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 βT≈0.5. In the counterhelicity (Type-B) merging, an oblate FRC formed by two merging spheromaks with opposing toroidal field Bt, was transformed into an ultra-high-beta (βT≈0.8) ST by applying external toroidal field Bt. 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 βN of thus produced STs as large as 6–17 for comparison with the Troyon scaling and a promising B2 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.