Double null merging start-up experiments on UTST

The University of Tokyo Spherical Tokamak (UTST, Univ. Tokyo) is a unique device in the world that demonstrates merging start-up of spherical tokamak (ST) by using double null merging (DNM) method. Unlike the other plasma merging devices, such as MAST (UKAEA) and TS-3/4 (Univ. Tokyo), the poloidal field coils of UTST are located outside the vacuum vessel, for the purpose of more reactor-relevant merging startup. The goals for the UTST experiment are to generate a high-beta ST through magnetic reconnection, and to sustain it by external heating such as neutral beam injection. Using the present power supply under construction, we successfully demonstrated the DNM start-up whose plasma current and the duration time are 40 kA and 0.6 ms. We increased them to 120 kA and 1.2 ms, respectively, by using a central solenoid coil in addition to the DNM discharge. Our 2D magnetic probe array composed of 290-channel pickup coils directly measured field line reconnection of two merging ST plasmas and its heating power of 1.7 MW using jxB plasma pressure.