Degradation Characteristics of a 20 T REBCO Insert Magnet After a 4.2 K Standalone Test

A 20-T/17-mm High-Temperature-Superconducting (HTS) magnet was developed and tested, comprising a 15-T no-insulation (NI) REBCO insert (Coil 1) nested in a 5-T metal-insulation (MI) REBCO coil (Coil 2). Coil 1 consists of 20 NI double pancakes (DP), while Coil 2 consists of 28 MI DPs. The 20-T magnet was cooled in liquid helium and charged for five runs, the maximum power supply current reached 228.7 A. Multiple quenches occurred during the charging process, causing the power supply current to drop to zero within one second. The rapid current variation induced a large voltage inside the coil, which damaged Coil 1. After re-warming, Coil 1 was tested in liquid nitrogen, and its critical current was significantly degraded. To investigate the degradation, three representative DPs from Coil 1 were examined using continuous I _c measurement and observed with optical microscope and scanning electron microscope (SEM). Most degradation areas consisted of a bubble with a current breakdown point distributed throughout Coil 1. Probable causes were proposed in this article.