Effect of cross-field drifts on flows in the main scrape-off-layer of DIII-D L-mode plasmas

Abstract

The flow velocities of deuterons and low charge-state carbon ions have been measured simultaneously in the main scrape-off-layer (SOL) in low-density plasmas in DIII-D, and the dependences of these flow fields on the direction of the cross-field drifts (E × B and B × ∇B) have been investigated. These measurements were taken poloidally localized in the SOL region vertically opposite the divertor X-point. The carbon ion flows do not necessarily match those of the deuterons either in the direction with respect to the magnetic field lines or in magnitude, suggesting that physics effects apart from entrainment play a significant role in the impurity response. In configurations with the ion B × ∇B drift towards the divertor X-point, the parallel-B deuteron velocities at the plasma crown are high (−20 to −30 km s−1 in the direction of the high field side (HFS) divertor), while they are nearly zero in configurations with the opposite B × ∇B drift direction. The flow direction of singly and doubly charged carbon ions is independent of the ion B × ∇B drift direction, and the ions flow at approximately −5 to −10 km s−1 towards the HFS divertor. Simulations with the UEDGE code have been carried out to better understand the underlying physics processes. Inclusion of cross-field drifts in the simulations produced divertor solutions for density and temperature that agree significantly better with measured divertor parameters. These simulations do not, however, reproduce the measured flow fields at the crown for the configuration with the ion B × ∇B drift towards the divertor X-point. The UEDGE code has also been used to understand the influence of pumping at the HFS divertor plate, and a poloidal dependence in the radial transport coefficient.