First plasma experiment on spherical tokamak device UTST

Abstract

SUMMARYThe UTST (University of Tokyo Spherical Tokamak) device was constructed for the purpose of exploring the formation of ultrahigh-beta ST (spherical tokamak) plasma using the double null plasma merging method. When two plasmas merge together to form a single plasma, magnetic field lines reconnect, and magnetic field energy is converted to plasma kinetic energy, increasing the plasma beta. Merging start-up has been demonstrated in the TS-3/4, START, and MAST devices using coils inside the vacuum vessel, and the TS-3 plasma obtained a 50% beta. In order to demonstrate start-up in a more reactor-relevant situation, UTST has all poloidal field (PF) coils outside the vacuum vessel. The first plasma experiment on the UTST was begun in December 2007. In the results, the plasma obtained 10 kA by using only the outer PF coils and a single ST was generated in the lower area (z = -0.3 to -1.0 m) close to a washer gun. This result suggests that another washer gun on the top of the UTST can allow the generation of ST in the upper area and merging start-up by using outer PF coils.

Centre-solenoid-free merging start-up of spherical tokamak plasmas in UTST

“…Therefore the vacuum vessel should have short penetration time for the magnetic field induced by the outer PF coils. The upper and lower sections of the vessel are made of thin stainless steel with wall thickness of 1.5 mm, whose penetration frequency of about 80 kHz is much higher than the typical frequency of the coil current of about 1 kHz, whereas the middle section is made of thicker conducting walls with thickness of 6 mm [24]. Typical major/minor radii of the merged ST plasma in the UTST are Upper & lower washer-guns 3.3 kV/100 mF TF R ∼ 0.35 m and a ∼ 0.24 m, respectively; the aspect ratio A is about 1.5.…”

“…In this paper, a CS-free merging start-up condition of ST plasmas established in the University of Tokyo spherical tokamak (UTST) device [24][25][26] by using outer PF coils located outside the vessel is presented. In section 2, the experimental setup of the UTST device and its numerical model will be described.…”

Solenoid-free plasma start-up in spherical tokamaks

“…As with outer PF start-up, the effect of the vessel walls on start-up needs to be understood to extrapolate these methods to future devices. This is now being studied in the UTST device [64] at the University of Tokyo.…”