Recent Publications

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

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.

Whole Plant Design, Integration, and Build

J. Morris

Global Gyrokinetic Simulations of Electromagnetic Turbulence in STEP

Daniel Kennedy, Facundo Sheffield, Tobias Görler, Colin Roach, Maurizio Giacomin, Arka Bokshi, David Dickinson, Harry Dudding, Bhavin Patel

This paper presents gradient-driven global electromagnetic gyrokinetic simulations for a conceptual burning flat-top operating point of STEP [1], STEP-EC-HD, and investigates how non-local effects influence the nonlinear saturation and transport of the electromagnetic turbulence at finite $β$. Local gyrokinetic simulations have shown that including $δB_{\parallel}$ is essential for the dominant hybrid kinetic ballooning modes, or hKBMs, to be unstable in STEP [2]. Using the long-wavelength $δB_{\parallel}$ solver [3] implemented in GENE [4], this work demonstrates that the linear mode spectrum can be accurately captured in global geometry, which results in good agreement with an ensemble of local flux-tube simulations. The global framework reproduces the hKBMs identified in [5], while microtearing modes remain challenging to resolve due to their shorter radial scales. Nonlinear simulations reveal clear evidence of an electromagnetic transition to states with extremely large heat fluxes, consistent with local simulations and with the predicted loss of zonal-flow regulation for this proposed operating point [6]. These findings establish the capability of global gyrokinetics to capture finite-$β$ dynamics in STEP-like plasmas and motivate future work to identify the conditions governing this transition.

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.

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.

Extension of the RAPDENS Control-Oriented Model with a Scrape-Off Layer Particle Balance Model for ASDEX Upgrade and ITER

Daniela Kropáčková, Francesco Pastore, Ondrej Kudlacek, Timo Ravensbergen, Olivier Sauter, Guillermo Suarez-Lopez, Andrei Pshenov, Emiliano Fable, Davide Silvagni, Maximilian Reisner

The plasma density is a key parameter in tokamak operation, as it influences plasma performance and stability. Hence, accurate real-time estimation and control are essential. Real-time estimation of the plasma electron density profile can be maintained even in the presence of corrupted or unavailable diagnostics by employing model-based electron density observers. This work extends one of these observers, RAPDENS [1], by incorporating a heuristic scrape-off layer model capable of predicting the separatrix electron density, thereby building upon previous work that implemented a non-zero boundary condition at the separatrix [2]. Implementations for both ASDEX Upgrade (AUG) and ITER are presented. For AUG, the RAPDENS adaptation is tested using offline Extended Kalman Filter simulations of existing AUG discharges. These simulations show that RAPDENS is capable of providing a reasonable reconstruction of the electron density profile, supporting its future real-time implementation and routine use at AUG. For ITER, the implementation is based on and compared against results of a high-fidelity simulation performed with JINTRAC, the integrated modeling suite, for a start of research operation ramp-up scenario, concluding that the improved version of RAPDENS can be used for ITER density controller design.

Mechanism of Ionization Avalanche in Tokamak Microwave Gas Breakdown

Jinwoo Gwak, Yeongsun Lee, Jeongwon Lee, Won Ik Jeong, Hyun-Tae Kim, Yong-Seok Hwang, Min-Gu Yoo, Yong-Su Na

Microwave breakdown driven by electron cyclotron (EC) waves provides a non-inductive route to plasma initiation in reactor-scale tokamaks. We introduce a three-dimensional Monte Carlo simulation that, for the first time, self-consistently treats nonlinear wave-particle interactions, atomic collisions, and guiding-center transport. The Monte Carlo simulation unveils the key role of parallel Brownian motion in the ionization avalanche mechanism. The predicted breakdown boundary is validated against KSTAR experiments. This work concludes that microwave gas breakdown will be successful under ITER-relevant conditions at a D$_2$ prefill pressure near 2 mPa with 1 MW of injected EC power.

Investigation of the magnetic flux pumping effect in MAST Upgrade

Sam Blackmore, Christopher J Ham, Daniele Brunetti, Clive A Michael, Bhavin S Patel, Koki Imada, Lucy Kogan, cameron Olde, Fulvio Auriemma, Chiara Piron, et al.

Stationary safety factor q profiles with qmin ≈ 1 and without sawtooth activity have been measured for the first time in MAST Upgrade (MAST-U) plasmas, indicative of the magnetic flux pumping mechanism. This anomalous current re-distribution phenomenon is observed in MAST-U plasmas with 2/1 tearing MHD instabilities. Simulations using the ideal MHD code MISHKA indicate that q profiles, measured using the motional Stark effect (MSE) diagnostic, with zero magnetic shear are also ideally unstable to n = 1, m = 1 MHD modes. Database analysis of MAST-U pulses demonstrate overlap in βN ≈ 2 in pulses which exhibit magnetic flux pumping and sawtooth activity. The operational space for robust access to the flux pumping regime on MAST-U is characterised by operating at q 95 > 6 and β p ≈ 0.6.

BLUEMIRA: a modular, open-source framework for designing tokamak fusion reactors

Matti Coleman, James Cook, Fabrizio Franza, Ivan Maione, Simon Mcintosh, Hudson Baker, Alexander Ian Blair, Shail Desai, Oliver Funk, Georgina Graham, et al.

As efforts across the world to deliver fusion power enter their respective conceptual design stages, many begin to encounter the difficulties of generating complete designs, evaluating them, performing trade-off studies, and comparing different alternatives. Many organisations successfully use fusion reactor systems codes with low-fidelity formulations of the critical physics and engineering aspects to inform the initial design stage and steer them towards a region of the design space they consider attractive, but struggle to explore the design space in more depth. This work is an overview of the open-source tokamak reactor design framework, BLUEMIRA, built from the BLUEPRINT [1, 2] and MIRA [3, 4] codes, which goes beyond the “classical” systems code paradigm and enables the user to parameterise their reactor designs at higher fidelity. Reactor design workflows ranging from 0-D “radial build” design, equilibrium design, to 3-D CAD and automated neutronics analyses can be run within minutes. We present the functionality implemented in the BLUEMIRA code and demonstrate its application to the conceptual design of a conventional aspect ratio fusion reactor, performing an indicative design study over aspect ratio and number of toroidal field coils.

Further development of JADE as a tool for validation and verification of nuclear data libraries and particle transport codes

Alex Valentine, Steven Bradnam, Jude Moorehead, Davide Laghi, Alberto Bittesnich, Marta Campos Fornes, Matteo Di Giacomo, Marco Fabbri, Allan Harte

Several international leading efforts are focused on the improvement of evaluations for nuclear data. This data captures all of the physics of nuclear interactions for which the applications are far ranging from medical applications to criticality simulations. Nuclear analysis in the field of fusion, commonly referred to as neutronics, is one such application, typically employing Monte Carlo methods that rely on continuous-energy, pointwise cross-section data. Historically, the most widely adopted and complete radiation transport code in this field is MCNP, however in recent years there has been an evident trend in exploration of alternatives, most notably OpenMC. Nuclear analysis is a critical driver of the design, operation, safety and decommissioning of future fusion power plants stressing the importance of confidence in the transport codes and underlying nuclear data used in prediction of the radiation environment. Here we present recent developments to the JADE tool - a framework originally conceived for verification and validation of nuclear data within MCNP, now extended to performing cross-code comparisons. In particular, the developments required for the complete integration of OpenMC are detailed. A demonstration of this capability is presented for several of the currently implemented computational and experimental OpenMC benchmarks, with results compared against MCNP with the FENDL-3.2c and ENDF/B-VIII.0 nuclear data libraries.

Neutron measurements with VERDI detectors in the long term irradiation station at JET tokamak during Deuterium-Tritium operation

Ion Evangelos Evangelos Stamatelatos, Marilia Savva, Theodora Vasilopoulou, K Mergia, S Messoloras, Steven Bradnam, Chantal Shand, Lee William Packer, Zamir Ghani, Paola Batistoni, et al.

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.

Investigation of impurity behaviour in three-ion ICRF scenarios in H-D and D-T plasmas at JET

Agata Chomiczewska, Yevgen Kazakov, Wojciech Gromelski, Irena Ivanova-Stanik, Agnieszka Jardin, Axel Jardin, Ewa Kowalska-Strzęciwilk, Kerry Lawson, Evie Litherland-Smith, Andy Meigs, et al.

This study investigates impurity behaviour during ion cyclotron resonance heating (ICRF) experiments, focusing on the application of two different three-ion heating schemes in H-D and D-T plasmas at JET. In the D-(³He)-H scenario, the phasing of the ICRF antenna straps was varied to modify the launched parallel wave number k||, enabling a systematic study of its effect on fast-ion generation, plasma dynamics and impurity transport. The results indicate dependence of impurity behaviour on antenna phasing, particularly for the nickel (Ni) in the main plasmas and for the beryllium (Be) source. Analysis of sawtooth oscillations using the symmetrised dot pattern method reveals correlations between sawtooth frequency and crash intensity, the applied ICRF power and antenna phasing. The lowest impurity levels are obtained with +90° antenna phasing, corresponding to maximised fast-ion generation. A comparative study of impurity behaviour in D-T plasmas is also presented for the three-ion T-(⁹Be)-D and hydrogen minority heating scenarios. The three-ion scheme produces the largest increases in ion temperature Ti, while hydrogen minority heating yields higher electron temperatures Te and slightly reduced impurity levels. These results suggest that impurity behaviour in three-ion ICRF scenarios depends on the chosen heating optimisation (fast-ion generation versus ion heating) and can be further controlled through appropriate selection of ICRF antenna phasing.

Real-time virtual circuits for plasma shape control via neural network emulators: experimental demonstration on MAST Upgrade

Nicola C. Amorisco, Kamran Pentland, Adriano Agnello, George K. Holt, Alasdair Ross, Matthew J. Marshall, Edward Jones, Graham J. McArdle, Charles Vincent, Timothy Nunn, et al.

Conventional plasma shape control in tokamaks relies on virtual circuits (VCs) that are computed offline from linearisations around a small, tailored number of reference equilibria, and deployed as expertly prepared schedules during the discharge. Here, we report on the first experimental deployment of real-time VCs. We replace pre-set look up tables with VCs updated in real time using surrogates of the plasma response. Both the existing control architecture and the interpretability of VC-based control are retained. Previous work showed that neural network emulators can produce accurate VCs, and validated their performance in closed-loop shape control simulations. Here, we report their first experimental validation on MAST Upgrade (MAST-U). Dedicated experiments spanning different scenarios, including prescribed shape perturbations, feedback-driven divertor-leg motion, and strongly evolving plasma configurations, show that real-time VCs can realise plasma shape control tasks within the MAST-U plasma control system. These results establish the experimental feasibility of real-time linearisations as a practical extension of conventional plasma shape control in tokamaks. The present implementation demonstrates a central step towards a simpler control workflow, in which manually constructed, phased VC schedules are replaced by VCs generated automatically online from a trained surrogate model, without scenario-specific retraining.

Analysis of neutron single event effect experiment on electronics during JET DTE-3 campaigns

Martin Felix Dentan, Soilihi Moindjie, Matteo Cecchetto, Jean-Luc Autran, Ruben Garcia Alia, Richard Naish, John Waterhouse, Alan R Horton, Xavier L Litaudon, Fernanda Rimini, et al.

We measured SEEs in 40 nm and 65 nm SRAMs exposed to the D–T fusion neutron environment of the JET tokamak during the DTE-3 campaign (September–October 2023), while also characterizing the neutron spectrum seen by the devices. Using consolidated JET neutron production data, the measured SEE rates show good agreement with predictions based on the measured spectra and independently determined SRAM sensitivity parameters from thermal and monoenergetic neutron tests. These results extend the validation of our spectrum-based SEE prediction methodology, previously demonstrated in a D–D fusion neutron environment in 2021, to a D–T fusion neutron field. The novelty of this work is therefore the first experimental validation of this complete prediction approach in a D–T fusion environment, using real-time neutron spectrometry and independent device sensitivity data, rather than a new SEE model. The experiments also show that a local B₄C shield reduces the bit-flip rate by absorbing thermal neutrons, and we discuss the conditions under which such shielding can improve electronics reliability in tokamak and accelerator neutron environments.

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.

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.

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.

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.

Identifying non-performing or Dud Plasmas for Burning Plasma Control: Insights from JET and TFTR Deuterium-Tritium Campaigns

Lidia Piron, Nicolò Ferron, Eric Fredrickson, Morten Lennholm, Alessandro Pau, Timo Ravensbergen, Olivier Sauter, Fulvio Auriemma, Matteo Baruzzo, Krassimir K. K Kirov, et al.

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 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 Deuterium-Tritium ITER scenario.

Impact of nitrogen injection on L-H transitions in JET with Be/W wall

Costanza F Maggi, Clarisse Bourdelle, Ephrem Delabie, Mathias Brix, Pedro Carvalho, Francis J Casson, Alex V Chankin, Nicholas Hawkes, Evie Litherland-Smith, Sheena Menmuir, et al.

L-H transition experiments in JET with Be/W wall (JET-ILW), with N2 injection into the divertor region, have revealed that the H-mode power threshold, PL-H, increases with nitrogen injection rate in the high-density branch and modifies the density at which the power threshold is minimum, ne,min. PL-H values approaching those obtained in JET with C wall are achieved for a sizeable increase in nitrogen concentration. At a given density in the high density branch of PL-H, both the total loss power (Ploss) as well as the power crossing the separatrix (Psep) at the L-H transition increase by a factor ~ 1.5 at high N2 levels. At a given line averaged density, while the edge density is similar in plasmas with and without N2 injection, the edge temperature at the L-H transition is higher in pulses with high nitrogen seeding levels and a larger shear of the edge radial electric field Er is measured. At low N concentrations, little change in PL-H is observed. Qualitative agreement is reported with Zeff impact on L-mode edge turbulence drive [1]. The enhanced L-mode edge turbulence drive with N2 seeding at higher Zeff implies that a higher PL-H is expected as, indeed, measured in experiment. It is assumed that the mean equilibrium Er×B shear is a key player for reduction of L-mode edge turbulence, leading to the L-H transition. In this framework, the L-H transition observations in JET-Be/W + N2 are interpreted by an increase in L-mode edge plasma turbulence with increased N concentration and the need for a larger stabilizing ErxB shear, thus higher Psep, to trigger the L-H transition. For future fusion reactors with scenarios with extrinsic low-Z impurity seeding for divertor heat load mitigation, our results suggest the need to predict Psep for H-mode access taking into account not only the high radiation fractions from the bulk plasma, but also the impact of low Z impurity concentration on Ploss and Psep at the L-H transition.

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.

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.

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.

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.

Establishing a Coherent Qualification Pathway for Fusion Technologies

Amanda Louise Allison