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The IPED predictive pedestal code has been used to determine the critical gradients for the onset of 1) separatrix ballooning modes and 2) global peeling-ballooning modes as a function of plasma shaping. This results in a scaling of the onset threshold of separatrix ballooning modes as a function of elongation and triangularity αedge,crit=0.64κ2.2(1+δ)0.9, while the critical gradient for global peeling-balloonig modes increases as ≈αedge,crit1.5. This implies that operational space for a ballooning unstable separatrix and stable peeling-ballooning modes exists at sufficiently high shaping. Applying a collisional broadnening based scaling of the separatrix gradients allows the critical separatrix density, required to drive the separatrix ballooning mode, to be derived from global plasma parameters for any operational scenario on any device. Evaluations for ASDEX Upgrade, JET, and the ITER 15 MA baseline plasma predict critical separatrix densities of 0.3–0.4 nGW for QCE access, making the QCE an attractive operational scenario for fusion devices.

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.

Understanding the transport processes that determine the plasma profile widths in the scrape-off layer (SOL) and divertor region of tokamaks is crucial for successful power and particle exhaust management in future devices. Plasma transport from the SOL into the private flux region (PFR) broadens the profiles and could mitigate the power exhaust challenge. Analysis of ion current profiles, measured by Langmuir probes in the ASDEX upgrade (AUG) tokamak, shows that the ion current width in the PFR, normalized by the flux expansion between outer target and midplane, is about 1.2 mm in L-mode and 0.9 mm in inter-ELM H-mode plasmas. The measured widths agree within a factor of two with predictions from an analytical model, based on Pfirsch-Schlüter flows. According to the model, the ion PFR width increases with the distance ΔR between the outer target and X-point major radii, and scales inversely with the poloidal magnetic field Bp. For the tokamaks ITER and SPARC the model predicts PFR widths below 0.3 mm.

In future fusion reactors, extended melt pools in combination with strong plasma-induced accelerations, suggest that the metallic melt could reach the gaps between castellated plasma-facing components, potentially accompanied by profound changes in their mechanical response. The first results of a combined experimental and modelling effort to elucidate the physics of melt transport across gaps are presented. Transient melting of specially designed tungsten samples featuring toroidal gaps has been achieved in ASDEX Upgrade providing direct evidence of gap bridging. Detailed modelling with the MEMENTO melt dynamics code is reported. Empirical evidence and simulations reveal that the presence of gaps can be safely ignored in macroscopic melt motion predictions as well as that the re-solidification limited melt spreading facilitates gap bridging and leads to poor melt attachment. The findings are discussed in the context of ITER and DEMO.

The 3D fluid edge code EMC3-EIRENE has been used to simulate three ASDEX Upgrade discharges with the same equilibrium but different upstream collisionality ν ∗ . In this scan, the plasma transits from an ELMy state at low collisionality to being in the quasi-continuous exhaust regime (QCE) at high ν ∗ . Particular attention was devoted to matching the experimental plasma profiles in front of the ion cyclotron resonance heating (ICRH) limiter, aiming to obtain a reliable estimation of the ICRH limiter peak heat flux in the QCE regime discharge. As a result, we observed that while moving from ELMy to QCE, the limiter peak heat flux increases, reaching values in the order of the MW m − 2 for a limiter-separatrix distance of 6 cm . These values were compared with those at the outer divertor, measured by infrared thermography, revealing that the heat flux perpendicular to the limiter surface can reach up to 90% of the one impinging onto the outer target. Finally, an attempt was made to extend the obtained results towards ITER. This simple analysis suggests that a large wall clearance and a small incidence angle of the magnetic field line, not only on the divertor target but also on ITER’s first wall, might have a crucial role in preventing wall damage during QCE discharges.

One of the major challenges for the design of future thermonuclear reactors is the problem of power exhaust—the removal of heat fluxes deposited by plasma particles onto the plasma-facing components (PFCs) of the reactor wall. In order for the reactor to work efficiently, the power loading of the PFCs has to stay within their material limits. A substantial part of these heat fluxes can be deposited transiently during the impact of edge localised modes (ELMs), which typically accompany the high confinement mode, a regime foreseen for tokamak ITER and next-step devices. One of the possible ways to mitigate the deposition of localised heat fluxes during ELMs is injection of impurities, which could similarly to inter-ELM detachment dissipate part of the energy carried by plasma particles, the so-called ELM buffering effect. In this contribution, we report on experimental observations in impurity seeded discharges in ASDEX Upgrade, where injection of argon is capable of reducing the ELM energy by up to 80 % (60 % without degradation of confinement). A simple model of ELM cooling is in some cases capable of providing quantitative prediction of this effect. The ELM peak energy fluence ϵ | | , p e a k was reduced by a factor 8 without a degradation of the pedestal pressure. Should such mitigation be achieved in ITER, the resulting power loading would satisfy the material limits of divertor tungsten monoblocks (Eich et al 2017 Nucl. Mater. Energy 12 84–90) and as such avoid the risk of their melting. The most favourable results in terms of confinement and divertor heat flux mitigation were achieved by use of a mixture of argon and nitrogen, where the later impurity helped to improve the confinement. The ELM frequency was identified as a scaling factor for ϵ | | , p e a k in discharges with impurity seeding, suggesting that high frequency ELMs are favourable for future devices.

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.

In order to extend the enhanced D-Alpha H-mode to future devices, it is crucial to understand the properties of the main signature of this regime, the quasicoherent mode (QCM), that likely clamps the pressure gradient below the ideal magnetohydrodynamic limit. The turbulent character of the QCM is investigated with scanning probes in ASDEX Upgrade. Analysis reveals the multi-faced character of the mode that spans both the confined region (where the radial electric field is negative) and the near scrape-off layer (SOL) (where it is positive). Fluctuations of density and potential at the QCM frequency are more anti-correlated towards the confined region, which is a fingerprint of electromagnetic instabilities, while they become more correlated in the SOL, as expected for a drift-wave, inducing cross-field transport across the separatrix.

A set of dedicated H-mode discharges with constant heating power combining Neutral Beam Injection and Electron Cyclotron Resonance Heating have been executed at the ASDEX Upgrade tokamak using a high triangularity magnetic geometry in order to investigate the impact of filamentary transport to divertor and non-divertor components. The evolution of upstream scrape-off layer (SOL) profiles have been correlated with dedicated separatrix quantities, mostly with the turbulence control parameter αt (Eich and Manz 2021 Nucl. Fusion 61 086016) describing the turbulence level at the separatrix. With increasing αt , a broadening of the upstream density profiles in the near-SOL together with the formation of a density shoulder in the far-SOL have been observed. This phenomenon is associated with an enhanced filamentary transport dominating the radial turbulent transport in the far-SOL and confirmed by means of the cooling water calorimetry on non-divertor components. The probe measurements conducted with the ball-pen probe-head mounted on the midplane manipulator and a retarding-field analyzer close to the limiter surface indicate that the key mechanism increasing the radial filamentary transport to the first wall is an increase of the particle flux Γr,fil , caused primarily by the packing fraction fPF,fil and the filament density ne,fil . At the same time, the electron temperature Te and ion temperature Ti measured close to the limiter surface show only small variations above αt > 0.5. Both the filamentary heat flux and the gross erosion derived from the first wall probe measurements reach a magnitude that should be considered in the design of future fusion reactors.

The quasi-coherent mode (QCM), appearing in enhanced D α high confinement mode (EDA H-mode) and quasi-continuous exhaust (QCE) plasmas has been analysed in detail at ASDEX Upgrade via thermal helium beam spectroscopy under various discharge parameters. In both scenarios the QCM appears to be localized close to the separatrix and to propagate in ion diamagnetic direction in the plasma frame. The poloidal wavenumber of the QCM is about 0.025

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