Equilibrium selection via current sheet relaxation and guide field amplification.

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 of a disequilibrated current sheet under a finite guide field. It is shown via phase-space analyses and particle-in-cell simulations that the current sheet relaxes in such a way that the guide field is locally amplified, yielding a mixed equilibrium from the spectrum. Comparisons to spacecraft observations and solar wind current sheet statistics demonstrate that such mixed equilibria are ubiquitous and exist as underlying local structures in various physical environments.

Radiation measurement in plasma disruption by thin-foil infrared bolometer.

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 practical ways by which the radiation power and energy can be estimated from the cooling time scale of the bolometer signal. A useful way of evaluating the linear response of the system is also introduced. The analysis is complemented by 2D heat transfer simulations. The bolometer signals from the shattered pellet injection experiments in the 2020 KSTAR campaign are analyzed and interpreted according to the heat transfer models.

Collisionless relaxation of a disequilibrated current sheet and implications for bifurcated structures.

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 analyses of phase-space distributions of single-particle orbit classes and particle-in-cell simulations, that collisionless transitions among the orbit classes are responsible for this process. Bifurcated current sheets, which are readily observed in geospace but whose origins remain controversial, are shown to naturally arise from the equilibration process and thus are likely to be the underlying structures in various phenomena; comparisons of spacecraft observations to particle-in-cell simulations support this fact. The bearing of this result on previous explanations of bifurcated structures is also discussed.

Effects of plasma turbulence on the nonlinear evolution of magnetic island in tokamak.

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 further understanding. Here, we provide comprehensive observations such as turbulence spreading into an MI and turbulence enhancement at the reconnection site, elucidating intricate effects of plasma turbulence on the nonlinear MI evolution.

Catalytic effect of laser-combined atmospheric pressure plasma in lowering the reduction temperature of hematite.

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 of hydrogen (0.5 vol%) lowers the oxide's reduction temperature. A near-infrared laser is used for localized heating to control the surface temperature. Controlled experiments without plasma confirm the catalytic effect of the plasma. Raman, XRD, SEM, and XPS analyses show that the plasma treatment changed the chemical state of the hematite to that of magnetite without sintering.

Probing electron-atom collision dynamics in gas plasma by high-order harmonic spectroscopy.

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 probing the complex electron-atom collision (EAC) dynamics in a laser-induced gas plasma. These measurements were carried out by using an elliptically polarized pump and a time-delayed linearly polarized probe. The HH spectra from argon and krypton plasmas were recorded by scanning the time delay up to hundreds of picoseconds. We found that the delay-dependent HH yield contains three distinct regions, i.e., the first enhancement, the subsequent suppression, and the final restoration regions. A qualitative analysis shows that these features are clear signatures of the EAC processes and interactions involved in the delay-dependent HH spectroscopy.

Nonlinear Interaction of Edge-Localized Modes and Turbulent Eddies in Toroidal Plasma under n=1 Magnetic Perturbation.

The effect of static n=1 resonant magnetic perturbation (RMP) on the spatial structure and temporal dynamics of edge-localized modes (ELMs) and edge turbulence in tokamak plasma has been investigated. Two-dimensional images measured by a millimeter-wave camera on the KSTAR tokamak revealed that the coherent filamentary modes (i.e., ELMs) are still present in the edge region when the usual large scale collapse of the edge confinement, i.e., the ELM crash, is completely suppressed by n=1 RMP. Cross-correlation analyses on the 2D images show that (1) the RMP enhances turbulent fluctuations in the edge toward the ELM-crash-suppression phase, (2) the induced turbulence has a clear dispersion relation for wide ranges of wave number and frequency, and (3) the turbulence involves a net radially outward energy transport. Nonlinear interactions of the turbulent eddies with the coexisting ELMs are clearly observed by bispectral analysis, which implies that the exchange of energy between them may be the key to the prevention of large scale crashes.