Formation of Second-Stable, Ultra-High-Beta Spherical Tokamaks by Use of Field-Reversed Configuration

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Novel CS(center solenoid)-less formations of ultra-high-beta Spherical Tokamaks (ST) have been developed in the TS-3 and TS-4 merging experiment using high power heating of magnetic reconnection. In Type-A merging, two STs were merged together to build up the plasma beta up to β_T【approximately equal】0.5. High power heating of reconnection transformed the low-beta merging STs into a high-beta ST within a short reconnection time. In Type-B merging, an oblate FRC was first formed by two merging spheromaks with .opposing toroidal field, and was transformed into an ultra-high-beta (β-_r【approximately equal】0.8) ST by applying external toroidal magnetic field. The ultra-high-beta STs by Type-B merging were found to have diamagnetic toroidal field and absolute minimum-B type configurations. We made (1) the BALLOO code stability analyzes of the produced STs for the first time and concluded formations of the first-stable / marginally ined ballooning-stable regime was almost consistent with measured high-n instabilities. The unstable STs by both merging formations suffered from high-n magnetic fluctuation and disrupted within a 10-30 Alfven time. Our experimental observation identified the ballooning stability window consistent with 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 (2) normalized betas β_N of thus produced STs as large as 6-17 for comparison with the Troyon scaling and (3) a promising B^2 scaling of the reconnection heating. Those facts indicate that the axial merging is one of the most efficient startup methods for high-beta STs without powerful CS.