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The quasi-continuous exhaust (QCE) regime, formerly known as either type-II ELM or small ELM regime is studied in ASDEX Upgrade. The regime is a natural type-I ELM-free H-mode. The operational space of QCE discharges in ASDEX Upgrade with respect to their separatrix conditions and their power exhaust capabilities are presented. A significant broadening of the power fall-off length is observed, correlating to an increased separatrix density and pressure. Moreover, the possible reactor relevance of this regime is demonstrated by expanding the operational space to low edge safety factor and demonstrating the benign tungsten impurity behaviour. A discharge without any type-I ELM from start to end reaching a partially detached divertor at high normalised energy confinement time is presented.

We investigated isotope effects during the tokamak à configuration variable (TCV) ohmic discharge of a diverted positive triangular shape configuration of deuterium (D) and hydrogen (H) plasmas. The transition from the linear ohmic confinement (LOC) regime to the saturated ohmic confinement (SOC) regime was clearly identified from the shot-by-shot density scan experiments. The transport characteristics were almost identical in the H and D plasmas in the LOC regime, and clear improvements were observed in the heat and particle transports in the D plasma compared with the H plasma in the SOC regime. In the SOC regime, the global energy confinement was higher in the D plasma than in the H plasma. Improvements in the SOC regime were evident in the ion channel of the heat transport and the diffusion term of the particle transport. Intrinsic toroidal rotation was found. Its profiles were identical in the H and D plasma in the LOC regime. However, the steeper gradient of toroidal rotation was found in the D plasma than in the H plasma in the SOC regime. The gyrokinetic modeling of switching ion species and keeping identical input profiles showed no difference of the heat flux in the LOC regime and a clear reduction in the D plasma heat flux in the SOC regime. Additionally, collisionality is shown to play an important role in in the heat flux reduction in D plasmas relative to H plasmas. The gyrokinetic validation of the heat transport against the experimental profiles showed a qualitative agreement regarding the heat and particle fluxes. Quantitative agreement was better for the ion heat channel than for the other transport channels.

The impact of plasma shaping on the properties of high density H-mode scrape-off layer (SOL) profiles and transport at the outer midplane has been investigated on Tokamakà configuration variable. The experimental dataset has been acquired by evolving the upper triangularity while keeping the other parameters constant. The scan comprises δup values between 0.0 and 0.6, excluding negative triangularity scenarios. Within this study, a transition from type-I edge localised modes to the quasi-continuous exhaust regime takes place from low to high δup . The modification of the upstream SOL profiles has been assessed, in terms of separatrix quantities, within the αt turbulence control parameter theoretical framework (Eich et al 2020 Nucl. Fusion 60 056016). The target parallel heat load and the upstream near-SOL density profiles have been shown to broaden significantly for increasing αt . Correspondingly, in the far SOL a density shoulder formation is observed when moving from low to high δup . These behaviours have been correlated with an enhancement of the SOL fluctuation level, as registered by wall-mounted Langmuir probes as well as the thermal helium beam diagnostic. Specifically, both the background and the filamentary-induced fluctuating parts of the first wall ion saturation current signal are larger at higher δup , with filaments being ejected more frequently into the SOL. Comparison of two pulses at the extremes of the δup scan range, but with otherwise same input parameters, shows that the midplane neutral pressure does not change much during the H-mode phase of the discharge. This indicates that indirect effects of the change in geometry, linked to first wall recycling sources, should not play a significant role. The total core radiation increases at high δup , on account of a stronger plasma–wall interaction and resulting larger carbon impurity intake from the first wall. This is likely associated to the enhanced first wall fluctuations, as well as a smaller outer gap and the close-to-double-null magnetic topology at high shaping.

Plasma state reconstruction methods combining measurements and physics modeling improve the estimation of physics quantities in real-time and in post-discharge analysis. We present a workflow to reconstruct the dynamic evolution of a set of internal tokamak plasma profiles with consistent equilibria, as applied in this paper for experimental data from TCV L and H-mode discharges. Plasma profile estimates for electron temperature Te, electron density ne and parallel current density jpar are obtained by data assimilation of Thomson scattering (TS) measurements into RAPTOR modeling, using an Extended Kalman Filter (EKF). A new kinetic equilibrium reconstruction method ensures mutual consistency of free-boundary equilibrium reconstruction, core plasma profile estimates and mapping of the TS measurements to flux surface coordinates. The RAPTOR code captures the coupled dynamics of electron heat, particle and current density transport and includes a model for the onset conditions of sawtooth instabilities and the resulting profile relaxations after a sawtooth crash, enabling a realistic q profile reconstruction even in the absence of direct measurements. During ohmic phases with sawtooth instabilities, the sawtooth period and inversion radius inferred from soft x-ray measurements are in excellent agreement with RAPTOR EKF inner q profile reconstructions and the predicted sawtooth dynamics, even for transient phases. In addition to the plasma profiles, the EKF allows to infer unknown physics quantities such as the effective charge Zeff and the on-axis ion-to-electron temperature ratio Ti0/Te0, as well as transport model parameters. Continuously updating the transport model parameters for electron heat and density transport, based on the available measurements, is an effective way to reduce the model-to-reality gap, as required for real-time model-based control of fusion reactor plasmas.

Fast ions have been observed to be accelerated in the presence of Edge Localised Modes (ELMs) and MHD activity on the Tokamak á Configuration Variable. The acceleration time, velocity-space and frequency dynamics have been resolved by analysing the fast-ion losses measured using a unique Fast Ion Loss Detector (FILD) that allows microsecond velocity-space mapping. The findings presented herein complement and extend previous studies done at the Asdex Upgrade Tokamak and show a decorrelation between the acceleration and the ELM crash. The experimental scenario is a high-confinement mode (H-mode) plasma, characterised by low density, high electron temperature and, hence, long slowing down times of the fast-ion population. Significant MHD activity has been observed in the inter-ELM-crash period with frequencies ranging from 50 to 250 kHz. These modes exhibit strong down-chirping and burst signatures. The empirical dependence of the modes’ frequency upon the plasma density identifies them as Alfvén Eigenmodes (AEs) and locates them in the outer region of the plasma, where the resonance conditions between the fast ions and modes are fulfilled. Their resonant interaction with the fast-ion, generated using a Neutral Beam Injector, changes during the ELM cycle according to the changes in the plasma parameters, such as density and temperature. The fast-ion losses are also identified using orbit following simulations, allowing us to distinguish the different contributions to the FILD signal. The pre-ELM AEs’ frequencies and the velocity-space of the MHD-induced fast-ion losses are preserved on the FILD signal during the first ∼800 microseconds of the ELM crash, suggesting a complex interaction between AEs, fast ions and ELMs.

We present the first edge Er measurements in negative triangularity (NT) plasmas in the Tokamak à Configuration Variable (TCV). The Doppler backscattering measurements of v⊥≈Er/B reveal a significant impact of triangularity on the Er well: in Ohmic, neutral beam injection, and electron cyclotron resonance heated discharges, the Er well and associated Er×B shear are stronger in NT-shaped plasmas compared to their positive triangularity (PT) counterpart. This suggests a connection to the concomitant NT performance gain relative to PT L-mode.

The quasi-continuous exhaust (QCE) regime is a regime that is naturally type-I ELM-free. It combines the high density at the plasma edge needed for power exhaust with the high normalised energy confinement typical for H-mode operation. In the QCE regime large-scale ELMs are avoided and high-frequency, low-amplitude filaments are present leading to the name-giving quasi-continuous edge transport of particles and energy. This contribution reports that for the first time the QCE regime was successfully achieved in JET with a metal wall. Moreover, it was demonstrated in the recent JET deuterium-tritium campaign DTE3 that the regime is compatible with D–T operation. Porting the QCE regime to JET strongly benefited from the experimental and modelling efforts at the medium sized tokamaks ASDEX Upgrade and TCV. Using the physics picture developed from the ASDEX Upgrade experimental results, the route to the QCE regime in JET reported here is following closely the approach that was successful in ASDEX Upgrade. First, strong plasma shaping—large elongation and triangularity and the highly correlated closeness to double null—is developed. Second, sufficient fuelling to achieve high enough density at the pedestal foot, close to the separatrix, is applied. In addition, neon seeding proved to be very beneficial to avoid type-I ELMs when reducing the main ion fuelling.

Edge Localised Modes (ELMs) induced fast-ion losses have been characterised in the energy, E, and pitch, λ=v∥/v, space using a unique Fast Ion Loss Detector (FILD) on the tokamak à configuration variable. The FILD is equipped with a 128 Avalanche Photo Diodes (APDs) camera measuring scintillator emission with a 1 MHz bandwidth. The well-defined view lines of the APDs allow for obtaining velocity space information from the emission pattern with unprecedented temporal resolution. Making use of the enhanced FILD capabilities, ELM-induced fast-ion losses were investigated in the ITER Baseline Scenario, mimicking its shape and normalised plasma parameters. Significant fast-ion losses were observed, notably before and during the ELM crashes induced by the presence of a 2/1 MHD instability localised at the plasma’s normalised radius, ρ∼0.7. The fast-ion interaction with the plasma instability is concluded to be unconnected to the presence of the ELMs. During the ELM crashes, there is an observed spread in both the fast-ion pitch and energy that reaches higher values (∼ 70 keV) than expected from the pre-ELM fast-ion population, generated by Neutral Beam Injection at ∼27 keV. This implies a fast-ion acceleration during the ELM’s crash. Full orbit neoclassical simulations are used to calculate the neoclassical fast-ion velocity space lost to the FILD and to quantify the neoclassical fast-ion losses.

Operation with beam heating is prone to central accumulation of high- Z impurities in present-day tokamaks due to a dominant neoclassical pinch. Fusion reactors are expected to have a different core transport regime for high- Z impurities, where turbulent transport completely dominates in both diffusion and convection, causing flat impurity profiles. In this work, we present integrated simulations of plasmas that approach this regime of less dominant neoclassical and stronger turbulent high- Z impurity transport in three tokamaks: ASDEX Upgrade, JET and Alcator C-Mod. The modelling is first validated against a suite of diagnostics measuring both the main plasma and the impurity profiles. The high- Z impurity densities are found to be flat in both experiments and simulations. The impurity transport coefficients as calculated by theory-based turbulent and neoclassical transport models are then analysed, showing comparable or even dominant turbulent components instead of the more typical dominant neoclassical high- Z impurity convection. The implications for a reactor are discussed.

Screening of high- Z (W) impurities from the confined plasma by the temperature gradient at the plasma periphery of fusion-grade H-mode plasmas was demonstrated for the first time in JET with the Be/W wall (Field et al 2023 Nucl. Fusion 63 016028). Additional experiments have been performed in JET during 2023, including in deuterium–tritium (DT) during the DTE3 campaign, to further optimise the impurity screening in such plasmas, as well as our bolometric measurements of the W impurity fluxes between and during edge-localised modes. A decrease in plasma current from 2.3 MA to 2.1 MA reduced the electron density and thereby increased the ion temperature at the H-mode pedestal top, resulting in stronger impurity screening behaviour. The scenario was then successfully transferred to operation in DT by increasing the toroidal field, in order to compensate the lower L/H-threshold power in DT compared to D plasmas. Here, results of detailed analysis and modelling of the neoclassical (NC) W transport in four pulses from these experiments are presented, two in D at 2.3 MA and 2.1 MA plasma current and a matched pulse pair at 2.1 MA in D and DT. Using the FACIT code (Fajardo et al 2023 Plasma Phys. Control. Fusion 65 035021) to model the NC W transport for these more recent pulses, the outward convection just inside the pedestal top found in our earlier study could not be reproduced. Possible reasons for this discrepancy between experimental observations and our modelling results are discussed, including potential deficiencies in our measurement technique and/or incompleteness of the NC transport modelling.