Assessment of KSTAR Nb3Sn superconducting magnet property after long term operation since 2008

KSTAR has been operated using Nb₃Sn superconducting magnets since 2008, which are capable of generating high magnetic fields and are ideal for high-performance plasma research. The KSTAR PF superconducting magnets have undergone repeated thermal cycling over many campaigns (16 cycles) and have been subjected to significant mechanical and thermal stress during over 30,000 plasma experiments conducted in pulse mode. To monitor any changes, a performance test is conducted on the PF1UL magnets before each campaign. These tests annually apply 4 kA and 15 kA currents, with heat generation steadily decreasing until 2024, where it has now stabilized. The KSTAR PF coils were designed to handle up to 25 kA. However, they typically operate in the range of ± 15 kA during the plasma experiments. In 2012, a 25 kA current test was conducted for the first time following the upgrade of the magnet power supply and the motor generator. Subsequently, a second 25 kA evaluation was performed in 2024 to investigate potential degradation of superconducting magnet. In this paper, we compare the thermal load on the PF1UL magnet at 25 kA charging current between 2012 and 2024 and analyze AC losses to confirm changes in its properties.