Phase Alignments between MHD Modes Followed by Minor Collapses on TST-2

MHD instabilities followed by minor collapses in the TST-2 plasma are studied. Precursors with toroidal mode numbers n = 1 and n = 2 localized in the plasma's core become phase aligned so that their mode amplitudes strengthen each other at a certain spatial point. During phase alignment, the total mode amplitude on the soft X-ray radiation profile does not show significant change, and a minor collapse occurs but does not terminate the discharge. This behaviour is similar to the results during a non-linear growth phase in three-dimensional MHD simulations [N. Mizuguchi et al., Phys. Plasmas 7, 940 (2000)]. However, predicted pressure-driven modes are not confirmed in these experiments.