Fulvio Militello

The HL-3 tokamak program addresses critical challenges in developing integrated high-performance scenarios compatible with power exhaust demands for ITER and future reactors. Through systematic facility enhancements including auxiliary heating upgrades to 19.5 MW and AI-enabled control systems achieving 95.5% disruption avoidance, HL-3 finished a new round of exploration and validation for high-performance operation and power exhaust solution. Experiments at mega-ampere plasma currents demonstrated a reactor-relevant hot-ion regime with core ion temperatures exceeding 10 keV and a fusion triple product of . High normalized beta scenarios featuring internal and double transport barriers were successfully established. To address the compatibility of the plasma boundary, various small- or no-edge localized mode (ELM) regimes were achieved, including the enhanced D-alpha (EDA) H-mode, quiescent H-mode (QH-mode), and the quasi continuous exhaust (QCE) regime. Investigations into power exhaust integration highlighted the efficacy of advanced divertor configurations, specifically snowflake and tripod geometries, in significantly reducing peak heat fluxes. Active ELM control was demonstrated via resonant magnetic perturbations (RMPs), lower hybrid waves (LHW), and impurity supersonic molecular beam injection (SMBI), complemented by real-time feedback control of divertor detachment. These developments establish the fundamental physics basis and technical foundations necessary for demonstrating and extrapolating high-performance operations to ITER and next-step devices.

Far scrape-off layer fluctuations, also known as blobs/filaments, are compared for the first time across six tokamaks: Alcator C-Mod, DIII-D, TCV, KSTAR, MAST, and MAST-U, under the context of stochastic modeling in Ohmic and low confinement mode scenarios. In this model, which well describes these fluctuations, measurements from Langmuir probes are modeled as a superposition of uncorrelated, two-sided exponential temporal pulses. The key parameters of this model are the mean waiting times between consecutive pulses, the pulse duration times and the mean of the pulse amplitudes. The study examines the influence of line-averaged density, plasma current, magnetic field and aspect ratio. The mean amplitudes and pulse waiting times increase with the line-averaged density across devices, while the pulse duration times remain constant. Whereas the variation in waiting times is gradual across the density scan, the mean amplitudes increase abruptly as the density profile broadens and flattens for Alcator C-Mod. The blob waiting times decrease with increasing plasma current and the toroidal magnetic field, while they increase with aspect ratio for conventional tokamaks. The duration times follow the same trends as the waiting times in these parameters but vary more slowly, resulting in the trends in the waiting times dominating the intermittency level of the fluctuations.

The research programme performed on the Mega Amp Spherical Tokamak (MAST) Upgrade experiment has made significant advances in developing the physics understanding of low aspect ratio tokamaks in support of the operation of ITER and design of fusion powerplants. High performance plasma scenarios have been developed to facilitate a broad programme of experiments, in which confinement is constrained by the presence of m / n = 2/1 modes that cause substantial losses of fast ions. The onset of these modes coincides with the q = 2 surface residing in a local minimum in the toroidal current density profile. The maximum electron temperature at the pedestal top, T _e,ped is limited with gas fuelling to ∼350 eV to maintain regular ELMs; higher T _e,ped results in a transition to a non-stationary ELM-free regime. The operational space of spherical tokamaks has been expanded into small and ELM-free regimes. Strong shaping of the last closed flux surface can induce a transition from large to small ELMs, and ELM suppression with resonant magnetic perturbations has been observed for the first time in a low aspect ratio tokamak. Negative triangularity shaping has induced a transition from ELMy H-mode to a high-performance L-mode regime for the first time in a low aspect ratio tokamak. In studies of fast ion confinement, losses of fast particles due to Global Alfvén Eigenmodes have been identified. Interactions between fast ions generated by off-axis neutral beam injection and thermal neutrals can result in significant losses of fast ions. Experiments with on- and off-axis neutral beam injection exhibit a flux pumping mechanism, where the central safety factor is held to ∼1 in the absence of sawteeth. In studies of pedestal physics, it has been found that elevated main chamber neutral pressures result in an increase in the electron density and reduction in the temperature at the pedestal top. Advances in understanding plasma exhaust include the integration of a high-performance plasma core with detached outer divertors in the X -point target configuration. A newly commissioned lower divertor cryopump reduces the lower divertor neutral pressure by up to 50%, with minimal effect on the main chamber or upper divertor. New measurements and SOLPS-ITER simulations emphasise the importance of plasma–neutral interactions on divertor detachment in the conditions accessible in experiments. Real-time control of the ionisation front location in both divertor chambers independently has been demonstrated in double null experiments, enabled by the tightly baffled divertor chambers.

Edge localised modes (ELMs) are quasi-periodic relaxation events, during which plasma is ejected to the walls, which is a major concern for future fusion reactors. Each ELM crash is preceded by transient density and magnetic field fluctuations inside the last closed flux surface, known as precursors. This paper presents a spectral wavelet analysis of ELM-precursors in mega ampere spherical tokamak. Type-I (totalling 77) ELMs and type-III (26) ELMs have been analysed. The analysed data originate from beam emission spectroscopy, Outboard Mirnov Vertical coils, and Outboard Mirnov Array High-frequency Acquisition coils. The precursor mode is detected at f ∼ 20 ± 6 kHz. The wavelet cross-correlation analysis could indicate two types of precursors: a linear type with low wavenumbers k θ ∼ 0.01 − 0.1 cm ⁻¹ , length scales k θ ρ L ≪ 0.1 and mode numbers n ∼ 1 − 10 ; a non-linear type with distinct ranges k θ > 0.1 cm ⁻¹ , k θ ρ L ∼ 0.1 and n > 10. Poloidal velocities tend to be of the order of v θ ∼ 1 − 10 km·s ⁻¹ . The wavelet bi-coherence analysis suggests that precursors could arise from two types of non-linear couplings: f 1 ∼ 15 ± 6 kHz and f 2 ∼ 5 ± 1 kHz; f 1 ∼ 17 ± 7 kHz and f 2 ∼ − ( 7 ± 3 ) kHz. The wavelet statistical analyses demonstrated that different tokamaks share generally similar precursor parameters.

The plasma energy and particle flows in double-null configurations during MAST L-mode discharges are investigated using the 3D plasma turbulence code STORM. The modelling reproduces key phenomena, such as in-out and up-down heat load asymmetries. Turbulent energy transport, driven by ballooning-like instabilities, dominates the radial energy flux across the last closed flux surface, with over 90% entering the scrape-off layer on the low-field side (LFS). In disconnected geometry (the separation between the two X-points in double-null configurations δr _sep ≠0), part of that LFS radial flux is transported to the high field side targets via the secondary X-points, causing the in-out power asymmetry to peak in connected geometries (δ r _sep = 0). Differences between lower double null (LDN) and upper double-null (UDN) configurations arise due to the upward electron ∇B drift and clock-wise poloidal E × B drift, leading to higher collisionality and stronger turbulence near the separatrix in LDN but a shorter heat flux decay length. Poloidal energy fluxes to different divertors exhibit in-out asymmetries, with more energy flowing to the primary outer divertor in LDN and UDN. Additionally, the clockwise E × B drift in the primary private flux regions (PFRs) redistributes energy between primary inner-outer divertors, reducing the heat load on LDN’s primary outer target while increasing it in UDN. Thus, for the same |δ r _sep |, the total heat loads on primary outer targets in LDN and UDN become comparable. These findings provide insights into plasma and energy transport in double-null configurations, with implications for optimizing divertor performance in fusion reactors.

By use of Lagrangian tracers propagated on 2D simulations of scrape-off layer turbulence, we are able to determine the non-local fractional-advection, fractional-diffusion equation (FADE) coefficients for a number of equilibrium cases. Solutions of the resultant FADEs shows good agreement with the simulated mean density profiles. We detail that how the FADE is derived: the stochastic flux equation is introduced, and it is shown how it is used to find general forms of Fick's first and second laws, dependent on the jump function. We show for spatially homogeneous jump functions, which belong to the Levy-α Stable distribution—that transport may be approximated by a non-local FADE with four parameters. This work demonstrates the sound basis for FADEs to act as reduced models of transport in systems dominated by coherent structures and so justifies the development of a first-principles approach to calculating FADE parameters.

Plasma-impurity reaction rates are a crucial part of modelling tokamak scrape-off layer (SOL) plasmas. To avoid calculating the full set of rates for the large number of important processes involved, a set of effective rates are typically derived which assume Maxwellian electrons. However, non-local parallel electron transport may result in non-Maxwellian electrons, particularly close to divertor targets. Here, the validity of using Maxwellian-averaged rates in this context is investigated by computing the full set of rate equations for a fixed plasma background from kinetic and fluid SOL simulations. We consider the effect of the electron distribution as well as the impact of the electron transport model on plasma profiles. Results are presented for lithium, beryllium, carbon, nitrogen, neon and argon. It is found that electron distributions with enhanced high-energy tails can result in significant modifications to the ionisation balance and radiative power loss rates from excitation, on the order of 50%–75% for the latter. Fluid electron models with Spitzer-Härm or flux-limited Spitzer-Härm thermal conductivity, combined with Maxwellian electrons for rate calculations, can increase or decrease this error, depending on the impurity species and plasma conditions. Based on these results, we also discuss some approaches to experimentally observing non-local electron transport in SOL plasmas.

Advances in edge and scrape-off layer diagnostics have shown that often the ion temperature is higher than the electron temperature in scrape-off layer filaments. We have therefore extended the STORM model beyond the cold ion limit commonly used, to include hot ion effects. We find that filaments are both faster and more coherent than in a cold ion model, resulting in increased particle transport. We trace the differences back to the effect of individual hot ion terms, compared using an isothermal ion model. Evolving ion temperature has a modest additional effect on net particle transport, but an ion temperature perturbation in a filament can counteract the instability caused by an electron temperature perturbation.

Recent results from MAST Upgrade are presented, emphasising understanding the capabilities of this new device and deepening understanding of key physics issues for the operation of ITER and the design of future fusion power plants. The impact of MHD instabilities on fast ion confinement have been studied, including the first observation of fast ion losses correlated with Compressional and Global Alfvén Eigenmodes. High-performance plasma scenarios have been developed by tailoring the early plasma current ramp phase to avoid internal reconnection events, resulting in a more monotonic q profile with low central shear. The impact of m/n = 3/2, 2/1 and 1/1 modes on thermal plasma confinement and rotation profiles has been quantified, and scenarios optimised to avoid them have transiently reached values of normalised beta approaching 4.2. In pedestal and ELM physics, a maximum pedestal top temperature of ∼350 eV has been achieved, exceeding the value achieved on MAST at similar heating power. Mitigation of type-I ELMs with n = 1 RMPs has been observed. Studies of plasma exhaust have concentrated on comparing conventional and Super-X divertor configurations, while X-point target, X-divertor and snowflake configurations have been developed and studied in parallel. In L-mode discharges, the separatrix density required to detach the outer divertors is approximately a factor 2 lower in the Super-X than the conventional configuration, in agreement with simulations. Detailed analysis of spectroscopy data from studies of the Super-X configuration reveal the importance of including plasma-molecule interactions and D₂ Fulcher band emission to properly quantify the rates of ionisation, plasma-molecule interactions and volumetric recombination processes governing divertor detachment. In H-mode with conventional and Super-X configurations, the outer divertors are attached in the former and detached in the latter with no impact on core or pedestal confinement.

Tokamak edge turbulence is crucial for the cross-field transport of particles and energy away from the separatrix. A better understanding of what affects the turbulence helps to control the heat flux to the divertor targets and the wall. One potentially important factor is the ion particle source in the divertor, as the neutral pathways and the ionisation source distributions are different depending on the divertor geometry, e.g. vertical- and horizontal-target configurations. Numerically, how to represent the sources and mimic the effects on the SOL in the simulations is still an open question. In this paper, we use a 3D turbulence code STORM, based on drift-reduced Braginskii equations, to study the effects of the divertor particle source distribution on turbulence in a simplified 3D slab geometry. The results show that it requires a large amount of divertor particle source to be peaked near the separatrix to alter the heat flux deposited on the target in attached conditions. This large non-uniform particle source can locally enhance the turbulence in the divertor volume, which redistributes the energy flux to the target and reduces the maximum amplitude. Meanwhile, the plasma profiles evaluated at the outboard midplane, such as the amplitudes and fluctuations of the density and temperature, are marginally changed. Another consequence of our results is that the prediction of the temperature difference between the outboard midplane and the target would be underestimated, if the calculation only considers the conductive heat flux and ignores this enhanced cross-field transport in the divertor.