Three-Fluid Axisymmetric Equilibrium Model and Application to Spherical Torus Plasmas Sustained by RF Electron Heating

Abstract

Radiofrequency (RF) waves could be used for plasma current start-up in spherical torus (ST) reactors, where plasma formation and current drive without the ohmic heating solenoid is required. In such a plasma, the electrons can be represented by two temperature components, i.e. high-temperature low-density electrons and lowtemperature high-density electrons. In order to describe the equilibrium of such plasmas, we develop a three-fluid (two electron fluids and one ion fluid) axisymmetric equilibrium model with toroidal and poloidal flows. This model has been applied for the first time to a recent TST-2 discharge, and we have obtained an equilibrium which is consistent with experimentally observed results. It is found that (1) the toroidal current density and pressure are dominated by the high-temperature low-density electron (eh-electron) fluid and (2) the radial force balance for each fluid species is quite different, i.e. the ion fluid is confined by the electric force due to the negative electrostatic potential while the eh-electron fluid pressure gradient force is balanced by the Lorentz force (its toroidal current density times the poloidal magnetic field). These results are different from previous speculations.