Nonlinear MHD simulation of magnetic relaxation during DC helicity injection in spherical torus plasmas

Recently, the intermittent plasma flow has been observed to be correlated with the fluctuations of the toroidal current $I_{t}$ and $n$=1 mode in the HIST spherical torus device. During the partially driven phase mixed with a resistive decay, the toroidal ion flow velocity ($\sim $ 40 km/s) in the opposite direction of $I_{t}$ is driven in the central open flux region, and the oscillations in $n$=1 mode occur there, while during the resistive decay phase, this flow velocity reverses and results in the same as that of $I_{t}$, and the oscillations in $n$=1 mode disappear there. The purpose of the present study is to investigate the plasma flow reversal process and the relevant MHD relaxation by using the 3-D nonlinear MHD simulations. The numerical results exhibit that during the driven phase, the toroidal flow velocity ($\sim $ 37 km/s) is in the opposite direction to $I_{t}$, but in the same direction as the \textbf{\textit{E}}$\times $\textbf{\textit{B }}rotation induced by an applied voltage. This flow is driven by the magnetic reconnection occurring at the X-point during the repetitive process of the non-axisymmetric magnetized plasmoid ejection from the helicity injector. The oscillations of poloidal flux \textit{$\Psi $}$_{p}$ are out of phase with those of toroidal flux \textit{$\Psi $}$_{t}$ and magnetic energy for the dominant $n$=1 mode, indicating the flux conversion from \textit{$\Psi $}$_{t}$ to \textit{$\Psi $}$_{p}$. The effect of the vacuum toroidal field strength on the plasma dynamics is discussed.