Asymmetric divertor plasma distribution observed in Heliotron J ECH discharges

Abstract An asymmetric divertor plasma distribution observed in the standard configuration of Heliotron J is reported. The divertor plasma profiles were investigated with two Langmuir probe arrays, which were installed at the geometrically up–down symmetric positions, for three different heating schemes of ECH with 53.2 or 70 GHz microwaves. Although the position of the divertor plasma flux was almost consistent with the footprint position of the divertor field lines, the existence of two types of up–down asymmetry was revealed in the divertor plasma density and floating potential profiles. The first type of asymmetry was mainly observed near the boundary to the ‘private region’. This asymmetry seems to be independent of the heating schemes of the toroidal position of the heating source. As the direction of the confinement field was reversed, the feature of the plasma profile on the top array came to appear on the bottom array, and vice versa. This field-direction dependence indicates that the asymmetric B ×∇B drift motion of charged particles might cause this type of asymmetry. The second type of asymmetry was observed in the region away from the boundary and seemed to depend on the heating schemes.