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The effects of a poloidally localized bulge at outer midplane are investigated with regards to the stability of ideal MHD peeling-ballooning modes (PBMs) and n=∞ ballooning modes. The effect of the bulge is that the n=∞ ballooning mode stability at the bottom of the H-mode pedestal is significantly degraded, while the PBM stability is only slightly affected. The ratio of separatrix to pedestal density at which the n=∞ ballooning mode limit is reached before the peeling-ballooning limit, decreases with the bulge size. The reduction of the required separatrix density would extend the access to quasi-continuous exhaust (QCE) operating mode without large edge localised modes to a wide range of scrape-off layer conditions. The free boundary equilibrium calculation shows that a bulge required for a significant stability change for a Spherical Tokamak for Energy Production fusion reactor can be achieved with 600 kA current in a midplane shaping coil.

The effect of resistive MHD in the pedestal predictions for JET-ILW has been investigated, using the Europed workflow [S Saarelma et al, Plasma Phys. Control. Fusion 2018] with the resistive MHD stability code CASTOR [W Kerner et al, J. Comput. Phys. 1998]. The inclusion of resistivity in the MHD stability calculations leads to a destabilisation of the peeling-ballooning modes. The effect of resistivity is shown for different plasma conditions with scans of the following parameters: the electron pedestal top density, the electron separatrix density and the normalised plasma pressure. The effect of resistivity is largely dependent on the input electron density pedestal profile, and less dependent on the normalised plasma pressure. Overall, at high temperatures, the predictions performed with ideal and resistive MHD are comparable. At low temperature, resistive effects are not negligible and the resistive MHD predictions produce a pedestal pressure height and gradient lower than ideal MHD predictions. Europed predictions with resistive MHD are also compared to experimental values from power scans and gas scans in JET-ILW. While at low gas both ideal and resistive predictions are reasonable, at high gas the resistive Europed predictions show a better agreement with the experimental pedestal height than ideal predictions. The improvement in the predictions of pedestal gradient and width using resistive MHD is instead much smaller. Due to the sensitivity of the resistive predictions to the stability threshold, the work shows that quantitative conclusions are very uncertain but, nonetheless, a qualitative improvement in the agreement with experimental results compared to ideal MHD has been observed.

As part the DTE2 campaign in the JET tokamak, we conducted a parameter scan in T and D-T complementing existing pulses in H and D. For the different main ion masses, type-I ELMy H-modes at fixed plasma current and magnetic field can have the pedestal pressure varying by a factor of 4 and the total pressure changing from β N = 1.0 to 3.0. We investigated the pedestal and core isotope mass dependencies using this extensive data set. The pedestal shows a strong mass dependence on the density, which influences the core due to the strong coupling between both plasma regions. To better understand the causes for the observed isotope mass dependence in the pedestal, we analysed the interplay between heat and particle transport and the edge localised mode (ELM) stability. For this purpose, we developed a dynamic ELM cycle model with basic transport assumptions and a realistic neutral penetration. The temporal evolution and resulting ELM frequency introduce an additional experimental constraint that conventional quasi-stationary transport analysis cannot provide. Our model shows that a mass dependence in the ELM stability or in the transport alone cannot explain the observations. One requires a mass dependence in the ELM stability as well as one in the particle sources. The core confinement time increases with pedestal pressure for all isotope masses due to profile stiffness and electromagnetic turbulence stabilisation. Interestingly, T and D-T plasmas show an improved core confinement time compared to H and D plasmas even for matched pedestal pressures. For T, this improvement is largely due to the unique pedestal composition of higher densities and lower temperatures than H and D. With a reduced gyroBohm factor at lower temperatures, more turbulent drive in the form of steeper gradients is required to transport the same amount of heat. This picture is supported by quasilinear flux-driven modelling using TGLF -SAT2 within Astra . With the experimental boundary condition TGLF -SAT2 predicts the core profiles well for gyroBohm heat fluxes > 15 , however, overestimates the heat and particle transport closer to the turbulent threshold.

We present a comparative transport analysis of the isotope mass dependence in the pedestal of two pairs of deuterium/hydrogen type I ELMy H-mode discharges in JET with ITER-like wall, one characterized by the same input power, the other one by the same stored energy. The investigation, carried out using the gyrokinetic code GENE, focuses on the steep profile region of the pedestal. While large wavenumber modes mainly contribute to the electron heat flux and are scarcely influenced by the main gas isotope, an effect of the ion mass in agreement with the experimental (so called anti-gyro-Bohm) scaling is revealed in the low wavenumber range. In this context, the major role played by the E × B shear in regulating the ion-temperature-gradient turbulence is analyzed in some detail. The competing level of turbulent and neoclassical transport is quantified to shed light on the experimental features of the pedestal profiles at different ion mass, with the particle transport found to be consistent with a higher pedestal top density for increasing isotope masses, and the heat transport shown to match the roughly unaltered observed temperature profiles.

Plasmas with negative triangularity (NT) shape have been recently shown to be able to achieve H-mode levels of confinement in L-mode, avoiding detrimental edge localised modes. Therefore, this plasma geometry is now studied as a possible viable option for a future fusion reactor. Within this framework, an NT option is under investigation for the full power scenario of the Divertor Tokamak Test (DTT) facility, under construction in Italy, with δtop=−0.32/δbottom≃0.02 top/bottom triangularity values at the separatrix. The transport properties of this scenario are studied in this work. Gyrokinetic GENE simulations and integrated modelling using ASTRA with the quasi-linear trapped gyro-Landau fluid (TGLF) model have been performed. The emerging picture from the ASTRA-TGLF runs with boundary conditions at ρtor=0.94 is that, in the L-mode NT option, the larger peaking of the kinetic profiles in the edge region is not sufficient to recover the loss of the PT H-mode pedestal, and reach similar central temperature values. Two additional shapes are also considered, obtained by flipping the triangularity of the scenarios, to single out the effect of the triangularity sign. A negligible ‘direct’ effect of the triangularity is found for the L-mode, while a small beneficial effect is observed for the H-mode. The ASTRA-TGLF results are validated by GENE and TGLF stand-alone at two selected radii. GENE shows ITG dominant micro-instability and explains the small beneficial effect of the NT for the H-mode as due to a strong reduction of the heat fluxes, when reversing the triangularity, with a relatively high Ti stiffness. An improvement of the predicted performances of the NT DTT scenario could come from ρtor≳0.9 , as indicated by some recent experiments at the tokamak à configuration variable (TCV) and ASDEX Upgrade.

This paper describes the extensive progress that has been made in the understanding of tokamak pedestal physics since the 2007 publication of ‘Progress in the ITER Physics Basis’ (Ikeda 2007 Nucl. Fusion 47 E01–S500). It serves as Chapter 3 of the 2025 Nuclear Fusion Special Issue titled ‘On the Path to Tokamak Burning Plasma Operation’ (Campbell et al 2025 Nucl. Fusion). This review was compiled by the pedestal and edge physics (PEP) community affiliated with the International Tokamak Physics Activity organization. It attempts to collect in one place citations to the majority of published literature on the pedestal physics topics that will be most important for the operation of a future power producing burning plasma tokamak. These include citations to publications describing the physics of the pedestal plasmas in many operating tokamaks worldwide and the pedestal physics projections for several near-term future devices including ITER. Descriptions of experimental results, interpretive modeling and predictive extrapolations are integrated together and comprehensive references are provided. This review is organized around four primary technical sections, viz.: pedestal structure, edge localized mode (ELM) characteristics, ELM control and regimes without large ELMs. Key results from many of the references are described briefly and set into the tokamak burning plasma power plant context. In addition, different perspectives on pedestal physics topics that are currently under debate within the community are also described, to provide guidance on needs for future research. Finally, attempts are made to describe conclusions from all of this progress consistent with discussions by the pedestal physics community at this time. The goal of this review is to provide a useful reference document for pedestal physics researchers going forward toward operation of a burning tokamak fusion plasma.

The neutral ionisation model proposed by Groebner et al (2002 Phys. Plasmas 9 2134) to determine the plasma density profile in the H-mode pedestal, is extended to include charge exchange processes in the pedestal stimulated by the ideas of Mahdavi et al (2003 Phys. Plasmas 10 3984). The model is then tested against JET H-mode pedestal data, both in a ‘standalone’ version using experimental temperature profiles and also by incorporating it in the Europed version of EPED. The model is able to predict the density pedestal over a wide range of conditions with good accuracy. It is also able to predict the experimentally observed isotope effect on the density pedestal that eludes simpler neutral ionization models.

For more than a decade, an unprecedented predict-first activity has been carried in order to predict the fusion power and provide guidance to the second Deuterium–Tritium (D–T) campaign performed at JET in 2021 (DTE2). Such an activity has provided a framework for a broad model validation and development towards the D–T operation. It is shown that it is necessary to go beyond projections using scaling laws in order to obtain detailed physics based predictions. Furthermore, mixing different modelling complexity and promoting an extended interplay between modelling and experiment are essential towards reliable predictions of D–T plasmas. The fusion power obtained in this predict-first activity is in broad agreement with the one finally measured in DTE2. Implications for the prediction of fusion power in future devices, such as ITER, are discussed.

In this work, the effect of neon seeding in the JET-ILW JET-ITER baseline scenario on pedestal structure, global stability and local stability is studied. Increased neon seeding leads to an increase in the pedestal electron and ion temperature while reducing the density. The combined effect leads to an increase in the pedestal top pressure. The dataset exhibits a mix of small edge localized mode (ELM)-like events and large ELMs. As the neon seeding is increased the frequency of the large ELMs goes down until the ELMs disappear completely. Infinite-n ballooning stability analysis shows that the seeded shots are all unstable to infinite-n ballooning modes at the bottom of the pedestal which could explain the small ELM-like events. Both ideal and resistive magnetohydrodynamic (MHD) stability analysis of the pedestal is performed and it is shown that resistive MHD can well describe the pre-ELM pedestal profiles for the large ELMs. The impact of the seeding on the resistive MHD stability is mainly tied to two competing effects. Firstly, increasing Zeff leads to an increased resistivity which destabilizes the resistive modes. Secondly, the diamagnetic frequency of the pedestal changes due to changes in the pedestal profiles that has a stronger impact on the resistive modes compared to the ideal ones.

The work describes the pedestal structure, transport and stability in an effective mass ( A eff ) scan from pure deuterium to pure tritium plasmas using a type I ELMy H-mode dataset in which key parameters that affect the pedestal behaviour (normalized pressure, ratio of the separatrix density to the pedestal density, pedestal ion Larmor radius, pedestal collisionality and rotation) are kept as constant as possible. Experimental results show a significant increase of the density at the pedestal top with increasing A eff , a modest reduction in the temperature and an increase in the pressure. The variations in the pedestal heights are mainly due to a change in the pedestal gradients while only small differences are observed in the pedestal width. A clear increase in the pedestal density and pressure gradients are observed from deuterium to tritium. The experimental results suggest a reduction of the pedestal inter-edge localized mode (inter-ELM) transport from deuterium to tritium. The reduction is likely in the pedestal inter-ELM particle transport, as suggested by the clear increase of the pedestal density gradients. The experimental results suggest also a possible reduction of the pedestal inter-ELM heat transport, however, the large experimental uncertainties do not allow conclusive claims on the heat diffusivity. The clear experimental reduction of η e (the ratio between density and temperature gradient lengths) in the middle/top of the pedestal with increasing A eff suggests that there may be a link between increasing A eff and the reduction of electron scale turbulent transport. From the modelling point of view, an initial characterization of the behaviour of pedestal microinstabilities shows that the tritium plasma is characterized by growth rates lower than the deuterium plasmas. The pedestal stability of peeling-ballooning modes is assessed with both ideal and resistive magnetohydrodynamics (MHD). No significant effect of the isotope mass on the pedestal stability is observed using ideal MHD. Instead, resistive MHD shows a clear increase of the stability with increasing isotope mass. The resistive MHD results are in reasonable agreement with the experimental results of the normalized pedestal pressure gradient. The experimental and modelling results suggest that the main candidates to explain the change in the pedestal are a reduction in the inter-ELM transport and an improvement of the pedestal stability from deuterium to tritium.