Formation of quasi-single helicity state from a paramagnetic pinch in KTX regime

Abstract

The formation of quasi-single helicity (QSH) state from a paramagnetic pinch in the KTX-RFP regime has been observed in recent NIMROD simulations. The quasi-single helicity state has a dominant helical component of the magnetic field that is known to improve the RFP confinement. For the initial paramagnetic pinch, linear calculations indicate that the tearing mode growth rate decreases with the plasma $β$. The initial QSH state arises from the dominant linear instability of the initial force-free paramagnetic pinch. The plasma's self-organization towards the second QSH state after the relaxation of the initial QSH state is found to depend on $β$. Specifically, when $β<4\%$, the plasma relaxes to an MH state; when $4\% \leq β\leq 8\%$, the plasma first transitions from a double axis (DAx) to a single helical axis (SHAx) state, and eventually return to the DAx state. The existence of such an optimal $β$ regime that is beneficial to the formation and maintenance of the QSH state, suggests an experimental scheme for the QSH formation based on $β$ tuning and control.