Formation of Ultra High Beta Spherical Tokamak by Use of Merging Spheromak Plasmas

We formed ultra-high beta spherical tokamak plasmas using both of two spherical plasma merging and ramp-up of external toroidal field.\footnote{Y. Ono et al. Nucl. Fusion 43, 789, (2003).} The merging and reconnection heats ions significantly during magnetic reconnection in TS-3 experiments. The maximum ion temperature $T_{i} \sim$ 250 eV is obtained in two merging spheromaks with counter-helicity and $T_{i} \sim$ 120 eV in those with co-helicity. While the reconnection heating decreases with the external guide toroidal field $B_{t}$, the confinement time of toroidal plasma tends to increase with external $B_{t}$. In order to confine the maximum ion thermal energy, we applied external $B_{t}$ to merging low-$q$ plasmas such as spheromaks with co- and counter-helicity after completion of reconnection, transforming the high-beta low-$q$ toroid to an ultra high-beta ST in TS-3$^{[1]}$ and TS-4. This transformation increases the life times of low-$q$ toroids by fact 2-3. It is noted that the high-beta ST has an absolute minimum $B$ profile with deep magnetic well. With increasing the ramp-up speed of external $B_{t}$, the magnetic well increases, but stays longer in low-$q$ state unstable to MHD modes.