Gunsu S. Yun

Gunsu S. Yun
Pohang University of Science and Technology | POSTECH · Department of Physics

PhD

About

199
Publications
19,213
Reads
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2,159
Citations
Citations since 2017
88 Research Items
1426 Citations
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2017201820192020202120222023050100150200250
2017201820192020202120222023050100150200250
2017201820192020202120222023050100150200250
Introduction
My research on high-temperature magnetized plasma includes (1) development of advanced plasma diagnostic tools and (2) non-normal hydrodynamics of magnetized plasmas with strong flows. My research on high-pressure (> 1 atm) plasmas includes (1) source technology, (2) development of a suite of diagnostic techniques, (3) transport phenomena in plasmas with high Coulomb-coupling constant, and (4) bio-medical and industrial applications.
Additional affiliations
September 2012 - present
Pohang University of Science and Technology
Position
  • Professor (Assistant)
Description
  • - High-pressure plasma sources and diagnostics - Wave-particle interaction, phase transition in high-density plasma - Advanced diagnostics for magnetized fusion plasma - Non-steady state solutions to magneto-hydrodynamic system
June 2009 - August 2012
Pohang University of Science and Technology
Position
  • Professor
Description
  • - Imaging diagnostics for magnetized high-temperature fusion plasmas based on mm-wave technology - Dynamic equilibria in magneto-hydrodynamic systems - Topological transport in plasmas
June 2009 - May 2017
Pohang University of Science and Technology
Position
  • Professor (Associate)
Education
October 2012 - July 2017
California Institute of Technology
Field of study
  • Plasma Physics
March 1994 - February 1998

Publications

Publications (199)
Article
Full-text available
Although there is a continuous spectrum of current sheet equilibria, how a particular equilibrium is selected by a given system remains a mystery. Yet, only a limited number of equilibrium solutions are used for analyses of magnetized plasma phenomena. Here we present the exact process of equilibrium selection, by analyzing the relaxation process o...
Article
Using a global model for atmospheric pressure plasma, we investigated general dependence of plasma properties on power density and plasma size. We built a global simulation for a pure argon cylindrical plasma and observed changes in plasma properties with the power density and plasma size. The study of the power dependence shows that the density of...
Article
We have studied the energy transport process in pulsed microwave argon plasmas at atmospheric pressure with a focus on the optical emission burst during the pulse-off time called afterpeak. Guided by experimental observations using nanosecond time resolution imaging and spectroscopic diagnostics, we have developed a global simulation model consider...
Article
A robust theoretical framework for describing the explosive growth of the magneto-hydro-dynamic (MHD) and turbulence-based instabilities in toroidal plasmas can only be achieved through a conclusive validation of the physics model with undisputable experimental information. Recently new techniques for high precision measurements of their 2-D/3-D st...
Article
We present that a statistical method known as the complexity–entropy analysis is useful to characterize a state of plasma turbulence and flux in the resonant magnetic perturbation (RMP) edge localized mode (ELM) control experiment. The stochastic pedestal top temperature fluctuation in the RMP ELM suppression phase is distinguished from the chaotic...
Article
A generalized formula for wave instability is developed for an anisotropic non-uniform plasma with finite flows and temperatures. Six-moment fluid equations are solved to give the analytic expression for wave instability in arbitrarily non-uniform plasmas. The analytic formula explicitly states the dependence of wave instability on non-uniformities...
Article
We discovered a hysteresis in a microwave-driven low-pressure argon plasma during gas pressure change across the transition region between α and γ discharge modes. The hysteresis is manifested in that the critical pressure of mode transition depends on the direction of pressure change. As a corollary, the plasma would attain different discharge pro...
Article
Hydroxyl radical (OH•) plays an important role in advanced oxidation processes (AOPs), which are employed to decompose organic pollutants in wastewater treatment. OH• is predominantly produced in AOPs for wastewater treatment via ultraviolet photolysis of hydrogen peroxide (H 2 O 2 ) or ozone, which is a costly and difficult process. This paper int...
Article
Strongly coupled plasmas (SCPs) exist in various places throughout the Universe, examples of which are inertial confinement fusion experiments, Jovian planets’ cores, neutron stars, and white dwarf stars. In recent decades, theoretical and numerical studies have been pursued to characterize the equation of states and thermodynamic properties of SCP...
Preprint
Turbulent transport near the marginal stability of tokamak plasmas is dominated by non-diffusive avalanche transport events in gyrokinetic simulations. The avalanche transport events are found to interact with self-organized shear flow layers, or the E X B staircase generating a staircase-like pressure corrugation. Various models have been suggeste...
Article
Full-text available
During bursty MHD events, transient ion cyclotron emission (ICE) is observed from deuterium plasmas in the Large Helical Device (LHD) heliotron-stellarator. Unusually, the frequencies of the successive ICE spectral peaks are not close to integer multiples of the local cyclotron frequency of an energetic ion population in the likely emitting region....
Preprint
Full-text available
Although there is a continuous spectrum of current sheet equilibria, how a particular equilibrium is selected by a given system remains a mystery. Yet, only a limited number of equilibrium solutions are used for analyses of magnetized plasma phenomena. Here we present the exact process of equilibrium selection, by analyzing the relaxation process o...
Preprint
We discovered a hysteresis in a microwave-driven low-pressure argon plasma during gas pressure change across the transition region between $\alpha$ and $\gamma$ discharge modes. The hysteresis is manifested in that the critical pressure of mode transition depends on the direction of pressure change. As a corollary, the plasma would attain different...
Article
In a time-dependent flow, nonlinear harmonics can be excited by coupling between linear waves and flow-induced harmonic waves. Examining the dispersion relations and selection rules for the coupling, we investigate nonlinearly coupled harmonics for waves propagating along the magnetic field line in a magnetized plasma as well as waves in an unmagne...
Article
Full-text available
Atmospheric pressure plasma (APP) generates highly reactive species that are useful for surface activation. We demonstrate a fast regeneration of iron oxides, that are popular catalysts in various industrial processes, using microwave-driven argon APP under ambient conditions. The surface treatment of hematite powder by the APP with a small portion...
Article
Full-text available
We report the design, performance, and characterization of a tunable microwave plasma resonator based on a coaxial transmission line structure operated at atmospheric pressure. The adjustable tuners of the compact device enable over 20 dB enhancement of reflection coefficient by counterbalancing the impedance mismatch. The in situ tuning capability...
Article
Full-text available
In their supercritical state simple fluids are generally thought to assume a homogeneous phase throughout all combinations of pressures and temperatures, although various response functions or transport properties may exhibit anomalous behavior, characterizing a state point as either more gas-like or liquid-like, respectively. While a large body of...
Article
Exact moments of the Boltzmann collision operator are calculated in the irreducible Hermitian moment expansion written in terms of the random-velocity variable of each species. The formulas are presented in closed, algebraic form and can be straightforwardly implemented in computer algebra systems. They are valid for two arbitrary masses, temperatu...
Article
Full-text available
Current sheets are ubiquitous plasma structures that play the crucial role of being energy sources for various magnetic phenomena. Although a plethora of current sheet equilibrium solutions have been found, the collisionless process through which a disequilibrated current sheet relaxes or equilibrates remains largely unknown. Here we show, through...
Article
A mechanism is presented whereby relativistic electron beams localized in phase space are deterministically scattered by coherent circularly polarized electromagnetic waves without stochastic processes. It is shown via an exact single-particle analysis that the condition for maximal scattering is an off-resonant condition, contrary to previous kine...
Article
A thin-foil infrared bolometer has been developed to measure the plasma radiation quantitatively during plasma disruptions in the KSTAR tokamak. We present analytic solutions of a 0D heat transfer model, which enable the estimation of the plasma radiation from the bolometer signal. The analytical solutions for the linear response regime give practi...
Article
Ion cyclotron emission (ICE) driven by perpendicular neutral beam-injected (NBI) deuterons, together with the distinctive ICE driven by tangential NBI, have been observed from heliotron–stellarator plasmas in the large helical device (LHD). Radio frequency radiation in the lower hybrid range has also been observed Saito K. et al (2018 Plasma Fusion...
Preprint
Full-text available
Current sheets are ubiquitous plasma structures that play the crucial role of being energy sources for various magnetic phenomena. Although a plethora of current sheet equilibrium solutions have been found, the collisionless process through which a disequilibrated current sheet relaxes or equilibrates remains largely unknown. Here we show, through...
Preprint
Atmospheric pressure plasma (APP) generates highly reactive species that are useful for surface activations. We demonstrate a fast regeneration of iron oxides, that are popular catalysts in various industrial processes, using microwave-driven argon APP under ambient condition. The surface treatment of hematite powder by the APP with a small portion...
Preprint
The stochastic layer formation by the penetration of the resonant magnetic perturbation (RMP) field has been considered as a key mechanism in the RMP control of the edge localized mode (ELM) in tokamak plasmas. Here, we provide experimental observations that the fluctuation and transport in the edge plasmas become more stochastic with the more pene...
Article
Full-text available
Experimental observations assisted by 2-D imaging diagnostics on the KSTAR tokamak show that a solitary perturbation (SP) emerges prior to a boundary burst of magnetized toroidal plasmas, which puts forward SP as a potential candidate for the burst trigger. We have constructed a machine learning (ML) model based on a convolutional deep neural netwo...
Article
Full-text available
Magnetic islands (MIs), resulting from a magnetic field reconnection, are ubiquitous structures in magnetized plasmas. In tokamak plasmas, recent researches suggested that the interaction between an MI and ambient turbulence can be important for the nonlinear MI evolution, but a lack of detailed experimental observations and analyses has prevented...
Article
Full-text available
Nonlinear coupling of cold and hot waves in a flowing magnetized plasma is analyzed with the Vlasov equation. An analytical solution is obtained for cold waves of a small amplitude (weak flow) and a long wavelength. The distribution function is obtained by integrating the kinetic equation along a perturbed phase-space trajectory for a time-varying...
Preprint
Full-text available
This study describes the discovery of a phase separation phenomenon in supercritical fluids (SCFs). An SCF is technically a single-phase fluid with two sub-domains separated by the Widom line. A pseudo phase transition occurs between liquid-like (LL) and gas-like (GL) states, similar to the gas-liquid phase transition across the coexistence line in...
Article
Full-text available
Perm-selective ion transportation in a nanoscale structure such as nanochannel, nanoporous membrane or nanojunction has been extensively studied with aids of nanofabrication technology for a decade. While theoretical and experimental advances pushed the phenomenon to seminal innovative applications, its basic observation has relied only on an indir...
Preprint
Magnetic islands (MIs), resulting from a magnetic field reconnection, are ubiquitous structures in magnetized plasmas. In tokamak plasmas, recent researches suggested that the interaction between the MI and ambient turbulence can be important for the nonlinear MI evolution, but a lack of detailed experimental observations and analyses has prevented...
Article
Full-text available
WEST is an upgrade of the Tore Supra tokamak to test ITER like divertor elements over very long pulses. For the studies of MHD instabilities affected by tungsten impurities, two microwave imaging diagnostics have been developed to obtain density and temperature fluctuation maps. The core reflectometer was refurbished and now measures the core densi...
Article
Full-text available
Intense bursts of suprathermal radiation, with spectral peaks at frequencies corresponding to the deuteron cyclotron frequency in the outer midplane edge region, are often detected from deuterium plasmas in the KSTAR tokamak that are heated by tangential neutral beam injection (NBI) of deuterons at 100 keV. Identifying the physical process by which...
Article
The plasma healing of acute wound on 56 hairless mice is examined and initiated by several externally controlled physical parameters such as driving frequency and power, duration and interval of plasma treatment, a fraction of air mixture, the gap distance between plasma and wound, and others. The variation of these physical parameters results in a...
Article
Full-text available
Ion cyclotron emission (ICE) is detected from all large toroidal magnetically confined fusion (MCF) plasmas. It is a form of spontaneous suprathermal radiation, whose spectral peak frequencies correspond to sequential cyclotron harmonics of energetic ion species, evaluated at the emission location. In ICE phenomenology, an important parameter is th...
Article
Time-dependent nonlinear simulation using the BOUT++ 3-field model is performed for a study of edge localized mode (ELM) dynamics in KSTAR H-mode plasmas. The simulated results of the ELM evolution are directly compared with the 2D ELM structures, measured by electron cyclotron emission imaging systems, from the initial growth to crash phase. The s...
Article
Full-text available
Semi-analytical expressions for the electron cyclotron emission (ECE) shape function are developed for arbitrary high harmonics. The integrand of the mth harmonic ECE shape function is fitted with the readily integrable test function parameterized by plasma temperature Te, harmonic number m, and emission angle θ. Semi-analytical formulae for high h...
Article
Full-text available
An Electron Cyclotron Emission Imaging (ECEI) diagnostic system for the WEST tokamak has been developed under the UNIST-WEST collaboration. This diagnostic system is designed to overcome accessibility and thermomechanical constraints for long pulse operation. The first O-mode channel will be installed in the first trimester of 2019 to probe the low...
Article
The microwave imaging reflectometer (MIR) is the leading diagnostic tool for study of density fluctuations in KSTAR. For last three years since 2014, major components such as the multi-frequency probe beam source, multi-channel detector array, signal processing electronic system, data acquisition system, and optical system have been gradually upgra...
Article
Korea Superconducting Tokamak Advanced Research (KSTAR) program is strongly focused on solving the scientific and technological issues in steady-state high performance plasma operation in preparation for ITER operation as well as the design basis for DEMO. In this regards, KSTAR has made significant advances in developing long pulse and high perfor...
Article
Dynamical properties of argon plasma jet at atmospheric pressure driven by microwave pulses with high repetition rate and fast rise time are investigated using time-resolved optical emission measurements in combination with fast imaging. Spectroscopic measurements reveal an emission burst at the pulse rise including electron continuum emission, hyd...
Article
The generation of reactive species in argon/oxygen plasma driven by a pulsed microwave has been investigated using global simulation. The applied power was given for varying pulse periods and duties with a constant average power. The differences in the trends for species densities with regard to the pulse parameters are analyzed based on the timesc...
Article
Fast sawtooth crash of m = 1 mode (tens of μs) is a common MHD phenomenon in the core of tokamak plasma. In KSTAR plasmas with localized heating and current drive close to the sawtooth inversion radius (ρ inv), very slow sawtooth crashes are observed with time scales of a few ms. Scan of heating deposition along with power modulation revealed two d...
Article
Quasi-coherent mode (QCM) is a widely studied ion-gyroscale micro-instability with characteristics similar to those of trapped electron mode in tokamak plasmas. The effect of the normalized collisionality on the QCM amplitude is quantitatively studied in low-confinement plasmas assisted by electron cyclotron resonant heating in the KSTAR machine us...
Preprint
Semi-analytic expressions for the electron cyclotron emission (ECE) shape function are developed for arbitrary high harmonics. The integrand of the ${m}^{\mathrm{th}}$ harmonic ECE shape function is fitted with the readily-integrable test function which is parameterized by plasma temperature $T_e$, harmonic number $m$ and emission angle $\theta $....
Article
Full-text available
The radio frequency detection system on the KSTAR tokamak has exceptionally high spectral and temporal resolution. This enables measurement of previously undetected fast plasma phenomena in the ion cyclotron range of frequencies. Here we report and analyse a novel spectrally structured ion cyclotron emission (ICE) feature in the range 500 MHz to 90...
Preprint
Full-text available
Long range avalanche-like electron heat transport events are observed in the L-mode tokamak plasma experiment. Detail avalanche dynamics and characteristics of the electron temperature ($T_\mathrm{e}$) fluctuations have been revealed by virtue of the advanced $T_\mathrm{e}$ imaging diagnostics in the KSTAR tokamak. Initially, the jet-like $T_\mathr...
Article
Full-text available
Plasma is a complex system involving diverse collisional processes and interactions, such as electron-impact excitation, ionization, recombination, etc. One of the most important methods for studying the properties and dynamics of plasma is to analyze the radiations from plasma. Here, we demonstrate the high-order harmonic (HH) spectroscopy for pro...
Article
A large axial volume expansion of microwave driven plasma at atmospheric pressure is attained with a low power radio frequency (RF) field at an axial location well isolated from the original plasma bulk. The gradual development of volume expansion visualized by high speed ICCD imaging suggests that the free electrons drifting toward the bias electr...
Article
The central safety factor (q 0) during sawtooth oscillation has been measured with a great accuracy with the motional Stark effect (MSE) system on KSTAR and the measured value was However, this measurement alone cannot validate the disputed full and partial reconnection models definitively due to non-trivial off-set error (∼0.05). Supplemental expe...
Article
Full-text available
Using a high-speed and broadband radio frequency (RF) (0.1-1 GHz) spectrum analyzer developed on the KSTAR tokamak, it is found that several distinct stages of RF emission appear at the pedestal collapse in high confinement discharges. Comparison with 2-D electron cyclotron emission (ECE) images has revealed that each stage is related to the instan...
Article
Broadband density fluctuations with peak frequency ranging from 150 to 400 kHz were measured using a multichannel microwave imaging reflectometer in core region of the lowdensity L-mode plasmas heated by neutral beam injection on KSTAR. These fluctuations have been studied by comparing the dominant mode scales estimated from the measurement with th...
Article
Quasi-coherent (QC) modes, known as a type of the trapped electron mode (TEM) turbulence, have been measured in the outboard core region of low-density electron cyclotron resonant heating (ECH) injected and ohmically heated L-mode plasmas on the Korea Superconducting Tokamak Advanced Research (KSTAR) device. It appears that QC modes of 20–60 kHz oc...
Article
The evolutions of toroidally coupled radially-distant and radially-adjacent tearing modes are visualized in 2D in detail on the Korea superconducting tokamak for advanced research. The coupled tearing modes are in-phase on the out-board mid-plane and become destabilized or compete with each other depending on their spatial separation. When two coup...
Article
Full-text available
Integral (nonlocal) closures [J.-Y. Ji and E. D. Held, Phys. Plasmas 21, 122116 (2014)] are combined with the momentum balance equation to derive electron parallel transport relations. For a single harmonic fluctuation, the relations take the same form as the classical Spitzer theory (with possible additional terms): The electric current and heat f...
Article
Full-text available
Pulsed microwave driven microplasma at atmospheric pressure is proposed as an efficient source of energetic electrons for low-temperature biomedical applications. Energetic electrons are required to break the covalent bonds of nitrogen, oxygen, and water molecules in humid air and produce reactive species. Particle-in-cell (PIC) simulations demonst...
Article
Multiscale interaction between the magnetic island and turbulence has been demonstrated through simultaneous two-dimensional measurements of turbulence and temperature and flow profiles. The magnetic island and turbulence can mutually interact via coupling between the electron temperature (T e ) gradient, the T e turbulence, and the poloidal flow....
Article
Full-text available
We show, with the help of a generalized Ginzburg-Landau model, that the time-varying perpendicular sheared flow is crucial to the dynamics of fluid instabilities observed in the boundary of toroidal magnetized high-temperature plasmas. The model reproduces key dynamical features of the boundary instabilities (called edge-localized modes) including...
Article
The explanation of the existence of the rotating MHD modes in the pedestal region before Type I edge localized mode (ELM) crash and in the inter-ELM periods (ELM precursors) observed in KSTAR is provided for the first time in the present paper. The dynamics of ELMs, observed using electron cyclotron emission imaging (ECEI) in KSTAR tokamak, is comp...
Article
Full-text available
We study a complex Ginzburg-Landau (GL) type model related to fluid instabilities in the boundary of magnetized toroidal plasmas (called edge-localized modes) with a prescribed shear flow on the Neumann boundary condition. We obtain the following universal results for the model in a one-dimensional interval. First, if the shear is weak, there is a...
Article
Surveys of the ion and electron heat transports of neutral beam (NB) heating plasma were carried out by power balance analysis in He and H rich plasma at LHD. Collisionality was scanned by changing density and heating power. The characteristics of the transport vary depending on collisionality. In low collisionality, with low density and high heati...
Article
Full-text available
This article assesses current understanding of hysteresis in transport relations, and its impact on the field. The rapid changes of fluxes compared to slow changes of plasma parameters are overviewed for both core and edge plasmas. The modulation ECH experiment is explained, in which the heating power cycles on-and-off periodically, revealing hyste...
Article
Coupled evolution of the electron temperature ($T_e$) profile, the poloidal flow profile, and the $T_e$ turbulence near the magnetic island has been observed through a high resolution simultaneous two-dimensional imaging of the turbulence and flow. As the radial $T_e$ gradient is increased outside the magnetic island due to the magnetic flux pertur...
Article
Large aspect ratio plasma jets with sub-mm diameter are produced by a microwave coaxial resonator electrode. The jet length shows a sharp asymmetric dependence on the drive frequency: the plasma jet suddenly turns off below a critical frequency while the jet length slowly decreases above the frequency. A general mechanism is proposed to explain the...