Siwon Jang

Control of core argon impurity profile by ECH in KSTAR L-mode plasmas

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.

Design and fabrication of a multi-purpose soft x-ray array diagnostic system for KSTAR

A multi-purpose soft x-ray array diagnostic system was developed for measuring two-dimensional x-ray emissivity profile, electron temperature, Ar impurity transport, and total radiation power. A remotely controlled filter wheel was designed with three different choices of filters. The electron temperature profile can be determined from the ratio of two channels having different thickness of Be layer, and the Ar impurity concentration transport can be determined from a pair of Ar Ross filters (CaF(2) and NaCl thin films). Without any filters, this diagnostic system can also be used as a bolometer.

Development of a particle injection system for impurity transport study in KSTAR.

A solid particle injection system is developed for KSTAR. The system has a compact size, compatibility with a strong magnetic field and high vacuum environment, and the capability to inject a small amount of solid particles with a narrow injection angle. The target flight-distance of 10 cm has been achieved with a particle loss rate of less than 10%. Solid impurity particles such as tungsten and carbon will be injected by this system at the midplane in KSTAR. The impurity transport feature will be studied with a soft X-ray array, a vacuum ultra-violet diagnostic, and Stand Alone Non-Corona code.

Installation of soft X-ray array diagnostics and its application to tomography reconstruction using synthetic KSTAR X-ray images

Four-array system of soft X-ray diagnostics was installed on KSTAR tokamak. Each array has 32 viewing chords of two photo-diode array detectors with spatial resolution of 2 cm. To estimate signals from the soft X-ray radiation power, typical ne, Te, and argon impurity line radiation profiles in KSTAR are chosen. The photo-diodes were absolutely calibrated as a function of the incident photon energy in 2-40 keV range with a portable X-ray tube. Two-dimensional Te image properties by multi-energy method were simulated and visualized with six combinations of beryllium filter sets within the dynamic range of signal ratio.