Quantitative measurements of ion orbit loss from gyrokinetic simulations

Ion-orbit loss is considered to have important impact on the radial electric fields Er in the tokamak edge. In neoclassical equilibria, collisions can scatter ions into the loss orbits and generate a steady-state radial current, which may drive the edge Er away from the confined-region neoclassical value without orbit-loss. To quantitatively measure this effect, an ion-orbit-flux diagnostic [1,2] has been implemented in the axisymmetric version of the total-f gyrokinetic particle-in-cell code XGC [3,4]. The validity of the diagnostic is demonstrated by studying the collisional relaxation of Er in the core plasmas. Then, the ion orbit-loss effect is numerically measured in the edge plasmas in the DIII-D geometry. It is found that the effect of the collisional ion orbit loss is more significant for an L-mode plasma compared to an established steady H-mode plasma.