A Metallurgical Inspection Method to Assess the Damage in Performance-Limiting Nb3Sn Accelerator Magnet Coils

TL;DR

This paper introduces a new metallurgical inspection method combining non-destructive and destructive techniques to identify damage causes in Nb3Sn accelerator magnet coils, improving quality assurance and performance reliability.

Contribution

The paper presents a novel, comprehensive inspection approach for Nb3Sn coils, enabling precise root-cause analysis of quenches and coil damage in accelerator magnets.

Findings

Identification of transverse broken sub-elements as quench causes

Effective micro-optical and deep etching techniques for coil analysis

Application to CERN and US coils demonstrating method efficacy

Abstract

The design and production of Nb3Sn-based dipole and quadrupole magnets is critical for the realization of the High-Luminosity Large Hadron Collider (HL-LHC) at the European Organization for Nuclear Research (CERN). Nb3Sn superconducting coils are aimed at enhancing the bending and focusing strengths of accelerator magnets for HL-LHC and beyond. Due to the brittle nature of Nb3Sn, the coil fabrication steps are very challenging and require very careful QA/QC. Flaws in the Nb3Sn filaments may lead to quenches, and eventually, performance limitation below nominal during magnet testing. A novel inspection method, including advanced non-destructive and destructive techniques, was developed to explore the root-causes of quenches occurring in performance-limiting coils. The most relevant results obtained for MQXF coils through this innovative inspection method are presented. This approach allows for precise assessment of the physical events associated to the quenches experienced b y magnet coils, mainly occurring under the form of damaged strands with transversely broken sub-elements. Coil-slice preparation, micro-optical observations of transverse and longitudinal cross-sections, and a deep etching technique of copper will be illustrated in the present work, with a focus on the results achieved for a CERN coil from a non-conforming quadrupole magnet prototype, and two coils fabricated in the US, in the framework of the Accelerator Upgrade Project (AUP) collaboration, from two different non-conforming quadrupole magnets, respectively. The results obtained through the proposed inspection method will be illustrated.