First plasma experiment on spherical tokamak device UTST

The 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.