Recent Publications

Development and high-power validation of a 476 MHz twin-disk RF window for the KSTAR helicon current drive system

Jeehyun Kim, Hyunho Wi, Sonjong Wang

Korea Institute of Fusion Energy

Real-time feedback control of ELM frequency using divertor gas puffing and its effects on tungsten-induced radiation and plasma performance in KSTAR

Minseok Kim, Young-Ho Lee, SangKyeun Kim, Minwoo Kim, Sang-hee Hahn, Hiro J. Farre-Kaga, Ricardo Shousha, Juhyeok Jang, SooHyun Son, Yoon Seong Han, et al.

The edge-localized mode (ELM) frequency ($f_{\mathrm{ELM}}$) was successfully controlled in real time on KSTAR using a proportional-integral (PI) feedback controller, employing a $\mathrm{D}_2$ divertor gas puff as the actuator under tungsten lower-divertor conditions. The controller accurately tracked a two-step target---a 30 Hz increase in $f_{\mathrm{ELM}}$ for 4 s, followed by a 30 Hz decrease for 3 s---yielding mean and median absolute percentage errors of approximately 13% and 12%, respectively. Compared to a reference discharge, the actively controlled shot did not exhibit a significant drop in volume-integrated core radiation, confirming that excessive gas use merely degrades overall plasma performance. However, when contrasted with the exponential increase in core radiation observed in the absence of divertor gas puffing, these results underscore the critical need for real-time optimization. Specifically, divertor gas commands must be actively managed to maintain an $f_{\mathrm{ELM}}$ sufficient for flushing tungsten from the core while maximizing global plasma performance.

FEM-based plasma equilibrium reconstruction for transient and off-normal phases in KSTAR

Jeongwon Lee, Jayhyun Kim, Jun-Gyo Bak, Sang-hee Hahn, Heung-Su Kim, Chanyoung Lee, Jeongyeon Nam, Hyunsun Han, YongUn Nam

Korea Institute of Fusion Energy, University of Science and Technology

A theory of E×B staircase stability: favorable role of fast ions

Gyungjin Choi, Mikhail Angel Torio, Taik-Soo Hahm

KAIST, Seoul National University

We present an analytic theory for the E×B staircase stability against intrinsic collapse with a bump-hole pair growth. The obtained scale-dependent critical E×B shearing rate required to sustain the staircase indicates that finer-scale flow components are inherently more susceptible to collapse. Extending our model to include fast ions, we find a drastic reduction in the critical shearing rate. It originates from the reduction of the radial drift wave group velocity due to fast ion population. The result suggests a highly favorable role of fast ions in maintaining the sharp, step-wise E×B staircase structure, consistent with recent experimental observations of long-lived staircases in KSTAR FIRE mode plasmas.

Validation of Integrated Modelling of KSTAR Plasmas with Carbon Divertor using JINTRAC and Prediction for Tungsten Divertor

Beomsu Kim, Sun Hee Kim, Florian Koechl, Francis J Casson, Romain Futtersack, Chan-Young Lee, SeulChan Hong, JAEGON LEE, Boseong Kim, Yong-Su Na

Korea Institute of Fusion Energy, Seoul National University, ITER Organization, CCFE, UKAEA Culham

The JINTRAC integrated modelling suite has been adapted for the KSTAR tokamak, with its primary goal being the establishment of a core-edge integrated modelling framework for predictive analysis of KSTAR plasmas. This framework was validated by benchmarking JINTRAC's physics modules against both established codes and experimental data from a KSTAR carbon (C) divertor discharge (#25458), demonstrating high fidelity. Based on the validation, predictive core-edge coupled simulations were performed to investigate the effects of changing the divertor material from C to tungsten (W). The simulations predicted a significant increase in core radiation and a corresponding decrease in plasma temperatures, consistent with the behavior of high-Z materials. Specifically, the high tungsten influx causes radiative cooling in the pedestal and core regions, leading to a reduction in edge bootstrap current and a compensatory increase in core inductive current. This redistribution of current density drops the central safety factor (q 0 ) below unity, triggering sawtooth instability which serves to suppress core impurity accumulation. Sensitivity analysis indicated that while toroidal rotation exhibits a non-monotonic correlation with tungsten profile peaking, the total tungsten inventory and radiated power (P rad ) increased with higher rotation. Additionally, increasing the separatrix electron density effectively suppressed the tungsten source at the divertor target, yet the reduction in core tungsten inventory was limited compared to the extent of source reduction. These findings imply that achieving high-performance operation requires a comprehensive control strategy that minimizes the absolute impurity influx across the separatrix through combined source suppression and transport control.

Automated Outlier-Robust Bayesian Profile Fitting for Magnetically Confined Plasmas with Modified Tanh Profiles and Good-and-Bad Gaussian Mixture Likelihoods

Jaewook Kim, Jekil Lee, Laurent Jung, Sang-hee Hahn, Sehyun Kwak

Korea Institute of Fusion Energy, University of Science and Technology, Max-Planck-Institut für Plasmaphysik

We present an outlier-robust Bayesian approach for automated kinetic profile fitting in magnetically confined plasmas with the modified tanh (mtanh) parametrisation and demonstrate its implementation on KSTAR. The method addresses two systematic obstacles: anomalous diagnostic channels can bias least-squares fits, and multimodality of the mtanh cost surface can trap deterministic optimisers in secondary minima. The deployed workflow uses a good-and-bad Gaussian mixture likelihood based on the Box--Tiao formulation as the default outlier-robust likelihood for fitted diagnostic channels, with posterior outlier probabilities retained as channel-level quality indicators. The posterior is sampled with an affine-invariant ensemble MCMC sampler initialised near the result of deterministic maximum a posteriori (MAP)-seeking optimisation, reducing sensitivity to secondary minima on the multimodal mtanh surface. A batch automation layer retrieves diagnostic data from MDSplus and fits arbitrary time slices in parallel for the quantities \(n_e\), \(T_e\), \(T_i\), and \(v_T\) for which the relevant diagnostics are available. Results are written in formats suitable for MDSplus upload and downstream analysis. Representative KSTAR H-mode cases show that the mixture likelihood downweights contaminated measurements while preserving plausible pedestal profiles. The workflow provides a practical basis for future large-scale kinetic profile production for kinetic-EFIT, TRANSP, FASTRAN, and data-driven analysis workflows.

From image-based detection to parameter-based prediction of UFO events in KSTAR

Kyumin Kim, Jaemin Seo, Yuseop Kim

Hallym University, Chung-Ang University

Plasma disruption prediction is a central challenge for reliable tokamak operation, yet the predictive value of intermediate off-normal phenomena, particularly UFO (Unidentified Flying Object) events that may precede or trigger disruptions, remains largely unexplored in the context of scalar parameter-based modeling. Here, using visible-camera sequences and frame-aligned scalar equilibrium and operational parameters from KSTAR (Korea Superconducting Tokamak Advanced Research), we show that UFO events can be systematically characterized and predicted from routine diagnostic signals alone. We first establish ground-truth event labels by manual annotation and demonstrate that a ResNet-18 image classifier reproduces this reference with a UFO F1-score of 0.911 under shot-aware evaluation, while also quantifying that event-level splitting substantially inflates apparent performance. Building on this ground truth, we show that a Random Forest classifier distinguishes UFO from Non-UFO frames using only 12 scalar parameters, achieving an accuracy of 0.96 and a UFO F1-score of 0.94, and that the most discriminative variables include upper triangularity ($\delta_u$), internal inductance ($l_i$), and on-axis safety factor ($q_0$). A shot-level event-aligned analysis further shows that discriminative power and temporal precedence are distinct: among these, only the plasma shape parameters (notably $\delta_u$ and elongation $\kappa$) change significantly before onset and thus act as genuine precursors, whereas $q_0$ responds only after onset and $l_i$ shows no significant pre-onset trend. We interpret these precursor signatures as correlational operational states rather than causal drivers of UFO occurrence. Taken together, these findings demonstrate that data-driven prediction can be extended beyond terminal disruption outcomes to actionable precursor phenomena, suggesting a deployable detect--classify--predict pathway for real-time UFO monitoring in KSTAR.

Real-time observation of toroidal current redistributions induced by three-dimensional MHD phenomena triggering vertical displacement events in tokamak plasmas

Matthew Tobin, Steve A Sabbagh, Veronika Zamkovska, Guillermo Bustos Ramirez, Hankyu Lee, Joseph R Jepson, Juan Riquezes, Frederick C Sheehan, Grant A Tillinghast, Keith Erickson, et al.

Columbia University, Korea Institute of Fusion Energy, Culham Centre for Fusion Energy, Princeton Plasma Physics Laboratory, UKAEA

Three-dimensional MHD instabilities, including edge-localized modes (ELMs) and internal reconnection events (IREs), have been observed to precipitate loss of vertical stability in tokamak plasmas, resulting in vertical displacement events (VDEs). This vertical destabilization can occur due to toroidal current redistributions and/or shape changes resulting from these phenomena. Using a recently introduced method for rapidly reconstructing the two-dimensional toroidal plasma current density profile in real-time, results are presented that demonstrate the specific current distribution changes that occur during ELMs (on KSTAR) and IREs (on MAST-U) that lead to loss of vertical control. The method most efficiently reconstructs the toroidal current density profile by doing so on a basis of principal components of historical profiles. These principal components isolate dominant current profile dynamics, improving interpretability, increasing speed, and reducing dimensionality of the profile computation. On KSTAR, this computation is executed in the real-time plasma control system at a rate of 10 kHz (limited by available CPU cycle times), allowing the current profile evolution to be assessed at several times over the course of each ELM event. Further, by incorporating the reconstructions into a novel vertical stability metric, the contribution of specific current profile dynamics to the loss of vertical stability can be assessed in real-time for VDE avoidance and improved understanding of the causal relationship between three-dimensional MHD phenomena and VDEs. The success of this method in approximating toroidal current density profiles from kinetic equilibrium reconstructions is also presented ($R^2=0.990$), along with its capability to produce other equilibrium quantities of interest in real-time at high time resolution.

First reduced model for integrated computations of helicon wave heating and current drive in magnetic fusion plasmas

Zi-Chen Kan, Lei Chang, Zhen-Yu Wang, Hua-Sheng Xie, Ping-Wei Zheng, Lai Wei, Qi-Bin Luan, Xue-Mei Zhai, Zhao-Qing Hu, Zheng-Xiong Wang, et al.

Fast predictive modelling of radio-frequency heating and current drive is important for integrated tokamak scenario design, yet kinetic calculations of helicon-wave absorption remain too computationally expensive for large-scale parameter scans. We present a reduced model for helicon-wave heating and current drive that retains the dominant parallel electron Landau-damping channel. The wave response is evaluated on the cold-plasma dispersion root, and a single-Landau-pole correction is introduced to obtain compact expressions for the local damping rate and current-drive efficiency. The model is benchmarked against the Chiu-Chan heating model using approximately 1.6 million samples covering representative conditions of EAST, HL-3, DIII-D and KSTAR. The reduction error is found to be governed primarily by the electron Landau parameter and electron beta. Within an identified sub-lower-hybrid-frequency validity window, results from different devices collapse onto a common error curve, which enables an empirical correction that is further tested using ITER-like and BEST-like extrapolation cases. Near and above the lower-hybrid frequency, the agreement deteriorates rapidly owing to changes in the cold-dispersion root structure and the breakdown of the single-branch WKB description. When coupled to a reduced current-drive source, the corrected heating model gives a median deviation of 10.8 percent from the Landau-channel Ehst-Karney reference and reproduces published CFETR current-density profiles. The resulting model provides a computationally efficient reduced closure for helicon-wave heating and current-drive calculations, together with physically interpretable limits on its range of validity.

Investigating long-duration plasma operation with the international multi-machine CICLOP database

Xavier L Litaudon, Ernesto A Lerche, Olaf Grulke, Christopher Thomas Holcomb, Juan Huang, Marcin Jakubowski, Hyun-Seok Kim, Pierre Manas, Tomohiro Morisaki, Francesca Turco, et al.

IRFM, Max-Planck-Institut für Plasmaphysik, Chinese Academy of Sciences, General Atomics, Culham Science Centre

Combined high-fusion performance and long-pulse operation is one of the key integration challenges for fusion energy development in magnetic devices. Addressing these challenges requires an integrated vision of physics and engineering aspects with the purpose of simultaneously increasing time duration and fusion performance. Since the previous 2023 IAEA Fusion Energy Conference, significant progresses have been made in tokamaks and stellarators including very recent achievement in duration and/or performance. These progresses are reviewed by analyzing the experimental data provided by 10 tokamaks and two stellarators. The published database [Litaudon X. et al 2024 Nucl. Fusion 64 015001], which initially included data up to January 2022, has been significantly updated for the 2025 IAEA Fusion Energy Conference to incorporate the latest 2023-2025 experiments (up to May 2025) including recent records performance with new entries provided by DIII-D, EAST, JET, KSTAR, WEST, and W7-X. The update dataset has been gathered and coordination have been provided by the IEA-IAEA international CICLOP group (Coordination on International Challenges on Long duration OPeration). An overview of the recent progress toward long pulse operation analysing the CICLOP database is provided in this publication.

Spatiotemporal Structure of Edge Harmonic Oscillation and Its Role in ELM-free QH-Mode at KSTAR

Jaehyun Lee, SangKyeun Kim, YoungMu Jeon, Minwoo Kim, Dong-Kwon Kim, Gunsu S Yun

Korea Institute of Fusion Energy, Princeton Plasma Physics Laboratory, Korea National University of Science and Technology (UST), Pohang University of Science and Technology

In Quiescent H-mode (QH-mode), edge-localized modes (ELMs) are naturally replaced by a low- n edge harmonic oscillation (EHO), yet the self-regulating transport mechanism driven by the EHO remains insufficiently understood. Using high-spatiotemporal-resolution imaging diagnostics on KSTAR—electron cyclotron emission imaging (ECEI) and broadband ECE—we resolve the eigenmode structure of the EHO and elucidate its regulatory role in edge transport. The EHO is localized within the pedestal near the maximum pressure gradient, propagates in the ion-diamagnetic direction, and its radial envelope expands with increasing shear, suggesting that rotational shear is closely associated with the structural evolution of the EHO. Information-theoretic Transfer Entropy (TE) analysis identifies a distinct 'dual-stabilization' regulatory interaction pattern: the EHO is associated with enhanced outward energy transport to limit the pressure gradient, while the background shear flow is associated with preferential regulatory influence on the EHO saturation amplitude and energy flux. These results demonstrate that the QH-mode pedestal is sustained by a shear-associated regulation mechanism, where the coupling between rotational shear and mode structure is central to maintaining the ELM-free state.

An integrated multi-diagnostic visualization platform for KSTAR tokamak

J.K. Lee

Korea Institute of Fusion Energy