Double Null Merging Start-up Experiments in the University of Tokyo Spherical Tokamak

Full text: University of Tokyo Spherical Tokamak (UTST) is a unique device in the world that demonstrates merging start-up of the spherical tokamak (ST) plasma by using the poloidal field (PF) coils located outside the vacuum vessel. When two ST plasmas merge together to form a single ST, their magnetic field lines reconnect, converting the magnetic field energy to the plasma kinetic or thermal energy and increasing the plasma beta up to 50% within the short reconnection period. The ST merging experiments have been demonstrated in the START/MAST (UKAEA) and TS-3/4 (Univ. Tokyo) devices, by using the PF coils located inside the vacuum vessels. Here, we present the successful generation of the spherical tokamak started-up using the external PF coils outside the vacuum vessel. This double null merging (DNM) method has been successfully performed in UTST. Two initial STs with aspect ratio of 2.4 were formed at the null-points and were merged to form a single ST with aspect ratio of 1.5. The plasma current of 40 kA and the pulse width of 0.6 ms were obtained. After the completion of the plasma merging, the thermal energy reached 80 J (central pressure of 200 Pa), and the heating power of reconnection was as high as 1.7 MW. In UTST, a central solenoid (CS) coil (95 turns, 45 kJ) is installed to improve the plasma parameters for incoming neutral beam injection and radio-frequency heating. In the case that using CS in addition to the DNM discharge, the plasma current and discharge time increased to 90 kA and 1.2 ms, respectively. The thermal energy reached 230 J (central pressure of 400 Pa) after the completion of the merging, and the heating power was as high as 4.5 MW. Note that the effect of CS is included in this power. After the merging completion, the thermal energy continued to increase and reached 650 J (central pressure of 1000 Pa). The DNM method is more reactor-relevant for developing the future fusion reactor. The UTST experiment has developed a useful CS-less start-up scheme, which is world-widely studied in the spherical tokamak research. (author)