Recent Publications

The geometry of flux surfaces with quasi-poloidal symmetry

Rishin Madan, Wrick Sengupta, Elizabeth J. Paul, Mohammed Haque, Richard Nies, José Luis Velasco, Amitava Bhattacharjee

Quasi-poloidal (QP) magnetic fields have desirable properties for confining plasma: no radial drift of guiding centres (with positive implications for neoclassical transport), zero Pfirsch–Schlüter current and a lower level of damping for poloidal flows. Despite their attractive properties, QP fields are not amenable to the near-axis expansion, a major theoretical tool for understanding toroidal fields. In this paper we provide a novel framework for defining and understanding QP flux surfaces. This framework relies on a simplification that transforms the task of finding a QP flux surface from a three-dimensional problem to a two-dimensional (2D) problem. This simplification also applies to asymmetric magnetic mirrors with desirable properties. We sketch how this 2D problem can form the basis of an efficient optimisation problem for finding QP flux surfaces. We leverage this 2D problem for theoretical understanding: for instance, we identify a route to finding QP flux surfaces that are naturally flat mirrors (Velasco et al. 2023, Nucl. Fusion , 63, 126038). The reduced model is qualitatively checked against numerically optimised QP equilibria. These numerical solutions only satisfy QP approximately, but we predictably find that local discrepancies with the reduced model correspond to significant local QP errors, anomalous parallel currents and field lines deviating from geodesics.

Physics-Informed Neural Networks to Infer the Perpendicular Energy Conductivity in the Scrape-Off Layer of Stellarator Devices

J. Gallego, P. Protopapas, A. Bustos, A. Alonso, S. Barquero, A. Baciero, I. Rivera, J. A. Moríñigo, R. Mayo-García

In this work, we develop an inverse Physics-Informed Neural Network (PINN) framework to infer the dependence of the scrape-off layer (SOL) perpendicular heat conductivity on plasma density and temperature, $κ_\perp(n,T)$. The method combines radial profile measurements of electron density and temperature with the residual of a reduced one-dimensional SOL transport equation, so that the inferred conductivity is constrained by both the measurements and the underlying transport model. Three neural networks are trained simultaneously: two reconstruct the temperature and density profiles as functions of the radial coordinate and transported power, while a third represents the effective conductivity as a function of the local density and temperature. The framework is first validated using synthetic data generated from a prescribed conductivity function, allowing the inferred $κ_\perp(n,T)$ to be compared directly with the ground truth. The model recovers the imposed functional dependence with errors below $10~\%$ in the data-constrained region. Bootstrap resampling is shown to provide a practical indicator of prediction reliability and consistency. A scan in the number of plasma profiles used for training and the number of radial measurement positions per profile identifies a practical trade-off between reconstruction accuracy and data availability. Finally, the method is applied to an experimental dataset from the TJ-II stellarator obtained with the helium-beam diagnostic. This exploratory application provides an initial estimate of the effective SOL conductivity and illustrates the potential of inverse PINNs for extracting transport information from plasma edge measurements.

Overview of Achievements and Outlook of the IFMIF/EVEDA Project

Kazuo Hasegawa, Atsushi Kasugai, Keitaro Kondo, Kai Masuda, Satoshi Sato, Kentaro Ochiai, Hervé Dzitko, Fabio Cismondi, Yann Carin, Dominique Gex, et al.

The Engineering Validation and Engineering Design Activities for the International Fusion Materials Irradiation Facility (IFMIF/EVEDA) project have been conducted as one of the three projects (IFMIF/EVEDA, IFERC and JT60SA) within the Broader Approach (BA) agreement between EURATOM and the Japanese government since 2007. The IFMIF is intended to deliver accelerator-based deuterium-lithium (D-Li) neutrons at energies and intensities to sufficient to enable the qualification of candidate materials for future fusion energy reactors, such as DEMO. The primary objective of the IFMIF/EVEDA project is twofold: (i) to develop a detailed engineering design of the IFMIF and (ii) to validate its major components, namely the Accelerator Facility, the Lithium Target Facility and the Test Facility. During Phase I of the BA, which concluded in March 2020, the Engineering Validation Activity (EVA) for the Lithium Target Facility and the Test Facility were successfully completed through the construction and testing of prototypes. In contrast, the EVA for the Accelerator Facility, implemented through the Linear IFMIF Prototype Accelerator (LIPAc), remains on-going. The current phase (Phase II) focuses on the continued commissioning of the LIPAc and the enhancement of some sub-systems to support the development of the Fusion Neutron Source Design (FNSD). This article presents an overview of the progress achieved in the LIPAc commissioning and FNSD activities and outlines the future directions of the activities.

Diagnostics for large tokamaks: from JET to JT-60SA 1

Carlo Sozzi, A Jokinen, G Phillips, K Tanaka, Juan Ayllon-Guerola, Andrea Belpane, Attila Buzás, Santiago Cabrera, mario - cavinato, Daniel Carralero, et al.

The main scientific purpose of JT-60SA is complementing ITER in the preparation of the operation of a DEMOnstration fusion reactor, in particular investigating the conditions for a controllable high beta steady-state regime able to optimize the fusion gain. In order to accomplish this task, a sequence of operation and machine enhancement periods in the next few years are planned to reach the target performance of the machine before a transition to a full tungsten wall. EUROfusion and Fusion for Energy are jointly contributing to the enhancement plan of JT-60SA, in particular, for what concerns the present contribution, to provide JT-60SA with state-of-art diagnostics in support of its scientific and technical objectives. This paper reports the status of the projects being implemented in view of the next scientific campaigns or under consideration through the various stages from feasibility to detailed design.

First wall particle fluxes and Be erosion in high-performance JET-ITER baseline plasmas

Eduardo de la Cal, Carine Giroud, Henri Aaron Kumpulainen, Juri Romazanov, Itziar Balboa, Scott Alan Silburn, Pedro Carvalho, Juuso Karhunen, Beth Thomas, Alex Tookey, et al.

In next step large fusion devices with a tungsten (W) first wall such as ITER, impurity control will be a challenge to avoid strong core radiation from highly ionized metals. Here we analyse the deuterium (D) fluxes impacting on the beryllium (Be) first wall limiters and the resulting surface erosion in high-performance neon (Ne) seeded ITER-baseline plasmas with up to 35 MW heating power in JET tokamak. Visible cameras are used to quantify the fluxes using the spectroscopic S/XB method at the regions of strongest plasma-wall interaction: the Outer Midplane (OMP) and the Upper Dump Plates (UDP). We first describe how Ne seeding, which significantly improves core plasma performance and power exhaust control, modifies the plasma flux dynamics and ELM properties at the walls. It is then shown, that the global Be erosion by sputtering is not significantly affected by Ne seeding. This is because the time-averaged fluxes are dominated by the inter-ELM phase, where the main erosion precursor is deduced to be D+. Furthermore, we observe a beneficial strong decrease of the fluxes at the OMP when slightly increasing the Separatrix-limiter clearance, indicating short far scrape-off layer (SOL) decay lengths. We also describe, how the fluxes change with average plasma density and toroidal magnetic field and plasma current. Regarding the fluxes at the UDP, we show the critical effect of the magnetic topology at the top of the chamber. The formation of a secondary Separatrix in this region enhances the parallel plasma fluxes to the UDP, which eventually becomes the main plasma-wall interaction area. Finally, we compare the experimentally estimated Be effective sputtering yields at the OMP for the inter- and intra-ELM phases with the values calculated using the SDTrim code to infer information on the mean ion impinging energies Ei and to show that the main global erosion precursor is D+. The obtained Ei values are in the range or at least compatible with those expected for the plasmas analysed: approximately 30 eV for the inter- and 1 keV for the intra-ELM periods respectively.

Accomplishment of high duty cycle beam commissioning of Linear IFMIF Prototype Accelerator (LIPAc) at 5 MeV, 125 mA D+

Tomoya Akagi, Florian Benedetti, Yann Carin, Janic Chambrillon, Fabio Cismondi, Andrea De Franco, Hervé Dzitko, Takashi Ebisawa, Dominique Gex, Kazuo Hasegawa, et al.

The Linear IFMIF Prototype Accelerator (LIPAc) is being commissioned under the Broader Approach agreement between Europe and Japan to validate the low-energy section of the accelerator for the International Fusion Materials Irradiation Facility (IFMIF). LIPAc is designed to accelerate a 125 mA deuteron beam up to 9 MeV in continuous-wave (CW) operation. This paper reports the results of high-duty cycle beam commissioning at 5 MeV in the Phase B+ configuration using the RFQ. A maximum duty cycle of 8.75% and a beam current of 119 mA were achieved, corresponding to an average RFQ beam power of 40–45 kW, which is among the highest average beam powers achieved by operational RFQs. The commissioning also identified the RFQ RF couplers as the bottleneck for further duty cycle increases, leading to the preparation of brazed high-duty couplers. These achievements establish a solid foundation for the next commissioning phase, including SRF linac integration and a path toward CW operation to demonstrate the IFMIF accelerator concept.

Overview of the European Breeding Blanket programme for Helias 5-B: Dual Coolant Lead-Lithium design, integration and tools

Iole Palermo, Javier Alguacil, Gaetano Bongiovi, Juan Pablo Catalan, Ilenia Catanzaro, Iván Fernández-Berceruelo, Salvatore Giambrone, Guillermo Gómez Fonfría, Jose Ángel Noguerón, Vicente Manuel Queral Mas, et al.

As part of EUROfusion’s mission to bring stellarators to technological maturity, the Stellarator Power Plant Studies (SPPS) WPPRD began in 2021 to develop a HELIAS-class power plant. Building on DEMO tokamak experience, European teams are designing a Dual Coolant Lead-Lithium (DCLL) breeding blanket (BB) for HELIAS. This concept uses liquid PbLi as breeder/coolant and decoupled helium cooling for the first wall (FW). Two key adaptations address HELIAS’s complex geometry: a detached FW using Capillary Porous System (CPS) technology, and a quasi-toroidal segmentation (QTS) with PbLi flow aligned to magnetic field lines. QTS reduces magnetohydrodynamic (MHD) pressure drop by up to two orders of magnitude, potentially eliminating electrical insulation needs. Remote handling (RH) is rethought for 3D stellarators, where traditional vertical-port extraction is impractical. Alternatives include enlarged fixed coils, movable coils for temporary large ports, and detachable vessel periods. The detached FW strategy shifts maintenance from large BB segments to smaller, easily replaceable FW panels, extending BB lifetime. CPS designs with Li in tungsten matrices were analysed thermally, hydraulically, and neutronically, showing potential to lower displacement-per-atom (dpa) damage while maintaining tritium breeding. Be-based moderators behind the FW CPS matrix improved TBR while reducing back-structure damage. To accelerate design and analysis, HeliasGeom and SHANE tools were developed to rapidly generate realistic 3D parametric geometries for CAD, neutronic, and thermal-hydraulic coupling. Preliminary 3D studies addressed TBR optimisation, shielding, MHD in non-uniform fields (via GridapMHD), and multi-scale thermal–mechanical assessments. These innovations collectively advance stellarator blanket technology, integration, and maintainability toward viable power plant concepts.

The influence of NBI-driven plasma current on ablation light profiles of cryogenic pellets injected into the stellarator TJ-II

Kieran McCarthy, Isabel Garcia-Cortes, Boudewijn Ph Van Milligen, Alfonso Baciero, Alvaro Cappa, Ricardo Carrasco, Teresa Estrada, Belén López-Miranda, Daniel Medina Roque, Nerea Panadero, et al.

Efficient plasma core fuelling is critical for magnetic confinement fusion (MCF). The envisaged technology is cryogenic pellet injection where small cylinders of fuel ice are injected at high speed into the confined plasma. Indeed, injectors are operated currently on many tokamak and stellarator devices. As a pellet penetrates into plasma its surface is ablated by particle impacts, the pellet’s ablation rate and, hence penetration depth, being determined primarily by plasma electron temperature and, to a lesser extent, by electron density. During ablation, Balmer light emitted from the neutral cloud that surrounds a pellet provides a means to quantify ablation and to determine its penetration depth. Reconstructed emission profiles often exhibit overlying structures such as striations. In the stellarator TJ-II, ablation light has been recorded for injections into a broad range of its magnetic configurations. After reviewing signals in its pellet database for selected magnetic configurations heated by balanced/unbalanced neutral beam injection, large reproducible transient structures, that cannot be explained by plasma parameter variations alone, are found occasionally in signals. It is argued here that their occurrence can be associated with the radial locations of low-order rational surfaces in the core region. Moreover, it is shown that structures appear in, or disappear from, ablation light signals as the rotational transform profile is modified by the net plasma current. Examples of ablation emission profiles with and without structures are presented for selected magnetic configurations and NBI heating set-ups and the radial locations of significant structures are compared with predicted locations for low-order rational surfaces. Finally, implications of these observations for pellet injection are discussed.

Turbulence analysis with the spectroscopic gas puff imaging diagnostic in the TJ-II stellarator

Boudewijn Ph Van Milligen, Igor Voldiner, Eduardo de la Cal, Arnold Alonso Alvarez, Ibere Luiz Caldas, Zwinglio Guimarães Filho

In this work, we use the new Spectroscopic Gas Puff Imaging diagnostic of the TJ-II stellarator to study edge turbulence. This diagnostic allows measuring electron density and temperature fluctuations simultaneously in a two-dimensional region in the plasma edge. We compare the properties of turbulence in two heating phases, namely the low-density Electron Cyclotron Resonance Heating (ECRH) phase and the high-density Neutral Beam Injection (NBI) phase. In the plasma edge region, 0.8 < ρ <1, turbulent structures have a diameter of ∼1 cm in ECRH and ∼3 cm in NBI, while ne and Te are strongly correlated and in phase. The structures are elongated and not aligned with flux surfaces. Rescaled range analysis shows that electron density and temperature fluctuations are persistent, indicating the presence of some turbulence regulation mechanism. Notably, electron temperature fluctuations are more persistent than electron density fluctuations. Simultaneously, causal analysis, based on the transfer entropy, reveals a net causal information flow from electron temperature to electron density fluctuations. These two results suggest that a turbulent mechanism is regulating edge electron temperature fluctuations, while electron density fluctuations follow. Also, Entropy-Complexity analysis shows that edge fluctuations exhibit nontrivial deterministic behavior, especially in the high-density phase.

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

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.

Combination of quasi-isodynamic and piecewise omnigenous magnetic fields

Jose Luis Velasco, Ivan Calvo, Víctor Fernández-Pacheco, Hengqian Liu, Misha Padidar, Edilberto Sanchez, Guodong Yu, Caoxiang Zhu

Due to their simultaneous optimization for radial and parallel neoclassical transport, quasi- isodynamic fields have been the main choice of stellarator magnetic configuration for most fusion reactor candidates in recent years. However, achieving a high degree of quasi-isodynamicity often comes at the cost of a strong shaping of the flux surfaces of the stellarator and complex coil geometries. In this work, the concepts of quasi-isodynamicity and piecewise omnigenity are combined to form QI-pwO fields. These fields are quasi-isodynamic in the low-field region of the magnetic surface, whereas they significantly depart from quasi-isodynamicity in the high-field region without sacrificing the neoclassical transport properties of quasi-isodynamic fields. This departure could make it easier to integrate the optimization of neoclassical transport with other physical and technological aspects of a stellarator reactor.

First validation experiments for the radar-based diagnostic system of the IFMIF-DONES lithium target

N. Roy, C. de la Morena, D. Jimenez-Rey, V. Villamayor, D. Regidor, J. Martínez-Fernández, F. Arranz, E.J. Blanco, B. Brañas, S. Cabrera, et al.

Developments & applications of the electrochemical hydrogen sensor in IFMIF-DONES research facilities

Nils Holstein, Wolfgang Krauss, Jürgen Konys, Francesco Saverio Nitti, Belit Garcinuño, Jorge Maestre

Selling a dream? Fusion experts’ perspectives on the challenges of fusion deployment: Evidence from conference workshops

Piotr Stankiewicz, Gaston Meskens, Christian Oltra, Vanja Čok, Alessio Giacometti, Chiara Bustreo

Transport in high-performance plasmas of the TJ-II stellarator: From first-principles simulations to experimental validation

José Manuel García-Regaña, Daniel Alegre, Arturo Alonso, Régulo Anton, Enrique Ascasibar, Alfonso Baciero, Alejandro Banon Navarro, Jose-Miguel Barcala, Michael Barnes, M. Scherezade Barquero Balsera, et al.

We provide an overview of activities carried out at the TJ-II stellarator aimed at understanding transport from first principles and power balance analysis. These tasks include gyrokinetic simulations with the codes stella and EUTERPE, neoclassical simulations with the code SFINCS, particle deposition calculations with the code HPI2, and heat source estimates with ASCOT5. All these numerical simulation efforts converge, together with transport analyses, to address the transport mechanisms in plasmas with improved confinement through pellet injection—a scenario studied during the past few TJ-II campaigns bearing resemblance to the pellet-fueled high-performance plasma scenarios of observed in W7-X.

Sn-Sn+collisional processes and heat flux dissipation in near-surface Tin plasmas

Alfonso de Castro, P Fernández-Mayo, David Tafalla, Kieran McCarthy, Daniel Alegre, Eider Oyarzabal

This work investigates the evolution of dense tin vapor/plasma structures generated during divertor-relevant high-heat-flux irradiation of a liquid tin Capillary Porous System target in the OLMAT facility. The target was exposed to 125 ms hydrogen particle-beam pulses (effective heat fluxes ~33 MW m⁻². The near-surface plasma was diagnosed using an embedded Langmuir probe and optical emission spectroscopy, while the target thermal response was monitored by infrared pyrometry. The addition of transient type-I ELM-like 2 ms laser loads (~500 MW m⁻² heat flux) enabled access to previously unexplored plasma/liquid-metal boundary conditions at a quasi-stationary target-temperature plateau near 1775 ± 100 K. The generated, nearly pure Sn plasmoid was characterized up to target temperatures of ~1700 K, corresponding to peak Sn⁺ densities ~2 × 10²⁰ m⁻³. The measured density increase is qualitatively consistent with the strong exponential rise of the evaporative Sn neutral source. However, the plasma evolution cannot be explained solely by vapor production. A simplified Sn⁺ ionization-efficiency analysis reveals a progressive decrease above target temperatures of ~1500 K despite electron temperatures remaining within 2-5- eV. Simultaneously, the radial expansion velocity decreases while the characteristic plasma size saturates, suggesting an increasing influence of collisional processes. A phenomenological Sn-Sn⁺ collisionality parameter was introduced by comparing inferred plasma length scales with ion-neutral mean free paths (Langevin-type collision model estimates). Although affected by significant uncertainty, collisionality increases by more than two orders of magnitude over the target temperature range driven by the strong rise in neutral tin density. Ion-neutral collision times also become substantially shorter than characteristic plasma expansion times against the particle beam. The inferred onset of moderate and strong collisionality occurs within the same target-temperature and plasma-density range associated with heat-flux mitigation at the target surface. These observations are qualitatively consistent with a power exhaust-transition toward a dissipative, collisional vapor-plasma regime

Implementation of Pfirsch-Schlüter Parallel Flow in X-ray Imaging Crystal Spectrometer Inversion Analysis

Courtney Lynn Johnson, Novimir Antoniuk Pablant, Andreas Langenberg, Jaime de la Riva Villén, Arturo Alonso, Tomas Gonda, Oliver Patrick Ford, Andreas Dinklage, Craig D Beidler

The x-ray imaging crystal spectrometer (XICS) tomographic inversion code for Wendelstein 7-X has been modified to consider the effects of parallel flows and has been applied to analyze measurements taken during recent experimental campaigns. Previous analysis neglected the effects of parallel flows due to the primarily perpendicular geometry of the sightlines and the small magnitude predicted by neoclassical theory. To reconsider these effects, the incompressibility condition for plasma flows is used to calculate the parallel Pfirsch-Schlüter flow component for the equilibrium configuration. By incorporating this condition along with the geometry of the sightlines - i.e., the fractional contributions of perpendicular and parallel flows -, an updated expression for the measured flow is used for the profile inversion. Application of this modified inversion code to data from W7-X shows that the magnitude of the radial electric field and the flux surface averaged perpendicular flow are reduced by approximately a factor of 2 and brought into better agreement with neoclassical predictions and charge exchange recombination spectroscopy measurements.

Effect of neutral and ion temperatures in the 2D fluid model for the RF heated ion source SPIDER

Roman Zagorski, Iacopo Regoli, Daniel Lopez-Bruna, Isabella Mario, Antonio Pimazzoni, E Sartori, Alastair Shepherd, Gianluigi Serianni

This paper presents the basic physical and numerical principles of a fluid model implemented into numerical code FSFS2D (Fluid Solver For SPIDER in 2D) used for simulating the SPIDER negative ion source. It gives self-consistent 2D description of the source, including the neutral gas flow, plasma chemistry, RF coupling in the source driver and plasma transport through the magnetic filter. This paper focuses on the recent developments of the plasma model and the first simulations showing the role of the neutrals and ions temperature equations on the source performance are presented. The upgraded model is validated against SPIDER Langmuir-probe measurements and shows reasonable agreement for the axial plasma profiles. The new heavy-species temperature equations reveal strong differences between atoms, molecules, and ions, with D- temperatures increasing in low-density regions near the plasma grid. The simulations provide insight into the energy exchange mechanisms between plasma species and show that only about one third of the RF power transferred to electrons is consumed by plasma-chemical reactions. Atomic deuterium and positive ions are heated mainly through chemical processes, whereas molecules and negative ions are heated predominantly by elastic collisions. Parametric studies demonstrate the sensitivity of heavy-species temperatures to RF power, gas pressure, and magnetic-filter strength.

Strong gradient neoclassical transport in the plateau regime

Silvia Trinczek, Felix I. Parra, Peter J. Catto, Iván Calvo

Strong gradient regions in tokamaks such as the pedestal or internal transport barriers are regions of reduced turbulence where neoclassical transport can play a dominant role. In pedestals, gradient lengths comparable to the ion poloidal gyroradius have been measured. Standard neoclassical theory can miss important strong gradient effects in these regions because it assumes that the gradient length scales of density, temperature and potential are larger than the ion poloidal gyroradius. We extend plateau regime neoclassical theory into regions of gradients of the order of the ion poloidal gyroradius to capture strong gradient effects on transport processes in the pedestal and internal transport barriers. The fundamental idea behind our new framework is to keep a scale separation between the orbit widths and the gradient length scales by performing a large aspect ratio expansion. In the plateau regime, strong gradients cause poloidal variation that is in–out as well as up–down asymmetric. We study two different test cases assuming either radial force balance or the absence of turbulence and show that strong gradient effects can enhance or reduce standard neoclassical theory predictions in the plateau regime in strong gradient regions.

Comparison between OTSG and UTSG for DEMO nuclear fusion facility under pump trip scenarios

Jaime Valverde-Hernández, César Queral, Francisco Martín-Fuertes, Kevin Fernández-Cosials