Effects of collisional ion orbit loss on neoclassical tokamak radial electric fields

TL;DR

This study quantifies how ion orbit loss influences the radial electric field in tokamak edge plasmas, using gyrokinetic simulations, revealing a mild effect in a DIII-D H-mode plasma and suggesting further research for ITER conditions.

Contribution

First implementation of an ion-orbit-flux diagnostic in gyrokinetic simulations to measure collisional ion orbit loss effects in tokamak edge plasmas.

Findings

Ion orbit loss current balances ion pressure gradient in the edge.

Radial electric field is only mildly affected by orbit loss in the studied plasma.

Diagnostic validation through core plasma collisional relaxation study.

Abstract

Ion orbit loss is considered important for generating the radially inward electric field $E_r$ in a tokamak edge plasma. In particular, this effect is emphasized in diverted tokamaks with a magnetic X point. In neoclassical equilibria, Coulomb collisions can scatter ions onto loss orbits and generate a radially outward current, which in steady state is balanced by the radially inward current from viscosity. To quantitatively measure this loss-orbit current in an edge pedestal, an ion-orbit-flux diagnostic has been implemented in the axisymmetric version of the gyrokinetic particle-in-cell code XGC. As the first application of this diagnostic, a neoclassical DIII-D H-mode plasma is studied using gyrokinetic ions and adiabatic electrons. The validity of the diagnostic is demonstrated by studying the collisional relaxation of $E_r$ in the core. After this demonstration, the loss-orbit current is numerically measured in the edge pedestal in quasisteady state. In this plasma, it is found that the radial electric force on ions from $E_r$ approximately balances the ion radial pressure gradient in the edge pedestal, with the radial force from the plasma flow term being a minor component. The effect of orbit loss on $E_r$ is found to be only mild. How ion orbit loss will affect $E_r$ in the full-current ITER plasma pedestal is left as a subsequent study topic.