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

Ion-orbit loss is considered important to the radial electric fields Er of tokamak edge plasmas. In neoclassical equilibria, collisions can scatter ions onto the loss orbits and generate a steady-state radial current, which may drive the edge Er away from its neoclassical value. To quantitatively measure this effect, an ion-orbit-flux diagnostic [1,2] has been implemented in the axisymmetric version of the gyrokinetic particle-in-cell code XGC [3]. 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 at the edge for an H-mode plasma in the DIII-D geometry. It is found that the steady-state collisional ion orbit loss is not significant enough to drive Er away from its neoclassical value with the given plasma parameters.