Recent Publications

Sep 28

First measurements of ion-scale turbulence with the upgraded beam emission spectroscopy diagnostic in MAST Upgrade

Steven Thomas, Istvan Cziegler, Daniel Dunai, Christopher Bowman, Yorick W Enters, Bhavin S Patel, Rory Scannell

Plasma Physics and Controlled Fusion·Sep 28, 2026Plasma & ConfinementControl & Diagnostics

Beam emission spectroscopy (BES) is used to diagnose ion-scale (k ⊥ ρ i <1) plasma turbulence in MAST Upgrade (MAST-U) by analysing the Doppler shifted D α light emitted from the on-axis neutral deuterium heating beam. These are the first results of ion-scale turbulence measured in MAST-U, and the first results from the upgraded BES diagnostic system which has an 8 × 8 array of avalanche photo-diodes (APDs) in the R−z plane, increased from 4 poloidal channels in the old BES system on MAST, spaced ≈2cm apart at up to 4MHz sampling frequency. A series of three repeatable L-mode discharges are analysed, with the BES sampling different locations in each shot, to obtain measurements over almost the entire low-field side (LFS) minor radius, 0.15≤Ψ N ≤1.1, giving radial profiles of key statistical quantities. The 2D coverage of the BES diagnostic (R×z≈13×15cm) is exploited to give simultaneous measurements of radial and poloidal correlation lengths and wavenumbers, as well as tilt angles of turbulent eddies. Compared with the system’s predecessor, the increased poloidal coverage allows for significant improvements in the inference of poloidal quantities (correlation lengths and wavenumbers), as well as improved turbulence decorrelation time and poloidal velocity inferences. Local maxima are seen mid-radius (R≈1.3m), which corresponds closely with a local maximum in the plasma rotation velocity and a flattening of the electron density gradient. The apparent z-velocity from the BES is used in tandem with other plasma profiles (i.e., electron temperature and density, ion temperature, plasma rotation velocity, and magnetic field components) to infer the E × B velocity profile. These are compared with incomplete E × B velocity estimates from the charge exchange diagnostic. Some features of the velocity profiles are present in both inferences, giving good qualitative agreement, but the absolute values do not agree. As there is no poloidal velocity measurement from charge exchange in MAST-U, we assume it is small in our inference of the radial electric field, E r . For MAST plasmas, however, it has been shown that the poloidal flow contribution to E r can be large, and we suggest this may be the cause of the quantitative difference in our profiles.

United Kingdom Atomic Energy Authority

Sep 24

Steady-state and dynamic validation of DIV1D with SOLPS-ITER and MAST-U experiments

Kuan-Wei Lee, Livia Casali, K Flesch, Gijs Lukas Derks, Stijn Peter Kobussen

Plasma Physics and Controlled Fusion·Sep 24, 2026Plasma & ConfinementControl & DiagnosticsAI, Modeling & Simulation

Future fusion reactors will require real-time detachment control to maintain divertor heat and particle fluxes within material limits. Here, we assess the time-dependent 1D scrape-off-layer model DIV1D as a reduced-order plant model for detachment controller design by benchmarking its steady-state predictions and control-relevant dynamics for a MAST-U H-mode scenario, respectively, against SOLPS-ITER simulations and experimental data. After tuning a small set of free parameters, DIV1D reproduces the main quantities from 1D mapped SOLPS profiles. Density-ramp and molecular-puffing scans show that DIV1D captures the trends in key target quantities. Multi-sine system-identification simulations show that baseline DIV1D responds faster than MAST-U system-identification experiments [1]. The agreement improves substantially when the total particle throughput is reduced to experimentally representative levels. This result demonstrates that the total particle throughput in the global particle balance is a key determinant of the modeled control-relevant dynamics and must be represented accurately for reliable plant-model behavior. These findings establish DIV1D as a useful tool for model-based detachment controller design on MAST-U, with direct relevance to next-step devices, where experimental system-identification opportunities will be limited and reduced-order models will be essential for pre-operational controller development.

Sep 21

The poloidal fuelling location and its effect on the pedestal and H-mode plasma performance in MAST Upgrade

Steven Thomas, Jerry W Hughes, Alex Tookey, Bartosz Lomanowski, Davis Easley, Jack Lovell, Christopher Beckley, James R Harrison, Edward DeWit, Saskia Mordijck, et al.

Plasma Physics and Controlled Fusion·Sep 21, 2026Plasma & ConfinementControl & DiagnosticsTritium & Fuel Cycle

This paper presents the first results exploiting the high-speed video (HSV) diagnostic on MAST Upgrade to infer neutral deuterium quantities from D α emission. We detail the process for absolute calibration of HSV which now produces 2D line-of-sight-integrated brightness (ph sr -1 m -2 s -1 ) of D α emission in the MAST-U main chamber, showing excellent quantitative agreement with other D α and neutral diagnostics. A typical workflow to combine HSV data with a collisional-radiative model, and electron density, n e , and temperature, T e , measurements to produce 1D radial profiles of neutral density, n 0 , and cold deuterium ionisation source rate, S 0 , is documented. The analysis is applied to a series of double null, neutral beam heated H-mode discharges in MAST-U changing the poloidal fuelling location. It is shown how the low-field side (LFS) pedestal electron density is resilient to, and remains largely unaffected by, the choice of poloidal fuelling location, as does n 0,sep despite differences in neutral pressure measured at the wall. LFS fuelling is seen to increase S 0 , reducing the pedestal electron temperature which decreases edge collisionality, ν*, and allows for core n e to be increased. S 0 is used to constrain inferences of deuterium ion flux, Γ, in a 1D radial transport model. When fuelling from the LFS, Γ becomes increasingly negative at the pedestal top, indicating an inward flux of particles, and is used with the diffusive-convective ansatz showing a negative convective transport coefficient, v, implying an inward particle pinch.

United Kingdom Atomic Energy Authority·Massachusetts Institute of Technology·Oak Ridge National Laboratory

Sep 20

VEQDB: A Compact and Reconstructible Multi-Device Tokamak Equilibrium Database

Huasheng Xie, Ruohan Zhang, Xingyu Li, Feng Zhang, Zhengxiong Wang

arXiv (physics.plasm-ph)·Sep 20, 2026Plasma & ConfinementAI, Modeling & Simulation

Tokamak equilibria are commonly exchanged as gridded G-EQDSK files whose conventions, resolutions, and machine-specific formats impede cross-device comparisons and data-driven modeling. Here, we present VEQDB, an open, compact, and reconstructible fixed-boundary equilibrium database built on continuous MXH--Chebyshev geometry and independent physical-profile roots. By decoupling authoritative equilibrium physics from rectangular meshes, VEQDB enables continuous evaluation and metric differentiation at arbitrary application-demanded resolutions. Backed by an automated numerical validation pipeline, VEQDB is structured as an extensible repository for ongoing community expansion. Its inaugural release provides 13,291 accepted equilibria across 267 conventional and spherical tokamaks, encompassing parameter-sampled Grad--Shafranov solutions, G-EQDSK projections spanning EAST, MAST-U, and ITER scales, and controlled variation families with explicit provenance. Benchmark projections reproduce normalized flux maps with RMS errors between $1.09 \times 10^{-3}$ and $1.45 \times 10^{-3}$, while compact JSON representations achieve an 89--96-fold size reduction relative to standard $129 \times 129$ G-EQDSK files. The complete initial release occupies 41~MB in raw JSON and 18~MB in compressed archives, and all records successfully passed independent reload and evaluation tests. VEQDB establishes an extensible, provenance-preserving foundation for equilibrium studies, reduced-order surrogate modeling, and cross-machine workflows.

Sep 11

Sensitivities of electron-scale core transport in MAST Upgrade

B. S. Patel, T. Adkins, S. Blackmore, D. Kennedy, C. Vincent

arXiv (physics.plasm-ph)·Sep 11, 2026Plasma & ConfinementAI, Modeling & Simulation

Integrated modelling of the Mega Ampere Spherical Tokamak Upgrade (MAST-U) indicates that turbulent electron heat transport can dominate over ion transport across a range of operating regimes. Only a limited set of instabilities are able to produce this behaviour, with the primary candidates being microtearing modes (MTMs) and electron temperature gradient (ETG) driven modes. This work investigates electron-scale core transport in two L-mode plasmas and one H-mode plasma on MAST-U using local gyrokinetic analysis. The linear and nonlinear sensitivities of ETG modes are examined, with particular focus on their dependence on electron temperature gradients and ExB shearing rates. In L-mode discharges, ETG modes are found to be linearly unstable over a broad radial region and can drive experimentally relevant levels of electron heat transport, particularly towards the outer core. The transport is strongly sensitive to both the electron temperature gradient and the ExB shear, with nonlinear simulations showing stiff transport and good agreement with experimental estimates within uncertainty. Towards the core, however, ETG-driven transport is reduced and is insufficient to fully explain the observed anomalous heat flux. In contrast, in the H-mode plasma ETG modes are found to be stable or only weakly unstable in the core, resulting in negligible electron-scale transport. This is attributed to higher equilibrium pressure gradients and reduced electron temperature gradients, both of which stabilise ETG turbulence. Following the collapse of the core rotation profile, ETG modes can become unstable towards the outer core, but typically remain insufficient to account for the full level of transport. Overall, ETG turbulence can play a significant role in setting electron heat transport in L-mode plasmas, particularly at larger radii, but is unlikely to dominate in high-performance H-mode conditions.

Sep 9

Machine learning tomography for sparse-view soft X-ray diagnostics in MAST-U

Brian Steward, Marco Cecconello

Plasma Physics and Controlled Fusion·Sep 9, 2026Plasma & ConfinementControl & DiagnosticsAI, Modeling & Simulation

Understanding magnetohydrodynamic (MHD) instabilities is crucial for advancing magnetic confinement fusion. Soft X-Ray (SXR) tomography enables reconstruction of the spatial structure of these instabilities from one dimensional line integrated measurements, however traditional tomography methods struggle with sparse diagnostic arrays. The MAST-U tokamak has a sparse camera array of 28 non-intersecting lines of sight (LoS), making traditional tomography techniques challenging. We present a machine learning (ML) tomographic method using a 50 layer residual neural network (ResNet) trained on synthetic data simulated by the transport code TRANSP. ML tomography is compared with traditional minimum Fisher information tomography, and with second derivative smoothing tomography. The ML technique shows a 78 to 94 times median improvement in the mean square error (MSE) over traditional 28 LoS reconstructions of synthetic data, at a computational speed which is 144 to 294 times greater (25 ms on GPU hardware). Notably, ML tomography with 28 LoS outperforms traditional methods even when provided with three times as much data in the form of 84 intersecting synthetic LoS. Validation on experimental data from MAST-U demonstrates accurate reconstructions of peaked and broad plasma SXR profiles, and the evolution of sawtooth instabilities are visualized with ML tomography. The sub-30 ms reconstruction time enables real-time plasma diagnostics, opening new possibilities for MHD monitoring in fusion devices.

Sep 7

Neoclassical tearing mode seeding by Alfvén eigenmode coupling in MAST-U

Kitt Cameron Medley Thomas, Laszlo Bardoczi, Kieran Gibson, Juan Ruiz Ruiz, Mykola Dreval, David Anthony Ryan, Kenneth G McClements, Clive A Michael

Nuclear Fusion·Sep 7, 2026Plasma & ConfinementControl & Diagnostics

We present evidence of tearing mode seeding by nonlinear three-wave coupling of Alfvén eigenmodes in a tokamak plasma. Here, a m,n=2,1 magnetic island appears to be seeded in a MAST-U discharge by coupling of n = 2 and n = 3 Alfvén eigenmodes, which is demonstrated by increased squared normalised bicoherence calculated from magnetic probes preceding tearing mode onset. This observation offers a new explanation for tearing modes that would be otherwise addressed as spontaneous, and indicates three-wave coupling can seed 2,1 magnetic islands in plasmas without pre-existing n ≥ 2 tearing modes. Further, this mechanism has the potential to be more prevalent in burning plasmas expected in future fusion devices.

General Atomics·Kharkiv Institute of Physics and Technology·Culham Centre for Fusion Energy

Aug 26

Real-time virtual circuits for plasma shape control via neural network emulators: integration and testing in the MAST-U PCS

Matthew J. Marshall, Edward Jones, Graham J. McArdle, Alasdair Ross, Kamran Pentland, Nicola C. Amorisco, Charles Vincent, Martin Kochan, Colin Hogben, Graham Jones, et al.

arXiv (physics.plasm-ph)·Aug 26, 2026Plasma & ConfinementControl & DiagnosticsAI, Modeling & Simulation

The deployment of advanced, AI-enabled control algorithms in tokamak experiments requires robust integration with existing plasma control system (PCS) architectures and extensive pre-experimental validation. In this contribution, we describe the integration and testing of neural-network-emulated virtual circuits for plasma shape control within the MAST Upgrade (MAST-U) PCS environment. The neural network models predict the plasma shape using the plasma current, poloidal field coil currents, and plasma profile parameters. In this paper, we explain how they are deployed via a real-time C++ inference server that interfaces with the PCS, returning the shape prediction and its Jacobian, and how, from the latter, virtual circuit matrices and updated coil current requests are computed for real-time actuation. Emphasis is placed on the validation workflow and best practices adopted to ensure confidence in the proposed control framework prior to experimental deployment. This work demonstrates practical AI-based shape control components for fusion control systems, with direct relevance for upcoming MAST-U experiments and future devices.

United Kingdom Atomic Energy Authority

Machine learning methods for modelling local, linear gyrokinetic simulations of MAST-U pedestal turbulence

Anna Niemelä, Daniel Jordan, Aaro Järvinen, Amanda Bruncrona, Adam Kit, Lorenzo Frassinetti, David Hatch, Leonhard Leppin, Samuli Saarelma, the MAST Upgrade team, et al.

arXiv (physics.plasm-ph)·Aug 26, 2026Plasma & ConfinementAI, Modeling & Simulation

Gyrokinetic (GK) stability strongly influences the performance of high-confinement-mode pedestals in spherical tokamak plasmas. High-fidelity gyrokinetic codes such as GENE can model microinstability-driven transport, but the computational cost limits their routine use in integrated pedestal modeling workflows. Instead, present workflows often rely on reduced transport assumptions, such as the ballooning-critical pedestal model used in EPED. This work investigates machine-learning surrogate models for local linear gyrokinetic simulations in a MAST-U-relevant pedestal parameter space, with the aim of providing faster gyrokinetic-based inputs to reduced pedestal models. A sampling workflow is developed in which pedestal profile parameters are varied within experimentally motivated bounds and used to generate physically self-consistent Grad-Shafranov equilibria. This reduces the dimensionality of the data-generation problem compared with sampling local gyrokinetic inputs directly, while maintaining physically plausible combinations of plasma profiles, geometry, and local stability parameters. The surrogate models are trained to predict linear growth rates, real frequencies, and diffusivity-ratio transport fingerprints from local linear GENE simulations. A multi-head multilayer perceptron accurately reproduces the growth rate, while the diffusivity ratios and real frequency exhibit more clustered, regime-dependent behavior. A multi-head classification-regression model using frequency-based regime classes reduces the mean absolute error for these clustered targets and better captures sharp transitions associated with changes in the underlying instability regime, although errors near mode-transition regions remain a limitation.

United Kingdom Atomic Energy Authority·VTT Technical Research Centre of Finland·KTH Royal Institute of Technology

Aug 24

Experimental Identification of the n=2 Error Field and its Interaction with MHD Activity and Plasma Rotation in MAST-U

Lidia Piron, David Anthony Ryan, Andrew Kirk, Alessandra Tonel, Paolo Zanca, Geoffrey Cunningham, Matteo Baruzzo, Sam Blackmore, Christopher J Ham, Scott Alan Silburn, et al.

Plasma Physics and Controlled Fusion·Aug 24, 2026Plasma & ConfinementControl & Diagnostics

Spurious magnetic field perturbations, known as error fields (EFs), with toroidal mode number n = 2 can have deleterious effects similar to those associated with n = 1 error fields. An n = 2 error field source is expected in MAST-U because the poloidal field coils P4 and P5, retained from the previous device MAST, exhibit n = 1 and n = 2 deformations due to coil manufacturing imperfections. This work presents the n = 2 error field identification studies carried out in MAST-U. The n = 2 compass scan indicates that, for 750 kA plasma current, double-null divertor H-mode plasmas, the n = 2 error field is relatively small when assessed using locked mode onset and rotation braking as metrics, suggesting that the n = 2 EF is effectively screened by the plasma in this scenario. Furthermore, during the n = 2 EF identification studies, an interesting interplay between the onset times of n = 1 and n = 2 rotating modes and plasma rotation was observed. This observation enables the identification of control strategies aimed at delaying the onset of n = 1 and n = 2 MHD modes while sustaining plasma rotation.

Consorzio RFX·United Kingdom Atomic Energy Authority·Culham Centre for Fusion Energy

Aug 21

Mitigation of sawtooth transient heat flux in the MAST-U Super-X divertor with deuterium and nitrogen

Rory Scannell, Jack Flanagan, Zhouji Huang, James R Harrison, Kevin Verhaegh, Scott Alan Silburn, Peter Ryan, Stuart Scott Henderson, Nicola Lonigro, Sidney Leigh, et al.

Nuclear Fusion·Aug 21, 2026Plasma & ConfinementMaterials & Plasma-Facing Components

The MAST Upgrade Super-X divertor protects plasma-facing components from heat fluxes during transient events. This paper examines heat loads from sawtooth crashes with energies ΔWsawtooth ≈ 2–9kJ and investigates the impact of deuterium and nitrogen gas pressure on their mitigation. In a D2 gas scan for sawteeth with energies up to 6kJ, the peak heat flux (q⊥,max) scales with the transient energy and inversely with the neutral pressure, with variations in ΔW/Pgas accounting for 70% of the observed variation in q⊥,max. Larger sawtooth transients (≈ 6–9 kJ) fall outside this trend and exhibit much higher heat fluxes. With D2 puffing in the divertor, the Super-X configuration exhibits substantially lower peak q⊥,max for comparable transient energy loads than observed in conventional divertor configurations. However, this reduction is broadly consistent with geometric expectations especially at higher pressures. Nitrogen gas seeding was applied to transients with energies below ≈ 6 kJ. Increasing the nitrogen pressure reduces q⊥,max, by 4.5MW/m2/Pa in Super-X while nitrogen seeding is found to have much less impact on transients in conventional divertor configuration. In the case of N2 seeding, the Super-X divertor outperforms conventional divertor configuration beyond geometric expectations. Direct Super-X divertor Te and ne measurements during transients were obtained from Thomson scattering. These indicate quiescent inter-transient temperatures of < 1 eV, corresponding to strong detachment. The temperatures rise to 6–8 eV during typical sawtooth transients and 10 eV where divertor heat loads exceed q⊥,max > 2MWm−2. The experimental data are compared to modelling results using a 1D exhaust code in the ReMKiT1D framework. The modelling highlights the importance of recycling on understanding the buffering of transients.

United Kingdom Atomic Energy Authority·Culham Centre for Fusion Energy·University of Liverpool

Aug 11

First experimental realization of robust negative triangularity plasma control in a spherical tokamak

Andrey Lvovskiy, Charles Vincent, Himank Anand, Anders S Welander, Sam Blackmore, Martin Kochan, Graham McArdle, Scott Alan Silburn, Andrew Oakleigh Nelson

Plasma Physics and Controlled Fusion·Aug 11, 2026Plasma & ConfinementControl & DiagnosticsAI, Modeling & Simulation

In this work, we present the experimental realization of a plasma with a steady negative triangularity 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 negative triangularity plasma in a spherical tokamak.

General Atomics·United Kingdom Atomic Energy Authority

Aug 4

Initial observations in X-point target divertor discharges on MAST-U

Nicola Lonigro, Kevin Verhaegh, James R Harrison, Bruce Lipschultz, Christopher Bowman, Fabio Federici, Jack Flanagan, Daniel Greenhouse, David Moulton, Peter Ryan, et al.

Nuclear Fusion·Aug 4, 2026Plasma & ConfinementMaterials & Plasma-Facing Components

The first high-power (≥ 3 MW) H-mode experiments using a double-null X-point-target (XPT) divertor configuration have been performed on MAST-U. The XPT geometry is obtained by combining a large strike point radius, similar to the Super-X divertor (SXD), with an additional X-point near the separatrix in the baffled outer divertor chambers and leads to additional exhaust benefits over the SXD. The broader electron density profile near the secondary X-point leads to additional plasma-neutral interactions, evidenced by a broader hydrogenic emission profile, and resulting in larger power and ion sinks. The increase in plasma-neutral interactions also leads to lower target electron temperatures and heat fluxes. These benefits appear to extend to transients, and preliminary evidence of improved ELM buffering in the XPT is presented. These results showcase how multiple alternative divertor configuration strategies can be combined to improve momentum, power, and particle losses, which may be required for the challenging exhaust conditions of future reactors.

Oak Ridge National Laboratory·Culham Centre for Fusion Energy·United Kingdom Atomic Energy Authority

Jul 24

Real-time observation of toroidal current redistributions induced by three-dimensional MHD phenomena triggering vertical displacement events in tokamak plasmas

Matthew Tobin, Steve A Sabbagh, Veronika Zamkovska, Guillermo Bustos Ramirez, Hankyu Lee, Joseph R Jepson, Juan Riquezes, Frederick C Sheehan, Grant A Tillinghast, Keith Erickson, et al.

Plasma Physics and Controlled Fusion·Jul 24, 2026Plasma & ConfinementControl & DiagnosticsAI, Modeling & Simulation

Three-dimensional MHD instabilities, including edge-localized modes (ELMs) and internal reconnection events (IREs), have been observed to precipitate loss of vertical stability in tokamak plasmas, resulting in vertical displacement events (VDEs). This vertical destabilization can occur due to toroidal current redistributions and/or shape changes resulting from these phenomena. Using a recently introduced method for rapidly reconstructing the two-dimensional toroidal plasma current density profile in real-time, results are presented that demonstrate the specific current distribution changes that occur during ELMs (on KSTAR) and IREs (on MAST-U) that lead to loss of vertical control. The method most efficiently reconstructs the toroidal current density profile by doing so on a basis of principal components of historical profiles. These principal components isolate dominant current profile dynamics, improving interpretability, increasing speed, and reducing dimensionality of the profile computation. On KSTAR, this computation is executed in the real-time plasma control system at a rate of 10 kHz (limited by available CPU cycle times), allowing the current profile evolution to be assessed at several times over the course of each ELM event. Further, by incorporating the reconstructions into a novel vertical stability metric, the contribution of specific current profile dynamics to the loss of vertical stability can be assessed in real-time for VDE avoidance and improved understanding of the causal relationship between three-dimensional MHD phenomena and VDEs. The success of this method in approximating toroidal current density profiles from kinetic equilibrium reconstructions is also presented ($R^2=0.990$), along with its capability to produce other equilibrium quantities of interest in real-time at high time resolution.

Culham Centre for Fusion Energy·Princeton Plasma Physics Laboratory·United Kingdom Atomic Energy Authority

Jul 22

Reduced fast-ion transport calculations of m = n = 1 fishbone-like instabilities in MAST-U

H. H. Wong, P. J. Bonofiglo, M. Podesta, C. A. Michael, S. Thomas, D. Dunai, A. R. Field, K. G. McClements, M. Cecconello, N. A. Crocker, et al.

Physics of Plasmas·Jul 22, 2026Plasma & ConfinementControl & DiagnosticsAI, Modeling & Simulation

Fast-ion transport associated with an m=n=1 fishbone-like burst in MAST-U discharge 47128 is investigated using a reduced guiding-center-based transport model (ORBIT-Kick) constrained by multi-diagnostic measurements. The two-dimensional beam-emission spectroscopy system provides measurements of the core poloidal mode structure and fluctuation amplitude, while EFIT++ reconstructions constrained by the motional Stark effect diagnostic indicate a flat q-profile with q0>1, indicating the absence of a resonant q=1 surface and supporting a pressure-driven infernal-mode interpretation. Analytic m=n=1 displacement profiles consistent with the measured core mode structure and equilibrium constraints are used as the mode structure inputs to ORBIT-Kick. The calculations show that the dominant resonances occur between the mode and co-passing fast ions, producing redistribution localized near the magnetic axis. Synthetic neutron camera signals from TRANSP-Kick recover up to 90% of the experimentally observed neutron deficit at the time of peak mode amplitude, indicating that the measured m=n=1 mode is a dominant contributor to core fast-ion transport. However, the synthetic neutron signals recover rapidly, whereas the measured neutron emission continues to decrease after the peak amplitude. The remaining discrepancy may arise from contributions not included in the present single-harmonic model, including higher-m and higher-n harmonics, multi-harmonic interactions, and additional transport mechanisms, motivating future diagnostic development and modeling efforts to resolve and incorporate these additional contributions.

United Kingdom Atomic Energy Authority·University of California Los Angeles·Princeton Plasma Physics Laboratory

Jul 21

Cryopump and fuelling location impacts on upstream density and detachment on MAST-Upgrade

Qian Xia, David Moulton, James R Harrison, Hongjuan Sun, Kevin Verhaegh, Nick Osborne, Nicola Lonigro, Peter Ryan

Nuclear Fusion·Jul 21, 2026Plasma & ConfinementTritium & Fuel CycleAI, Modeling & Simulation

A cryopump was installed in the MAST-U lower divertor at the start of its fourth scientific campaign (MU04) to enhance density control, detachment front management, and impurity removal, while expanding the operational space to lower-density regimes. The tightly baffled divertor chamber quasi-isolates the divertor neutral environment from the main chamber, enabling localised neutral pressure (P n ) tuning via divertor fuelling and cryopump operation, with minimal impact on upstream plasma density. MU04 experiments and SOLPS-ITER simulations both confirm that activating the cryopump reduces the outer lower divertor P n by 50-65%, while maintaining line-averaged density (n̄ e ) and midplane P n . The cryopump facilitates lower divertor P n , aiding plasma attachment and drawing the detachment front closer to the target. Modelling indicates a 25-40% increase in upstream density required for rollover onset in both CD and SXD configurations, enabling a broader operational scan from attached conditions through detached to radiative collapse. Divertor fuelling exhibits a low fuelling efficiency (~5% of injected neutrals reach the separatrix), resulting in a weak scaling between upstream separatrix density and divertor neutral pressure (n e,sep ∝P n 0.3 ), consistent with observations on other devices. In contrast, main-chamber fuelling achieves substantially higher efficiency (~15% for LFS and up to ~40% for HFS fuelling) and yields a stronger density scaling (exponent >0.6), reflecting the different plasma–neutral coupling pathways associated with the fuelling location. These findings demonstrate that the combined use of different fuelling locations and cryopumping provides complementary control of upstream density and divertor detachment, with direct relevance for exhaust optimisation in future fusion devices.

United Kingdom Atomic Energy Authority

Jul 7

Post-puff SOL broadening on MAST-U under high-recycling conditions: evidence consistent with cross-field transport changes

Y Damizia, S Mordijck, N Walkden, J Lovell, S Thomas, E Ozturk, P Ryan

Plasma Physics and Controlled Fusion·Jul 7, 2026Plasma & Confinement

Transient broadening of the scrape-off layer (SOL) density profile can modify main-chamber first-wall particle fluxes and divertor loading, yet its control parameters remain debated between divertor-regime transitions, neutral dynamics and changes in cross-field transport. We investigate fueling-driven SOL density-profile evolution and post-fueling relaxation on MAST-Upgrade (MAST-U) in ohmic L-mode, using two otherwise similar double-null Conventional Divertor discharges ( I p = 450 kA, B T = 0.33 T) with identical 50 ms low-field side gas puffs; the only intentional difference is the puff start time. Upstream Thomson scattering shows that both discharges develop a transient far-SOL density profile modification, expressed as an increased SOL-width metric and a far-SOL enhancement consistent with a shoulder-like signature. In the earlier-puff case, the SOL-width metric λ n e remains elevated after puff termination and the post-puff decay time scales are systematically longer across the analysed radii. Outer-target Langmuir probes indicate high-recycling conditions during the analysed window (few-eV T e with radially peaked j sat , ∥ and q ∥ profiles, without signatures of deep detachment). The outer-divertor collisionality proxy Λ div is elevated in both cases and does not discriminate between the different post-puff persistence. The target-integrated ion flux proxy ∫ J sat d A evolves nearly identically in both discharges when aligned to the puff start. Taken together, these observations suggest that the late/post-puff upstream SOL evolution is not set by parallel exhaust to the outer targetalone and is more consistent with upstream cross-field redistribution, with a possible role for plasma–neutral coupling and fueling geometry.

Oak Ridge National Laboratory·MIT Plasma Science and Fusion Center