Two-Fluid Equilibrium Considerations of <i>T</i>_e/<i>T</i>_i≫ 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); T e /T i 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 ∇p i , centrifugal, and electrostatic forces of nearly equal magnitudes combine to balance the J i × B force, differently from the massless electron fluid that satisfies ∇p e = J e × 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.

Non-inductive plasma start-up experiments on the TST-2 spherical tokamak using waves in the lower-hybrid frequency range

“…A study to identify the mechanism of this flow switch is under way. A two-fluid equilibrium formulation was developed recently to describe this type of equilibrium, consisting of ions with relatively low temperature and electrons dominated by the energetic component represented by high temperature so that T e /T i 1 [7]. In this model, orbit-confined energetic electrons carry substantial toroidal current outside the last closed flux surface (LCFS), as inferred earlier using different models [8,9].…”

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

“…To solve the equation for the magnetic flux function (Step 1 in Appendix A), we use the experimentally observed magnetic flux data on this boundary. Effects of the eddy currents flowing in various structures are excluded from the flux data [16], so the current which is allowed to flow inside the boundary is only the plasma current [8].…”

“…Such a technique is author's e-mail: frclab@cocoa.ocn.ne.jp crucial especially for spherical torus (ST) plasmas in order to realize a commercial fusion reactor. Recently a full two-fluid equilibrium model [3,4] was applied to describe a solenoid-free RF sustained ST plasma [8]. In the past equilibrium reconstruction of such a plasma has been carried out using the Grad-Shafranov formalism [9,10].…”

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

“…Initially, equilibrium reconstruction of such a plasma has been carried out using the Grad-Shafranov approach [2,3]. Then, a full two-fluid equilibrium model [7,6] was applied to describe a solenoid-free RF sustained ST plasma [13]. 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.…”