The role of ExB drifts on COMPASS-D divertor

An additional complication that must be faced in transport descriptions of the divertor plasma are the distortions and asymmetries in divertor profiles. On Compass-D , distortions in the density profile at the divertor target are observed in most discharge phases. These distortions are strongly affected by the toroidal field direction, and for BT>0 appear as a large asymmetry between the inner and outer divertor regions. The high resolution profiles from the divertor probes have proven to be valuable for understanding these distortions due to the complicated spatial structure of the divertor profiles. An interesting observation reported in this paper are the complicated electric field and current patterns observed across the divertor target profiles. Large currents are observed in the divertor region, which change the plasma potential from its current-free value and result in large electric fields across the divertor target. The resulting ExB drift is suggested as a possible mechanism causing the observed distortions. For BT>0, the correlation between ErxB drift direction and the observed distortions is manifestly clear. Furthermore, the magnitude of the pressure imbalance arising from the ErxB drift is in good agreement with the measured pressure asymmetry. For BT>0, both the radial and poloidal components of the ExB drift are directed towards the outer divertor region, explaining the large asymmetry observed for this field direction. In conclusion, it has been shown that divertor target profiles are strongly influenced by drifts and SOL currents, and therefore, these effects should not be ignored in transport analysis of the divertor region. (author)