrid magnets for high-energy particle colliders. In high-field dipole magnets, the conductors experience strong mechanical loads transverse to their longitudinal axis. The current-carrying capability of the superconductor may decrease under these transverse loads and eventually degrade due to cracks caused by excessive loading. Therefore, understanding the response of superconducting wires to transverse loading is a crucial factor in the design of high-field accelerator magnets. The mechanical performance of STAR® wires under transverse compressive loads has not yet been fully investigated, and the results of the test campaign will help clarify the load limits of these wires and prepare the ground for possible fabrication improvements. STAR® wires with a diameter of 1.35 mm have been fabricated by AMPeers with four REBCO tapes, where the width of the first three tapes is to 2 mm and the final one is 2.6 mm. The diameter of the copper former is 0.8 mm. To estimate the forces representative of the room temperature prestress that could cause irreversible degradation, STAR® wires are mounted on a dedicated impregnation mold that recreates the conditions experienced by the wires in the grooves of the magnet support structure. For this test campaign, paraffin wax filled with aluminum oxide has been selected as the impregnation material to protect the REBCO tapes from delamination, which could compromise the magnet performance. As a soft impregnation medium, filled wax does not apply a strong pulling force on the tapes. The design of the impregnation mold ensures that the wire is surrounded by a square matrix of filled wax. The critical current of STAR® wire is measured before and after the impregnation procedure at 77 K to establish the baseline performance and asses any potential degradation. Then, after each transverse compressive load is applied to the sample at room temperature, the wire is cooled-down to 77 K, and the critical current is measured to identify the irreversible load limit. The machine used to apply the compressive loads during the test campaign is the Instron 6800 Series. A 3D finite element model is also under development to estimate the stress on the REBCO tapes, and to analyze and compare the experimental data.
