Dynamics of plasma waves at the pedestal collapse in KSTAR plasmas

Abstract

As the entire world gradually increases its energy consumption, its high dependency on fossil fuels causes problems such as the depletion of fossil fuel supplies and increase in carbon emissions. Nuclear fusion, which is free from such problems, is thus the most promising solution for a possible future energy resource. To accomplish nuclear fusion, high temperatures and pressures are necessary to satisfy the conditions for self-sustainable ignition of plasmas. To achieve these conditions, magnetically confined fusion plasma has been studied for several decades, and various operation scenarios have been investigated in fusion plasma experiments. A representative example of such an operation scenario is the high-confinement (H-) mode, which can notably improve temperature, density, and confinement time. In H-mode plasmas, the steep pressure gradient at the edge region (pedestal) causes instabilities known as edge-localized modes (ELMs). ELMs drive quasi-periodic relaxations of the transport barrier, referred to as pedestal collapse, which accompanies magnetic reconnection at the plasma boundary. A pedestal collapse can deteriorate plasma performance and stability. Additionally, particle and heat fluxes are expelled toward the outer region of the plasmas, which can damage plasma-facing components (PFCs) in International Thermonuclear Experimental Reactor (ITER)-size plasmas. Understanding the mechanism of and controlling ELM crashes are essential for future H-mode stationary operations. All studies in this thesis were conducted in the Korea Superconducting Tokamak for Advanced Research (KSTAR) at the National Fusion Research Institute (NFRI). The KSTAR team routinely performed ELMy H-mode operations, and was equipped with advanced diagnostics for measuring the plasma parameters and properties of ELMs. In this thesis, we propose a novel diagnostic system for measuring various plasma waves in KSTAR experiments. In particular, the system can measure radio frequency (RF) emissions at pedestal collapses in KSTAR H-mode plasmas. At the tokamak plasma boundary, the RF ranges from that of an ion cyclotron harmonic wave (<1 GHz) to that of a whistler wave (~GHz). These waves can provide experimental evidence for investigating fast particle dynamics and plasma wave–particle interactions in collapse events. For this purpose, we installed a fast RF diagnostic system in KSTAR, which has sufficient temporal and spectral resolutions (Δf~1 MHz with 1 μs time range) for measuring RF waves at ELM crash events. A fast RF spectrometer consists of two different types of receivers, bandpass filters, amplifiers, and two different types of spectrometers for post-processing RF signals. For the receivers, we utilized traditional RF antennas and mm-wave heterodyne mixer antennas. The bandpass filters and amplifiers enhanced the signal-to-noise ratios of the acquired signals. To handle the RF signals, we applied two different spectrometers: (1) a filter-bank spectrometer, to measure the overall trends of RF activity, and (2) a fast digitizer, to obtain high-resolution RF spectra within short time ranges (0.1 s at 10 GSa/s). Using the fast RF diagnostic system, we observed RF emissions (<1 GHz) with the RF antenna at a pedestal collapse. Comparisons between the RF signals and electron cyclotron emission imaging data confirmed that the plasma status at the outer edge region is strongly correlated with distinct changes in RF emissions in the range of deuterium cyclotron harmonics. With the aid of simulation collaboration, it was discovered that neutral beam injections can intensify fast ion distributions, which can drive high harmonic ion cyclotron emissions, at the edge region. These results suggest that ELM dynamics and collapses can affect fast ion dynamics at the plasma boundary. Afterward, we measured the modulated ECE radiation at the pedestal collapse using mm-wave mixer antennas. These modulations are in the GHz range, which corresponds to the whistler-frequency range. The modulated ECE signals are localized at the edge regions, which can be confirmed based on the second harmonic ECE frequency. Utilizing the spatial localization of ECE radiation, we analyzed the features of the ~GHz modulations. During the pedestal collapse, (1) broadband emissions, with an upper boundary of ~3 GHz, and (2) narrowband emissions, with a bandwidth in the tens of MHz, coexist. Bispectral analysis reveals that nonlinear interactions occur between these two types of emissions, indicating that the two different types of waves can coexist. The decreases in narrowband emission intensity and bispectrum value occur within tens of microseconds, which is clear evidence of rapid energy transfer between the narrowband waves and background particles. Additionally, we confirmed coherent spectra from two adjacent mixer antennas located in the same poloidal plane. Measuring the phase differences of the spectra, we confirm the perpendicular wave number (k_y) in the poloidal plane. A comparison between the measured k_y and dispersion relation of whistler waves verifies that k_y is in the range of a whistler-wave dispersion curve. This result indicates that whistler-frequency waves may contribute to the generation of energetic electrons during a magnetic reconnection.