The critical point for the onset of divertor energy flux asymmetry in tokamaks

The heat flux to the divertor target plates in tokamaks is typically unbalanced. Several theories have been proposed to explain this asymmetry based on physical effects, such as the E x B drift, which produce a definite direction for the asymmetry. New mechanisms, which do not produce a preferred direction, are also shown to generate asymmetric heat fluxes spontaneously. The critical point for the onset of the asymmetric solution is calculated analytically and compared with numerical solutions. The two principle mechanisms for spontaneous asymmetry generation are divertor radiation and the parallel current flow in the scrape-off layer. Electrically floating the divertor target plates eliminates the parallel current drive and increases the radiated power fraction required to reach the critical point for the onset of an asymmetric heat flux. (author). 19 refs, 15 figs