Spontaneous formation of toroidal field reversal and quasi-single helicity state from a paramagnetic pinch

The formation of a quasi-single helicity (QSH) state from a paramagnetic pinch in the Keda Torus eXperiment-reverse field pinch (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 toward 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%≤β≤8%, the plasma first transitions from a double axis (DAx) to a single helical axis (SHAx) state, and eventually returns to the DAx state. Whereas in our simulation setup, the implicitly introduced source of the induced electric field likely differs significantly from those in actual QSH experiments, the demonstrated existence of such an optimal β regime in our simulations 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.