Threshold effects of tungsten impurity accumulation on core ion temperature in EAST tokamak

Abstract The critical threshold of the tungsten impurity concentration on the ion temperature in the core region is experimentally identified and studied in this paper. Sufficient experiments have demonstrated that the effect of tungsten impurity concentration on ion temperature in the core region is steep, exhibiting distinct threshold behavior. In the regime above the threshold, the core ion temperature is significantly lower than that below the threshold. The core ion temperature increases by over 50% with an abrupt phase transition when crossing this threshold, which is around 0.85 MW for radiation power in discharges heated by 3.5 MW NBI (neutral beam injection) and 1 MW LHW (Lower hybrid wave), but is minimally influenced by tungsten impurity concentration either below or above the threshold. The corresponding line-averaged tungsten concentration lies between <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mn>1.5</mml:mn> <mml:mo>×</mml:mo> <mml:msup> <mml:mn>10</mml:mn> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>4</mml:mn> </mml:mrow> </mml:msup> </mml:mrow> </mml:math> and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mn>2</mml:mn> <mml:mo>×</mml:mo> <mml:msup> <mml:mn>10</mml:mn> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>4</mml:mn> </mml:mrow> </mml:msup> </mml:mrow> </mml:math> . Modeling results from HD7 indicate that the likely mechanism involves the impurity drift mode being driven by the presence of tungsten impurity. This mode requires a certain impurity concentration and gradient to be excited and can contribute significantly to ion thermal transport, leading to a reduction in ion temperature. These findings offer new insights into the impact of tungsten impurity accumulation on ion temperature, which is crucial for improving the understanding of the impurity effect on plasma confinement.