Numerical Study of Two-fluid Flowing Equilibria of Helicity-driven   Spherical Torus Plasmas
  Two-fluid flowing equilibrium configurations of a helicity-driven spherical torus (HD-ST) are numerically determined by using the combination of the finite difference and the boundary element methods. It is found from the numerical results that electron fluids near the central conductor are tied to an external toroidal field and ion fluids are not. The magnetic configurations change from the high-$q$ HD-ST ($q$>1) with paramagnetic toroidal field and low-$\beta$ (volume average $\beta$ value, ($<\beta>\approx$ 2 %) through the helicity-driven spheromak and RFP to the ultra low-$q$ HD-ST (0<q<1) with diagmagnetic toroidal field and high-$\beta$ ($\beta\approx 18\%) as the external toroidal field at the inner edge region decreases and reverses the sign. The two-fluid effecs are more significant in this equilibrium transition when the ion diagmagnetic drift is dominant in the flowing two-fluid. 
T. Kanki, M. Nagata, T. Uyama