Recent Publications

Comparing HSX Reflectometer Measurements with a Full-Wave Synthetic Diagnostic

Henrique Oliveira Miller Hillebrecht, Michael Gerard, Michael Richardson, Gavin W Held, Gavin McCabe Weir, Benjamin J Faber, M J Pueschel, Xiang Han, Benedikt Geiger

A synthetic reflectometer diagnostic has been developed to model the reflectometer used at the Helically Symmetric Experiment (HSX) stellarator. The main purpose of the HSX reflectometer is to measure density fluctuations in order to study turbulence and coherent modes in HSX. A full-wave 2-D synthetic diagnostic modeling the HSX reflectometer has been used to investigate its radial localization, characterize its sensitivities, and relate gyrokinetic simulations to physical measurements. The synthetic diagnostic results show that signal localization peaks slightly outboard of the cutoff surface, with the main localization lobe spanning between 15% and 30% of the minor radius. The poloidal and radial wavenumber sensitivities are k θ < 1.7 cm -1 and k r < 6 cm -1 , respectively, with minimal variation in k r sensitivity across probing regions and density profiles. Conversely, k θ sensitivity increases to > 2 cm -1 in the core of the device, in accordance with an increase in local flux-surface curvature. Synthetic reflectometer signals scale linearly with density fluctuation amplitude over a large range of fluctuation levels. Multiple density fluctuation level estimators are tested to identify the optimal choice for use in HSX, with an equation from G.D. Conway Plasma Phys. Control. Fusion 1999 consistently performing best overall. Synthetic measurements of rescaled density fluctuations from a gyrokinetic simulation are then compared against experimental measurements, with coherent mode activity at 13 -15 kHz and 22 kHz observed in both experimental and synthetic frequency spectra. Synthetic spectra overestimated experimental spectra at low frequencies and underestimated at high frequency.

Coupling time-dependent equilibrium evolution to 1D current diffusion equation towards integrated modeling of stellarators

Emiliano Fable, Fabian Solfronk, Elisa Buglione-Ceresa, David Kulla

It is presented a framework in which time-dependent simulations of a 3D toroidal magnetized plasma (stellarator device) are self-consistently taking into account the variation of the magnetic equilibrium together with the plasma current and a background varying pressure inside a given plasma boundary shape (prescribed boundary simulations). The key result is to solve the issue of prescribing the degree of freedom present when performing prescribed boundary simulations with a 3D equilibrium solver, that is the anchor vacuum field not usually considered in this kind of framework.

Uncertainties of magnetic equilibrium reconstructions

Rainer Fischer, Michael G Dunne, Joerg Hobirk, Tilmann Lunt, Wolfgang Suttrop

Uncertainties of quantities of magnetic equilibrium reconstructions are essential for the validation and quantification of estimated plasma parameters and their uncertainties. The uncertainties of a few equilibrium quantities can be evaluated analytically. A Monte-Carlo method is proposed to estimate the uncertainty of any equilibrium quantity. The Monte-Carlo method was verified using analytic formulas for the uncertainty of some equilibrium quantities and applied to various scalar, profile, flux-surface averaged and integral plasma quantities. Additionally, the uncertainty of the category of negative triangularity plasmas and alternative divertor configuration plasmas were evaluated.

Identifying non-performing or Dud Plasmas for Burning Plasma Control: Insights from JET and TFTR Deuterium-Tritium Campaigns

Lidia Piron, Nicolò Ferron, Eric Fredrickson, Morten Lennholm, Alessandro Pau, Timo Ravensbergen, Olivier Sauter, Fulvio Auriemma, Matteo Baruzzo, Krassimir K. K Kirov, et al.

Among the burning plasma controllers for research fusion reactors, the dud detector will be of primary importance as it determines whether the plasma is performing well or if it is a dud. In the latter case, the discharge needs to be terminated to remain within tritium and neutron activation limits. To this scope, monitors which track the plasma performance will be integrated in the plasma control system. In this work, we present a novel dud detector that has been empirically identified based on Deuterium-Tritium campaigns carried out at JET and TFTR. This controller estimates a proxy of the neutron rate using a combination of the diamagnetic energy and the density peakedness. If the predicted neutron rate deviates from the real-time measurement, then the dud detector will trigger an alarm leading to a safe plasma termination if plasma recovery is not expected or, to actuator requests when adjustment is possible. This monitoring function can also be coupled with equilibrium solver and control-oriented models, such as RAPDENS, as proposed in the 15 MA plasma current, 5.3 T toroidal magnetic field baseline Deuterium-Tritium ITER scenario.

On Phase Transition of ITG Turbulence in the Dimits shift

L. N. Marquant, P. Morel, Ö. D. Gürcan

The transition between turbulent and zonal flow dominated states is investigated by varying the ion temperature gradient in nonlinear gyrokinetic simulations. Independent gradient scans reveal three distinct regimes: a turbulent regime at high gradients, a zonal flow dominated regime with strongly reduced heat transport at low gradients, and an intermediate regime characterized by intermittent switching between these two states. These different regimes can be classified using an order parameter, defined as the fraction of the zonal to total free energy in the system. To assess the memory effects, the temperature gradient is first lowered gradually from high values that result in fully developed turbulence, to lower values in the Dimits shift region that form strong zonal flows, and then is slowly increased back. Once the zonal flows form, and efficiently suppress turbulence, they can persist at higher gradients, leading to an asymmetric response implying a hysteresis loop. It is observed that, in the zonal flow dominated state, the free energy is mostly condensated in the largest radial scale, with a steep slope of the $k_x$ spectrum, while in the turbulent state, it exhibits a wider spectrum with two distinct slopes.

DKEKAN: A single-parameterized KAN surrogate for Drift Kinetic Equation Toward Fast Neoclassical Toroidal Viscosity Torque Modeling in Tokamaks

Jinpeng Huang, Xingting Yan, Mingyu Zhang, Nana Bao, Zixuan Song, Yuetao Meng, Weiyong Zhou, Youwen Sun

The neoclassical toroidal viscosity (NTV) torque is a critical driver of toroidal rotation in tokamaks, profoundly influencing plasma stability and performance. Consequently, incorporating NTV effects is essential for modern integrated modeling frameworks that aim to self-consistently unify multiple physical processes. However, the high computational cost of NTV modeling precludes its self-consistent integration within such frameworks. This bottleneck arises because NTV calculation requires solving its governing equation--the drift kinetic equation (DKE)--in high-dimensional phase space. To address this issue, this study develops DKEKAN, a single-parameterized Kolmogorov-Arnold Network (SKAN) surrogate for solving DKE, to realize fast NTV modeling in tokamaks. The research process consists of the following steps: Firstly, a large dataset mapping DKE equation parameters to solutions is generated based on first-principle simulations under plasma parameters of the Experimental Advanced Superconducting Tokamak (EAST); Secondly, a surrogate model for solving DKE is developed based on the SKAN framework, which also incorporates a modular expert network design; Finally, the DKEKAN surrogate model is integrated with the NTV modeling framework to realize fast NTV calculation. With its physics-grouped expert layer and SKAN backbone, DKEKAN outperforms the tested MLP, KAN, and neural-operator baselines in overall prediction accuracy, while reducing the standalone DKE-solving time from 35.85s to 3.74s, corresponding to a speedup of approximately 9.6x, and reducing the total coupled NTVTOK runtime from 38.24s to 5.58s, corresponding to an overall speedup of approximately 6.9x. This work effectively overcomes the computational bottleneck in NTV simulations, thus supporting further integrated modeling that incorporates NTV effects.

Tunable betatron X-ray radiation by spatially varying magnetic fields in laser wakefield accelerators

Anandu Ramachandran Sushama, Bhuvanesh Ramakrishna

Indian Institute of Technology Hyderabad

Betatron X-ray radiation from laser wakefield accelerators provides a compact source of ultrafast, broadband, high-brightness photons, but its tunability is fundamentally limited by the lack of direct control over transverse electron dynamics. Here, we demonstrate controlled manipulation of betatron oscillations using an externally applied transverse magnetic field with a Gaussian longitudinal profile. The spatially varying magnetic field shifts the electron equilibrium orbit and excites transverse motion through a driven-oscillator mechanism. We develop an analytical model that predicts the induced betatron amplitude and identifies a dimensionless interaction parameter governing the excitation. The final oscillation amplitude is shown to arise from coherent interference between the initial and induced oscillations, enabling both enhancement and suppression of transverse motion. Particle-in-cell simulations quantitatively validate the theoretical predictions and demonstrate tunable betatron X-ray emission with controllable photon yield, spectral distribution, and angular divergence. These results establish externally tailored magnetic fields as a practical approach for phase-selective control of electron dynamics and radiation in plasma-based accelerators.

Ionization and charge-exchange cross sections in H⁺ + H2 and D⁺ + D2 collisions based on the classical trajectory Monte Carlo method

Iman Ziaeian, Reza Ramazani-Sharifabadi, Karoly Tokesi

Nuclear Science and Technology Research Institute, HUN-REN Institute for Nuclear Research (ATOMKI)

We present ionization and charge exchange cross sections for H + + H2 and D + + D2 collisions at impact energies ranging from 10 to 200 keV/amu. A three-body Classical Trajectory Monte Carlo approximation is used to calculate these cross sections. In our approach, H2 and D2 molecules are modeled as hydrogen-type atoms with one active electron bound to a central core of effective charge and binding energy. Our classical results are compared with available experimental data and quantum mechanical results. We found that the charge-exchange cross sections for hydrogen molecules impacted by protons, as well as for deuterium molecules impacted by D + , are in good agreement with experimental data and semiempirical results. For the ionization channel, our calculated cross sections for H + + H2 collisions align well with semi-empirical results. We demonstrate that the classical treatment can effectively describe the ionization and charge exchange cross sections in molecular targets.

The role of data quality and alignment in cross-tokamak disruption prediction

Chengshuo Shen, Wei Zheng, Fengming Xue, Xinkun Ai, Bihao Guo, Dalong Chen, Zhongyong Chen, Yong Hua Ding

Huazhong University of Science and Technology, Institute of Plasma Physics, Chinese Academy of Sciences

Reliable disruption prediction across tokamaks is needed for next generation devices such as ITER, SPARC, and BEST, where disruptive target-machine data will be scarce by design. For a transferred predictor, target performance is limited by both the source-domain error and the mismatch between the source and target feature distributions. We use this distinction to revisit our previous J-TEXT to EAST study and to add three components for few-shot and zero-shot operation. An upgraded physics-guided feature extraction (PGFE-U) reduces geometry-dependent differences in Mirnov, soft x-ray and absolute extreme ultraviolet array features. A floating labelling strategy (FLS) replaces a fixed pre-disruption window with shot-dependent precursor onset labels. An estimation of the feature distribution (EFD) supplies target-machine z-score statistics from outside the classifier training set. With the supervised CORAL (S-CORAL) backbone, the area under the receiver operating characteristic curve (AUC) on the same EAST test set reaches 0.957 in a few-shot setting using only 10 disruptive EAST discharges, whereas the previous work required 110 EAST discharges to reach 0.890, and the partial AUC over false positive rates up to 10% (pAUC) rises from 0.428 to 0.858. The zero-shot AUC reaches 0.892 with no EAST discharge used for classifier training, against 0.592 for the same model normalised with J-TEXT statistics, and the zero-shot pAUC is 0.619 against 0.084. The EFD statistics are estimated from real EAST shots; a per-feature Monte-Carlo scan shows that the sensitivity is concentrated in a few global discharge parameters, the quantities most reliably estimated at the design stage. Shapley additive explanation (SHAP) based attribution shows that both transferred models preserve most of the qualitative feature-to-disruption trends of the abundant-data baseline. These results indicate that improving feature quality, labels and normalization statistics can recover a large fraction of abundant-data performance when target-machine shots are limited.

Sheared poloidal plasma flows in the island divertor scrape-off layer of Wendelstein 7-X

Sean Bozkurt Ballinger, Seung Gyou Baek, Olaf Grulke, Carsten Killer, Floris Scharmer, Jim L Terry, Adrian von Stechow

Massachusetts Institute of Technology, Max-Planck-Institut fur Plasmaphysik, Technical University of Denmark

Gas puff imaging (GPI) has been routinely operated since the installation of the water-cooled high heat flux divertor in the Wendelstein 7-X (W7-X) stellarator. GPI provides the first systematic, two-dimensional experimental characterization of perpendicular plasma flows in the magnetic island divertor scrape-off layer (SOL) on W7-X. Using spatiotemporal analysis of turbulent fluctuations, we find that, across magnetic configurations and plasma scenarios, poloidal flows ubiquitously dominate over radial flows, with typical poloidal velocities of order km/s. In contrast to tokamaks, radially propagating blob filaments are not observed, consistent with the predominantly normal fluctuation statistics measured in the island SOL. Net radial motion is detected only in the vicinity of an X-point of the magnetic island, which is accessed in a low-iota magnetic configuration by the present GPI field-of-view. In general, the poloidal flow profiles in the island SOL exhibit a complex structure, with multiple counter-propagating flow channels as narrow as 1 cm. The experimental parameters that order the observed poloidal flow structure near island O-points are investigated. Both the magnitude of the poloidal velocity and the number of shear layers, as well as their radial location, depend sensitively on the size and position of the magnetic islands, as well as on the toroidal current, plasma density, and toroidal field direction. These results demonstrate that cross-field transport in the W7-X island divertor SOL is correlated primarily with topology-driven ExB sheared flows, rather than by radial filamentary transport, and provide the first experimental documentation of the complex, shear-layer-dominated flow dynamics in the island divertor over a wide range of plasma conditions.

High energy X-ray generation via nanosecond laser irradiation of metal wire-array targets

Chao Tian, Tiankui Zhang, Minghai Yu, Lianqiang Shan, Zhimeng Zhang, Peilin Yao, Feng Zhang, Zongqiang Yuan, Zhongjing Chen, Weimin Zhou

China Academy of Engineering Physics

Experiments were conducted at the SG-III prototype laser facility to generate high-energy X-rays via interaction of focused nanosecond laser beams with wire-array targets of varying materials. X-rays exceeding 10 keV were successfully produced. The energy spectrum was measured using a filtered stack spectrometer and a transmission crystal spectrometer, determining the laser-to-X-ray conversion efficiency. Backlighting radiography of test pattern targets was performed, with image clarity enhanced through source distribution deconvolution, achieving improved spatial resolution of the X-ray source. Additionally, areal density resolution was evaluated through backlighting radiography of stepped objects.

Challenges of Locked Mode Avoidance in the Low-ν* ITER Baseline Scenario Plasmas in ASDEX Upgrade

Lidia Piron, Thomas Pütterich, Tomas Markovic, Olivier Sauter, Wolfgang Suttrop, Paolo Bettini, M Bonotto, Athina Kappatou, Marc Maraschek, Leonardo Pigatto, et al.

Max-Planck-Institut für Plasmaphysik, Universita degli Studi di Padova, Czech Academy of Sciences Institute of Plasma Physics, Ecole Polytechnique Federale de Lausanne, Universita di Udine

In AUG, the requirement to explore the low-ν* regime within the same ITER baseline scenario parameters, i.e. a Greenwald fraction of ~0.85, q₉₅ ≈ 3, βN ≈ 1.8, and H98 ≈ 1, has been addressed by inducing density pump-out through n=2 magnetic field perturbations. This approach required additionally relaxing the high-δ plasma shape to δ ≈ 0.1 to enable sufficiently strong density pump-out. However, at low density also the generation of locked modes was observed which deteriorates the plasma performance. Several strategies were therefore tested, including model-based error field correction combined with different heating schemes, as well as a core–edge decoupling metric derived from the GPEC code. Nevertheless, these attempts were unsuccessful, largely because the gap between the onset of density pump-out and the excitation of the core mode is very small.

Accuracy of COMPASS Upgrade equilibrium reconstruction

Marie Vanakova, David Fridrich, Ondrej Kovanda, Tomas Markovic, Stefano Marchioni, Lukas Kripner, Martin Imrisek, Pavel Bělina, Viktor Veselovsky, Petr Vondracek, et al.

Institute of Plasma Physics of the Czech Academy of Sciences, Czech Academy of Sciences, Czech Technical University in Prague, Universitaet Innsbruck, University of West Bohemia in Pilsen

We carry out an analysis of the reconstruction accuracy of plasma equilibrium during the plasma current flat-top phase of tokamak COMPASS Upgrade, utilizing the codes EFIT and LIUQE. We do so for the nominal set of input synthetic magnetic diagnostics signals, as well as those with varying degrees of introduced measurement errors (normal distribution with 2 % standard deviation) within 75 Monte Carlo data sets inputs. The analysis was performed for three COMPASS Upgrade baseline scenarios using the measurements of standard and backup magnetic diagnostics set, and for a case study of missing magnetics sensors on low field side. We show that both in the absence of the measurement errors as well as for conservative estimates of 2 % error the reconstruction accuracy of LIUQE code is within the 1 cm limit, albeit approaching it on the top and bottom of the plasma column. EFIT also performs well in the absence of the input measurement errors, but with errors included the inaccuracy of the reconstruction reaches slightly above the error limits on several positions. We conclude that the utilization of LIUQE for tokamak COMPASS Upgrade plasma control will be viable, albeit real-time implementation of the reconstruction algorithms will have to be subject of our future work.

Numerical modeling of plasma focus device dynamics using FLASH validated with shadowgrams

CHRISTOS KARVOUNIS, Alexandros Skoulakis, Ioannis Tazes, Christos Vlachos, John Chatzakis, Evaggelos Kaselouris, Vasilis Menelaos Dimitriou, Nektarios Andrea Papadogiannis, Michael Tatarakis, Ioannis Fitilis

Hellenic Mediterranean University

This paper presents a detailed 2D resistive magnetohydrodynamic (MHD) study of a miniature deuterium-filled plasma focus (PF) device using the FLASH code. The simulations capture the full plasma evolution, from initial breakdown ("cold start") to pinch compression and disruption. A novel contribution is the implementation of a dynamic magnetic-field updating algorithm which tracks the evolving plasma sheath and self-consistently reconstructs the return-current path, including skin-effect corrections. To overcome limitations of optical probing diagnostics, an inhouse developed ray-tracing post-processing tool generates synthetic shadowgrams directly derived from the simulated plasma parameters validating the proposed modeling approach. Numerical results -including plasma density, velocity, current density, magnetic fields, and ion/electron temperatures -show strong agreement with experimental data from a 600 J (~80 kA) miniature PF device. This validation demonstrates the capability of FLASH to provide highfidelity physical insights into dense plasma focus dynamics.

Development of a high-current, high fusion performance scenario on JET: physics insights and operational challenges

Luca Garzotti, Domenico Frigione, Peter J Lomas, Fernanda Rimini, Dirk Van Eester, Vito Konrad Zotta, Spyridon Aleiferis, Edoardo Alessi, Fulvio Auriemma, Rennan Bianchetti Morales, et al.

Culham Centre for Fusion Energy, United Kingdom Atomic Energy Authority, Institute of Plasma Physics and Laser Microfusion, Consiglio Nazionale delle Ricerche, Sapienza University of Rome

Since the installation of a beryllium-tungsten(Be/W) first wall on JET an intense scenario development programme has taken place to realize a high-current (3.5-4 MA) scenario, denominated JET baseline scenario, with $q_{95} \sim 3$, $\beta_N \sim 1.8$ and capable of delivering high fusion performance ($P_{fusion} > 10$ MW) for 5 s in the D-T campaigns conducted in 2021 and 2023. The JET baseline scenario was realized in D-T at 3.5 MA in 2021 (DTE2 campaign), but, despite achieving transitorily $P_{fus} \sim 8$ MW, it could not be sustained for more than 3 s. In this paper we will revisit and expand the analysis of the baseline scenario at 3.5 MA and present new results obtained in D-T at 3.0 MA in DTE3, where we were able to sustain the scenario for 5 s. We will also present recent modelling results showing how available semi-empirical and first-principles transport models can explain a variety of phenomena ranging from fusion performance, impurity transport and control of the fuelling mix. Many of these points are potentially relevant for bigger machines such as ITER. Finally, we will discuss open questions and improvements to our present modelling capabilities required to extrapolate the results to power plant devices.

Observation of fast ion acceleration under EGAM in EAST pure ECRH plasma with tearing mode

Chaowei Mai, liqing xu, Shiyao Lin, Tianfu Zhou, Tonghui Shi, Kangning Geng, Yifei Jin, Yanmin Duan, Kaiyun Chen, Yan Chao, et al.

Institute of Plasma Physics, Chinese Academy of Sciences, Chinese Academy of Sciences, Guangdong Ocean University, Chinese Academy of Sciences - Hefei Institutes of Physical Sciences

In magnetically confined fusion plasmas heated only by Electron Cyclotron Resonance Heating (ECRH), ion temperature typically saturates in the 1–3 keV range. The EAST tokamak, equipped with an ITER-like tungsten divertor, has operated deuterium H-mode discharges with high-power pure ECRH. In a specific discharge, a continuous ion acceleration process is observed, with ion energy ranging from an off-axis supra-thermal component above 6 keV to an isolated Gaussian peak reaching 1.8 MeV. The acceleration occurs at the location of a large 2/1 magnetic island, and an EGAM-like electrostatic mode at ∼ 23 kHz is detected by microwave diagnostics. The MeV-range cutoff energy, the mode power, and the island width exhibit pairwise strong linear correlations (R2 > 0.9), and the three are spatially coincident. Transfer entropy analysis and a 0D collisional-radiative model suggest a mechanism from island to mode to ion acceleration, excluding a purely electric-field-driven process. These observations provide experimental evidence of MeV-range ion acceleration accompanied by an EGAM-like mode in a pure ECRH plasma without auxiliary ion heating, offering a new data point for understanding ion acceleration under ECRH operation in tungsten-wall devices, particularly with degraded boron wall coating

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.

Fast-ion enhanced modeling of neoclassical tearing modes at NSTX and DIII-D

James Yang, Eric Fredrickson, John W Berkery, Robert John La Haye, Mario Podesta

Princeton University Plasma Physics Laboratory, General Atomics, Ecole Polytechnique Federale de Lausanne

A new framework for the solution of modified Rutherford equation including fast ions is successfully applied to interpret the tearing mode stability of two discharges NSTX #134020 and DIII-D #135861. The simulated island width growth rates are in better agreement with the measured island growth rate when the fast ions are included. While constants are multiplied to the polarization current contribution terms for NSTX #134020, no constants are necessary to match the simulated and measured island width growth rates for DIII-D #135861. The estimated island frequencies appear to provide an explanation of the different constants used in the two discharges. The gradient scale lengths suggest that the fast ion contribution can become significant in plasmas with flat thermal ion density profile and steep fast ion density profile.

Simulation results of the multi-mode islands on runaway electron suppression in J-TEXT

Zhifang Lin, Yang Yang, Junhui Yang, Haocheng Wang, Xiping Jiang, Zhonghe Jiang, Zhongyong Chen, Wei Yan

Huazhong University of Science and Technology, China University of Mining and Technology, Jiangsu Normal University, Jilin Electric Power Co. Ltd

Locked modes are known as one of the major causes of disruptions, and investigating their impact on runaway electron (RE) generation is essential for the development and implementation of disruption mitigation systems. The effect of multiple mode islands (including the 2/1 mode and the 3/1 mode islands) on the RE suppression is simulated in the J-TEXT tokamak. The O-points of the 2/1 and 3/1 mode islands are initially phase-locked in the poloidal direction. NIMROD and DREAM simulations show that the RE suppression is significantly dependent on the relative phase between the O-points of multi-mode islands and the Massive Gas Injection (MGI) port. When the relative phase is 0^° or 180^°, the loss of RE seeds reaches its maximum in the NIMROD simulation. In contrast, the efficiency of RE mitigation is lower when the relative phase is 90^° or 270^°. This distinct behavior can be attributed to differences in the degree of magnetic-field stochasticity, which arise from impurity penetration and the evolution of magnetic perturbations. The transport coefficients calculated from the stochastic magnetic fields are incorporated into DREAM to evaluate the runaway current evolution during the current quench phase. In the cases with relative phases of 0^° and 180^°, the transport-induced losses are sufficiently strong to overcome the avalanche generation, resulting in a negative net RE generation rate and effective suppression of RE current. These findings demonstrate that the topology of multi-mode islands prior to disruption can significantly modulate the RE loss during disruption. For the locked-mode disruptions, effective runaway current mitigation may be achieved through the optimization of the relative phases between MGI and locked-mode islands.

Gas pressure dependence of stray particles in the dual-driver RF negative ion source of CRAFT NNBI

Yuwen Yang, Zhengkun Cao, Na Wang, Yongjian Xu, Wei Liu, Yuming Gu, Chundong Hu, Yahong Xie, Lizhen Liang, Jianglong Wei

Chinese Academy of Sciences Hefei Institutes of Physical Science

The negative-ion-based neutral beam injection (NNBI) system is a reliable plasma heating and current drive method for large-scale magnetic confinement fusion devices. In the negative ion source, the problem of stray particles is a serious and unavoidable issue. These stray particles are mainly produced through particle-gas interaction in the accelerator region and can be subsequently accelerated, leading to high-voltage breakdown and substantial thermal deposition on components. The gas pressure plays a key role in determining the frequency of particle-gas collisions and simultaneously influences plasma and beam parameters, which in turn affect the generation and transport of stray particles. A NNBI test facility has been constructed in the Comprehensive Research Facility for Fusion Technology (CRAFT) in China. To investigate the dependence of stray particles behaviour on gas pressure, three sets of experiments have been conducted on CRAFT NNBI test facility. As the gas pressure increased, the power load on the ground grid (GG) first decreased and then increased, exhibiting an unexpected minimum value in the pressure range of 0.40~0.45 Pa. A similar trend was observed for the vertical beam divergence, whereas the horizontal beam divergence increased monotonically with gas pressure. In contrast, the ejecting electron power, stripping loss, and backstreaming positive ion power were positively correlated with gas pressure. These results provide experimental insight into the complex role of gas pressure in stray particle generation and beam quality in NNBI systems.