Abstract The VERDI detector is a passive neutron detector using the multi-foil activation technique to measure neutron fluence in extreme fusion conditions. It features a low-activation capsule that contains a defined concentration of metallic elements. The robustness of the detector is ensured by the capsule material, while neutron detection relies on activation of the metallic elements. Post-irradiation gamma spectrometry combined with computational unfolding enables reconstruction of the neutron fluence and energy spectrum. This study reviews the development of the VERDI detector and reports the first deployment of VERDI detectors in an operational Deuterium-Tritium (D-T) fusion environment in the Long-Term Irradiation Station (LTIS) of JET tokamak during the DTE2 campaign, featuring a neutron spectrum representative of future fusion devices such as ITER and DEMO. The results provide benchmarks for validating neutronic simulations and nuclear data, supporting the design optimization and safety assessment of future fusion power plants.
Abstract Laser wakefield acceleration offers ultra-high accelerating gradients, enabling compact sources of multi-GeV electron beams with femtosecond duration and quasi-monoenergetic spectra. In this work, we investigated the nonlinear evolution of plasma bubbles and their effect on electron injection and energy gain, using high-fidelity, multi-dimensional particle-in-cell (PIC) simulations performed with the open-source code SMILEI. A relativistically intense laser pulse (a 0 =7.7, λ 0 =0.8 μm, w 0 =20 μm, E=30 J, and τ=30 fs) propagated through a helium gaseous medium with a plateau density of n e =7×10 18 cm -3 after complete ionization. The simulations revealed complex bubble dynamics, including initial formation, disruption, destruction, and bubble merging, strongly influenced by local longitudinal electric field E x approaching or exceeding the cold wave-breaking limit E WB . Exceeding the wave-breaking field E WB triggered bubble destabilization, while the bubble merging at the end of the plateau plasma density region extended the acceleration length, sustaining high longitudinal fields on the order of 1 TV/m and boosting continuous electron injection. This mechanism produced a dense, quasi-monoenergetic electron bunch with a charge of 1 nC, a peak energy of ~1 GeV, femtosecond-scale duration, energy spread below 10%, rms angular divergence θ y of 7 mrad, and a longitudinal rms beam size of σ x =10 μm. Comprehensive analysis of the wakefield structure evolution demonstrated that exploiting nonlinear bubble evolution and interactions near the wave-breaking limit can enhance both energy gain and electron beam quality, providing actionable strategies for next-generation compact LPAs.
Abstract In this work, beam emission spectroscopy (BES) is applied to the investigation of beam divergence and stripping losses in the full-scale ITER negative ion source prototype SPIDER, operating in a multibeamlet configuration. A semi-analytical model is developed to simulate the emission spectra produced by overlapping beamlets, accounting for realistic beamlet divergence, aiming, and beam composition along the accelerator and drift regions. The comparison between synthetic and experimental spectra shows that the Doppler broadening measured in multibeamlet operation cannot be interpreted solely in terms of single-beamlet divergence, but results from the combined effects of beamlet overlap, residual magnetic deflections, electrostatic repulsion, and halo contributions. Using beamlet parameters independently obtained from calorimetric diagnostics, the model reproduces a significant fraction of the experimentally measured divergence. Moreover, the possibility of displacing a line-of-sight allows the detection of variations in the populations of particles with different divergences, namely the core and halo components, across the beam. As a phenomenological characterization, stripping losses are systematically quantified over a wide range of operational parameters, showing an approximately linear increase with source pressure and values consistent with previous single-beamlet studies. These results demonstrate the capability of BES, combined with multibeamlet modelling, to support beam optimization and performance assessment in ITER-relevant negative ion sources.
Abstract To improve the modeling of laser ablation of structured high-Z materials, we extended the hybrid ablation-expansion framework of Hudec et al (Physics of Plasmas 30, 042704, 2023) to Ta 2 O 5 foam targets, with a key improvement of the explicit coupling of radiation energy from the macroscopic radiation hydrodynamic module into the pore-scale ablation-expansion model. The material thermodynamic properties and radiation–matter interaction are described using tabulated equations of state and opacities. The model is recalibrated using updated Ta 2 O 5 equation-of-state and opacity tables and is quantitatively benchmarked against published x-ray self-emission measurements. It is found that the foam microstructure modifies laser coupling in a non-monotonic manner. The effects of foam density and laser intensity on the laser absorption efficiency (LAE) and x-ray conversion efficiency (XCE) are investigated. The LAE exhibits a non-monotonic dependence on the foam density, reaching its maximum at a foam density of ρ ― = 35 mg ⋅ cm − 3 under a laser intensity of I = 10 15 W ⋅ cm − 2 . The XCE also varies non-monotonically with the foam density, but its maximum is shifted to 40 mg ⋅ cm − 3 . This offset indicates that XCE is not controlled by laser absorption alone; it is also affected by the density-weighted radiation generation and optical-depth-limited radiation escape. In addition, the XCE decreases with increasing laser intensity, suggesting enhanced radiation losses and reduced conversion efficiency at higher drive conditions. Higher laser intensity, however, produces a larger total x-ray energy output. A maximum XCE of 24.8% is achieved at 40 mg ⋅ cm − 3 and 10 15 W ⋅ cm − 2 . The hybrid model provides a physically consistent description of laser–Ta 2 O 5 foam interaction and an effective simulation tool for laser ablation of high-Z foam materials in inertial confinement fusion research.
Abstract Meter-scale, low plasma density ( ∼ 10 17 c m − 3 ) hydrodynamic plasma waveguides generated by Bessel beams have forged a path for development of all-optical laser wakefield acceleration (LWFA) to tens of GeV. At low density, the mechanism and appropriate conditions for development of a well-confining waveguide cladding are of particular interest. Here, we apply a simple scaling model to estimate these conditions and use them to contextualize recent plasma waveguide generation experiments from several groups. We show that plasma waveguide generation initiated in low-density gases using a single Bessel beam pulse typically requires an auxiliary laser pulse to generate a well-confining cladding. We present simulations and measurements of low-density optical field ionized plasma channel evolution that confirm the need for such an auxiliary pulse. We also discuss meter-scale supersonic gas jet targets in which waveguides are formed, a key development leading to the recent progress in multi-GeV LWFA.
Abstract Based on the drift MHD model, we investigate kinetic effects on the Alfvén continuum, including the finite parallel perturbed electric field ($\delta E_\parallel$), ion finite Larmor radius (FLR), and plasma diamagnetic drifts. The $\delta E_\parallel$ arises from electron diamagnetic drift and electron Landau damping. It is shown that electron Landau damping couples the kinetic Alfvén wave (KAW) and the electron drift wave (EDW). The electron diamagnetic drift has a much weaker effect compared to the ion diamagnetic drift, as the latter can enhance the frequency of the ion mode. We find that ion FLR increases the continuum frequency in the core region, whereas its influence weakens towards the edge. These kinetic effects are also studied for the toroidal Alfvén eigenmode (TAE) within the self-consistent model. While $\delta E_\parallel$ has negligible impact on the mode structure and real frequency of TAE, it introduces radiative damping through electron Landau damping. These findings emphasize the importance of including kinetic effects in the analysis of Alfvén continuum.
Abstract The dynamics of a pressure-gradient driven plasmoid (p-plasmoid) are investigated using numerical simulations based on a two-fluid model of magnetized plasmas, including ion and electron diamagnetic effects. The p-plasmoid is devoid of a pressure gradient confined by locally concentric magnetic flux and is generated in a host magnetic island produced by a resistive interchange mode driven by the pressure gradient across a neutral magnetic sheet. Through a series of parameter scans, the p-plasmoid formation and its ejection from the host island are found to be controlled by the electron diamagnetic velocity inside the island. The ion skin depth and plasma resistivity scans show that the p-plasmoid is formed and ejected from the island for low-density and low-temperature plasmas. By a magnetic configuration scan, a higher magnetic shear and a favourable averaged curvature are found to be effective in avoiding the formation and ejection of the p-plasmoid. It is also found that a strong monopolar vortex flow is induced in the p-plasmoid, which can be in the clockwise and counter-clockwise directions, contributing to the sustainment of the p-plasmoid.
Abstract Uncertainties of quantities of magnetic equilibrium reconstructions are essential for the validation and quantification of estimated plasma parameters and their uncertainties. The uncertainties of a few equilibrium quantities can be evaluated analytically. A Monte-Carlo method is proposed to estimate the uncertainty of any equilibrium quantity. The Monte-Carlo method was verified using analytic formulas for the uncertainty of some equilibrium quantities and applied to various scalar, profile, flux-surface averaged and integral plasma quantities. Additionally, the uncertainty of the category of negative triangularity plasmas and alternative divertor configuration plasmas were evaluated.
Abstract Among the burning plasma controllers for research fusion reactors, the dud detector will be of primary importance as it determines whether the plasma is performing well or if it is a dud. In the latter case, the discharge needs to be terminated to remain within tritium and neutron activation limits. To this scope, monitors which track the plasma performance will be integrated in the plasma control system. In this work, we present a novel dud detector that has been empirically identified based on deuterium–tritium (DT) campaigns carried out at JET and TFTR. This controller estimates a proxy of the neutron rate using a combination of the diamagnetic energy and the density peakedness. If the predicted neutron rate deviates from the real-time measurement, then the dud detector will trigger an alarm leading to a safe plasma termination if plasma recovery is not expected or, to actuator requests when adjustment is possible. This monitoring function can also be coupled with equilibrium solver and control-oriented models, such as RAPDENS, as proposed in the 15 MA plasma current, 5.3 T toroidal magnetic field baseline DT ITER scenario.
Space plasmas like the solar wind or the Earth's space environment offer unique opportunities to observe fundamental plasma processes and their impact in situ. With modern space instrumentation, we measure the velocity distribution function of the plasma particles as well as the electromagnetic fields at high resolution and with minimal perturbation of the observed plasma systems. Plasma measurements like this are often not possible in laboratory settings on Earth. This review article focuses on modern diagnostic methods for the in-situ detection of plasma particles in space. It presents the detection principle of top-hat electrostatic analysers and highlights recent examples of scientific discoveries based on data from the heliospheric space missions Parker Solar Probe and Solar Orbiter. These examples demonstrate the capabilities of modern space plasma instrumentation. The article then discusses future directions in space plasma physics as well as the involved challenges in terms of the required plasma diagnostics. These new developments include, for example, upcoming and proposed space missions such as the operational space-weather mission Vigil, the multi-spacecraft mission HelioSwarm, the Mars mission M-MATISSE, and the electron-astrophysics mission Debye.
Abstract For tokamaks like the HL-3, which operates at reactor-grade parameters, major disruptions under high-performance conditions are intolerable. High-parameter tokamaks require not only algorithms and engineering solutions for disruption mitigation but also systematic analysis of disruption causes. Traditional disruption analysis relies on various diagnostic data, requiring both temporal evolution analysis and diagnostic cross-channel comparison, which demands substantial expert knowledge and manual effort. To address these limitations, a Plasma Event Identification System have been developed to detect key plasma events in the HL-3 tokamak. This system integrates artificial intelligence, threshold-based judgments to identify disruptions and pre-disruption plasma events including vertical displacement event, magnetohydrodynamics instability, low- q disruption and high density. The performance of each module in the system was evaluated: the disruption identification module achieved 99% accuracy; The disruption time identification module reached 95% accuracy; The tearing mode and locked mode identification attained 95% overall accuracy, demonstrating the system’s robust performance. The system enables rapid statistics and analysis for HL-3 experiments, including disruption cause analysis based on expert-logic judgments and 200 ms pre-disruption event-chain. Furthermore, this system has been deployed in the HL-3 plasma display for disruption analysis. The analysis results are also upload to the Dig Data analysis database to support the research needs of engineering and physics researchers.
Abstract 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 acceleration, 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 is equipped with 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.
Abstract The complex spatial structure of energetic-particle-driven geodesic acoustic modes (EGAMs) has been recently observed experimentally in the TCV tokamak (Dreval et al 2025 Nucl. Fusion 65 016037). In the present work, we model the spatial structure of EGAM modes in the TCV equilibrium using a realistic fast ion distribution function, implemented in the gyrokinetic particle-in-cell code ORB5. For a peaked radial profile of fast ions, the EGAM spatial structure coincides with that of the conventional geodesic acoustic mode, characterized by an nm −1 = 0/0 plasma potential perturbation and an m = 1 standing density perturbation, where the amplitude of the density oscillations is proportional to the sine of the poloidal angle. In contrast, for a hollow fast-ion radial profile and a marginally stable mode, our simulations reproduce a complex EGAM spatial structure similar to that observed experimentally in TCV, with two coexisting density fluctuation frequencies at different radial locations.
Abstract Fast and accurate solutions of the Grad–Shafranov (GS) equation are essential for equilibrium analysis, integrated modeling, and surrogate model construction in magnetic confinement fusion. Motivated by the fundamental question of what is the minimum number of free parameters required to represent solutions of the GS equation, we propose a highly compact spectral representation framework for fixed-boundary equilibria. We demonstrate that, for most practical applications, GS equilibria can be represented using only 3–5 free parameters while maintaining relative errors below 5%. For higher-accuracy requirements, we introduce a unified spectral representation based on the Miller extended harmonic (MXH) expansion in the poloidal direction combined with shifted Chebyshev (Cheb) polynomials in the radial direction. This MXH-Cheb basis exhibits rapid convergence for two-dimensional GS equilibria. For configurations where three geometric moments (shift, elongation, and triangularity) are specified at the last closed flux surface, relative errors on the order of 10 − 2 – 10 − 3 can be achieved using as few as 15–20 parameters. In more general cases, including up–down asymmetric equilibria, X -point configurations, and stiff pressure and current profiles (e.g. H-mode pedestals), high-fidelity representations can be obtained with fewer than 100 parameters. The resulting equilibrium configurations and profile functions are fully analytical, with smooth derivatives of all orders. These results provide a systematic foundation for developing future high-fidelity, ultra-fast GS solvers and enable efficient reduced-order and AI-based surrogate modeling of tokamak equilibria.
Abstract Experiments were conducted at the SG-III prototype laser facility to generate high-energy X-rays via interaction of focused nanosecond laser beams with wire-array targets of varying materials. X-rays exceeding 10 keV were successfully produced. The energy spectrum was measured using a filtered stack spectrometer and a transmission crystal spectrometer, determining the laser-to-X-ray conversion efficiency. Backlighting radiography of test pattern targets was performed, with image clarity enhanced through source distribution deconvolution, achieving improved spatial resolution of the X-ray source. Additionally, areal density resolution was evaluated through backlighting radiography of stepped objects.
Abstract In this work, we present the experimental realization of a plasma with a steady negative triangularity (NT) in a spherical tokamak (MAST-U) for the first time. An average triangularity of − 0.1 was maintained for 100 ms in a double-null plasma with a plasma current of 600 kA, a neutral beam injected power of 3.2 MW, a normalized β of 2, and an elongation of 2 on the MAST-U spherical tokamak. Despite many constraints, this was made possible due to the development of a dedicated plasma shape control scheme and the validation and assessment of magnetic shape controllers in simulations. This effort lays the foundation for physics exploitation of the NT plasma in a spherical tokamak.
Abstract Kinetic-magnetohydrodynamic hybrid simulations have been carried out to investigate the transition between different types of Alfvén eigenmodes (AEs) based on the Experimental Advanced Superconducting Tokamak. Two evolution processes of q -profile, starting with a reverse shear q -profile with minimum q min < 2 have been analyzed: an ascending one based on the magnetic diffusion effect and a descending one inferred from the coexistence of the double tearing mode. The experimental phenomenon, where the type of AE transit from the beta induced AE (BAE) to the reverse shear AE (RSAE) and up-sweeping of RSAE frequency with multiple toroidal number n is reproduced in both processes. In the ascending process, the transition from BAE to RSAE happens at q min = 2 when the q = 2 rational surface vanishes. In the descending process, the transition happens at q min = 1.92 without the vanishing of q = 2 rational surface and is accompanied by a sudden shift in mode location. By comparing frequency and growth rate patterns with experimental observations during the mode transition, the descending process satisfies experiment better. In addition, different physical factors affecting the mode transition have been investigated, including the q -profile shape and energetic particle (EP) pressure. It is found that smaller magnetic shear at rational surface favors the excitation of BAE, and the RSAE frequency is influenced by second order derivative of the q -profile at its minimum. Moreover, the excitation of BAE is not sensitive to the peak location of EP pressure gradient, but the radial phase variation of the mode structure is positively linked to the distance between the EP drive location and the mode location, which is also influenced by the local magnetic shear. These systematic studies on the effects of different physical factors on AEs can provide guidance for controlling Alfvén instabilities in the future.
Abstract At the ASDEX Upgrade tokamak, a new fast method for the evaluation of the Fast-Ion D α (FIDA) diagnostic has been developed. Up to now, the analysis of FIDA data required forward-modelling with the FIDASIM code, whose long calculation times restricted the analysis to single time points within a discharge. We developed a faster forward model, which makes it possible to study the evolution of the fast-ion distribution over whole discharges. The computational time to evaluate a single time point has been reduced from minutes to near real-time. The new method allows the reproduction of the FIDA spectrum using the FIDASIM weight functions approach: the spectral intensity is obtained by convolving the fast-ion distribution function with a weight function. The latter can be seen as the product between a wavelength-independent FIDA intensity function, called R , and an analytic probability of emission in a determined λ range. The computational times are strongly reduced by adopting a look-up table (LUT), in which the R functions computed by FIDASIM are stored as a function of the most relevant plasma parameters, line of sight geometry and neutral components (beam components and halo). The LUT requires the previous knowledge of the neutral densities inside the tokamak, for this purpose, a fast model has been implemented within the RABBIT code. A detailed evaluation of the new method’s approximations is provided, highlighting the discrepancies identified relative to the FIDASIM reference.
Abstract Predictive, fast reduced-order models are essential for fusion devices like ITER, where real-time transport forecasting, optimisation, and control cannot depend on costly first-principles turbulence simulations alone. We tackle this problem with the LaQuey Mahajan, Rutherford, and Tang equation, a reduced trapped-ion mode turbulence model equivalent, after nondimensionalisation, to the Kuramoto–Sivashinsky equation. We use a geometry-aware reduced-order framework over a two-parameter space of damping and dissipation. Local proper orthogonal decomposition bases extracted from direct numerical simulation are used to build Galerkin reduced models. Frozen bases remain accurate when turbulent structures vary smoothly, but they deviate near transitional regimes where fluctuation content reorganises. Grassmann-manifold analysis shows that subspace proximity does not always guarantee physical accuracy: transport may be reproduced even when fine-scale structure is not. Adaptive, parameter-dependent bases obtained by manifold interpolation significantly improve robustness and recover both large-scale and fine-scale fidelity. Overall, the results show that transport accuracy, structural fidelity, and basis geometry are complementary validation criteria for plasma surrogates, and they support efficient predictive models for ITER-scale optimisation and control.
Abstract A tokamak model with a coupling surface (CS) (Villone et al 2013 Plasma Phys. Control. Fusion 55 , 095008) separating the regions with two-dimensional and three-dimensional (3D) descriptions of the magnetic field B is considered. In this approach, it is assumed that the 3D 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 3D 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.
