A seminar by Choongki Sung
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Fast ions, produced by fusion reactions and external heating, play a crucial role in achieving self-sustained plasmas for fusion energy production. At the same time, they can drive plasma instabilities, making a detailed understanding of fast ion physics essential for future fusion reactors. In addition, growing attention has been directed toward the potential of fast ions to improve plasma performance through turbulence suppression. In line with this trend, the FIRE (Fast Ion Regulated Enhancement) mode, an internal transport barrier (ITB) discharge characterized by a high fast ion fraction, has been discovered in KSTAR. In this study, we employ gyrokinetic simulations to investigate the influence of fast ions on turbulence within the ITB region of a FIRE mode discharge. Our results show that the turbulent energy flux is substantially reduced by the inclusion of fast ions in both ion scale and electron scale simulations, in qualitative agreement with experimental observations. However, the underlying physical mechanisms are distinct between the two scales. At the ion scale, the reduction in energy flux is primarily attributed to dilution effects associated with fast ions. In contrast, at the electron scale, the dominant mechanism is the enhancement of the Shafranov shift induced by fast ions. In addition, we observe a long wavelength mode that becomes destabilized in the presence of fast ions. This mode is identified as a fast ion driven kinetic ballooning mode (KBM). Although this KBM does not directly contribute to the reduction of turbulent energy flux in this case, it generates strong zonal flows, suggesting a potential indirect role in turbulence suppression. This presentation will provide a detailed analysis of the gyrokinetic simulation results, elucidating the comprehensive effects of fast ions on turbulence in FIRE mode discharges.
Choongki Sung is an associate professor in the Department of Nuclear and Quantum Engineering at KAIST. Before joining KAIST in 2020, he worked at Lam Research Corp. as a process engineer from 2018-2020, where he developed etching processes for semiconductor manufacturing. He also worked at UCLA as a postdoctoral researcher from 2015 to 2018. During this time, he developed a diagnostic for turbulence measurements in magnetized high-temperature plasmas relevant to nuclear fusion, i.e., fusion plasmas, and studied the effects of turbulence on the confinement of the fusion plasmas. He received his B.S. and M. S. degrees from Seoul National University in 2008 and 2010, respectively, and his Ph.D. degree from MIT in 2015. His main research focus involved turbulence and transport in fusion plasmas. He has concentrated on plasma physics research for fusion energy production, particularly in studying turbulence behavior and its effects on macroscopic instability and overall fusion plasma performance through measurements, data analysis, and modeling studies using simulations. He has also explored alternative fusion concepts to find a shorter pathway to a commercial fusion reactor compared to conventional fusion reactor concepts. Furthermore, he is expanding his research portfolio from fusion to other plasma applications, such as plasma processing for semiconductor manufacturing and neutron source development.
