Parity Transition of MHD Instabilities in LHD Experiments

This study reviews experimental observations of parity transitions in the mode structures of magnetohydrodynamic (MHD) instabilities in magnetically confined plasmas. The transition between even and odd-parity modes, which is closely related to changes in the topology of a magnetic vessel, suggests the formation and healing of the magnetic island. While the transitions from even to odd parity have been reported in several tokamak devices, recent experiments in the Large Helical Device (LHD) have revealed conventional even-to-odd parity transitions due to various instabilities and a novel transition from odd to even parity. These include transitions associated with energetic ion-driven resistive interchange instabilities, locked-mode-like instabilities, ”Edge” MHD instabilities, and double-odd-parity modes. These findings highlight the need for advanced diagnostics and theoretical modeling to further clarify the mechanisms governing parity transitions.