James R. Harrison

A spectrally filtered fast visible camera viewing the lower X-point is installed on the Mega Ampere Spherical Tokamak - Upgrade (MAST-U) to study the filamentary plasma transport in standard and advanced divertor configurations. The X-point camera has a complementary metal–oxide–semiconductor sensor with high light sensitivity, capable of 12-bit, 16 500 fps operation at full frame resolution of 1024 × 1024 pixels. It is coupled to the front lens via a coherent fiber optic bundle and a pair of relay lenses. Spatial camera calibration is performed to map the real space onto 2D pixel coordinates of the camera image, which enables identification and tracking of plasma filaments. For the first time, the X-point camera is operated in synchronization with other fast cameras, one viewing the main chamber plasma tangentially and the other in the Super-X divertor chamber, at a 100 kHz frame rate with a time lag <1 μs. This technique enables us to compare the filamentary transport in the Super-X divertor chamber in MAST-U with the midplane and X-point regions along the same flux tube.

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.

The application of certain configurations of resonant magnetic perturbations (RMPs) has been shown to cause a plasma pump-out in both L- and H-mode discharges at Mega-Ampere Sphecical Tokamak (MAST) (Kirk et al the MAST team 2010 Nucl. Fusion 50 034008). In this paper we discuss the impact of neutral fueling on this density reduction. The neutral fueling and average particle confinement time τ p of the main ion species were calculated using a 0-D particle balance analysis. When the RMPs were applied, it was found that there was an increase in ionizations and a 15 % reduction in τ p for L-mode discharges and a similar result for inter-ELM (edge localized modes) periods of H-mode discharges. A time-dependent global multi-reservoir particle balance (MRPB) was developed, which included atomic and molecular reservoirs, to further investigate the role neutrals had on the density change. We discuss how this model was able to accurately reproduce the experimentally measured density reduction and ionization increase due to either a reduction in τ p or a reduction in particle fueling efficiency. Results from EMC3-EIRENE modeling indicate this change could be attributed to neutral particle fueling occurring in locations with now-opened field lines due to the chaotic edge-region from RMP applications.

The MAST Upgrade Super-X divertor (SXD) protects plasma-facing components from heat fluxes during transient events. This paper examines heat loads from sawtooth crashes with energies Δ W sawtooth ≈ 2 – 9   kJ and investigates the impact of deuterium and nitrogen gas pressure on their mitigation. In a D 2 gas scan for sawteeth with energies up to 6   kJ , the peak heat flux ( q ⊥ , max ) scales with the transient energy and inversely with the neutral pressure, with variations in Δ W / P gas 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 D 2 puffing in the divertor, the Super-X configuration exhibits substantially lower peak q ⊥ , max for comparable transient energy loads than observed in conventional divertor (CD) 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 4.5 MW m − 2 Pa − 1 in Super-X while nitrogen seeding is found to have much less impact on transients in CD configuration. In the case of N 2 seeding, the SXD outperforms CD configuration beyond geometric expectations. Direct SXD T e and n e 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 > 2 MW m − 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.

Three-dimensional magnetohydrodynamic (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.

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 Edge Localized Mode 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.

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.

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.

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.

Plasmas with low collisionality and with the pedestal limited by peeling modes have been achieved in JET-ILW, MAST-U and TCV. The achieved electron-electron pedestal collisionality and the achieved ratio between electron separatrix density to pedestal density are in the ranges ν_e^(*ped)=0.1-0.4 and n_e^sep/n_e^ped=0.3-0.9, approaching ITER values. In these conditions, the experimental results show that the pedestal pressure increases with increasing density and no degradation of the pedestal pressure with increasing n_e^sep/n_e^ped has been observed. These behaviours are opposite to what observed in ballooning limited pedestals. However, in all the three machines, both the increase of the pedestal density and the increase of the separatrix density destabilize the ballooning modes and tend to move the pedestal from being peeling limited to being limited by coupled peeling –ballooning modes. The Europed pedestal predictions have been validated against the experimental results in all the three machines, showing a good qualitative agreement and a reasonable quantitative agreement when approaching ITER-relevant parameters of ν_e^(*ped) and ρ_ ^(*ped) (normalized ion Larmor radius). The Europed predictions have been applied to ITER showing that the instabilities that will limit its pedestal strongly depend on the values of n_e^ped and n_e^sep. Nonetheless, assuming type I ELMy H-modes, the ITER pedestal in the Q=10 scenario will reach p_e^ped≈60kPa, with a pedestal pressure that will increase with increasing density and with no degradation at high n_e^sep/n_e^ped.