High-beta characteristics of first and second-stable spherical tokamaks in reconnection heating experiments of TS-3

Since 1986, the centre solenoid (CS)-less formations of ultra-high-beta CT/ST plasmas have been developed in the TS-3 merging experiment using high power heating of magnetic reconnection. In the cohelicity (Type-A) merging, two STs were merged together to build up the plasma beta to β T ≈0.5. In the counterhelicity (Type-B) merging, an oblate FRC formed by two merging spheromaks with opposing toroidal field B t , was transformed into an ultra-high-beta (β T ≈0.8) ST by applying external toroidal field B t . We made the BALLOO code stability analyses of the produced STs and concluded that formations of the first-stable/marginally second-stable STs were obtained by Type-A merging and the second-stable STs by Type-B merging and also unstable STs by both mergings. The ballooning-stable regime calculated from the experimental data was almost consistent with the measured high- n instabilities. The stable regime became larger significantly by increasing the hollowness of current profile and broadness of pressure profile. This paper also addresses normalized betas β N of thus produced STs as large as 6–17 for comparison with the Troyon scaling and a promising B 2 scaling of the reconnection heating. These facts indicate that the axial merging is one of the most efficient startup method for high-beta ST without powerful CS.