Divertor heat flux mitigation and internal transport barrier formation by krypton and argon seeding in KSTAR plasmas

Abstract

Improvement of fusion reactions efficiency and development of optimal operational scenarios for control of plasma heat flux are major challenges for realization of fusion power plants. An impurity gas seeding is considered as one of the promising techniques to achieve this goal. An optimal scenario should be developed considering each species of impurities because the radiative cooling efficiency over electron temperature varies depending on their atomic structures. As much higher radiated power fraction is required in the next generation fusion device, high-Z impurities such as krypton and xenon are expected to be used in a wide temperature region. Therefore, in order to analyze the effects of these impurities on plasma, impurity studies must be carried out based on experiments, which requires investigation of impurity behavior with appropriate diagnostic systems. In this work, the effects of krypton impurity gas seeding in KSTAR plasmas on divertor heat flux mitigation and ion internal transport barrier (ITB) formation are investigated. By krypton seeding, the particle flux into both divertor targets significantly reduced and also the ion ITB with high core temperature was formed. Tomographically reconstructed two-dimensional radiation distributions were measured by an infrared image video bolometer to evaluate the degree of temperature in divertor plasmas and the distribution of krypton in the core plasma. In the radiation pattern from krypton, the temperature near the divertor was significantly reduced and poloidal asymmetry was identified due to the centrifugal force effect caused by toroidal rotation. Using GKW, a gyrokinetic simulation code, the maximum linear growth rate γmax was calculated in the 0.3 < ρtor <0.7 and the results during the H-mode and ITB phase cases were compared with the E×B shearing rate, which is usually described as the mechanism of the ITB formation. Ion temperature gradient mode was found to be the most dominant microinstability in all areas where the calculation was performed and in the wide radial domain 0.3 < ρtor < 0.5, ωE×B exceeds the γmax. The location of the lowest γmax/ωE×B ratio was at ρtor = 0.48 which is near the that the highest krypton ion concentration. In addition, an argon seeding experiment was conducted to demonstrate the compatibility of impurity-seeded detachment and magnetic resonance perturbation (RMP)-driven edge localized mode (ELM) control through magnetic resonance perturbation during the H-mode. Heat and particle fluxes were successfully reduced through argon seeding during ELM control by RMPs, and the localized radiated power near the X-point was observed by the imaging bolometer, which is the sign of divertor detachment. To investigate impurity-seeded scenario, appropriate diagnostics for monitoring behaviors of impurity ions in the plasma is essential. In this study, a processing code for automatic identification of impurity line spectra was developed and evaluated by utilizing the ITER-relevant vacuum ultraviolet (VUV) spectroscopic diagnostic system in KSTAR. For real-time analysis of a spectrum, it is necessary to reduce the noise level caused by high neutron and gamma fluxes reaching the detector such as a charge-coupled device. The code adopts a high-order derivative method that distinguishes relatively spiky noises from spectra. Tests using synthetic spectra showed successful noise reduction of approximately 90%. In addition, an in-situ wavelength calibration algorithm was developed by using representative carbon emission lines (C III and C IV) as markers that appear during the current ramp-up phase in carbon-wall devices. This algorithm is followed by a matching algorithm that enables the annotation of an ionic state of an impurity on the peaks of spectra by referencing the NIST atomic database. From the integrated processing algorithm, simulated VUV spectra were tested and the results showed successful automatic annotation of the spectral lines with noise reduction.