Simultaneous measurements of plasma parameters and blob characteristics at the far-SOL region using a hybrid probe in KSTAR

Abstract

We developed a hybrid probe comprised of an electrical probe and optical probe to investigate the properties of the far scrape-off layer (SOL) plasma and the blob in Korea Superconducting Tokamak Advanced Research (KSTAR). The hybrid probe is attached to a horizontal manipulator, and the radial profiles of plasma parameters such as electron temperature, ion flux, and the Dα signals are measured with time resolution (100 μs) by varying the radial position (0.15–0.4 m away from the last closed flux surface (LCFS)). In ELMy H-mode discharge, the blob radial velocity is obtained from the time delay between the start of the peak Dα signal and the peak electrical probe current and the corresponding radial position. The obtained blob radial velocity is in good agreement with the radial velocity of the filament obtained using the two radially separated electric probes at the far-SOL in the KSTAR. The profiles of the blob radial velocity for five discharges are obtained depending on the radial position in similar experimental parameters. The blob radial velocity is estimated as 44.1–250 m/s and 25.4–135 m/s at the radial position of 0.15 m (near the LCFS) and 0.4 m (near the outer wall), respectively. It is observed that the blobs are propagated to the outer wall with various radial velocities, and the radial velocity slows down as they get closer to the outer wall. The hybrid probe measurement found that the blob properties are about 2 to 10 times larger than the background plasma, and these show the same trend with the blob radial velocity. The hybrid probe is expected to contribute to the analysis and understanding of far-SOL plasma research in fusion reactors.