The impact of divertor configuration on the structure of the radial electric field
has been simulated by the B2SOLPS5.0 transport fluid code. It is shown that
the change in the parallel flows in the scrape-off layer, which are transported
through the separatrix due to turbulent viscosity and diffusivity, should result in
variation of the radial electric field and toroidal rotation in the separatrix vicinity.
The modelling predictions are compared with the measurements of the radial
electric field for the low field side equatorial mid-plane of ASDEX Upgrade
in lower, upper and double-null (DN) divertor configurations. The parallel
(toroidal) flows in the scrape-off layer and mechanisms for their formation are
analysed for different geometries. It is demonstrated that a spike in the electric
field exists at the high field side equatorial mid-plane in the connected DN
divertor configuration. Its origin is connected with different potential drops
between the separatrix vicinity and divertor plates in the two disconnected
scrape-off layers, while the separatrix should be at almost the same potential.
The spike might be important for additional turbulent suppression. |