Improvement of the Quench Detection System for the PF Coils of JT-60SA

The JT-60SA superconducting coil system comprises 6 Equilibrium Field coils, 4 Central Solenoid modules, and 18 Toroidal Field coils. Detecting a quench correctly in superconducting coils is essential to avoid damage of the coil. During the last integrated commissioning test, spike-shaped voltage fluctuations were observed on the EF and CS quench detection (QD) voltages. The voltage fluctuations occurred when the currents in the EF and CS coils rapidly changed. Following a circuit analysis of the EF coils, it was found that the spike voltage originated at the input of the QD differential amplifier from a slight mismatch of the time constants of the resistance-capacitance (RC) noise filter circuit. The RC filter circuit had to be adjusted precisely to achieve the same time constant between the winding and the pickup coil. The reduction of spike noise was experimentally confirmed by adjusting the variable capacitor or resistor in the filter circuit.