Recent Publications

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.

CIEMAT, Universita degli Studi di Palermo, UNED

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.

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.

CIEMAT, Universidad Politécnica de Madrid, Sener

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

CIEMAT, Universidade de São Paulo

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.

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.

Laboratorio Nacional de Fusión, CIEMAT

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.

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.

Culham Centre for Fusion Energy, VTT, Consorzio RFX, Forschungszentrum Juelich GmbH, Centro de Investigaciones Energeticas Medioambientales y Tecnologicas

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.

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.

Fusion for Energy, Consorzio RFX, Hungarian Academy of Sciences Centre for Energy Research, Centro de Investigaciones Energeticas Medioambientales y Tecnologicas, Consiglio Nazionale delle Ricerche

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.

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.

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.

National Institutes for Quantum and Radiological Science and Technology (QST) Rokkasho Institute for Fusion Energy, Fusion for Energy, CEA Paris-Saclay, Ciemat

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.

Strong gradient neoclassical transport in the plateau regime

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

Princeton Plasma Physics Laboratory, Massachusetts Institute of Technology, CIEMAT

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

Universidad Politécnica de Madrid, CIEMAT

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

CIEMAT, University of Science and Technology of China, Flatiron Institute

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.

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.

CIEMAT, Max-Planck-Institut für Plasmaphysik, Universidad Carlos III, National Institute for Fusion Science, University of Oxford

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

CIEMAT

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

Numerical investigations of surface negative ion production in the giant NBI source SPIDER

Iacopo Regoli, Roman Zagórski, Daniel López-Bruna, Isabella Mario, Antonio Pimazzoni, Emanuele Sartori, Alastair Shepherd, Gianluigi Serianni

Consorzio RFX, University of Pisa, National Centre for Nuclear Research, Centro de Investigaciones Energéticas Medioambientales y Tecnològicas, University of Padova