Experimental Evaluation of the Influence of External Current on the KSTAR TF Quench Detection System

The quench detection system (QDS) for 16 toroidal field (TF) coils of the Korea Superconducting Tokamak Advanced Research (KSTAR) is using Wheatstone bridges to naturally reduce induced voltage in the quench detection circuits. The TF coils have to be quickly discharged to save the superconductors from thermal runaway if quench occurs; on the other hand, the fast discharge brings about many side effects. Thus, malfunction of the TF QDS should be avoided. According to the results of the TF QDS operation in KSTAR campaigns, the single Wheatstone bridges can sufficiently reduce self-induced voltage even though the induced voltage may remain due to unbalancing the bridges by ferromagnetism of the cable-in-conduit conductor (CICC) jackets at small TF coil current. In contrast, mutually induced voltage by central solenoid (CS) and poloidal field (PF) coils may not be sufficiently reduced by using a single Wheatstone bridge comparing voltages across a number of TF coils due to larger electric circuits in the cryostat. On the other hand, a single Wheatstone bridge comparing single TF coil voltages could not effectively reduce the sharp impulse voltage caused by locally induced current on the vacuum vessel at the end of plasma current with vertical displacement event (VDE).