This letter reports on the successful demonstration of the second harmonic electron cyclotron heating (ECH)-assisted startup in the first plasma experiments recently completed in the fully superconducting Korea Superconducting Tokamak Advanced Research (KSTAR) device whose major and minor radii are 1.8 m and 0.5 m, respectively. For the second harmonic ECH-assisted startup, an 84 GHz EC wave at 0.35 MW was launched before the onset of the toroidal electric field of the Ohmic system. And it was observed that this was sufficient to achieve breakdown in the ECH pre-ionization phase, allow burn-through and sustain the plasma during the current ramp with a low loop voltage of 2.0 V and a corresponding toroidal electric field of 0.24 V m−1at the innermost vacuum vessel wall (R = 1.3 m). This is a lower value than 0.3 Vm−1 which is the maximum electric field in ITER. Due to the limited volt-seconds and the loop voltage of the Ohmic power system, the extended pulse duration of the ECH power up to 180 ms allowed the plasma current to rise up to more than 100 kA with a ramp-up rate of 0.8 MA s−1.
Abstract. MHD instabilities in the core and edge of the KSTAR plasmas have been visualized in 2D using an electron cyclotron emission imaging (ECEI) system with sufficient time and space resolutions for the study of the underlying dynamics. In the core region where the ECE optical depth is large, the ECEI has provided localized measurements of fast MHD phenomena such as the crash of internal kinks and coalescence of dual flux tubes. In the edge pedestal region of H-mode plasmas where the optical depth is marginal, the ECEI measurements were found to be still localized and were able to provide detailed 2D images of edge localized modes (ELMs) such as the growth of multiple filamentary structures and the crash of the pedestal characterized by fast localized bursts of the filaments [3]. The effect of electron temperature and density fluctuations on the ECE signals has been analysed to understand the limitations of the edge ECEI measurements.
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