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

Abstract

A non-relativistic multi-fluid plasma axisymmetric equilibrium model 1 was developed recently to account for the presence of an energetic electron fluid. The equilibrium formulation of a multi-fluid plasma with relativistic energetic electrons is developed and reported in this paper. The formulation is then applied to a four-fluid plasma composed of a relativistic energetic electron fluid, one thermal electron fluid, and fluids of two thermal ion species. This equilibrium model is relevant to the analysis of a toroidal hydrogenic plasma with a dominant impurity species (e.g., carbon) and an energetic electron component (e.g., runaway-like electrons).

Investigation of the effectiveness of non-inductive `multi-harmonic' electron cyclotron current drive through modeling multi-pass absorptions in the EXL-50 spherical tokamak

“…• The model implemented in CQL3D can not calculate current flowing along open field lines outside of the LCFS, which may although be a dominant fraction in experiment as estimated by a different physical model [12]. Increasing density at low plasma temperature scenarios reduces I p to a substantially low value inside the LCFS, but it doesn't affect much the current outside the LCFS, mostly carried by very high velocity electrons being less impacted upon collision with all background plasma species.…”

“…Based on experimental finding, one's intuition could be that the thermal and energetic population of electrons perhaps behave as two separate fluids identity of electrons to maintain momentum balance with ions. This ideology has been implemented in a multifluid equilibrium code to reconstruct flux information, and thereby estimate plasma current in force balance [12]. Such calculation results in a broadened current density profile of energetic electrons in plasma covering both closed and open field line regions as shown in figure 8(b) of reference [11].…”

“…This scenario changes for a new generation low aspect ratio ST which, under purposeful matching of toroidal magnetic field profile and EC wave frequency, contains several ECR harmonics. Such a device named EXL-50, built and owned by the ENN (Energy iNNovation) science and technology development private limited company stationed at Langfang in China, starting its high power operation in the beginning of year 2020 until now, has routinely achieved high current (I p ) drive efficiency (over 1 A/W) in fully non-inductive plasma discharges solely powered by ECRH [11,12]. There are five number of cold plasma ECR harmonics present inside its vacuum vessel if the toroidal field (TF) coil current is set close to 100 kA.…”

Solenoid-free current drive via ECRH in EXL-50 spherical torus plasmas

“…8b shows the current profile and last close flux surface (LCFS) for a 120kA discharge shown in Fig. 2, computed via the multi-fluid equilibrium model [30]. A significant fraction of the plasma current (52% in this case) is flowing outside the LCFS.…”

“…7 shows the orbit analysis for energetic electrons in velocity-energy space. A three-fluid equilibrium reproducing a 50kA EXL-50's plasma was obtained via the multi-fluid equilibrium model [30] for the computation for the guiding-center orbits [31]. A strongly asymmetric distribution in toroidal direction, in the v||, v, and energy space, is obtained, accentuated as the electron energy increases toward the limiting energy of orbit containment.…”