The electron density profile measurement with a fast time-resolution in the KSTAR tokamak

Abstract

A frequency-modulation reflectometer is installed in the KSTAR tokamak to measure the plasma density profile. The reflectometer operates in the frequency range of 32.3–107.3 GHz. A plasma density up to 7 × 10¹⁹ m⁻³ can be measured when the toroidal magnetic field is 2 T. The reflectometer measures the electron density profile with a time resolution of 25 μs for 0.16 s. This fast time-resolution is essential for the study of the transport barrier formation during the L-H transition. In this paper, the measured time evolutions of the sawtooth oscillation, the density increase due to supersonic molecular beam injection (SMBI), and the pedestal relaxation due to an edge-localized mode (ELM) are presented. The spatial profile of slow fluctuations (< 10 kHz) may be extracted from these fast profile measurements. In addition, the dependences of the pedestal structure, such as its width and height, on the plasma parameters are characterized.