Wonho Choe

The developed KAIST impurity modelling (KIM) code analyses impurity transport in the poloidal cross-section of tokamak plasma in both prediction and reconstruction modes. The prediction mode utilizes a numerical solution of the one-dimensional radial continuity equation to simulate impurity transport based on the given plasma parameters and extends the resulting radial impurity density distribution to a two-dimensional poloidal distribution by incorporating centrifugal-force-driven asymmetry. Synthetic diagnostic data, such as radiation power distribution and spectral emissions, are generated using atomic databases for direct comparison with experimental measurements. In reconstruction mode, the code reconstructs the impurity transport coefficient profiles from the experimentally obtained impurity density evolution over time, to overcome the limitations of forward-modelling-based transport coefficient inference. The developed reconstruction algorithm is based on numerical impurity flux calculations and Bayesian techniques and was validated using six reference impurity transport coefficient profiles from the KSTAR, JET, and ASDEX-U tokamaks. The reconstruction accuracy evaluated using the R 2 value, demonstrated good agreement ( R 2 > 0.87) between the reconstructed and reference profiles in the core regions ( r / a < 0.8), even in the presence of 5% random noise. The code was applied to a stationary KSTAR H-mode discharge to demonstrate its capability in analysing experimental W transport. The reconstructed transport coefficients were compared with neoclassical predictions to assess their physical validity. The results show that the total diffusion coefficient follows a radial trend similar to the neoclassical prediction but at a higher level consistent with the expectation of turbulent transport enhancement. This trend confirms that the KIM code yields physically reasonable transport coefficients from experimental data. This study details the theoretical framework, numerical validation, and experimental efficacy of the KIM code in advancing impurity transport research.

Energy-efficient and low-waste air disinfection is essential for protecting public health and promoting sustainable development. Here, we present an electrostatic precipitator–surface dielectric barrier discharge (ESP–SDBD) filter that integrates low-power electrostatic collection with plasma-assisted pathogen inactivation. The filter exhibited a 0.1 Pa pressure drop at 600 L/min, continuously captured > 99.994% of airborne Escherichia coli and Micrococcus luteus, and inactivated > 99.98% of collected bacteria within 1 h. Operational analysis indicates that the ESP–SDBD filter can reduce energy consumption by 30% relative to HEPA filtration. In situ reactive species measurements with electrohydrodynamic simulations revealed a synergistic mechanism enhancing inactivation efficacy and energy efficiency by factors of 17.3 and 12.4, respectively, compared with SDBD-only ozonation. These findings highlight the importance of reactive-species utilization, in addition to reactive-species generation, in plasma-assisted disinfection and demonstrate the potential of ESP–SDBD filtration as an energy-efficient and sustainable approach to air purification.

Achieving core-edge compatible divertor detachment is a critical requirement for stable operation in future fusion devices. This study compares nitrogen and neon seeding in KSTAR H-mode plasmas with carbon walls, combining experiments and SOLPS-ITER modelling to evaluate their radiative dissipation and core-edge compatibility. Experimentally, N seeding achieved stronger divertor detachment, with larger reductions in target particle and heat fluxes, a higher divertor radiation fraction, and a lower core radiation fraction than Ne. In contrast, Ne seeding triggered a significant rise in core radiation followed by H-L back transitions, limiting the maximum total radiated power fraction to roughly half that of N. SOLPS-ITER simulations reproduced the experimental trends and revealed that the better core-edge compatibility of N arises from its higher divertor retention in addition to its higher cooling factor. The relative positions of the stagnation points of impurity poloidal velocity and ionization sources did not explain the different divertor compression. Instead, in the present modelling, the higher impurity parallel particle flux, resulting from the higher parallel impurity velocity, explains the stronger nitrogen impurity compression in the divertor region. The parallel temperature distribution with N was more favourable for achieving higher impurity parallel velocity than with Ne, because the impurity velocity is governed by modifications of the main ion flow due to friction and thermal forces, both of which strongly depend on the temperature. Ultimately, this behaviour is attributed to the strongly divertor-localized radiation of N. These results demonstrate that N is more effective than Ne in achieving radiative divertor detachment while maintaining low core contamination in KSTAR, consistent with observations in other present tokamaks.

The electron energy distribution function (EEDF) is a fundamental indicator of the nonequilibrium nature of low-temperature plasmas that govern electron-driven physical and chemical processes. In this study, an optimization-based inversion framework was developed to reconstruct parameterized EEDFs, described by a generalized functional form, in lowpressure argon plasmas; these EEDFs are reconstructed from optical emission spectra. A collisional-radiative model was coupled with a genetic algorithm to determine the parameters of the adopted generalized EEDF expression by fitting the intensities of the selected emission lines. Numerical tests demonstrated that the proposed method could accurately reproduce the prescribed EEDF shapes, and it exhibited a certain degree of robustness against spectral noise. The framework was further validated experimentally in two different argon-based inductively-coupled plasma systems; the retrieved EEDFs showed good agreement with those obtained from the Langmuir probe measurements, particularly in the low-energy region. The role of the metastable-assisted stepwise excitation from the 1s manifold in enhancing the spectral sensitivity to low-energy electrons was analyzed. These results indicate that the proposed approach provides a feasible and physically consistent solution for detailed diagnostics of non-equilibrium electron kinetics in low-pressure plasmas, and its applicability and limitations are clearly defined.

Precise control of radiation losses from impurities remains a critical challenge for achieving stable operation in fusion plasmas. Infrared video bolometers (IRVBs) are an important diagnostic tool for visualizing the spatial distribution of radiated power in such plasmas. However, conventional IRVB signal-processing methods exhibit inherent limitations, as they typically assume a linear temporal variation of dynamically changing background radiation, which restricts the accurate reconstruction of radiated power profiles. To overcome this limitation, this study proposes a novel signal-processing algorithm based on the heat balance equation. The distinct feature of the proposed approach is its ability to physically model time-dependent background radiation, thereby effectively isolating its contribution to the measured signal. The proposed algorithm’s performance was validated using phantom datasets with known ground truth. Synthetic tests simulating various discharge scenarios demonstrate that the proposed method reconstructs radiated power profiles with significantly higher accuracy (coefficient of determination, R2≥0.99 in all cases) compared with conventional methods. In addition, the algorithm was applied to representative Korea Superconducting Tokamak Advanced Research high-confinement mode discharges, confirming that the reconstructed total radiated power and its temporal evolution remain physically consistent under realistic plasma conditions. These results indicate that the developed algorithm provides a robust tool for enhancing the reliability of impurity transport and power balance analyses in fusion plasmas.

This study investigates the performance enhancement of a cylindrical Hall thruster through ion-focusing induced by Xe and Kr mixture discharges. The thruster operated at a fixed anode flow rate for both Xe and Xe–Kr mixture gases, with the anode voltage varying from 250 to 450 V and the anode power from 350 to 700 W. Results indicate that for anode voltages higher than 375 V, the performance of the Xe-Kr mixture discharge with 46% Xe surpasses that of the pure Xe discharge. At an anode voltage of 400 V, the 46% Xe discharge demonstrates improvements in thrust (20.3 mN vs. 18.6 mN), anode specific impulse (2455 s vs. 2263 s), and anode efficiency (40.2% vs. 33.8%). Using laser-induced fluorescence diagnostics, an upstream shift in the ion velocity profile is observed, indicating a corresponding shift in the acceleration region within the discharge cavity. This upstream shift of the acceleration structure is related to ion-focusing, which narrows the plume angle and enhances the thrust compared with the pure Xe discharge.

Two-dimensional (2D) electron temperature and density profiles were obtained for a cylindrical Hall thruster plasma by incorporating optical emission tomography and a collisional-radiative (CR) model. Using an inverse Radon transform-based tomographic reconstruction of 1350 lines of sight, 2D profiles of Xe neutral optical emission intensities of 10 distinct wavelengths (992.3, 979.9, 916.2, 904.5, 895.2, 881.9, 834.6, 828.0, 823.1, and 788.7 nm) were obtained at 4 mm from the thruster channel exit. Local electron temperatures and densities were subsequently derived using the CR model. The developed CR model, combined with tomographically reconstructed optical emission spectroscopy, demonstrated electron parameters comparable to those measured using a double Langmuir probe. This study highlights the essential role of optical emission tomographic reconstruction in accurately capturing the localized electron parameters for Xe-based Hall thruster plasmas. Additionally, a notable similarity in the spatial distribution was observed between the normalized 2D emission profiles of 992.3, 979.9, 916.2, and 881.9 nm and the Xe ionization rate. This finding demonstrates the potential of using reconstructed emission profiles at specific wavelengths as diagnostic tools to qualitatively infer 2D ionization regions without relying on a complex CR model.

Plasma, an ionized gas composed of charged particles, has shown therapeutic potential in enhancing biological processes such as wound healing and tissue integration. Implants, such as silicone and human acellular dermal matrix (hADM), are commonly used in reconstructive surgery, but improving their biocompatibility and integration remains a challenge. This study investigated the effects of vacuum plasma treatment on silicone and hADM implants using an in vivo rat model. Plasma-treated and untreated implants were inserted subcutaneously, and tissue samples were collected at 1, 4, and 8 weeks post-implantation. Histological and immunohistochemical analyses were performed to assess inflammation, cellular infiltration, collagen formation (neocollagenesis), and angiogenesis. Results showed that plasma-treated silicone and hADM implants had significantly reduced capsule thickness at weeks 4 and 8 compared to untreated controls, indicating a lower chronic inflammatory response. Plasma treatment also promoted greater fibroblast infiltration and enhanced neocollagenesis within the hADM implants. Furthermore, immunohistochemical staining revealed a notable increase in blood vessel formation around and within the plasma-treated hADM implants, suggesting improved vascularization. In conclusion, vacuum plasma treatment enhances the biocompatibility and tissue integration of implants by reducing inflammation and promoting cellular and vascular responses, offering promising potential for improving outcomes in reconstructive surgery.

Liquid metal (LM) offers exceptional potential for stretchable and flexible applications due to its outstanding stretchability, flexibility, and electrical conductivity. The inherent characteristics of LM–such as high surface tension, low viscosity, poor wettability, and sintering challenges–pose significant obstacles for use in commercial printing. To overcome these, a self‐sintering liquid metal composite particle (LMCP) ink with tunable surface tension, viscosity, and wettability is developed. LMCP inks are tailored for commercial printing processes requiring low surface tension and high viscosity, enabling printing on diverse substrates. The ink achieves coffee‐ring‐free, crack‐free, bilayer‐free, and post‐processing‐free deposition through simple evaporation under ambient conditions, while preserving electrical conductivity. Key innovations include: i) utilizing polyvinylpyrrolidone (PVP)‐capped liquid metal particles (LMPs) to reduce surface tension and enhance wettability; ii) incorporating Laponite as a viscosity enhancer and leveraging LMP sedimentation to control viscosity; iii) forming LMCPs encapsulated by PVP and Laponite through co‐self‐assembly on LMP surfaces to promote self‐sintering; and iv) achieving uniform deposition via solutal‐Marangoni‐driven mixing and particle settling. The resulting LMCP electrodes exhibit over 1200% stretchability, patternability into complex configurations, and stability for nearly one year in air. Additionally, their application as stretchable metamaterial absorbers is highlighted, showcasing their potential in advanced printable electronics.

Measuring the in situ erosion rate of tungsten using an optical emission spectroscopic diagnostic technique, so called the S/XB method, has recently gained attention in current fusion research. This method calculates the erosion rate by multiplying the S/XB value, which represents ionization per photon emission, with the intensity of the specific spectral line. The S/XB value was given by the ionization rates (S) and the excitation rates (X) from the ground and the metastable levels and the branching ratio (B) for the specific radiative transition of neutral tungsten (W I), assuming a corona equilibrium among the ground, metastable, and upper resonance levels. The conventional S/XB method assumes a Boltzmann distribution for metastable level densities and introduces the free parameter of the tungsten atomic characteristic temperature ( T W ). Herein, we propose an improved S/XB method to measure the erosion rates of tungsten in a divertor to eliminate the ambiguity caused by T W . Our approach directly calculates the densities of ground and metastable levels using a set of photon emission equations that relate their densities to multiple spectral line intensities within the framework of coronal equilibrium. Subsequently, the sputtered flux is obtained from the matrix operator form of S/XB and the multiple spectral line intensities given by the measurement. Our proposed method was experimentally tested using the plasma beam irradiation facility by comparing the gross tungsten erosion rates calculated using our method with those obtained from conventional approaches and those determined through the sputtering yield by the Ar⁺ ion beam flux. From the experiment, we obtained the sputtered tungsten flux of 4.0 × 10²⁰ m⁻² s⁻¹ ( ± 2.1 × 10²⁰) using our method at a discharge current of 150 A from the plasma source and an ion incident energy of 100 eV, while the previous method yielded a range from 1.4 × 10²⁰ m⁻² s⁻¹–1.9 × 10²¹ m⁻² s⁻¹ by varying T W and spectral lines. Using the sputtering yield method, a sputtered flux of 4.1 × 10²⁰ m⁻² s⁻¹ ( ± 1.1 × 10¹⁹) was measured as a reference value. The results demonstrate that our S/XB method significantly reduces ambiguity in the selection of spectral lines and the T W parameter, and closely agrees with the reference values, offering a more accurate and consistent approach for measuring tungsten erosion in fusion reactors.