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Woong-Ryol Lee, Taehyun Tak, Luca Zabeo, Timo Ravensbergen, Anze Zagar, Piotr Perek, Walter Van Herck, Peter De Vries, Seung-Ju Lee, Lana Abadie, et al.
J.H. Jeong, S.C. Hong, B.S. Kim, J.Y. Jang, W. Cho, J.G. Kwak, S.J. Wang
Young-Hoon Lee, Chanyoung Lee, Hyun-Seok Kim, Minho Woo, Sang-Hee Hahn, S.S. Kim, Jae-Min Kwon
Hyungkeun Nam, Jaemin Seo
We study equilibrium reconstruction for the Korea Superconducting Tokamak Advanced Research (KSTAR) device under magnetic sensor dropout, with a diffusion model as the prior. Magnetic measurements leave most components of the toroidal current density $J_φ$ undetermined, and sensor loss leaves more of them to the prior. The diffusion model learns only $J_φ$, conditioned on the coil currents, the plasma current and the product of major radius and toroidal field, which do not depend on the dropout. The physics enters through a linear forward operator built from sensor response matrices of the LIUQE code. Because the observations are linear, a variant of decoupled annealing posterior sampling fits them with a closed-form linear correction. On measured signals of 37 KSTAR shots, we switch off a random fraction (0.0 to 0.9, ten settings) of the 124 magnetic channels in use and compare with LIUQE under the same masks, taking the full-sensor LIUQE reconstruction as the label. Both methods fit the remaining channels to a similar level. The median relative distance of $J_φ$ from the label stays at 3.22--4.07\% for the diffusion reconstruction in all settings, whereas that of LIUQE reaches 11.89\% at dropout 0.9. At dropout 0.5--0.9, the diffusion reconstruction is closer to the label on 27--37 of the 37 shots. At low dropout, LIUQE is closer on most shots. For the derived flux, the diffusion reconstruction is closer on 21--36 shots at dropout 0.5--0.9, and its error at high dropout comes from the vessel current estimate, not from the prior.
Gwang-Geun Seo, Seongjun Han, Jayhyun Kim, Yeongsun Lee, SeongMoo Yang, June-Woo Juhn, Jong-Kyu Park
Error field (EF) correction against n=1 intrinsic errors has been revised in KSTAR in three respects. First, a local penetration timing criterion is defined from electron cyclotron emission (ECE) and charge exchange spectroscopy (CES) signatures near the reconstructed q = 2 surface. This timing determines the in-vessel control coil current used as the penetration threshold in the compass scan analysis. Second, the compass scans using the MID and OFFMID coils in 1.8T L-mode plasmas indicate an intrinsic EF level larger than the 2015 KSTAR reference under the conditions examined here. This implies that intrinsic error fields can vary after extended periods of device operations and hardware modifications. Third, the inferred EF direction is assessed by comparing the MID and OFFMID results in a GPEC overlap field basis, where the two configurations give nearly the same direction despite their different poloidal spectra. A rotating resonant magnetic perturbation (RMP) experiment provides an additional dynamic check of this direction, with penetration observed near the phase predicted from the static MID scan. The reversed-B t comparison constrains the possible source symmetry and is more compatible with a PF related contribution than with a TF dominated contribution, although the specific hardware source is not identified. These results provide an updated EF reference for KSTAR 3D-field operation and motivate periodic reassessment of intrinsic EF after extended device operation.
E. M. Bursch, A. Xie, J. L. Barr, S. K. Kim, N. C. Logan, S. M. Yang, J. G. Bak, W. Choi, Q. Hu, J. W. Juhn, et al.
Non-disruptive compass scan error field identification via island healing is demonstrated on KSTAR in a single H-mode discharge. This method can allow for up to a 75% reduction in the operational time necessary for a complete compass scan by reducing the required discharges from four to one. Previous experiments on DIII-D, JET, and MAST-U have demonstrated the technique in Ohmic and L-mode but required multiple discharges and strong real-time n=1 magnetics diagnostics close to the plasma that are unlikely to be viable for future devices. The results presented here were made possible by implementing a new set of triggering algorithms relevant to fusion pilot plant operation into the KSTAR plasma control system, along with KSTAR's strong RMPs and long pulse lengths. This represents a significant step towards the disruption-free error field identification method being demonstrated for deployment on future disruption-averse tokamaks, including ITER and fusion pilot plants. Steps needed to close remaining gaps to viability are addressed.
Jinwoo Gwak, Yeongsun Lee, Jeongwon Lee, Won Ik Jeong, Hyun-Tae Kim, Yong-Seok Hwang, Min-Gu Yoo, Yong-Su Na
Microwave breakdown driven by electron cyclotron (EC) waves provides a non-inductive route to plasma initiation in reactor-scale tokamaks. We introduce a three-dimensional Monte Carlo simulation that, for the first time, self-consistently treats nonlinear wave-particle interactions, atomic collisions, and guiding-center transport. The Monte Carlo simulation unveils the key role of parallel Brownian motion in the ionization avalanche mechanism. The predicted breakdown boundary is validated against KSTAR experiments. This work concludes that microwave gas breakdown will be successful under ITER-relevant conditions at a D$_2$ prefill pressure near 2 mPa with 1 MW of injected EC power.
Yoon Seong Han, Junhyeok Yoon, Inwoo Song, Junghoo Hwang, H.H. Lee, Si-Woo Yoon, Jaemin Kwon, Wonho Choe
The developed KAIST impurity modelling (KIM) code analyzes 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 phantom 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 phantom 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.
Gyungjin Choi, Mikhail Angel Torio, Taik-Soo Hahm
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.
Beomsu Kim, Sun Hee Kim, Florian Koechl, Francis J Casson, Romain Futtersack, Chan-Young Lee, SeulChan Hong, JAEGON LEE, Boseong Kim, Yong-Su Na
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.
Jeehyun Kim, Hyunho Wi, Sonjong Wang
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.
Jeongwon Lee, Jayhyun Kim, Jun-Gyo Bak, Sang-hee Hahn, Heung-Su Kim, Chanyoung Lee, Jeongyeon Nam, Hyunsun Han, YongUn Nam
Jaewook Kim, Jekil Lee, Laurent Jung, Sang-hee Hahn, Sehyun Kwak
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.
Kyumin Kim, Jaemin Seo, Yuseop Kim
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.
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.
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.
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.
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.
Yong Un Nam, Hyunsun Han, H.H. Lee, Jinil Chung, YoungMu Jeon, Gunyoung Park, Jayhyun Kim, Kimin Kim, Minwoo Kim, Jinseok Ko, et al.
The Korea Superconducting Tokamak Advanced Research (KSTAR) device has recently entered a new phase of operation following the installation of a tungsten mono-block divertor, providing a unique platform to investigate high-performance, long-pulse plasma scenarios for future reactors. The 2023–2024 experimental campaigns demonstrated significant progress in commissioning the tungsten divertor, developing plasma operation scenarios, and achieving real-time control. Comparative studies revealed increased core radiation losses and associated performance degradation relative to the carbon divertor environment. Alleviation strategies, such as optimized neutral beam injection timing, boron powder seeding, and impurity control through gas puffing, were shown to reduce tungsten accumulation and extend high-performance phases, including record H-mode operation exceeding 100 s. International collaboration enabled the first demonstration of high poloidal beta scenarios with an internal transport barrier on KSTAR. Advances in control included upgraded real-time resonant magnetic perturbation schemes and disruption forecasting via the DECAF system. Furthermore, divertor detachment control using a new surrogate-model-based approach with real-time radiation imaging demonstrated active impurity and heat flux management. Supported by enhanced heating, current drive, and diagnostic systems, these achievements offer valuable insights into tungsten impurity behavior, transport physics, and control strategies for future reactors including ITER. Planned upgrades toward a full-tungsten wall and ITER-aligned real-time plasma control will further establish KSTAR as a leading experimental platform for developing operational scenarios required to achieve steady-state, burning plasma conditions.
Jaehyun Lee, SangKyeun Kim, YoungMu Jeon, Minwoo Kim, Dong-Kwon Kim, Gunsu S Yun
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.
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.
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