Study of up–down poloidal density asymmetry of high- impurities with the new impurity version of XGCa

Abstract

Addition of multispecies impurity ions to the total-f gyrokinetic particle-in-cell code XGCa is reported, including a cross-verification of neoclassical physics against the NEO code. This new version of the neoclassical gyrokinetic code XGCa is used to benchmark and confirm the previous reduced-equation-based prediction that high-Z impurity particles in the Pfirsch-Schlüter regime can exhibit a significant level of up-down poloidal asymmetry, through the large parallel friction force, and thus influences the radial plasma transport significantly. The study is performed in a plasma with weak toroidal rotation. In comparison, when the impurity particles are in the plateau regime, the up-down poloidal asymmetry becomes weak, with the parallel friction force becoming weaker than the parallel viscous force. It is also found that the linearization of the perturbed distribution function, based on the small poloidal asymmetry assumption, can become invalid. Using the numerical data from XGCa, each terms in the parallel fluid force-balance equation have been analyzed to find that both the main ions and the electrons respond to the poloidal potential variation adiabatically when the high-Z tungsten possesses large poloidal variation. † Email address for correspondence: jdominsk@pppl.gov

An analytic model for the collisional transport and poloidal asymmetry distribution of impurities in tokamak plasmas

“…It is proportional to the ion temperature gradient and drives an inward pinch. We note that a significant contribution of the electrostatic potential asymmetry to the poloidal distribution of highly charged impurities was also outlined in PIC simulations with the XGCa code [37]. The amplitude of this effect decreases at low collisionality, and becomes subdominant as soon as a sufficiently large external source of poloidal asymmetry is considered, for example, when the ion Mach number exceeds few percents.…”

“…In addition to this horizontal asymmetry driven by rotation and ICRF heating, a small residual asymmetry of the electrostatic potential is spontaneously driven by collisions, and this residual is particularly important in the 'natural' case. This asymmetry is essentially vertical [36], as found with particlein-cell (PIC) simulations [37] and with NEO [32]. An analytic derivation by Wong et al [38] will be used here.…”

Electromagnetic total-f algorithm for gyrokinetic particle-in-cell simulations of boundary plasma in XGC

“…5. XGC has been extensively benchmarked against other codes, e.g., the drift-kinetic code NEO for neoclassical and impurity transport [6][7][8] , and the gyrokinetic codes GENE 9 , GTC 10 , GEM 11 , ORB5 12 , and EUTERPE 13 for nonlinear turbulence.…”

Improving Gyrokinetic Field Solvers toward Whole-Volume Modeling of Stellarators

“…The unstructured meshes are generated based on the flux function [6]. The whole-volume simulation is useful to investigate core-edge coupling phenomena such as L-H transition [7] and impurity transport [8]. The kinetic modeling of edge plasma considers anisotropic plasma flux, edge plasma turbulence [9], X-point orbit loss [10] and neutral recycling processes [11,12].…”