Spherical tokamak generation and merging on UTST using ex-vessel poloidal field coils only

We successfully generated two spherical tokamaks(ST) at two null points by using ex-vessel poloidal field(PF) coils only and succeeded in merging them. This scheme is called as a double null merging(DNM) scheme. First, two pairs of ex-vessel PF coils generate two null points where a poloidal field is zero at the upper and lower regions inside the vacuum vessel. Then a poloidal flux swing generates two STs at two null points, because a distance to a wall along the magnetic field is long at the null points. Finally, the coil currents push two STs toward the mid-plane and merge them into a single ST. Since a magnetic reconnection during merging transforms a magnetic energy into a thermal energy, this merged ST plasma is expected to have high beta. It must be noted that the DNM scheme generates a ST without a center solenoid coil. The DNM scheme was demonstrated on TS-3/4(JAPAN) and MAST(UKAEA). However, these devices have all PF coils inside the vacuum vessels, and initial plasmas were generated around PF coils, not null points. Since the internal coils are not feasible in a fusion reactor due to a high neutron flux, it is important to demonstrate the DNM scheme by using the ex-vessel PF coils.