In-out asymmetry of divertor temperatures in tokamaks

A 2-D numerical simulation of the edge plasma in ASDEX Upgrade is used to s how that above a critical density a bifurcation occurs, leading to strong p oloidal in-out asymmetry of the electron temperature in the divertor. At de nsities well below and above the critical one, the asymmetry is less pronou nced. Although drift terms are not included in the model, these findings ca n explain experimental observations from Alcator C-Mod and ASDEX Upgrade: ( a) the clamping of the temperature in the inner divertor after the bifurcat ion and (b) the large temperature asymmetry immediately above the critical density. The position of the colder divertor leg is determined by a small a symmetry 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 x B drifts. Comparison of X point top and bo ttom measurements then produces evidence that the E x B drift effects are d ominant. The effect of the difference in field line length is still present in the experiment but E x B produces a stronger effect.