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Validated integrated modelling of JET ITER-like wall experiments in which fusion performance is driven by reactions between fast ions and intrinsically present metal wall impurities is presented. A steady-state L-mode plasma with dominant proton-beryllium fusion and neutron yields of up to ≈ 6⋅10 ¹³s^-1 is developed in He and D, via radiofrequency heating of a H minority. The fusion drive is unambiguously confirmed by the neutral particle analyser, fast ion loss detector, and γ-ray diagnostics. Experiments are analysed via an integrated modelling framework, developed to model the two-stage proton beryllium-fusion chain and produce high-fidelity fusion product source terms. The modelling chain comprises TRANSP and JETTO for plasma core modelling, LOCUST for full orbit product tracking and collisional slowing-down, DRESS to resolve two- and three-body fusion kinematics, and MCNP for neutron transport calculations. Modelling shows that the primary ⁹Be(p,n) ⁹B reaction is the dominant neutron emitter at naturally present concentrations of beryllium in these experiments. The yield contribution of secondary reactions between fusion products and beryllium, ⁹Be(d,n) ¹⁰B and ⁹Be(α,n) ¹²C, is found to be negligible. The proton-deuteron knock-on effect in D plasmas is modelled, which is calculated to contribute ≈ 25% to the total neutron yield. For both He and D discharges the total computed neutron rates match fission chamber (FC) measurements within the combined experimental and computational uncertainty, with an average discrepancy of ≈ ± 20%. Realistic proton-beryllium neutron sources are propagated through JET's MCNP neutron transport model which shows that ²³⁵U FCs' response is sensitive to p-Be source changes, with up to ≈ 10% variation compared to a D-D neutron source. We show that the high-energy tail of the fast proton minority can be studied with multi-foil neutron activation. The framework is also applied to the study of interactions between fast protons and boron impurities, of relevance to ITER. We calculate that in JET conditions a significant alpha source with DT-like energies could be generated through ¹¹B(p, α)2 α fusion, and detected via gamma-emission in secondary interactions between fast alphas and boron. The work represents an important step towards validating predictive integrated modelling capabilities for non-standard fusion reactions.

In JET-ILW, beam-target reactions contribute to a large fraction of the fusion power generated in deuterium-tritium (D-T) plasmas, with core ion temperatures of 10-12 keV and large neutral-beam injection (NBI) power. Previous modelling done in preparation for the recent D-T campaigns in JET have shown that injecting D beam ions with energies of similar to 120 keV in T-rich plasmas produces larger 14 MeV fusion yield than in 50:50 D:T plasmas, but such scenario had never been tested in past D-T experiments. In addition, the simulations showed that fundamental ion cyclotron resonance heating (ICRH) of the D ions can significantly boost the net fusion reactivity, since both the D-bulk ions and the fast D-beam ions are accelerated to energy ranges that are optimal for the D-T reactions to take place. In the last JET D-T campaigns (DTE2 and DTE3), dedicated experiments confirmed-for the first time-the improved fusion performance of T-rich plasmas with high D-NBI power and highlighted the key impact of fundamental D ICRH on the fusion performance. This new scenario led to the world-wide D-T fusion energy record ever achieved in a fusion device and allowed to sustain more than 12 MW of fusion power averaged over 5 s. The main results of these unprecedented experiments will be presented and the NBI + ICRF physics responsible for the high fusion performance achieved will be highlighted through numerical modelling.

JET, the only tokamak capable of operating with deuterium-tritium (D-T) fuel (since TFTR was shutdown in 1999), has provided essential experimental data to support ITER and DEMO design and operation. Within the EUROfusion Tokamak Exploitation Work Package, JET completed its final campaigns (2022-2023), culminating in the third D-T campaign (DTE3). These experiments addressed key challenges in plasma scenarios, exhaust control, and tritium management under reactor-relevant conditions. Significant progress was achieved in demonstrating ITER-like integrated scenarios with impurity seeding, achieving partial divertor detachment and high confinement ( H98(y,2) approximate to 0.85) at 3 MA in D-T plasmas. Advanced exhaust regimes such as quasi-continuous exhaust (QCE) and X-point radiator (XPR) were successfully achieved first in D-D and then extended to D-T operation, confirming their relevance for mixed isotope operation. Operational milestones included a new world record of 69 MJ fusion energy in tritium-rich hybrid plasmas and long-pulse H-mode operation up to 60 s, contributing with unique data to the CICLOP database. Physics studies focused on peeling-limited pedestals in support of ITER and improved understanding of edge stability and impurity screening in metallic environments. Extensive usage of the shattered pellet injector (SPI) on JET provided critical information for the design of the ITER disruption mitigation system (DMS). Real-time control systems for D/T ratio control and plasma exhaust were deployed and demonstrated in D-D and D-T, while energetic particle physics investigations unfolded the role of fast ions in turbulence suppression mechanisms. Comprehensive tritium retention studies using gas balance method, post-mortem analysis, and ITER-relevant laser induced desorption spectroscopy (LIDS) diagnostics provided essential input for tritium accountancy strategies. These results are validating the ITER operational concepts, inform DEMO design, and deliver critical experience in nuclear operation and scenario integration.

In high-gain reactor-relevant fusion plasmas, alpha heating dominates the power balance. As alpha particles thermalize, they transfer kinetic energy to the bulk plasma via collisions. At the MeV collision energies characteristic of alpha thermalization, nuclear elastic scattering cross sections become comparable to or exceed the Coulomb cross section, greatly enhancing the probability of large-angle scattering events. These nuclear interactions can accelerate a fuel ion to several MeV in a single collision, producing distinctive non-Maxwellian fast "knock-on" tails in the kinetic distribution functions that are characteristic of thermonuclear burn. This effect has been experimentally confirmed by the observation of fast neutron spectral tails at the Joint European Torus and at the National Ignition Facility. Here, we present the first kinetic simulations of alpha particle knock-on ion generation for the SPARC tokamak's Primary Reference Discharge, a 12.2 T, 8.7 MA H-mode scenario projected to achieve a scientific gain of ∼ 11, deep in the burning plasma regime. We couple the bounce-averaged Fokker-Planck solver CQL3D with the Monte Carlo reaction integral code DRESS, implementing a thermally broadened alpha source term, large-angle scattering corrections to CQL3D's collision operator for deuterons and tritons, and a synthetic neutron spectrum diagnostic. The knock-on ion tails evolve toward a kinetic steady state after about two alpha slowing-down times (∼ 600 ms) and comprise approximately 0.1% of the total fuel ion population, with the fast-deuterium density slightly exceeding fast-tritium, consistent with nuclear resonance in the α,d cross section below the alpha birth energy. Equivalent Maxwellian fits to the fast-ion distribution functions, assessed with log₁₀-space coefficients of determination R²_log, are adequate for describing knock-on fuel ions (R²_log = 0.64-0.89), but are fundamentally inappropriate for the alpha slowing-down distribution (R²_log ≈ 0). Synthetic neutron emission spectra computed with DRESS predict a steady-state alpha knock-on neutron ratio of 8.8 x 10^-5 and demonstrate that SPARC's magnetic proton recoil neutron spectrometer will be sensitive to the alpha knock-on neutron tail, providing a direct observational diagnostic of alpha confinement and thermalization in a magnetically confined burning plasma.

This Roadmap article addresses the critical and multifaceted challenge of plasma-facing component (PFC) damage caused by runaway electrons (REs) in tokamaks, a phenomenon that poses a significant threat to the viability and longevity of future fusion reactors such as ITER and DEMO. The dramatically increased RE production expected in future high-current tokamaks makes it very difficult to avoid or mitigate REs in such devices when a plasma discharge terminates abnormally. Preventing damage from the intense localized heat loads they can cause requires a holistic approach that considers plasma, REs and PFC damage. Despite decades of progress in understanding the physics of REs and the thermomechanical response of PFCs separately, their complex interplay remains poorly understood. This document aims to initiate a coordinated, interdisciplinary approach to bridge this gap by reviewing experimental evidence, advancing diagnostic capabilities, and improving modeling tools across different scales, dimensionalities, and fidelities. Key topics include RE beam formation and transport, damage mechanisms in both brittle and metallic PFCs, and observed effects in major facilities such as JET, DIII-D, WEST and EAST. The Roadmap emphasizes the urgency of predictive, high-fidelity modeling validated against well-diagnosed controlled experiments, particularly in the light of recent changes in ITER's wall material strategy and the growing importance of private sector fusion initiatives. The modeling gaps include calculations of the magnetic equilibrium, RE distribution function and background plasma properties at the start of the beam termination, with a critical need to advance modeling of access conditions to benign terminations. This is to be followed by simulations of the termination itself which requires a self-consistent description of the 3D MHD fields together with the REs, which is now available thanks to recent developments. Finally, the damage assessment is enabled by modeling of the full thermo-mechanical response, encompassing the hydrodynamic and the deviatoric behavior, to RE impacts with characteristics as provided by the termination simulations. The full workflow is computationally heavy for scoping studies to assist design of future machines, calling for development of reduced or surrogate models. Each section of the Roadmap article is written to provide a concise overview of one area of this multidisciplinary subject, with an assessment of the status, a look at current and future challenges, and a brief summary. The ultimate goal of this initiative is to guide future mitigation strategies and design resilient components that can withstand the intense localized loads imposed by REs, thus ensuring the safe and sustainable operation of the next generation of fusion power plants.

At JET, Deuterium-Helium-3 (D-³He) experiments using ion cyclotron resonance heating (ICRH) with the three-ion scheme were conducted, involving thermal Deuterium (D), thermal Helium-3 (³He), and a fast D beam injected via neutral beam injection (NBI) and further heated using ICRH. In these plasmas, intense neutron emission at the level of ≈ 10¹⁶ ns^-1 was observed, and a non-thermal neutron spectrum was measured with the JET time of flight neutron spectrometer. Furthermore, the spatial profile of ≈ 16.4 MeV gamma rays originating from the ³He(D,γ)⁵Li reaction was observed. This is a weak branch (≈10^-5 relative probability) of the more common ³He(D,p)α reaction and it can be used to assess the production of alpha particles in these experiments. In this work we analyse a set of data from nuclear diagnostics in D-D_NBI-³He experiments at JET and compare them with synthetic diagnostics based on TRANSP simulations of the fast deuterium distribution function, using the so-called 'kick operator' to model power transfer from the wave to the beam fast deuterons. The neutron rate predicted by the TRANSP simulations broadly agrees with data measured with fission chambers. The shape of the neutron emission spectrum is also well understood, once finite Larmor radius effects are introduced into calculations. Data from the gamma ray camera are used to infer the alpha birth profile through tomographic reconstructions based on the TREVISO code. These were compared with corresponding calculations starting from TRANSP results and showing an overall good agreement with the experimental tomographies. The validated TRANSP results are finally used to discuss the modifications of the fast deuterium distribution function and of the alpha birth profile in a set of discharges.

We present a fully analytical model for calculating energy spectra of neutrons generated by fusion reactions involving a fast ion, or beam, and a stationary ion, or target, in magnetic fusion plasmas. For neutrons moving along the line-of-sight of a detector, the neutron spectrum is given by an analytical expression and the usual differential cross section. This makes the model several orders of magnitude faster than ordinary Monte Carlo simulations and free of any related statistical noise. Additionally, the analytical description of the reaction physics provides much more insight into the formation of the spectrum. An example of this is the bias of beam-target spectra towards high-energy neutron counts, which corresponds to forward-emission events. On the other hand, the fast-ion uniform gyro-angle distribution has an opposite effect, but is ultimately weaker than the preferential forward emission of neutrons. The model is validated against numerical calculations from the forward model code GENESIS to verify its validity and it is furthermore derived from a probabilistic viewpoint, adding further insight.

We present a method to analytically compute gamma-ray spectra generated via two-step fusion reactions, where a gamma-ray is emitted from the excited nucleus generated in the first step of the reaction. If one reactant is energetic and the other is at rest, the first step of the reaction can be treated analytically. The second step, which is the gamma-ray emission from the excited nucleus, can always be treated analytically. The model we derive is tested against the established forward-model code GENESIS, obtaining very satisfactory results. Our fully analytic treatment is a far less expensive technique than standard Monte Carlo methods, achieving several times faster computations. Fast calculations of spectra are especially beneficial when working with finely-resolved 3D-4D phase spaces. Furthermore, tractable analytical expressions give insight that is not provided by Monte Carlo methods. The formalism used for the first step of the reaction additionally allows the computation of birth distributions of fusion products from any beam-target reaction with one reactant at rest, e.g. fusion-born alpha distributions.

The fuel ion densities of tritium (T) and deuterium (D) are important parameters for reactor control in fusion experiments. The fuel ion densities typically need to be determined for a wide range of experimental conditions, anywhere between pure deuterium plasmas with trace amounts of T to pure tritium plasmas with trace amounts of D. Given the significantly larger cross section for D+T fusion reactions, compared to D+D or T+T, introducing small amounts of tritium to a deuterium plasma (or vice versa) quickly has a large impact on the neutron emission rates and the neutron energy spectrum. For experiments that use the neutron emission rate as a figure of merit, or for simulations, the time-resolved T and D densities often need to be considered. In this paper, we present a method for determining the densities using multiple neutron diagnostics together with simulations of supra-thermal fuel ions utilizing the plasma transport code TRANSP. The method is set up utilizing a Bayesian framework to estimate the most likely distribution of T and D densities given the available data and the modeled slowing-down ion velocity distribution, using the synthetic neutron diagnostics code DRESS to calculate the expected neutron emission for the given fuel ion distributions. The method is applied to experiments conducted at the JET tokamak, involving deuterium-dominated plasmas with tritium concentrations ranging from 0 to 10%, heated by D neutral beam injection and ion cyclotron resonance frequency heating.

High Resolution Neutron Spectrometer (HRNS) is one of the essential plasma diagnostics of ITER, whose operational role is the neutronic measurement of the nt/nd ratio in a plasma core. Coexisting with the other ITER diagnostics makes it a powerful tool for efficient and precise plasma diagnostics. The main goal of this work is to present the operating principles and key challenges associated with the High Resolution Neutron Spectrometer in ITER, with a particular focus on the Thin-Foil Proton Recoil (TPR) Spectrometer. The complexity of ITER tokamak brings with it many variables that had not been considered of primary importance until now, such as the magnetic field or high temperature in the detector region.