Transport processes around the magnetic X-point of tokamaks, such as turbulence and mean-field drifts, are scarcely understood. The assessment of the capability of turbulence codes to quantitatively reproduce these dynamics has been hampered by limitations in computational power and available experimental data. In this paper, we present a rigorous validation of full-scale simulations of a newly developed X-point scenario in the basic toroidal plasma device TORPEX, performed with the four state-of-the-art codes FELTOR, GBS, GRILLIX, and STORM. High-resolution Langmuir probe array measurements of various time-averaged and fluctuating quantities and across the entire cross section of TORPEX show that this X-point scenario features the key ingredients of X-point dynamics, such as small-scale fluctuations and background drifts. The codes are able to qualitatively reproduce some characteristics of the time-averaged fields, such as the ion saturation current profiles at mid-height, the plasma up-down asymmetry, and the blob trajectories. A quantitative agreement is found for the background E × B velocity pattern, while the fluctuation levels are generally underestimated typically by factors of 2 or more, and thus, background fluxes are found to dominate over turbulent ones in simulations. The sensitivity of the simulation results on the plasma collisionality and on the position of the sources is tested in GBS, showing a mild effect on the overall quantitative agreement with the experiment. Overall, this validation reveals the challenges to reproduce the plasma dynamics near an X-point and provides a clear path to a quantitative and computationally relatively inexpensive assessment of future developments in turbulence codes.

Self-consistent full-size turbulent-transport simulations of the divertor and scrape-off-layer (SOL) of existing tokamaks have recently become feasible. This enables the direct comparison of turbulence simulations against experimental measurements. In this work, we perform a series of diverted ohmic L-mode discharges on the tokamak a configuration variable (TCV) tokamak, building a first-of-a-kind dataset for the validation of edge turbulence models. This dataset, referred to as TCV-X21, contains measurements from five diagnostic systems from the outboard midplane (OMP) to the divertor targets-giving a total of 45 one- and two-dimensional comparison observables in two toroidal magnetic field directions. The experimental dataset is used to validate three flux-driven 3D fluid-turbulence models-GBS, GRILLIX and TOKAM3X. With each model, we perform simulations of the TCV-X21 scenario, individually tuning the particle and power source rates to achieve a reasonable match of the upstream separatrix value of density and electron temperature. We find that the simulations match the experimental profiles for most observables at the OMP-both in terms of profile shape and absolute magnitude-while a comparatively poorer agreement is found towards the divertor targets. The match between simulation and experiment is seen to be sensitive to the value of the resistivity, the heat conductivities, the power injection rate and the choice of sheath boundary conditions. Additionally, despite targeting a sheath-limited regime, the discrepancy between simulations and experiment also suggests that the neutral dynamics should be included. The results of this validation show that turbulence models are able to perform simulations of existing devices and achieve reasonable agreement with experimental measurements. Where disagreement is found, the validation helps to identify how the models can be improved. By publicly releasing the experimental dataset and validation analysis, this work should help to guide and accelerate the development of predictive turbulence simulations of the edge and SOL.

The tokamak à configuration variable (TCV) continues to leverage its unique shaping capabilities, flexible heating systems and modern control system to address critical issues in preparation for ITER and a fusion power plant. For the 2019–20 campaign its configurational flexibility has been enhanced with the installation of removable divertor gas baffles, its diagnostic capabilities with an extensive set of upgrades and its heating systems with new dual frequency gyrotrons. The gas baffles reduce coupling between the divertor and the main chamber and allow for detailed investigations on the role of fuelling in general and, together with upgraded boundary diagnostics, test divertor and edge models in particular. The increased heating capabilities broaden the operational regime to include T e / T i ∼ 1 and have stimulated refocussing studies from L-mode to H-mode across a range of research topics. ITER baseline parameters were reached in type-I ELMy H-modes and alternative regimes with ‘small’ (or no) ELMs explored. Most prominently, negative triangularity was investigated in detail and confirmed as an attractive scenario with H-mode level core confinement but an L-mode edge. Emphasis was also placed on control, where an increased number of observers, actuators and control solutions became available and are now integrated into a generic control framework as will be needed in future devices. The quantity and quality of results of the 2019–20 TCV campaign are a testament to its successful integration within the European research effort alongside a vibrant domestic programme and international collaborations.

The impact of triangularity on edge plasma transport and turbulence is addressed from full 3D turbulence simulations performed with TOKAM3X. Flux driven fluid simulations are run on analytical magnetic equilibria generated with positive and negative triangularity δ in a bottom limiter configuration. The conservation of the energy is assured by the increase of the bottom limiter radial position from δ>0 to δ<0. Changing the triangularity impacts both the plasma equilibrium and the turbulence. In particular, negative triangularity leads to a reduction of the density and electron temperature decay lengths in agreement with the literature. Concerning the turbulence, in all the simulations, it remains ballooned with an enhanced level of fluctuations at low field side in comparison to the high field one. Moreover, no clear trend is visible on the relative level of fluctuations of both density and electron temperature in the CFR whereas an enhancement (resp. reduction) is visible in the scrape-off layer at the low field side midplane for the negative (resp. positive) triangularity simulations. This behaviour differs from TCV and DIII-D measurements which show the benefit of negative triangularity in terms of turbulence reduction and increased confinement. However, no conclusion is drawn from our preliminary study concerning the impact of triangularity on the turbulent transport. Change in triangularity impacts many simulation control parameters, as in the experiments, and that the analysis of its impact alone on the dynamics of the plasma is not obvious in this configuration.