Development of a Quasi-Steady Equilibrium Field System for Plasma Merging ST Startup Experiments on the UTST Device

Abstract

A novel equilibrium field (EF) system has been developed for use in the merging start-up of spherical tokamak (ST) plasma in the University of Tokyo Spherical Tokamak (UTST) device. Since the UTST device utilizes external poloidal field (PF) coils to form two STs on the top and bottom of the device by huge induction voltages, it is necessary to decouple the slow EF coil system from the fast-swing PF coils. A new EF system using electric double-layer capacitors was constructed and evaluated through the merging start-up experiment in the UTST. Using a very long duration of the EF current waveform and a thick magnetic shield successfully reduced the induction voltages from the PF coils and prolonged the equilibrium time constant.