Two-fluid low-collisionality equilibrium model and application to spherical torus plasmas

Abstract

A two-fluid equilibrium model with low-collisionality is developed including a new flow-singularity condition. This description is applied to the rapidly rotating, high-performance National Spherical Torus eXperiment (NSTX) [Peng et al., Plasma Phys. Controlled Fusion 47, B263 (2005)]. The model replicates the primary features of an example equilibrium, such as the profiles of electron and ion temperatures, density, and toroidal flow of an example equilibrium. This is the first full-two-fluid computation of two-dimensional equilibrium with rapid ion flow near the thermal speed. In consequence several important features emerge: (1) the ion toroidal current exceeds the toroidal plasma current as a result of electron rotational flow reversal; (2) the ion flow in the core region is roughly parallel to the magnetic field, i.e., the ion fluid in the core is nearly force free; (3) E+u(i) x B differs considerably from zero, so that the E x B drift fails to describe the ion and electron flows perpendicular to the magnetic field. Simpler models of equilibrium, such as the static equilibrium or the one-fluid flowing model, cannot replicate these properties. These results suggest that the rapidly rotating, high-performance NSTX plasma may represent a new parameter regime of fusion plasmas.