The effect of plasma elongation on the second-stable spherical tokamak (ST)was numerically studied using the experimentally measuredpressure and current profiles of ultra-high beta STs. The maximum beta of ST over 50% was obtained in the TS-3 ST/CT experimentby applying an external toroidal field to an FRC. It was found that the marginal beta for the ballooning instabilityincreased with the plasma elongation κ of ST. The elongated STs with κ > 2 have the magnetic shear (S) -- pressure gradient(α) profiles located in the second-stable regime for the ballooning mode andthe stability margin increased with κ. The close relation between the absolute minimum-B profile andthe second stability was documented. The effect of elongation on maximum beta was observed to saturatewhen κ exceeded 3, indicating that the optimized elongationfor high-beta STs are located around 2 < κ < 3.
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