Experiments on trace argon impurity transport in L-mode discharges were performed on Korea superconducting tokamak advanced research (KSTAR) with electron cyclotron resonance heating (ECH). Ar emission was measured by soft x-ray (SXR) arrays and vacuum UV (VUV) diagnostics. A significant reduction in the core Ar emissivity was observed with core ECH. The reduction was the largest with on-axis heating and became smaller with outward heating positions. The diffusivity and convection velocity of Ar were obtained by analysis of the SXR data with the SANCO impurity transport code for the on-axis ECH and the non-ECH shots. In the on-axis ECH case, both diffusivity and convection velocity increased. Furthermore, the convection changed its direction from inward to outward in the plasma core (r/a < 0.3), resulting in a hollow profile of the total Ar density. Together with the reduction in the SXR signals, the hollow impurity profile in the core and the reversal of the convection velocity consistently confirm that ECH can reduce impurity accumulation in the core region. Neoclassical impurity transport and linear stability of micro-turbulence were calculated and discussed in relation to the possible transport mechanism.

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Experiments with a small amount of Ar gas injection as a trace impurity were conducted in the Korea Superconducting Tokamak Advanced Research (KSTAR) H-mode plasma (= 2.8 T, = 0.6 MA, and = 4.0 MW). 170 GHz electron cyclotron resonance heating (ECH) was focused along midplane with fixed major radial position of = 1.66 m and scanned its power from 0 to 800 kW. The emissivity of the Ar 16+ (3.949 Å) and Ar 15+ (353.860 Å) spectral lines were measured by the X-ray imaging crystal spectroscopy (XICS) and vacuum UV (VUV) spectrometer, respectively. The peak emissivity of Ar 15+ was reduced by ECH, an effect largest with 800 kW compared to 600 kW of ECH. The Ar 16+ emission increased with higher heating power. The ADAS-SANCO impurity transport code was used to evaluate the Ar transport coefficients. It was found that the inward convective velocity found in the plasma core without ECH was decreased with the ECH, while diffusion remained approximately constant resulting in a less-peaked Ar density profile. Theoretical results from the NEO code suggest that neoclassical transport is not responsible for the change in transport, while the microstability analysis using GKW suggests a dominant ITG mode during both ECH and non-ECH plasmas.

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