In-out asymmetry of divertor temperatures in tokamaks

A 2-D numerical simulation of the edge plasma in ASDEX Upgrade is used to show that above a critical density a bifurcation occurs, leading to strong poloidal in-out asymmetry of the electron temperature in the divertor. At densities well below and above the critical one, the asymmetry is less pronounced. Although drift terms are not included in the model, these findings can explain experimental observations from Alcator C-Mod and ASDEX Upgrade: (a) the clamping of the temperature in the inner divertor after the bifurcation and (b) the large temperature asymmetry immediately above the critical density. The position of the colder divertor leg is determined by a small asymmetry which is amplified by the bifurcation. In the simulation the basic asymmetry is due to different lengths of the flux tubes in the plasma edge. In the experiment the asymmetry effect of different field line lengths is combined with the action of E × B drifts. Comparison of X point top and bottom measurements then produces evidence that the E × B drift effects are dominant. The effect of the difference in field line length is still present in the experiment but E × B produces a stronger effect.