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.

Four-fluid axisymmetric plasma equilibrium model including relativistic electrons and computational method and results

“…Estimate of the 2 nd term of in Eq. (40a) using the non-relativistic three-fluid model 1 suggests that it is much less than the 1 st term . Therefore, we treat the 2 nd term as correction term including .…”

“…As dimensionless form is convenient in numerical computation we adopt dimensionless variables hereafter. The primary scales are (1) a reference length of a plasma, (2) a reference plasma current and (3) a reference density.…”

“…3,4 Until recently we have described these electrons using nonrelativistic fluid model. 1 Here we develop axisymmetric equilibrium formulation of relativistic multifluid plasma model and apply the formulation to four-fluid equilibrium.…”

Experimental study of the characteristics of energetic electrons outside LCFS in EXL-50 spherical torus

“…It is therefore appropriate, as a first approximation, to describe these plasmas using a multi-fluid equilibrium model that includes a high-energy electron component in addition to a low-energy electron and ion component. In order to describe the equilibrium of such type of plasmas, some models have been developed [3][4][5]. A three-fluid (two electron fluids and one ion fluid) axisymmetric equilibrium model with toroidal and poloidal flows was developed and first applied to the TST-2 spherical tokamak [3].…”

A simulation environment to simulate lower-hybrid-wave-driven plasmas efficiently

“…However, a certain part of the electrons being accelerated by the RF wave has higher temperatures compared to bulk electrons, i.e., there are low-density high-temperature electrons and high-density low-temperature electrons. To that end, Ishida et al [5] developed for the simulation of collisionless ST plasmas sustained by strong RF electron heating a 3-fluid approach including MHD ordering and neglecting the gyro-viscous cancellation. The plasma considered here consists of high-density low-temperature electrons, low-density high-temperature electrons, and highdensity low-temperature ions [8].…”