Ephrem Delabie

Disentangling the complex interplay of neutral fueling and transport considerations in the edge plasma pedestal region can be informed by direct measurements of the energy-resolved local neutral densities from passive Dα spectroscopy. In this work, we employed the MAST upgrade charge exchange spectroscopy system passive background viewing fan to assess the feasibility of resolving both the higher energy thermal neutral component due to charge exchange reactions with thermal ions and the cold component due to edge neutral recycling. The spectral coverage, resolution, and dynamic range of the CCD adequately capture the entire Dα line profile. The outer midplane tangential viewing fan of 19 lines of sight provides good spatial resolution of the edge plasma region. To infer the 1D radial profiles of the thermal and cold neutral densities, we used a modified version of the FIDASIM Monte Carlo neutral modeling package to develop a forward model of the line integrated spectroscopic signals. In combination with a numerical optimization scheme, the model enables a direct comparison of the spectrally resolved Dα profiles to the experiment data for a given range of assumptions on the cold neutral densities in the scrape-off layer. Quantitatively good agreement is obtained in recovering the 1D cold and thermal density profiles in the edge pedestal region as well as the spectral Dα line shape details, with typically observed cold and thermal neutral densities at the outer midplane separatrix of ∼3–5 × 1015 and 5–7 × 1014 m−3, respectively, for high-field side fueled plasmas. Inboard of the pedestal region the model consistently underestimates the measured cold component line-integrated intensity, suggesting that further characterization of the viewing geometry, reflections impact and/or FIDASIM model assumptions, is needed.

The (ITER-prototype) diagnostic residual gas analyzer (p-DRGA) was deployed in the W7-X OP2.3 experimental campaign, with emphasis on W7-X research applications, i.e., going beyond diagnostic capability demonstration focus of the previous campaigns. In addition to an initial validation of neutrals modeling in sub-divertor regions for helium exhaust applications and support of the associate ICRF-induced fast ion generation research program, the diagnostic system also provided early data relevant to an envisioned DRGA-enabled, stellarator reactor-relevant, fusion plasma burn control study, to be extended in OP2.4 and beyond. In this extended study concept, the fast ion generation would serve as proxy to the ICRF-enhanced DT burn approach used in JET-DTE2/DTE3, while the DRGA will provide the relevant species concentrations in real-time. Implicit in this concept is the correlation between core and sub-divertor concentrations, in ∼few sec equilibration time, which still needs validation for the case of stellarators.

The suite of spectrometers monitoring the lower divertor [Verhaegh et al., Nucl. Fusion 63, 016014 (2022)] on the MAST Upgrade tokamak has been extended before the start of the MU04 campaign. A new fiber bundle with good transmission in the near-Ultraviolet was installed to increase the number of instruments viewing the same radial profile in the divertor to a maximum of 4. This was done to accommodate a dedicated instrument to measure the Balmer series-limit recombination continuum around 365 nm. The ratio of the continuum above and below the recombination “edge,” as well as the spectral shape of the free–bound continuum, allows us to derive the electron temperature in recombining plasmas independently of the excitation pathways [Lomanowski et al., Plasma Phys. Controlled Fusion 62, 065006 (2020)]. The electron temperature is key to understanding heat exhaust processes in the divertor, which is essential for future fusion power plant design. A good diagnosis has been achieved of deeply detached, electron impact recombining plasmas, confirming Te = 0.05–0.3 eV [Verhaegh et al., Nucl. Fusion 63, 126023 (2023)]. The electron temperature derived from the wavelength dependence of the continuum below 365 nm is in good agreement with the electron temperature derived from the intensity ratios of the high-n Balmer lines for ne = 1018–1019 m−3. At higher divertor temperatures, molecular assisted recombination mechanisms dominate, and the emission of CD and C2 molecules dominates the spectra, making it difficult to extract the temperature from the continuum.

The ability to capture the isotope mass scaling of core confinement seen in experiment is validated with JETTO-TGLF for low power JET-ILW discharges across H, D and T. The cases analysed include Ohmic discharges spanning the linear and saturated Ohmic confinement regimes as well as a trio of L-modes. The TGLF saturation rules SAT1-SAT3 are seen to predict a similar isotope scaling across both the ITG- and TEM-dominated discharges simulated, despite for the latter case the inclusion of the TEM branch of SAT3. The models demonstrate good agreement with experiment for the scaling between D and T plasmas, however a discrepancy is observed for H in the ITG-dominated discharges of higher density, as well as a systematic overprediction of the confinement time on the order of 20% in most cases. A retuned version of the SAT3 model, which was fit to better recreate fluxes close to the transport threshold, is seen to improve the magnitude of confinement predictions across all shots owing to an increased transport stiffness. This retuning was not seen to influence the confinement isotope scaling however, and possible transport mechanisms responsible for the continued discrepancy of higher density Ohmic and L-mode discharges in H are discussed.

The interplay between externally induced locked modes and the plasma toroidal rotation is studied in JET L-mode plasmas, through the analysis of shots in which the non-disruptive compass scan technique (Piron et al 2024 Nucl. Fusion 64 066029) was performed. In these experiments, an external magnetic field perturbation was applied up to the onset of a locked mode, with toroidal mode number n = 1, at constant plasma density. In Ohmic plasmas, during the execution of the compass scan, it is observed that the toroidal rotation brakes in a wide range of the core. In the case of neutral beam injection-heated shots, the rotation braking is significantly reduced. The experimental analyses have been interpreted by the RFXlocking code adapted to the JET tokamak (Zanca et al 2015 Nucl. Fusion 55 043020), which also allowed to investigate the roles of the electromagnetic force and of the neoclassical toroidal viscosity, in the rotation braking phenomenon. Plasma rotation has been analysed also during a density variation in the absence of external magnetic field perturbations, highlighting that a profile transition occurs in the core.

The dimensionally matched deuterium–tritium pulse pair under JET L-mode conditions showed 13%–16% improvement in the energy confinement time in favour of the tritium pulse. This favourable isotope scaling can be seen clearly in the effective diffusion coefficients throughout the radius. The isotope scaling originates dominantly from the electron heat transport channel and from the edge part of the plasma. The phase and amplitude profiles in response to the gas puff modulation robustly show that there is no room for a large isotope scaling in the particle transport channel in the core plasma at ρ tor ⩽ 0.95 . This can also be seen in the derived particle transport coefficients between the deuterium and tritium pulses. EDGE2D-EIRENE simulations found that the radial ionisation profiles are very similar between the dimensionally matched deuterium and tritium identity pulses. A similar deviation from the gyro-Bohm scaling, i.e. strong isotope scaling favouring tritium in heat transport was found with gyrokinetic simulations in the edge at ρ tor = 0.95 under the JET L-mode conditions. The strong edge isotope scaling favouring tritium is consistent with the experimental observation. This edge isotope effect ρ tor = 0.95 in the GENE simulations is also much larger than found in the core plasma ρ tor = 0.6 when comparing similarly deuterium and tritium simulation results. Both the experimental results and the GENE simulations suggest that the isotope mass scaling is a nonlinear function of the isotope mass itself, being significantly stronger between deuterium and tritium than between hydrogen and deuterium, at least in JET L-mode conditions.

In JET-ILW, beam-target reactions contribute to a large fraction of the fusion power generated in Deuterium – Tritium (D-T) plasmas, with core ion temperatures of 10-12keV and large neutral-beam injection (NBI) power. Previous modelling done in preparation for the recent D-T campaigns in JET have shown that injecting D beam ions with energies of ~120keV in T-rich plasmas produces larger 14MeV fusion yield than in 50:50 D:T plasmas, but such scenario had never been tested in past D-T experiments. In addition, the simulations showed that fundamental ion-cyclotron resonance heating (ICRH) of the D ions can significantly boost the net fusion reactivity, since both the D-bulk ions and the fast D-beam ions are accelerated to energy ranges that are optimal for the D-T reactions to take place. In the last JET D-T campaigns (DTE2 and DTE3), dedicated experiments confirmed - for the first time - the improved fusion performance of T-rich plasmas with high D-NBI power and highlighted the key impact of fundamental D ICRH on the fusion performance. This new scenario led to the world-wide D-T fusion energy record ever achieved in a fusion device and allowed to sustain more than 12MW of fusion power averaged over 5s. The main results of these unprecedented experiments will be presented and the NBI+ICRF physics responsible for the high fusion performance achieved will be highlighted through numerical modelling.

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.

The low density and high temperature at the pedestal in hybrid scenarios on JET provide a unique test of the neo-classical screening in the periphery of the plasma that has previously been observed on JET in such plasmas. Hybrid scenario plasmas have been further developed to aid in the study of temperature gradient screening of high-Z impurities from the core of the plasma via plasma current, toroidal field, gas fuelling, density and isotope variations. The steps required to adapt the pulses from deuterium to deuterium-tritium are discussed and provide some guidance for future experiments that may operate in deuteriumtritium. The optimisation of the scenario allowed for improved bolometry data compared to the Deuterium Tritium Experiment 2 (DTE2) campaign on JET, hence this work provides vital evidence of impurity screening in a high fusion power, deuterium-tritium plasma.

A novel optical gas analyser, designed for isotope-resolved exhaust composition measurement, was field-tested at Wendelstein 7-X (W7-X) to validate its laboratory-proven concept under operational fusion experiment conditions. The system, Optix, comprises a cold cathode remote plasma generator and a high-resolution Fabry–Perot spectrometer and was deployed in the exhaust line of W7-X during the OP2.3 campaign. The injection of ³ He and ⁴ He for minority ion-cyclotron heating provided a test case for helium isotope discrimination. Despite limitations due to background gas and low partial pressures of the target species, isotope-resolved spectral signatures were successfully observed, demonstrating the fundamental viability of the Optix approach. Additionally, the spectrometer was evaluated for plasma emission measurements from both core and edge sightlines. While helium line emission was detectable, interpretation was hindered by complex background signals, highlighting the benefits of controlled remote plasma generators for spectroscopy. This first deployment provides critical insight into pressure requirements, spectral resolution, and operational constraints, informing future applications of optical exhaust diagnostics in fusion devices.