The influence of tungsten impurity accumulation on T-10 ohmic discharges stability

Abstract

During the experimental campaign of 2017-2018 the regularly repeated cyclic process (W-cycle) related to the tungsten accumulation in the plasma center is discovered on the T-10 tokamak with tungsten limiters and lithized conditions. This process occurs as the increase of tungsten density and its neoclassic peaking at the center. This results in the rise of central radiation losses PWrad (0) on W ions. The growth of PWrad (0) causes the flattening of electron temperature profile Te(r), the decrease of current density in the central area and the suppression of saw-tooth oscillations (SO). The latter results in addition peaking of W density and even greater displacement of current from the plasma center and as a result causes a «small» disruption by MHD mode m = 2 that removes heat and main and all impurity particles from the plasma center. The renewal of discharge parameters after «small» disruption accompanied as a rule by repeated accumulation of tungsten in the plasma center and a new «small» disruption. The detected W-cycles can be repeated many times during the discharge and eventually lead to the disruption of the discharge current.