Two-Fluid Equilibrium Considerations of Te/Ti » 1, Collisionless ST Plasmas Sustained by RF Electron Heating

Abstract

A solution of two-fluid (electron and ion), axisymmetric equilibrium is presented that approximates solenoidfree plasmas sustained only by RF electron heating that are recently studied in TST-2, LATE, QUEST. These plasmas indicate presence of orbit-confined energetic electrons carrying substantial toroidal current outside the last closed flux surface (LCFS); Te/Ti » 1 and low collisionality at modest densities within LCFS; and likely a positive plasma potential relative to the conductive vacuum vessel. A system of nonlinear second-order partial differential and algebraic equations constraining six functionals of poloidal magnetic flux or canonical angular momentum are solved. An example plasma measured in TST-2 is used to guide, by trial and error, the selection of these functionals to find appropriate solutions, while assuming peaked plasma profiles and 60% toroidal current within the LCFS. The numerical equilibrium obtained indicates a substantial ion toroidal flow and electrostatic potential so that the ion ∇pi, centrifugal, and electrostatic forces of nearly equal magnitudes combine to balance the Ji × B force, differently from the massless electron fluid that satisfies ∇pe = Je × B. The calculated properties suggest additional measurements needed to refine the choices of the functional forms and improve the two-fluid equilibrium fit to such plasmas.