Evaluation of the Parallel-Scheme Varistors as Energy-Extraction System for a Test Facility of Superconducting Accelerator Magnet

A varistor is a non-linear $V$-$I$ resistor and its feature helps to extract stored energy of the magnet quickly while suppressing the magnet voltage. High Energy Accelerator Research Organization (KEK) decided to install the varistor for the test facility in order to conduct quench training of a superconducting magnet safely. The new energy extraction (EE) system consists of varistors made of silicon carbide (SiC) disks and is designed to extract energy up to 5 MJ. These varistors were installed prior to the test of the HL-LHC D1 magnet in 2023 and validated throughout the power test. A current imbalance and turn-on time ($t_\text{ON}$) among the varistor units, which could affect performance of the magnet protection, are monitored and measured to be within $\pm \text{4}\%$ and 300 mu s, respectively.