The Generation of Edge Toroidal Momentum due to Ion Orbit Losses and its Effect on the Scrape-Off Layer
The collisionless ion orbit losses to the target in a single null divertor configuration is proposed as a mechanism for explaining the asymmetries in plasma parameters, such as density and electron temperature, between the strike zones. The latter show strong dependence on the direction of the ion B drift. Ion orbit losses occur from the layer of the order of a banana width inside the separatrix. Because of the anisotropic velocity distribution of the escaping ions, their loss generates a toroidal momentum of the plasma in the direction parallel to the main plasma current. The magnitude of the surface averaged toroidal velocity can become comparable to the ion sound speed. The edge toroidal momentum is transferred to the scrape-off layer (SOL) mainly due to perpendicular viscosity. Additional parallel plasma rotation in the SOL, induced by the influence of the toroidal momentum, compresses the plasma in the divertor branch which represents the ion drift side and increases the plasma pressure there. This process increases the density and recycling and reduces the electron temperature at the ion side. The ion side is located at the inner strike zone when the ion B drift is directed towards the target, and at the outer strike zone for the reversed B drift. The expected influence of the toroidal momentum on the SOL asymmetries qualitatively agrees with the experiment. Therefore, the proposed mechanism can be a candidate to explain the experimentally observed dependence of the asymmetries between the strike zones on the direction of the B drift.