Comparative study between flowing two-fluid and single-fluid equilibria of spherical torus plasmas

A two-fluid equilibrium with small but non-zero two-fluid parameter ε is regarded as a singular perturbation of a single-fluid (MHD) equilibrium. The reduction from the two-fluid equilibrium to the MHD one was an open problem for several years, but was recently solved by Hameiri¹ and Guazzotto². We have solved the problem by clarifying the relationship between electrostatic potential and total enthalpy. The relationship is derived from the appropriate selection of reference physical quantities for normalization that electrostatic potential (electric field) become relatively large, and the nearby-fluid model is required to eliminate the 1/ε singularities appearing in the Ampere’s equation and the combined Bernoulli equation. The flowing two-fluid equilibria observed in the HIST experiment have been reconstructed, and have the diamagnetic toroidal field in the central open flux column and the poloidal flow velocity with zonal flow like structure. In the toroidal ion flow velocity, the parallel flow along the magnetic field and the Hall effect are dominant. These can be almost cancelled, because their radial profiles are similar, and their signs are different each other. Therefore, the radial profile of the toroidal ion flow velocity is similar to that of the ExB drift velocity.