Ion Loss as Intrinsic Momentum Source in Tokamaks

Measurements of D$^+$ parallel velocity at the DIII-D edge are consistent with the kinetic loss of thermal ions as the mechanism for edge momentum generation. Edge velocity profiles exhibit a co-$I_p$ peak velocity of 40-60~km/s in OH, L- and H-mode. The flow layer acts as a robust boundary value not affected by NBI injection. D$^+$ velocity measurements are compared to a first-principles, collisionless, kinetic model predicting the existence of a loss-cone distribution in velocity space resulting in a co-$I_p$ directed velocity. A fine $E_r$ structure, found by probes, has 10-20~kV/m peaks in the scrape-off layer (SOL) and LCFS and when incorporated in the kinetic model, results in: 1) $\sim$30\%-50\% increase in the peak parallel velocity over the zero field case and, 2) broadened rotation profile into the SOL. The model-data agreement shows this mechanism is important, competing with pre-sheath acceleration and Pfirsch-Schluter drives. Computations with XGC0, a full-f particle-in-cell drift-kinetic solver with collisional kinetic ions and electrons, and NEO, a drift kinetic code with multiple species and linearized F-P collisions confirms the relevance of the ion orbit loss the impact of kinetic effects on $E_r$ and the measured C$^{6+}$ and D$^+$ velocities inside the LCFS.