Self-consistent full-size turbulent-transport simulations of the divertor and scrape-off-layer 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 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 5 diagnostic systems from the outboard midplane 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 outboard midplane -- 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 scrape-off-layer.

Using recently installed scrape-off layer diagnostics on the tokamak à configuration variable, we characterise the poloidal and parallel properties of turbulent filaments. We access both attached and detached divertor conditions across a wide range of core densities (f G ∈ [0.09, 0.66]) in diverted L-mode plasma configurations. With a gas puff imaging (GPI) diagnostic at the outer midplane we observed filaments with a monotonic increase in radial velocity (from 390 m s−1 to 800 m s−1) and cross-field radii (from 8.5 mm to 13.4 mm) with increasing core density. Interpreting the filament behaviour in the context of the two-region model by Myra et al (2006 Phys. Plasmas 13 112502), we find that they populate the ideal-interchange regime (C i) in discharges at very low densities, and the resistive X (RX)-point regime for all other discharges. The scaling of filament velocity versus size shows good agreement with this interpretation. These results are discussed and compared with previous probe-based measurements for similar conditions, which mostly placed filaments in TCV in the resistive ballooning (RB) regime (Tsui et al 2018 Phys. Plasmas 25 072506). In addition, for the first time in TCV, the parallel filament extension is studied by magnetically aligning the GPI measurements at the outboard midplane with a reciprocating probe in the divertor. In agreement with the filaments being in the ideal-interchange and the RX-point regimes, they are found to extend beyond the X-point into the outer divertor leg.

A new Gas Puff Imaging (GPI) diagnostic has been installed on the TCV tokamak, providing two-dimensional insights into Scrape-Off-Layer (SOL) turbulence dynamics above, at and below the magnetic X-point. A detailed study in L-mode, attached, lower single-null discharges shows that statistical properties have little poloidal variations, while vast differences are present in the 2D behaviour of intermittent filaments. Strongly elongated filaments, just above the X-point and in the divertor far-SOL, show a good consistency in shape and dynamics with field-line tracing from filaments at the outboard midplane, highlighting their connection. In the near-SOL of the outer divertor leg, short-lived, high frequency and more circular (diameter ∼ 15 sound Larmour radii) filaments are observed. These divertor-localised filaments appear born radially at the position of maximum density and display a radially outward motion with velocity ≈400 m/s that is comparable to radial velocities of upstream-connected filaments. Conversely, in these discharges (B×∇B pointing away from the divertor), these divertor filaments’ poloidal velocities differ strongly from those of upstream-connected filaments. The importance of divertor-localised filaments upon radial transport and profile broadening is explored using filament statistics and in-situ kinetic profile measurements along the divertor leg. This provides evidence that these filaments contribute significantly to electron density profile broadening in the divertor.

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. Highresolution 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 as well as 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 by typically factors of 2 or more, 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 are 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.

Multi-spectral imaging of helium atomic emission (HeMSI) has been used to create 2D poloidal maps of Te and ne in TCV’s divertor. To achieve these measurements, TCV’s MANTIS multispectral cameras simultaneously imaged four He I lines (2 singlet and 2 triplet) and a He II line (468nm) from passively present He and He+. The images, which were absolutely calibrated and covered the whole divertor region, were inverted through the assumption of toroidal symmetry to create emissivity profiles and, consequently, line-ratio profiles. A collisional-radiative model (CRM) was applied to the line-ratio profiles to produce 2D poloidal maps of Te and ne. The collisional-radiative modeling was accomplished with the Goto helium CRM code which accounts for electron-impact excitation and deexcitation (EIE), and electron-ion recombination (EIR) with He+. The HeMSI Te and ne measurements were compared with co-local Thomson scattering measurements. The two sets of measurements exhibited good agreement for ionizing plasmas: (5 eV ≤ Te ≤ 60 eV, and 2 × 1018 m-3 ≤ ne ≤ 3 × 1019 m-3) in the case of majority helium plasmas, and (10 eV ≤ Te ≤ 40 eV, 2 × 1018 m-3 ≤ ne ≤ 3 × 1019 m-3) in the case of majority deuterium plasmas. However, there were instances where HeMSI measurements diverged from Thomson scattering. When Te ≤ 10 eV in majority deuterium plasmas, HeMSI deduced inaccurately high values of Te. This disagreement cannot be rectified within the CRM’s EIE and EIR framework. Second, on sporadic occasions within the private flux region, HeMSI produced erroneously high measurements of ne. Multi-spectral imaging of Helium emission has been demonstrated to produce accurate 2D poloidal maps of Te and ne within the divertor of a tokamak for plasma conditions relevant to contemporary divertor studies.