Recent Publications

On Phase Transition of ITG Turbulence in the Dimits shift

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

Laboratoire de Physique des Plasmas

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

Anhui University, Hefei Institutes of Physical Science Chinese Academy of Sciences

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.

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

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

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

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

Gyrokinetic global simulation of Alfvenic ion temperature gradient mode in reversed magnetic shear

Gengxian Li, Zhixin Lu, Philipp Lauber, Matthias Hoelzl, Guo Meng, Yong Xiao

Max Planck Institute for Plasma Physics, Zhejiang University

In this work, a systematic study of electromagnetic instabilities driven by the temperature gradient in magnetically confined fusion plasmas with reversed magnetic shear is conducted using gyrokinetic particle-in-cell simulations. An electromagnetic instability arising in the low-beta regime is investigated, where beta=8*pi*nT/B^2 denotes the ratio of plasma pressure to magnetic pressure. Within a reversed shear safety factor (q) profile, when a mode rational surface coincides with the position of zero shear, an instability dominated by only one poloidal harmonic emerges, rather than the conventional ion-temperature-gradient (ITG) mode. Simulation results demonstrate that the instability exhibits pronounced electromagnetic polarization even in the low-beta regime, with a real frequency significantly higher than that of ITG modes, and show that it is destabilized by the temperature gradient and not by the density gradient. This instability can be observed even for a monotonic q profile with weak magnetic shear. Based on a systematic comparison with other typical electrostatic and electromagnetic instabilities, this instability is identified as a weak shear Alfvenic-ion-temperature-gradient (WSAITG) mode, which may provide an explanation for the low-frequency Alfven modes (LFAM) observed in experiments. Wave-particle resonance analysis in phase space reveals that, in contrast to the ITG mode, well-passing particles provide an additional resonant population that drives the WSAITG mode.

Evaluation of spatiotemporal tungsten density profiles using Unresolved Transition Arrays in the Large Helical Device

R. Nishimura, T. Oishi, I. Murakami, D. Kato, H. A. Sakaue, S. Gupta, H. Ohashi, C. Suzuki, M. Goto, Y. Kawamoto, et al.

National Institutes of Natural Sciences, The Graduate University for Advanced Studies, Sokendai, Tohoku University, Kyushu University, National Cheng Kung University

Tungsten spectroscopic studies have been conducted in the Large Helical Device with a pellet injection technique. Spatiotemporal profiles of tungsten density were evaluated using a space-resolved spectrometer, for plasmas with an electron temperature of below 1 keV and electron density of $10^{19}-10^{20}$ $m^{-3}$. Slice & Stack, a method for reconstructing emissivity, was applied to a line at 191.7 Å , which is a part of the Unresolved Transition Array (UTA) spectrum at 90-250 Å. Tungsten density was obtained using photon emission coefficients of $\mathrm{W}^{17+} - \mathrm{W}^{27+}$, evaluated from collisional-radiative model. The tungsten pellet injected from outside the plasma was first ablated in the edge plasma and subsequently diffused throughout the plasma. This behavior is typical of pellet injection experiments. Furthermore, after an event triggered by NBI breakdown, tungsten accumulated in the core plasma. The radiation power was estimated from the evaluated tungsten density profile and cooling factor dataset, and compared with bolometer measurement. This sequence of processes would be useful for validating atomic data of tungsten ions in low-to-intermediate charge states.

On the transition to large fluxes and access to second stability in gyrokinetic simulations of electromagnetic turbulence in STEP

Daniel Kennedy, Yujia Zhang, Toby Adkins, Plamen Ivanov, Francis Casson, Harry Dudding, Bhavin Patel, Colin Roach, Howard Wilson

United Kingdom Atomic Energy Authority, PPPL, UK Industrial Fusion Solutions Ltd

This work investigates the nonlinear transition to large heat fluxes observed in local gyrokinetic simulations of electromagnetic turbulence in STEP. Using the stress-balance framework of Zhang et al. (arXiv:2606.04616, arXiv:2607.11789), we confirm that the onset of extreme transport correlates with a critical value of $q^{2}β_{e}$, where $q$ is the safety factor and $β_{e}$ is the ratio of electron thermal pressure to magnetic pressure, and relate this to a limit on the poloidal beta $β_{\mathrm{pol}}$. Crucially, this critical value lies below any relevant linear stability limit in the ($q$, $β_{e}$) space (e.g., the onset of ideal or kinetic ballooning modes). Using an extensive set of nonlinear gyrokinetic simulations, we demonstrate that the transition to large fluxes in STEP is governed by a balance between the electrostatic and magnetic-flutter stresses. We argue, and also show numerically, that larger-major-radius tokamaks reach the electromagnetic non-zonal regime at lower $β_{e}$, making this MHD-controlled saturation limit more accessible in reactor-scale devices than in small spherical tokamaks. We also demonstrate that access to a second-stable regime enables re-saturation at larger values of $β^{\prime}$. We further show that the ideal ballooning mode (IBM) threshold serves as a useful proxy for delineating this second-stable region and also as a qualitative guide for the onset of large fluxes. These results provide a predictive framework for identifying no-go zone predictions from local gyrokinetics and offer new insight into the electromagnetic saturation physics relevant to STEP and other high-$β_{e}$ devices.

Design of DIVO: a diagnostic system for the ion velocity distribution function in fusion devices

Sara Molisani, Georgios Nicolaou, Daniel Verscharen, Matteo Zuin, Domenico Abate, Andrea Belpane, Maurizio Giacomin, Christopher Owen, Oreste Pezzi, Antonio Pimazzoni, et al.

Consorzio RFX, Istituto per la Scienza e la Tecnologia dei Plasmi ISTP CNR, University College London, University of Padova

In magnetized plasmas, many dynamical processes affect the ion velocity distribution function, both in laboratory and astrophysical environments. Measurements of this quantity can give useful insights for the study of phenomena such as magnetic reconnection, ion heating, and turbulence. For this purpose, we designed a new diagnostic system that evaluates the ion velocity distribution function at the edge of fusion plasmas. The proposed device, called DIVO (Diagnostic for Ion Velocity Observation), resolves the two components of the ion velocity, parallel and perpendicular to the magnetic field. DIVO will be mounted at the plasma edge in the RFX-mod2 experiment, a Reversed-Field Pinch device, that has been upgraded for operation in 2026. DIVO offers a direct and local measurement that will enhance our knowledge on the thermal and supra-thermal ion populations at the plasma edge, as RFX-mod was not equipped with any instrument able to evaluate locally the ion distribution functions in velocity space. The working principle of this diagnostic system is based on the force balance between the electric and the Lorentz force, with the aim of interrupting the Larmor gyration of the ions, which is associated with the velocity component perpendicular to the magnetic field. Since the balance depends on the ion's perpendicular velocity, a specific externally applied electric field allows for the selection of ions with v⊥ within a given range, while v∥ is evaluated by the ion's impinging position on a matrix of detectors. The instrument features a series of parallel thin metallic plates that filter the incoming ions, improving the resolution of the system. Individual particle simulations using Boris algorithm have been performed to optimize the instrument design and performances, assess its expected velocity resolution and operational range, and evaluate the transmission function needed to convert the detected counts into the original distribution function of the ions.

A Fiber-Optic Current Sensor for Fusion Plasma Diagnostics: Deterministic Error Compensation and Functional Validation on a Tokamak

Dongwei LIU, Xiaoyue Chang, Dong Guo, Hao Xiao, Boyang Liu, Jia Li, Li Wang, Qianglong Wang

Beijing SIO Technology Co., Ltd, ENN Science and Technology Development Co., Ltd

The precise measurement of plasma current is fundamental for the safe operation and advanced control of tokamak devices. Fiber-optic current sensors (FOCS) based on the Faraday effect offer inherent advantages such as galvanic isolation, immunity to electromagnetic interference, and DC measurement capability. However, their application to high-precision, large-dynamic-range plasma diagnostics faces two major challenges: nonlinear errors from optical imperfections and engineering reliability in harsh fusion environments. First, this paper proposes a novel deterministic error compensation method. By establishing an analytical model that directly links the nonlinearity to the physical parameters of the quarter-wave plate (QWP), the periodic error is precisely compensated over the full measurement range. On this basis, a piecewise linear empirical model is further developed to correct the residual error from linear birefringence. Laboratory tests demonstrate that this comprehensive compensation strategy achieves an accuracy better than ±0.1% in the range of 0-5.4 MA. Second, to address the harsh fusion environment, a high-temperature-resistant encapsulation with active cooling is designed, which limits the temperature rise to below 40°C under 160°C baking conditions. This enables the functional validation of the sensor on a tokamak. The sensor was integrated into the EXL-50U spherical tokamak for plasma current diagnostics. During actual plasma discharges (up to 140 kA), the FOCS output showed excellent agreement with conventional Rogowski coils (deviation <1 kA, relative error 1%), confirming its dynamic response and measurement consistency in a real tokamak environment. The proposed sensor provides a robust and accurate solution for plasma current measurement in future long-pulse and steady-state fusion devices such as ITER and CFETR.

Tungsten erosion and scrape-off layer transport modelling in L-mode helium plasma discharges in ASDEX Upgrade

Gabriele Alberti, Elena Tonello, Carlo Tuccari, Fabio Mombelli, Sebastijan Brezinsek, Timo Dittmar, Antti Hakola, Andreas Kirschner, Karl Krieger, Marcin Rasinski, et al.

Politecnico di Milano, Forschungszentrum Jülich GmbH, Istituto per la Scienza e Tecnologia dei Plasmi, CNR, École Polytechnique Federale de Lausanne (EPFL), VTT Technical Research Centre of Finland Ltd

Due to its unavoidable presence in thermonuclear DT plasmas and to its peculiar effects on materials, investigating the role of helium (He) in plasma-wall interaction (PWI) in current tokamaks is fundamental. In this work, PWI in L-mode He plasma discharges in ASDEX Upgrade (AUG) is modelled by exploiting simplified analytical approaches and two state-of-the-art codes. SOLPS-ITER is employed both to provide a suitable background plasma for erosion simulations and to interpret diagnostics measurements in terms of He+/2+ fraction. In particular, a 50-50% concentration of the two He ions is found in the proximity of the strike-points, while He2+ represents the dominant population farther in the scrape-off layer (SOL). The role of He ion fraction on AUG tungsten divertor erosion is first estimated by means of a simple analytical model and, afterwards, by exploiting ERO2.0, showing the major impact of He2+ in common AUG plasma temperatures. The potential influence of extrinsic impurities on divertor erosion is inferred from the preliminary comparison of ERO2.0 simulation results with experimental erosion measurements in the strike-point region. A comparison between the multi-fluid and kinetic approaches for simulating W erosion and migration reveals significant discrepancies in the predicted tungsten source and transport. In particular, he routinely adopted perfect entrainment assumption in SOLPS-ITER, i.e. setting the same velocity at the magnetic pre-sheath entrance for both main ions and impurities, is found to substantially overestimate the W source compared to ERO2.0 results. Moreover, ERO2.0 predicts a stronger tungsten transport towards the X-point than current SOLPS-ITER simulations without drifts. Comparable W influx into the core can, however, be reproduced in ERO2.0 by reducing the anomalous diffusivity.

Sideways forces in a tokamak model with a surface separating the region with a three-dimensional kink perturbation from the two-dimensional plasma inside

V D Pustovitov, Marina Evgenevna Sukhovitskaya

Moscow Center for Advanced Studies

A tokamak model with a coupling surface (CS) [Plasma Phys. Control. Fusion 55, 095008 (2013)] separating the regions with two-dimensional (2D) and three-dimensional (3D) descriptions of the magnetic field B is considered. In this approach, it is assumed that the three-dimensional magnetic perturbation b does not penetrate inside the CS torus, as if the CS would be a perfect conductor. Such shielding, when b ≠ 0 outside the CS, requires the presence of a sufficiently large current on this surface. Here, an analytical calculation of the sideways (horizontal) forces on the tokamak vacuum vessel wall and on the CS is presented for such a CS response to perturbations with toroidal mode number n = 1. It is shown that the force acting on the CS can be significant and may even greatly exceed the force acting on the wall. Therefore, the force balance in the real configuration and in the model with a CS turns out to be substantially different.

Full 3D Maxwell solver for Inertial Electrostatic Confinement devices

Victor-Otto de Haan, Niels Geerits

Results of a full 3D electromagnetic plasma fusion simulator are presented. It will enable investigations into the influence of structured electromagnetic potentials, anomalous magnetic moments, or charge cluster structures on the fusion yield. The simulator calculates fusion yields similar to those found in real world fusors. In addition, the ion energy and density are accurately simulated. Given that the ion properties arise from a full Maxwell solver there is ample evidence that this part of the simulation is accurate. Concluded with a recommendation for continued research.

First Observation of Fishbone-Driven Zonal Flows with Fine Reversed Structure in Tokamak Plasmas

Liutian Gao, Yuehao Ma, Huishan Cai, Adi Liu, Bin Zhang, Ming Xu, Haiqing Liu, Liqing Xu, Chu Zhou, Feifei Long, et al.

We present the first direct experimental observation of fishbone-driven zonal flows in the core of the EAST tokamak. In contrast to the global pattern predicted by previous models and simulations based on the energetic-particle-expulsion mechanism, the observed flows exhibit a fine-scale, radially reversed structure inside the q = 1 rational surface. The flow rises faster and saturates earlier than the fishbone within a single burst, indicating that a beat-driven nonlinear process dominates the early stage rather than the energetic-particle-expulsion mechanism. Global nonlinear gyrokinetic simulations quantitatively reproduce the observed radial profile and reveal that this structure arises from the cancellation of comparable but opposite contributions from thermal ions and electrons. This cancellation mechanism is not captured in previous theoretical frameworks. These findings establish that the fishbone can generate sheared flows with a distinct radial topology, offering a promising pathway for regulating turbulence and improving core confinement.

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

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

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

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

Physics-augmented surrogate model for linear instability analysis of high-n toroidal Alfvén eigenmodes in tokamaks

Zhaoyang Liu, Shiyang Liu, Guoyong Fu

Zhejiang University, Zhejiang Lab

We propose the physics-augmented surrogate model for linear instability analysis of high- n toroidal Alfvén eigenmodes (TAEs) in tokamaks. The database for high- n TAE linear instabilities is generated by the global initial-value simulations by GMEC, and also the local eigenvalue code ETEC. The equations solved by ETEC are much simplified from the first-principle equations solved by GMEC, with high- n ballooning representation. The local solutions from ETEC exhibit a strong correlation with the global solutions from GMEC. The multi-layer perceptron (MLP) based surrogate models are trained, taking local normalized equilibrium parameters as inputs, and the real frequency and growth rate of TAEs obtained from GMEC as outputs. Our results indicate that incorporating ETEC results as prior physical information into the inputs leads to a substantial improvement in the generalization performance of the surrogate model.

Modeling the effects of carbon impurity transport and E×B drift on detachment cliff in the HL-3 open X-divertor by SOLPS-ITER

Rui Wang, Hailong Du, Na Wu, Chaofeng Sang, Lei Xue, Jiaxian Li, Xiao Song, Guangzhou Hao, Dongmei FAN, Guoliang Xiao, et al.

Southwestern Institute of Physics, Dalian University of Technology

This work employs the SOLPS-ITER code package to investigate the effects of carbon impurity transport and E×B drift on the detachment cliff in the HL-3 open X-divertor configuration (OXD), and elucidates the physical mechanism behind the experimental observation that divertor detachment in the HL-3 OXD requires a higher upstream separatrix density ( n e,sep ) to occur than in the standard single null divertor configuration (SN). In this work, we find that: (1) The E×B drift more strongly suppresses divertor detachment in the HL-3 OXD under low n e,sep than in the SN; (2) A detachment cliff is observed in the HL-3 OXD with/without E×B drift. The open structure and large poloidal magnetic flux expansion of the HL‑3 OXD lead to a detachment cliff at the outer strike point under low n e,sep conditions without drifts, resulting in an inhomogeneous divertor state (strike point detached, far SOL high-recycling). With drifts included, the inhomogeneous state drives poloidal and radial E×B flows that together transport carbon impurities away from the outer strike point, making the HL‑3 OXD harder to detach—consistent with experiments. Besides, we find that the fundamental cause of the detachment cliff in the HL‑3 OXD with/without drifts: as n e,sep rises above a certain threshold, the abrupt reversal of carbon impurity transport direction and the resulting positive‑feedback loop in the divertor region. The difference lies in what drives the carbon impurity transport reversal: without drifts, it is driven by competition between thermal and frictional forces on carbon ions; with drifts, by changes in the poloidal E×B drift flow near the separatrix. These findings indicate that the open divertor structure of the HL‑3 OXD is the primary reason for its difficulty in achieving detachment. Enhancing its closure can significantly improve its detachment capability, providing a reference for the application of the X‑divertor in future devices (ITER/CFETR).

Experimental investigation of a closed vapour box module for a divertor-like configuration in Magnum-PSI

Fabio Romano, Victor Tanke, Jacob Schwartz, Robert James Goldston, Serge Brons, T W Morgan

DIFFER, Princeton University Plasma Physics Laboratory, Dutch Institute For Fundamental Energy Research

Efficient management of extreme heat fluxes in the divertor region to extend the lifetime of the components remains a critical challenge for the realization of nuclear fusion-based power plants. Among the alternative concepts explored for the divertor region, the use of liquid metals, particularly lithium, is of interest due its ability to dissipate the incoming plasma heat flux through the vapour shielding effect (VS). In this work, we experimentally investigated a "closed" configuration of a dedicated Vapour Box Module (VBM) in the linear plasma device Magnum-PSI. The goal of the experiments is to simulate the vapour box divertor environment conditions and assess its performance in terms of power mitigation and redistribution and lithium confinement. Initial testing without Li demonstrated the efficacy of a closed VBM structure in inducing detachment via neutral gas accumulation. Apertures which enabled non-condensing gas to be effectively pumped while ensuring lithium condensed on the inner surfaces were therefore added. With a lithium capillary porous structure target used, lithium is directly vaporized by the plasma, forming a dense lithium vapour cloud that interacts with the incoming plasma. This resulted in a significant reduction of the target temperature of at least 48 %, together with a temperature locking effect, a phenomenon typically observed in the VS regime. Lithium vapour confinement within the VBM was strongly correlated with the wall temperature. Relatively cold walls promoted Li re-condensation and therefore improved Li confinement, although with the expected trade-off of increased hydrogenic retention on lithium-wetted surfaces. As the wall temperature increased, the confinement efficiency decreased, consistent with reduced Li re-condensation and thermally activated Li--H chemistry and remobilization at the walls. Diagnostic measurements through embedded thermocouples and calorimetry revealed that lithium vaporization and re-condensation processes also played significant roles in plasma power dissipation. The results advance the case for a closed divertor chamber with direct lithium evaporation from the strike-points as a viable method to manage divertor heat fluxes in future fusion reactors.

MHD stability analysis of the Helios stellarator fusion power plant

M.F. Martin, S. Saxena, D.W. Dudt, E. Flom, C.P.S. Swanson, S.T.A Kumar, D.A. Gates

Thea Energy

Exploiting ICRH to maximise the neutron yield in VNS

Dirk Van Eester, Ernesto A Lerche, Mattia Siccinio, Emiliano Fable, M Vergote

Laboratorium voor Plasmafysica—Laboratoire de Physique des Plasmas, DEMO Central Team, Max-Planck-Institut für Plasmaphysik

While ITER will demonstrate that long duration pulses producing significant fusion power is possible in a fusion reactor relevant environment, the aim of the Volumetric Neutron Source (VNS) tokamak is to offer a testbed for studying the impact of long-time fusion neutron exposure to the plasma facing components. Presently, the auxiliary heating schemes foreseen for the VNS are electron cyclotron and neutral beam injection (NBI) heating while the main fusion power generation is intended to result from beam-target reactions between the fast deuterium NBI ions and the thermal Tritium ions in the plasma. The present paper discusses the potential of only using wave heating to maximise the fusion yield in modest scale fusion reactors. We focus here on ion cyclotron resonance heating (ICRH) while electron cyclotron heating (ECRH) provides the basic target plasma the IC waves couple to. More in particular, we exploit the idea that permitted the fusion performance to be boosted beyond what neutral beam injection alone could achieve in the JET DTE2 and DTE3 campaigns. Adopting IC heating at the fundamental cyclotron frequency allows to create a moderately fast ion tail that has a large fraction of particles of a high concentration minority tail near the energy where the D-T cross section peaks, similar to the NBI beam-target fusion scheme. Rather than adopting balanced D and T concentrations and count on fusion reactions produced by 2 thermal fuel ion thermal populations, the steepness of the cross section curve is exploited to boost the nuclear reactions between a thermal population and an IC created fast particle "beam". In the present wave-only exploratory study it is estimated that 20-25MW of fusion power can be achieved when adopting the 2025 reference parameters for the VNS device. More comprehensive study is, however, needed to assess the potential in detail.

Role of the radial electric field in the confinement of energetic ions in the Wendelstein 7-X stellarator

Marcos Arranz Jiménez, Jose Luis Velasco, Ivan Calvo, Daniel Carralero

CIEMAT

Good fast-ion confinement is an essential requirement for a fusion reactor. The magnetic configuration of the Wendelstein 7-X (W7-X) stellarator is partially optimized in this regard in a reactor-relevant scenario: it is expected to show improved fast-ion confinement when β is high and the effect of the radial electric field is negligible. The experimental validation of this optimization is difficult since, with the available power, achieving high β under appropriate conditions for the validation is challenging and the effect of the radial electric field is inevitable. In this work, the confinement of fast ions in W7-X has been studied numerically for a variety of scenarios via the ASCOT5 code. The effect of the radial electric field on fast-ion losses is confirmed to be equivalent to the one produced by β, and this is characterized by means of scans on both parameters. Through a preliminary study with experimentally-based profiles, a viable scenario is identified that takes advantage of this effect for the experimental validation of the optimization strategy of W7-X. Finally, the study provides an exhaustive analysis of the entire fast-ion phase space, helping to identify where the fast ions should be generated in such experimental exercise.

yancc: A GPU-accelerated, differentiable solver for neoclassical transport in tokamaks and stellarators

Rory Conlin, Matt Landreman

We present yancc, a new GPU-accelerated solver for the drift kinetic equation that computes neoclassical transport fluxes, flows, and currents in tokamaks and stellarators. The drift kinetic equation is challenging to solve numerically due to strong advection-dominance, recirculating flows, internal boundary layers, severe anisotropy, and high dimensionality. The code solves both the full four-dimensional drift kinetic equation (retaining speed-dependent collisions, energy scattering, and full interspecies coupling), and the reduced monoenergetic form. The discretization combines a Maxwell polynomial collocation grid in speed with finite differences in pitch angle and the flux surface coordinates, using a modified upwind stencil designed to improve diagonal dominance for multigrid efficiency. The resulting linear system is solved with a multigrid-preconditioned Krylov method. Built in JAX, yancc is fully differentiable, enabling gradient-based optimization and adjoint sensitivity analysis. Benchmarks against MONKES and SFINCS show agreement within 1\% across a range of collisionalities, geometries, and multi-species configurations. yancc achieves roughly an order of magnitude speedup over SFINCS on a per-scan basis while using an order of magnitude less memory, with runtime remaining nearly flat across the full range of collisionality. The combination of speed, low memory footprint, and differentiability makes yancc well suited for integration into stellarator optimization workflows, uncertainty quantification, and profile prediction.