A 2-D numerical simulation of the edge plasma inASDEX Upgrade is used to show that above a critical density a bifurcation occurs, leading to strong poloidal in-out asymmetry of the electrontemperature in the divertor. At densities well below and above thecritical one, the asymmetry is less pronounced. Although drift termsare not included in the model, these findings can explain experimentalobservations from Alcator C-Mod and ASDEX Upgrade: (a) the clamping ofthe temperature in the inner divertor after the bifurcation and (b) the large temperature asymmetry immediately above the criticaldensity. The position of the colder divertor leg is determined by asmall asymmetry which is amplified by the bifurcation. In thesimulation the basic asymmetry is due to different lengths of the fluxtubes in the plasma edge. In the experiment the asymmetry effect ofdifferent field line lengths is combined with the action of E × B drifts. Comparison of X point top and bottom measurements thenproduces evidence that the E × B drift effects are dominant. Theeffect of the difference in field line length is still present in theexperiment but E × B produces a stronger effect.
E × B flow driven electron temperature bifurcation in a closed slot divertor with ion B × ∇B away from the X-point in the DIII-D tokamak