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I.1-Barabaschi: Status of the ITER project

• Pietro Barabaschi (Iter)

I.5-Zarzoso: Energetic particle transport in theory, modelling and experiments in magnetically confined plasmas

• David Zarzoso (CNRS, Aix-Marseille Université)

O.1-Grondin-Exbrayat: Numerical Investigation of Fishbone Phase-space Nonlinear Dynamic : Impact of the MHD Nonlinear Effects

• Alodie Grondin-Exbrayat (Aix-Marseille Université, CNRS, PIIM UMR7345, Marseille, France)

Understanding the dynamics of Energetic Particles (EPs) is essential for optimizing the perfor- mance of fusion devices. EPs serve as the primary energy carriers in fusion plasmas, and their loss due to transport from the core to the edge results in a significant reduction in efficiency of the fusion device. One of the key phenomena driving the EPs transport is the precessional fishbone instability[1]. The precessional fishbone instability is a kinetic instability driven by the destabilization of a magnetohydrodynamic (MHD) mode due to an EP gradient [2]. In this work, we present a numerical study of the precessional fishbone instability using a sim- plified 2D phase-space model [3]. The model couples a reduced MHD framework in cylindrical geometry, which describes the thermal plasma, with a kinetic description of trapped energetic particles. This study builds on the reduced MHD code AMON [4], to which we have added a gyro- and bounce-averaged Vlasov equation for the EPs. We investigate the resonant interaction between particles and fields, which drives the nonlinear evolution of the EPs gradient, leading to their transport [5]. The question addressed in this study concerns the impact of MHD nonlinearities on the saturation level and the evolution of the mode frequency. While the linear phase of the fishbone instability is relatively well understood [3, 6], our work aims to investigate how MHD nonlinearities influence the level of saturation by preserving the resonant condition thus modifying the frequency chirping behavior [5], which are directly linked to the nature of EPs transport [7]. [1] R.B. White, et al., Physical review letters 62, 539 (1989) [2] L. Chen, et al., Physics of Plasmas 7, 1519-1522 (1994) [3] M. Idouakass, et al., Physics of Plasmas 23, 102113 (2016) [4] A. Poye, et al., Plasmas Physics and Controlled Fusion 56, 125005 (2014) [5] L. Chen, et al., Reviews of Modern Physics 88, 015008 (2016) [6] A. Fasoli, et al., Nuclear Fusion 47, S264 (2007) [7] F. Zonca, et al., Nuclear Fusion 45, 477 (2005)

I.6-Brochard: Transformation of energetic particle distributions into constants of motion coordinates for realistic simulations of EP-driven modes, and application to Alfvén eigenmodes stability in DIII-D plasmas

• Guillaume Brochard (CEA Cadarache)

0.2-Votta: Runaway electron dynamics in ITER shattered pellet injection mitigated disruptions

• Lorenzo Votta (KTH Royal Institute of Technology)

Disruptions represent a critical challenge to the safe and reliable operation of future fusion devices like ITER, as they impose severe thermal and mechanical loads on the tokamak structure and generate high-energy runaway electrons (REs). This study expands on the work of [Vallhagen et al, Nucl. Fusion 64 (2024)] and presents a significant upgrade of the DREAM [1] disruption simulation framework to investigate RE dynamics in ITER disruptions mitigated by Shattered Pellet Injection (SPI). The updated simulations account for four new key physical effects. The scraping-off of REs during Vertical Displacement Events is incorporated via a reduced model which reproduces the RE avalanche gain of higher-fidelity simulations [2]. The plasmoid drift effect, which affects the deposition location of the injected pellet material, is accounted for via an analytical model which has been validated against experimental data on DIII-D [3]. Additionally, a model for suppressing unphysical thin-current channels during the current quench phase is implemented. The Compton seed generation model has been updated with photon spectra reflecting the new ITER tungsten first-wall design. A wide range of realistic disruption scenarios are explored, including cases with low Neon SPI, scans of radial RE transport under varying magnetic perturbations, and trace tritium concentration scans to assess the impact of tritium beta decay RE source. The analysis of these scenarios guides the design of ITER SPI strategies, supporting the development of effective disruption mitigation techniques for ITER and future fusion devices. [1] M. Hoppe, O. Embreus, and T. Fülöp, “DREAM: A fluid-kinetic framework for tokamak disruption runaway electron simulations,” Computer Physics Communications, vol. 268, p. 108098, 2021. doi: 10.1016/j.cpc.2021.108098. [2] O. Vallhagen, L. Hanebring, T. Fülöp, M. Hoppe, and I. Pusztai, “Re- duced modeling of scrape-off losses of runaway electrons during tokamak disruptions,” arXiv preprint arXiv:2410.03512, 2024. Available: https: //arxiv.org/abs/2410.03512 [3] O. Vallhagen, I. Pusztai, P. Helander, S. L. Newton, and T. Fülöp, “Drift of ablated material after pellet injection in a tokamak,” Journal of Plasma Physics, vol. 89, no. 3, 2023. doi: 10.1017/S0022377823000466.

O.3-Edes: Resistive Wall Tearing Modes in View of Natural Disruption Dynamics

• Lili Édes (EPFL SPC)

Plasma disruption is one of the key factors limiting the stable and safe operation of future large tokamaks. It involves a sudden collapse of the plasma confinement and can cause severe heat loads and electromagnetic forces on surrounding structures. Understanding the chain of events leading to unintentional/natural disruptions remaining a critical goal in the pursuit of sustainable fusion energy [1]. The final phase of a disruption is commonly associated with the growth of magnetohydrodynamic (MHD) instabilities, particularly tearing modes. Recent studies indicated that the finite resistivity of the wall can modify the previously predicted stability boundaries of such modes [2], giving rise to what are termed as Resistive Wall Tearing Modes (RWTMs) [2,3]. If this is the main mechanism that triggers the termination of plasma, then predictions on the thermal quench duration must be revised and adjusted to the RWTM growth rates, which need to be studied in detail. In order to investigate these effects, we developed a linear solver that accounts for wall resistivity, a feature typically omitted in classical tearing mode analysis. This implementation was based on the methodology shown in [4]. The solver was employed to scan for unstable RWTM scenarios, specifically by varying current profile shapes in large-aspect-ratio equilibria, providing a baseline for nonlinear studies. Subsequently, simulations were carried out using the three-dimensional non-linear MHD code JOREK, coupled with STARWALL. Notably, this work marks the first identification of RWTMs in JOREK–STARWALL simulations. Good agreement was found between the linear solver and the JOREK-STARWALL code results in terms of mode growth rates. Further benchmarking are being performed against the CASTOR3D MHD code. The applicability of these findings are also being tested in a more realistic and experimentally relevant setup. Specifically, the effect of wall resistivity on mode growth are being analyzed with JOREK-STARWALL for a JET disruption case previously studied by Hank Strauss using the M3D code [2]. The sensitivity of the mode growth rate to wall resistivity are being examined, and comparison with previous M3D simulations will be discussed. Our findings offer new insight into the role of RWTMs in disruption dynamics and support the development of improved prediction and mitigation strategies, an essential step toward ensuring the stability and safety of next-generation fusion devices. References [1] G. Pucella et al., Nuclear Fusion 61 (2021): 046020 [2] H. Strauss et al., Physics of Plasmas 28 (2021): 032501 [3] H. Strauss et al., Plasma Physics and Controlled Fusion 65 (2023): 084002 [4] J.P. Graves et al., Plasma Physics and Controlled Fusion 64 (2021): 014001

I.7-Vallhagen: Semi-analytical model for the pellet rocket effect in magnetic confinement fusion plasmas

• Oskar Vallhagen (Chalmers University of Technology)

0.4-Ciraolo: Transport and turbulence in edge tokamak plasma: Hierarchy of models in SOLEDGE3X code and comparison with experiments

• Guido Ciraolo (CEA)

Accurate numerical modelling of turbulent transport in edge tokamak plasma remains a significant challenge. Many key experimental features, such as the formation of edge transport barriers, are still difficult to simulate, especially for ITER-sized tokamaks. Predicting the scrape-off layer (SOL) width or the power load imbalance between the inner and outer divertor legs remain an open issue, their characterization being essential to determine the plasma regimes to be developed in future fusion power plants. First-principle modelling of edge plasma turbulence is therefore a key area of research in the fusion community, as it allows to extrapolate from present day experiments to future tokamaks. Inspired by the hierarchy of models used to simulate turbulence in the neutral fluid community, the SOLEDGE3X fluid code incorporates a broad range of models with varying fidelity [1], which allows a stage approach analysis to the problem of edge turbulence. These range from empirical diffusivities, which are used to perform so-called "transport" simulations, to full-scale 3D first-principle turbulence modelling, where turbulent structures are self-consistently simulated. In between, a reduced approach inspired by the k-epsilon model [2], widely used in the computational fluid dynamics community, is proposed to capture key features of edge plasma turbulence and incorporate them into transport simulations [3]. Specifically, the growth of the turbulent energy "k" is governed by the primary interchange and drift wave instabilities, while turbulence saturation is achieved through a semi-empirical closure based on scaling laws [4]. Such approach allows for a quick assessment of the main turbulent characteristics of the plasma edge. In this contribution, we present a direct comparison of the various approaches to model TCV and WEST edge plasma transport and turbulence, ranging from empirical transport modelling, transport modelling with k-epsilon prediction, and first-principle modelling. The fluctuation levels and cross-field transport predicted by the k-epsilon model is directly compared with first-principle simulations and experimental measurements. Interestingly, the k-epsilon model is able to recover not only the ballooned feature of radial transport at the outer midplane but also an increased radial transport along the divertor leg. A special focus will be put on comparing the behaviour of divertor localized filaments observed experimentally in TCV with fast cameras and recovered by 3D turbulence modelling [5]. [1] H. Bufferand et al., Nucl. Fusion 61 (2021), 116052 [2] W.P. Jones, B.E. Launder, Int. J. Heat Mass Transfer 15, (1972), 301 [3] S. Baschetti et al., Nucl. Fusion 61 (2021), 106020 [4] R.J. Goldston et al Nucl. Fusion 52 (2012), 013009 [5] H. Bufferand et al., Nucl. Mat. Energy 41 (2024), 101824

O.5-Hallatschek: Non-gyrokinetic high-frequency mode instability for tokamak edge like gradients

• Klaus Hallatschek (Max-Planck-Institute for Plasma Physics)

The stability analysis of gyrokinetic slab ITG modes is well-established and results in temperature and density gradient stability thresholds that depend on the parallel and perpendicular wavenumbers. In a recent PRL [1], using 6D turbulence simulations with a specially optimized code that resolves the Larmor orbits, we found potentially significant non-gyrokinetic instabilities for steep, but not unrealistic, gradients in the tokamak edge with simplified slab geometry. These instabilities may be classified as unstable ion-Bernstein waves (IBW). To understand these phenomena, we extended the ITG analysis to high-frequency non-gyrokinetic modes. This extension requires modifying the zero-frequency gyrokinetic polarization using the Gordeyev function, as well as providing rigorous estimates of the magnitudes of the infinite sums appearing in the dispersion relation. Similar to the ITG modes, we derive a threshold criterion for the instability. However, unlike gyrokinetic ITGs, the non-gyrokinetic modes require only the presence of a temperature gradient and not a particularly high ratio of temperature to density gradients. In contrast, gyrokinetic ITGs are suppressed when the density gradient is too large relative to the temperature gradient (the $\eta_i$ criterion). Interestingly, the IBW growth rate tends to increase with the density gradient. It is likely that including additional physics (e.g., magnetic curvature or interactions with kinetic electrons) will significantly amplify the drive of similar non-gyrokinetic instabilities, potentially leading to an expansion of the field of strongly magnetized plasma turbulence. [1] M. Raeth, K. Hallatschek, "High-Frequency Nongyrokinetic Turbulence at Tokamak Edge Parameters", Phys. Rev. Lett 133, 195101

Poster Session #1 **Alfie Adhemar** [*Fluid and Kinetic Modelling of Sheath Transition Region with a Novel Anisotropic Ion Pressure Model and Enhanced Boundary Conditions*][1] **Benoit Clavier** [*Artificial Intelligence surrogate model for accurate long-time plasma turbulence simulations*][2] **Chiara De Piccoli** [*Modelling NBI and ICRH synergy in ITER plasmas*][3] **Guilhem Dif-Pradalier** [*Weakly driven machines of the future: a challenge for plasma turbulence modelling*][4] **Peter Donnel** [*Gysela-axi: a flux-driven full-F gyrokinetic code to simulate axisymmetric tokamak plasmas*][5] **Achilleas Evangelias** [*Effect of Aspect Ratio and Simplified Coil Winding Surfaces on Optimized Quasisymmetric Stellarator Configurations*][6] **David Korger** [*Toward a nonlinear Schroedinger equation for the description of geodesic-acoustic-modes in tokamaks: Analytic gyrokinetic studies of the nonlinear self-interaction*][7] **Markus Markl** [*Toward understanding the impact of ion species in the plasma response to magnetic perturbations*][8] **Yann Narbutt** [*Fully global simulations of electromagnetic turbulence in the stellarator W7-X*][9] **Vera Oberhauser** [*Self-consistent Fluid-kinetic Implementation of Flux Expansion in a 1D Hybrid Fluid-kinetic Code*][10] **Carl Wilhel Rogge** [*Modelling the heating of a pellet-produced plasmoid in the non-linear MHD Code Jorek*][11] **George Tzimopoulos** [*Orbit Bifurcations and Drift Frequencies Shifts in Shaped Tokamak Equilibria*][12] **Irina Voitsekhovitch** [*Modelling of the non-inductive current ramp up scenario on TCV: thermal electron transport in reversed magnetic shear configuration*][13] **Fiona Wouters** [*Avalanche source in a 3D hybrid fluid-kinetic model of runaway electrons in tokamaks*][14] **Björn Zaar** [*Criterion for significant runaway electron generation in activated devices*][15] **Xiaolong Zhu** *Energetic-particle transport induced by synergy of multiple AEs and MHDs and phase space engineering control techniques in tokamak plasmas* **Alessandro Zocco** [*Trapped-electron modification of kinetic ballooning instabilities in general geometry*][16] [1]: https://indico.global/event/13788/contributions/133572/ [2]: https://indico.global/event/13788/contributions/133555/ [3]: https://indico.global/event/13788/contributions/133578/ [4]: https://indico.global/event/13788/contributions/133556/ [5]: https://indico.global/event/13788/contributions/133562/ [6]: https://indico.global/event/13788/contributions/133567/ [7]: https://indico.global/event/13788/contributions/133563/ [8]: https://indico.global/event/13788/contributions/133576/ [9]: https://indico.global/event/13788/contributions/133577/ [10]: https://indico.global/event/13788/contributions/133570/ [11]: https://indico.global/event/13788/contributions/133568/ [12]: https://indico.global/event/13788/contributions/133560/ [13]: https://indico.global/event/13788/contributions/133573/ [14]: https://indico.global/event/13788/contributions/133565/ [15]: https://indico.global/event/13788/contributions/133557/ [16]: https://indico.global/event/13788/contributions/133559/

I.2-Alonso: Fundamental physics and technology considerations of stellarator reactor design

• Arturo Alonso (CIEMAT)

I.8-Goodman: Searching for SQuIDs: Developments in the optimisation of Stable Quasi-Isodynamic Designs

• Alan Goodman (Max Planck Institute for Plasma Physics)

O.6-Calvo: A new class of optimized stellarators with zero bootstrap current

• Ivan Calvo (Laboratorio Nacional de Fusion, CIEMAT, Madrid, Spain)

Typically, stellarator optimization relies on the notion of omnigenity [1, 2], which guarantees small radial neoclassical transport at the low collisionalities that are characteristic of fusion-grade plasmas. In omnigenous magnetic fields, collisionless particles do not move radially on average and, in the course of their motion along flux surfaces, they never undergo transitions between different types of orbits. Quasi-isodynamic fields are a subset of omnigenous fields that give zero bootstrap current at low collisionality [3], a property that makes them compatible with an island divertor, an advanced exhaust solution for stellarators. Quasi-isodynamicity is the approach Wendelstein 7-X is based on [4], and probably the most mature concept for stellarator reactors. The recent introduction of the notion of piecewise omnigenity [5] represents a major theoretical breakthrough. In piecewise omnigenous fields, the average radial displacement of collisionless particles is zero, but in their motion over magnetic surfaces, particles experience transitions between different types of trapped orbits. The discovery of piecewise omnigenous fields radically expands the space of known configurations that are optimized with respect to radial neoclassical transport. In particular, piecewise omnigenous fields are free from certain topological constraints obeyed by magnetic field strength contours in omnigenous configurations that often lead to complex coils. In this conference contribution, we will show that the potential of piecewise omnigenity goes far beyond the optimization of radial neoclassical transport by proving that there exist piecewise omnigenous fields that give zero bootstrap current at low collisionality [6]. We will analytically derive the mathematical condition for zero bootstrap current and will confirm the accuracy of this condition by means of neoclassical simulations. Our results imply that two fundamental properties of plasmas confined in quasi-isodynamic fields (small radial neoclassical transport and vanishing bootstrap current) can also be attained by piecewise omnigenous fields, providing a pathway to new, possibly simpler stellarator reactor designs. [1] J. R. Cary and S. G. Shasharina, Phys. Rev. Lett. 78, 674 (1997). [2] F. I. Parra, I. Calvo, P. Helander et al., Nucl. Fusion 55, 033005 (2015). [3] P. Helander and J. Nührenberg, Plasma Phys. Control. Fusion 51, 055004 (2009). [4] H. Wobig, Plasma Phys. Control. Fusion 35, 903 (1993). [5] J. L. Velasco, I. Calvo, F. J. Escoto et al., Phys. Rev. Lett. 133, 185101 (2024). [6] I. Calvo, J. L. Velasco, P. Helander et al., arXiv:2505.02546. Submitted to Phys. Rev. Lett.

I.9-Kazakov: Alpha-Particle Physics Studies in D-3He Plasmas at JET and JT-60SA in Support of the ITER Rebaseline

• Yevgen O. Kazakov (LPP/ERM, Bruxelles, Belgium)

O.7- De Gianni: Effect of the magnetic geometry on Trapped Electron Modes instability: linear and non linear analysis

• Ludovica De Gianni (CEA Cadarache)

Good plasma confinement is crucial to harness fusion energy. Experiments on the TCV [1] and DIII-D [2] tokamaks have shown that negative triangularity (NT) reduces the turbulent transport, hence improving confinement. Trapped Electron Modes (TEM) are thought to play an important role in this process. A full understanding of the underlying physics is necessary for assuring NT plasmas to be reactor relevant, but is still missing. To identify the key physical mechanisms driving the improvement of confinement in TEMs dominated NT plasmas, a reduced analytical model has been recently derived, focusing on TEMs linear stability [3]. In this contribution, the model of [3] is extended by including a simplified model for ions without accounting for their resonant response - thus excluding ITGs. This extension enables a qualitative agreement of the model with gyrokinetic (GK) simulations performed with GYSELA [4] and GENE [5] in the linear regime, and an evaluation of the impact of Finite Larmor Radius effects on stability. The model highlights the key role of "Finite Mode Width" (FMW) effects, which is the poloidal localization of the linear modes – ballooning character. A key quantity for understanding the importance of FMW effects is the precession frequency, the growth rate in the fluid limit scaling like its square root. The ballooning of the instability gives more weight to deeply trapped electrons, whose precession frequency is lower for NT than for positive triangularity (PT). On the other hand, the benchmark with GENE highlights the key role played by the trapped electrons parallel dynamic (related to their bouncing frequency) and by passing electrons in the shear dependence of the different linear stability of TEMs in negative and positive triangularity. Although it provides precious insights into the underlying physics, a linear analysis is not sufficient to explain the experimental results [1][2]. Therefore, a nonlinear analysis has been carried out using the gyrokinetic code GYSELA. In particular, the electric and diamagnetic components of the Reynolds stress, which drive the zonal flows, will be shown to exhibit significant differences in NT and PT configurations. References [1] Y. Camenen et al., Nucl. Fusion 47 (2007) [2] M. Austin et al., Phys. Rev. Lett. 122 (2019) [3] X. Garbet et al., Nucl. Fusion 64 (2024) [4] V. Grandgirard et al., Comput. Phys. Comm. 207 (2016) [5] G. Merlo et F. Jenko, Journ. Plasma Phys. 89 (2023)

O.8-Zestanakis: Geometric Identification of Mode-Particle Resonances and Transport Barriers in Realistic Tokamak Equilibria

• Panagiotis Zestanakis (National Technical University of Athens)

Symmetry‑breaking perturbations in fusion plasmas—whether produced by intrinsically excited magnetohydrodynamic modes such as Alfvénic Eigenmodes and Geodesic Acoustic Modes, or by externally applied fields such as Resonant Magnetic Perturbations and Toroidal‑Field ripples—interact with charged particles through resonances that govern the transport of particles, energy, and momentum. Building on our earlier analysis for large‑aspect‑ratio equilibria [1–5], we generalize Orbital Spectrum Analysis (OSA) to numerically reconstructed, realistic tokamak equilibria. A computationally efficient semi-analytical geometrical method yields Guiding Center orbital frequencies and the kinetic safety factor for any given unperturbed axisymmetric equilibrium. The resulting resonance diagrams pinpoint all mode-particle resonances and locate transport barriers at the extrema of the kinetic safety factor. Systematic comparisons with numerical particle tracing validate the predicted resonance and transport barrier positions with excellent agreement for thermal and energetic particles alike. The methodology therefore furnishes a computationally efficient tool for assessing the response of all particle species to multi‑scale symmetry‑breaking perturbations in realistic equilibria. **References** [1] P.A. Zestanakis, Y. Kominis, G. Anastassiou and K. Hizanidis, Phys. Plasmas 23, 032507 (2016) [2] Y. Antonenas, G. Anastassiou and Y. Kominis, J. Plasma Phys. 87, 855870101 (2021) [3] H.T. Moges, Y. Antonenas, G. Anastassiou, Ch. Skokos and Y. Kominis, Phys. Plasmas 31, 012302 (2024) [4] G. Anastassiou, P. Zestanakis, Y. Antonenas, E. Viezzer and Y. Kominis, J. Plasma Phys. 90, 905900110 (2024) [5] Y. Antonenas, G. Anastassiou and Y. Kominis, Phys. Plasmas 31, 102302 (2024)

I.10-Duarte: Shifting and Splitting of Resonance Lines due to Dynamical Friction in Plasmas

• Vinicius Duarte (Princeton Plasma Physics Laboratory)

O.9-Sanchez: CIEMAT-QI4X: a reactor-relevant stellarator configuration compatible with an island divertor

• Ivan Calvo (Laboratorio Nacional de Fusion, CIEMAT, Madrid, Spain)
• Edilberto Sanchez
• Claudia Salcuni (Laboratorio Nacional de Fusión, CIEMAT)
• José Luis Velasco (Laboratorio Nacional de Fusión, CIEMAT)
• Jose Manuel Garcia-Regana (CIEMAT)

The stellarator concept offers advantages for a fusion reactor compared to the tokamak, but stellarator magnetic fields require careful optimization to achieve a confinement quality comparable to that of tokamaks. The numerical optimization for reduced neoclassical transport has already been experimentally validated in W7-X [1] and HSX [2], and great improvements have been made in the last few years including fast-ion confinement and turbulent transport in the set of optimization criteria [3,4,5]. However, insufficient attention is usually paid to the rotational transform profile during the optimization process. In the optimization of quasi-symmetric configurations, few restrictions are commonly set to the rotational transform profile. In the optimization of quasi-isodynamic (QI) configurations, if an island divertor is pursued, the rotational transform is usually constrained only to avoid the lowest-order rational values in the plasma column and to approach a low-order rational value at the edge [3,4]. However, if the rotational transform profile is not optimized more carefully than this, several issues, such as the formation of low-order islands, the overlapping of neighboring islands, and the insufficient quality of the edge island for a divertor, can appear, which can be worsened by finite b effects [6]. Furthermore, these issues can be very sensitive to coil design details and construction errors. In this work, we show how including a strict control of the rotational transform profile and the magnetic shear in the optimization process can improve the flux surface quality and the structure of the divertor island, and propose new metrics to be included in the optimization with this purpose. Using this strategy we have obtained a new reactor-relevant QI configuration, named CIEMAT-QI4X, which keeps the physics properties of the QI configuration presented in [3] (reduced neoclassical and turbulent transport, low bootstrap current, and good fast ion confinement), while at the same time improves the flux surface quality and shows a prominent low-order island at the edge suited to design an island divertor. Furthermore, its optimized rotational transform profile makes it more robust to coil construction errors. A set of optimized magnetic coils for this configuration is also presented.

O.10-Jamann: Insights into the ExB staircase via synthetic reflectometry and gyrokinetic modeling

• Frederic Clairet (CEA, IRFM)
• Sébastien Hacquin (CEA, IRFM, F-13108 Saint-Paul-lez-Durance, France)
• Guilhem Dif-Pradalier (CEA, IRFM)
• Antoine Jamann (CEA, IRFM, F-13108 Saint Paul-lez-Durance, France)
• Anna Medvedeva (Aix-Marseille University, CNRS, Centrale Méditerranée, M2P2, France)

Tokamak plasmas are complex non-equilibrium systems where turbulence plays a critical role in the transport of heat and particles. These turbulent processes span a wide range of spatial and temporal scales, making their observation particularly challenging. Ultra-fast sweeping reflectometry is a diagnostic technique capable of measuring electron density fluctuations with very high spatial and temporal resolution, capturing phenomena driven by both MHD activity and microturbulence. This technique has been used to observe meso-scale structures, such as the ExB staircase — a successive radial pattern of poloidal flows and avalanches [1]. This tertiary structure emerges from the self-organization of turbulence and regulates turbulent transport by alternating spatially between phases of free energy accumulation (zonal mean flows) and bursts of outward transport. Such organization is believed to enhance plasma confinement and could be of significant importance for future fusion devices. In this work, we propose to probe turbulence maps generated by the gyrokinetic code GYSELA using the synthetic reflectometry diagnostic FeDoT [2]. FeDoT is based on a two-dimensional (2D) Finite Difference Time Domain (FDTD) full-wave numerical scheme with absorbing boundary conditions. It supports arbitrary antenna configurations, and can simulate both O-mode and X-mode polarizations at any probing incidence angle. This flexibility enables the extraction of key turbulence characteristics, including frequency spectra, fluctuation levels and radial correlation lengths. The turbulence data analyzed in this study originates from flux-driven simulations with kinetic electrons. The external source can be modulated to vary the distance to “marginality” (see G. Dif-Pradalier et al., this conference), allowing us to investigate its influence on the formation of the ExB staircase. This approach enables a direct comparison of synthetic diagnostic results obtained in scenarios both with and without staircase formation. The simulated reflectometry signals are then compared to experimental measurements to identify and validate ExB staircase signatures. Additionally, we propose new diagnostic signatures that may further enhance the detection and characterization of these meso-scale structures. This integrated approach, coupling synthetic diagnostics and flux-driven gyrokinetic simulations, advances the interpretation of experimental measurements and paves the way toward a deeper understanding of turbulent self-organization mechanisms in tokamak plasmas. References [1] G. Dif-Pradalier et al. Phys. Rev. Lett. 114, 085004 (2015) ; G. Hornung et al. Nuclear Fusion: 57.1 (2016). [2] A. Glasser et al. Plasma Phys. Control. Fusion 67 035022 (2025).

I.3-Hizanidis: A quantum computing approach to electromagnetic wave propagation in fusion plasmas

• Kyriakos Hizanidis (National Technical University of Athens (NTUA), Greece)

I.11-Ewart: Rapid thermalisation, and non-thermal equilibria in near-collisionless plasmas

• Robert Ewart (Princeton University)

O.11-Turica: Drift-kinetic electron-temperature-gradient turbulence and cascade suppression by density gradients

• Leonard-Petru Turica (Rudolf Peierls Centre for Theoretical Physics, University of Oxford, Oxford, OX13PU, UK, United Kingdom Atomic Energy Authority, Culham Science Centre, Abingdon, OX14 3DB, UK, University College, Oxford, OX1 4BH, UK)

In magnetic-confinement-fusion devices, turbulent transport is an essential constraint on the confinement of the plasma. In tokamak edge-pedestals formed in the high-confinement mode of operation (H-mode), electron-scale turbulence is found to be the main source of anomalous transport, and the parameter $\eta_e = L_{n_e}/L_{T_e}$ is strongly correlated with the structure of the pedestal (Turica et al. 2025). Here, $L_{n_e}$ and $L_{T_e}$ are the equilibrium gradient length scales of the electron density and temperature, respectively. Using a model for electron-scale, collisional, drift-kinetic turbulence (Adkins et al. 2022), we investigate the dependence of the nonlinear heat transport on $\eta_e$. Linearly, this model describes unstable drift waves driven by the electron-temperature gradients of the underlying equilibrium. The inclusion of a finite density gradient affects the linear modes by inducing a drift that changes the relative phase between the perturbed electron temperature $T_e$ and electron-density $n_e$, suppressing the instability. We study the nonlinear behaviour of this system using simulations in shear-less slab geometry with periodic boundary conditions (Adkins et al. 2023,Ivanov et al. 2025). The suppression of the instability by finite $\eta_e$ quenches the heat transport in the nonlinearly saturated state. This is also accompanied by a qualitative change in the turbulence: the decoherence of waves caused by the density-gradient drift increases the injection-scale anisotropy, which manifests itself by a longer radial coherence of streamers. In the marginal limit, where $\eta_e$ approaches $\eta_{e,\mathrm{crit}}$, the critical value for linear stability, the system saturates in a streamer-dominated low-Reynolds-number state that is nearly monochromatic around the injection scale, imitating the linear solution. We identify the root causes of the nonlinear suppression of turbulence by describing the turbulent cascade and anisotropy in the weakly suppressed limit ($\eta_e \rightarrow \infty$), and we show a solution describing the saturation of the system in the marginal limit ($\eta_e \rightarrow \eta_{e,\mathrm{crit}}$). By describing the system in the intermediate stages and identifying the $\eta_e$ value at which the radial correlation length of streamers diverges, we identify the physical roots of the transition our system undergoes.

I.12-Foster: Alpha-particle orbits near rational flux surfaces in stellarators

O.12-Rofman: Finite beta effects in global, electromagnetic, gyrokinetic, linear and nonlinear simulations of Alfvén eigenmodes and microturbulence

• Baruch Rofman (Ecole Polytechnique F\'ed\'erale de Lausanne, Swiss Plasma Center, CH-1015 Lausanne, Switzerland)

Future nuclear fusion reactors will have to magnetically confine burning plasmas. In such scenarios, even a small fraction of fusion-born energetic particles (EP), which are 100 times hotter than the electrons, will contribute greatly to the kinetic pressure and therefore to the shaping of the MHD equilibrium, mainly via the Shafranov Shift. Nonetheless, many numerical works still prefer to use simplified magnetic geometries (e.g. ad-hoc or concentric circular MHD) to draw operational conclusions. In this work we perform first-principles numerical simulations using the gyrokinetic, electromagnetic, global code ORB5 to study the effect of a self consistent high $\beta$ equilibrium on the arising Alfv\'en Eigenmodes (destabilized by EPs) and (electromagnetic) Ion Temperature Gradient (ITG) microturbulence. Exploring the parameter space of both bulk plasma profiles and EP fraction, we show the linear (mainly stabilizing) effects of accounting for the bulk plasma and EP kinetic pressure in the MHD equilibrium, on the unstable TAE and ITG modes in our system. Focusing on the early nonlinear phase of a single toroidal mode TAE and ITG (separately), we study the impact of self-generated zonal flows on the shearing rates, kinetic profiles and fluxes, in both consistent and not consistent MHD equilibrium. Overall, we show that a burning plasma with a self-consistent MHD equilibrium behaves significantly differently than if the bulk or EP pressure is not considered.

O.13- De Lucca: Electromagnetic suppression of drift-wave turbulence and the LH transition

• Zeno Tecchiolli (EPFL)
• Louis Stenger (EPFL)
• Brenno Jason Sanzio Peter De Lucca
• Paolo Ricci (EPFL)

Leveraging the results of a series of $3$D flux-driven $2$-fluid simulations in a diverted equilibrium with GBS, it is shown how a regime of high confinement can develop as the power crossing the separatrix exceeds a critical value. As the edge temperature increases, the resistive-ballooning turbulence characteristic of L-mode conditions becomes subdominant, and turbulence is mostly driven by the electron drift-wave instability. Electromagnetic effects then act to suppress drift-wave turbulence by enhancing the electron adiabatic response. For the resistive branch of the drift-wave instability in particular, the strength of suppression is proportional to the background gradients driving the instability. Under a set of specified conditions, the plasma can therefore become unstable to the spontaneous formation of an edge transport barrier. In this regime, a steepening of the edge profiles in $n, T_e$ leads to a further decrease of turbulent flux and a feedback loop develops, driving the transport barrier formation. Furthermore, the transition to a high-confinement regime is impeded when the toroidal magnetic field points in the unfavourable direction for H-mode access. The power required to access this regime $P_{\text{th}}$ is analytically derived in a simplified geometry, via a local quasi-linear estimate for the $E \times B$ turbulent transport rate in the electromagnetic drift-wave regime. The resulting scaling law for $P_{\text{th}}$ is compared with the ITPA experimental database for the threshold power $P_{\text{LH}}$ to access H-mode in tokamaks, yielding good overall agreement $R^2 \simeq 0.7$.

I.13-Garcia: Resilient Stellarator Divertor Characteristics in the Helically Symmetric eXperiment

I.4-Belli: Multiscale analysis of improved confinement regimes in D-T plasmas

• Emily Belli

I.14-Mishchenko: Global electromagnetic turbulence and waves in stellarator plasmas

• Alexey Mishchenko

O.15-Mariani: Predicting the transport of a DTT negative triangularity scenario

• Alberto Mariani (ISTP-CNR Milano)

Experiments at TCV [1–3], DIII-D [4–6] and AUG [7,8] indicate that Negative Triangularity (NT) plasmas [9] can achieve H-mode-like performance with negligible ELM activity. Therefore, NT is investigated as a possible scenario for fusion reactors [10, 11]. However, the physics of NT is still to be fully understood. An experimental and theoretical effort is ongoing to fill this gap. A NT scenario is being designed for the Divertor Tokamak Test (DTT) facility [13], under construction in Italy, which aims to test alternative designs and materials for the EU DEMO divertor. Its transport properties are being studied by performing integrated modelling (ASTRA [14]/TGLF [15]), gyrokinetic simulations (GENE [16]) and experiments with DTT-like shapes on actual tokamaks. The first results of DTT predictions [17], as well as of experiments performed at TCV [3] and AUG [8], indicated that with the first proposed DTT NT shape with relatively small average triangularity, a beneficial effect of NT was found in the edge/scrape-off layer [18]. The NT confinement levels were found intermediate between positive delta H-mode and L-mode and NT core pressure values which are similar to the ones predicted for the Positive Triangularity (PT) H-mode scenario, in absence of ELMs, making the NT option an attractive alternative. Then, a new DTT shape with larger upper NT and reduced volume was proposed, to further optimize the scenario. A newly developed ASTRA/TGLF interface that provides a realistic plasma shape was also adopted, since the standard ASTRA/TGLF Miller geometry was found a too strong approximation for the DTT ‘teardrop’-like NT shapes. In this way, the formation of a proto-pedestal in the edge pressure was observed in the simulations, further improving the NT performance almost reaching the PT H-mode. These ASTRA simulations provided input for more complex GENE-TANGO simulations, which are ongoing. References [1] Y. Camenen, et al., 2005 Plasma Phys. Control. Fusion 47, 1971. [2] S. Coda, et al., 2022 Plasma Phys. Control. Fusion 64, 014004. [3] A. Balestri, et al., 2024 Plasma Phys. Control. Fusion 66, 065031. [4] M.E. Austin, et al., 2019 Phys. Rev. Lett. 122, 115001. [5] A. Marinoni, et al., 2019 Phys. Plasmas 26, 042515. [6] A. Marinoni, et al., 2021 Nucl. Fusion 61, 116010. [7] T. Happel, et al., 2023 Nucl. Fusion 63, 016002. [8] L. Aucone, et al., 2024 Plasma Phys. Control. Fusion 66, 075013. [9] A. Marinoni, et al., 2021 Rev. Mod Plasma Phys. 5, 6. [10] S.Yu. Medvedev, et al., 2015 Nucl. Fusion 55, 063013. [11] M. Kikuchi, et al., 2019 Nucl. Fus. 59(5), 056017. [12] J. Ball on behalf of the TSVV 2 team, AAPPS-DPP 2023. [13] R. Ambrosino, et al., 2021 Fusion Eng. Des. 167, 112330. [14] G.V. Pereverzev and P.N. Yushmanov 2002 IPP Report 5/98. [15] G.M. Staebler, et al., 2016 Phys. Plasmas 23 062518. [16] F. Jenko, et al., 2000 Phys. Plasmas 7 1904. [17] A. Mariani, et al., 2024 Nucl. Fusion 64, 046018. [18] A. Mariani, et al., 2024 Nucl. Fusion 64, 106024.

I.15-Qiu: Nonlinear saturation of toroidal Alfvén eigenmode wia ion induced scattering in nonuniform plasmas

• Zhiyong Qiu (Zhejiang University, Hangzhou, China)

O.16-Singh: JOREK modelling of Runaway Electron beam benign termination in JET

• Matthias Hoelzl (Max Planck Institute for Plasma Physics, Garching, Germany)
• Vinodh Bandaru (Indian Institute of Technology Guwahati, Guwahati, Assam, India)
• Eric Nardon (CEA, IRFM)
• Hannes Bergström (Max Planck Institute for Plasma Physics, Garching, Germany)
• Cristian Sommariva (Ecole Polytechnique Fédérale de Lausanne (EPFL), Swiss Plasma Center (SPC), CH-1015 Lausanne, Switzerland)
• Lovepreet Singh (CEA IRFM)

The runaway electron (RE) beam benign termination observed in JET pulse 95135 has been modelled in [1] using the nonlinear MHD code JOREK [2]. The study demonstrated the role of magnetic stochasticity in causing RE loss. This demonstration was based on particular assumptions regarding the properties of the background plasma, such as resistivity, that might not be accurate due to the recombination following massive deuterium injection. The transport of REs was modelled through an ad-hoc diffusion due to the high computational demands of fully advective simulation. Moreover, an ideal wall boundary condition was used which tends to have a stabilizing effect on the MHD activity. In this work, the same RE beam termination event was modelled using RE advection at the speed of light. Different resistivity values were also considered to assess the influence of this uncertain parameter. In particular, the influence of the plasma resistivity on the RE footprint on the wall was studied. It was found that higher resistivity results in MHD modes growing faster and up to larger amplitudes. The intense magnetic stochasticity observed with high resistivities results in a larger wetted area on the wall, contributing to a more benign termination of RE. Similar results were found for ITER in [3]. In addition, the comparison between the RE transport modelled with diffusion and advection showed a slower RE termination event with the advection model with respect to the diffusion due to a slower MHD modes growth rate, consistent with what is shown in [4]. At the same time, a resistive wall boundary condition [5, 6] leads to a faster RE termination due to the absence of the stabilizing effects caused by an ideal wall. As a result, with a background plasma resistivity corresponding to an electron temperature of 10 eV, the RE current loss occurs in 50 µs, which is comparable to the experimentally observed value of 20 µs [1]. With a higher resistivity, the loss time gets even closer to the experimental value. Finally, in order to better understand the physics of RE beam termination, we also studied the evolution toward a highly MHD-unstable regime. In contrast to the above results, where the simulations were started from an already unstable state, this approach involves ramping up the plasma current from a stable equilibrium. This method provides a more self-consistent view of how benign terminations can arise. [1] V Bandaru et al 2021 Plasma Phys. Control. Fusion 63 035024 [2] M Hölzl et al 2021 Nucl. Fusion 61 065001 [3] V. Bandaru et al 2024 Nucl. Fusion 64 076053 [4] H. Bergström et al 2025 Plasma Phys. Control. Fusion 67 035004 [5] P. Merkel et al. arXiv:1508.04911 (2015) [6] M Hölzl et al 2012 J. Phys.: Conf. Ser. 401 012010

O.17-Lanzarone: Comparison of Neural Network and XGBoost Decision Trees as Reduced Linear Gyrokinetic Model Surrogates for Growth Rate and Stability Prediction

• Emil Fransson (CNRS, Aix-Marseille Univ. , PIIM UMR7345, Marseille, France)
• Guillaume Fuhr (CNRS, Aix-Marseille Univ. , PIIM UMR7345, Marseille, France)
• Yann Camenen (CNRS, Aix-Marseille Univ., PIIM UMR7345)
• Matisse Lanzarone (Universite Aix-Marseille)

Integrated modelling of tokamaks combines a host of different codes to self-consistently model plasma discharges with key applications in modelling plasma scenarios and reactor designs as well as analysing experimental discharges. Fast and accurate integrated modeling is crucial to enable rapid iteration and efficient use of limited computational resources. Currently, a major bottleneck in integrated modelling is the running of gyro-kinetic simulations to calculate transport fluxes for each time-step, with even the fastest reduced models using the quasi-linear approximation still being orders of magnitude too slow for routine use. To address this challenge, machine learning surrogate models have been explored as a means of significantly accelerating transport predictions. Previous work has demonstrated the use of real time capable Neural Network (NN) surrogate models trained on QuaLiKiz [1] simulations to predict core tokamak transport quasi-linear heat and particle fluxes [2,3] as well as local linear stability and mode eigenvalues [4]. We investigate the viability of another machine learning algorithm: Gradient Boosted Decision Trees (XGBoost [5]) which have been shown to have superior performance to neural networks in tasks involving tabular data such as that from simulations, and for predicting functions with sharp transitions [6] such as the threshold for the onset of mode instability in plasmas. We compare NNs and XGBoost models trained on linear QuaLiKiz simulations spanning a 22D JET experimental space [3] to predict the stability of a local simulation as well as the growth rates of unstable modes. The performance of both algorithms is examined in a focused comparison study using 100,000 training points and across a broader range of training set sizes, to capture scaling trends, ranging from 100 to 3 million training points. Both models achieve comparable accuracy using 100,000 data points, however XGBoost has a significant edge in inference speed by a factor of approximately 50 while maintaining smaller model variance due to random initialisation, as well as greatly reduced training and hyperparameter optimisation times. These last two aspects are particularly important when considering active learning within the training pipeline in which models are frequently retrained as new data is acquired. Early scaling results shown in Figure 1 suggest advantages of XGBoost in low-data regimes showing increased robustness to hyperparameter variation. References: 1] QuaLiKiz homepage: http://qualikiz.com [2] K.L van de Plassche, J. Citrin, C. Bourdelle, Y. Camenen et al., Phys. Plasmas, 27 (2020) 022310 [3] A. Ho, J. Citrin, C. Bourdelle, Y. Camenen, F. J. Casson, K. L. van de Plassche, H. Weisen, JET Contributors; Neural network surrogate of QuaLiKiz using JET experimental data to populate training space. Phys. Plasmas 1 March 2021; 28 (3): 032305. https://doi.org/10.1063/5.0038290 [4] E. Fransson, A. Gillgren, A. Ho, J. Borsander, O. Lindberg, W. Rieck, M. Åqvist, P. Strand; A fast neural network surrogate model for the eigenvalues of QuaLiKiz. Phys. Plasmas 1 December 2023; 30 (12): 123904. https://doi.org/10.1063/5.0174643 [5] Chen, T., & Guestrin, C. (2016). XGBoost: A Scalable Tree Boosting System. In Proceedings of the 22nd ACM SIGKDD International Conference on Knowledge Discovery and Data Mining (pp. 785–794). New York, NY, USA: ACM. https://doi.org/10.1145/2939672.2939785 [6] Léo Grinsztajn, Edouard Oyallon, Gaël Varoquaux. Why do tree-based models still outperform deep learning on typical tabular data?. 36th Conference on Neural Information Processing Systems (NeurIPS 2022) Track on Datasets and Benchmarks, Nov 2022, New Orleans, United States. hal-03723551v3

I.16-Ferraro: Extended-MHD Stellarator Modeling with M3D-C1

• Nathaniel Ferraro (Princeton Plasma Physics Laboratory, Princeton University, Princeton, USA)

I.17-Garcia-Regana: The role of impurities on electrostatic stellarator turbulence

• Jose Manuel Garcia-Regana (CIEMAT)

O.14-Varadarajan: Integrated Modelling of Tungsten Erosion, Transport, and Radiation in WEST Geometry

• Naren Varadarajan (IRFM)

Core contamination is a key issue for full Tungsten devices and will be a major concern for the operation of ITER. Today’s experiments on medium size tokamaks such as WEST or AUG are vital to further characterize the mechanisms at play that regulate Tungsten concentration. WEST for instance, is characterized by a near-constant radiated fraction, regardless of input power. To better understand experimental observations and predict core contamination, the computational domain of the edge plasma code SOLEDGE-3X is now extended to describe the plasma from the first wall to the core including Tungsten. It self-consistently models the main ion species (usually a Hydrogen isotope), as well as light and heavy impurities in the core and in the edge, performing integrated simulations of plasma scenarios. On the wall, the gross erosion is estimated using the Eckstein formula, supplemented by prompt redeposition from both an analytical formula [1] and a neural network [2]. In the core, a new 1D model is used that solves particle and energy balance for all species, and is time-evolved along with the 2D edge-SOL. Core turbulence is taken into account using QLKNN-10D [3], a neural network trained on a database of QuaLiKiz simulations [4]. In addition, Neoclassical transport of heavy impurities is accounted for in the core using a reduced model benchmarked against NEO, that takes rotational effects into account [5]. Using this framework, tungsten sources, transport and radiation are investigated. A special focus is put on understanding self-regulation mechanisms, in particular the feedback between Tungsten sources and core radiation, which impacts the power entering the scrape-off layer. REFERENCES: D. Tskhakaya and M. Groth, Journal of Nuclear Materials 463 (2015): 624–628 L. Cappelli et al., PPCF 65, no. 9 (2023): 095001 K. L. Van De Plassche et al., PoP 27, no. 2 (2020): 022310 Stephens, C.D., X. Garbet, J. Citrin, C. Bourdelle, K.L. van de Plassche, and F. Jenko., Journal of Plasma Physics 87, no. 4 (2021): 905870409. P. Maget et al, PPCF 62, 105001 (2020); D. Fajardo et al, PPCF 64, 055017 (2022)

Closing session