SPC - Physique des Plasmas de Base

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Simultaneous measurements of unstable and stable Alfven eigenmodes in JET

Ambrogio Fasoli, Mario Podesta

In this paper, we report the novel experimental observation of both unstable and stable toroidicity-induced Alfven eigenmodes (TAEs) measured simultaneously in a JET tokamak plasma. The three-ion-heating scheme (D-DNBI-He-3) is employed to accelerate deuterons to MeV energies, thereby destabilizing TAEs with toroidal mode numbers n = 3-5, each decreasing in mode amplitude. At the same time, the Alfven eigenmode active diagnostic resonantly excites a stable n = 6 TAE with total normalized damping rate -gamma/omega (0) approximate to 1%-4%. Hybrid kinetic-MHD modeling with codes NOVA-K and MEGA both find eigenmodes with similar frequencies, mode structures, and radial locations as in experiment. NOVA-K demonstrates good agreement with the n = 3, 4, and 6 TAEs, matching the damping rate of the n = 6 mode within uncertainties and identifying radiative damping as the dominant contribution. Improved agreement is found with MEGA for all modes: the unstable n = 3-5 and stable n = 2, 6 modes, with the latter two stabilized by higher intrinsic damping and lower fast ion drive, respectively. While some discrepancies remain to be resolved, this unique validation effort gives us confidence in TAE stability predictions for future fusion devices.

IOP Publishing Ltd2022

New insights on divertor parallel flows,E x B drifts, and fluctuations from in situ, two-dimensional probe measurement in the Tokamak a Configuration Variable

Hugo De Oliveira, Basil Duval, Holger Reimerdes, Christian Gabriel Theiler, Cedric Kar-Wai Tsui

In situ, two-dimensional (2D) Langmuir probe measurements across a large part of the TCV outer divertor are reported in L-mode discharges with and without divertor baffles. This provides detailed insights into time averaged profiles, particle fluxes, and fluctuation behavior in different divertor regimes. The presence of the baffles is shown to substantially increase the divertor neutral pressure for a given upstream density and to facilitate the access to detachment, an effect that increases with plasma current. The detailed, 2D probe measurements allow for a divertor particle balance, including ion flux contributions from parallel flows and E x B drifts. The poloidal flux contribution from the latter is often comparable or even larger than the former, and the divertor parallel flow direction reverses in some conditions, pointing away from the target. In most conditions, the integrated particle flux at the outer target can be predominantly ascribed to ionization along the outer divertor leg, consistent with a closed-box approximation of the divertor. The exception is a strongly detached divertor, achieved here only with baffles, where the total poloidal ion flux even decreases towards the outer target, indicative of significant plasma recombination. The most striking observation from relative density fluctuation measurements along the outer divertor leg is the transition from poloidally uniform fluctuation levels in attached conditions to fluctuations strongly peaking near the X-point when approaching detachment.

IOP Publishing Ltd2022

Validation of edge turbulence codes against the TCV-X21 diverted L-mode reference case

Claudia Colandrea, Hugo De Oliveira, Davide Galassi, Maurizio Giacomin, Nicola Offeddu, Holger Reimerdes, Paolo Ricci, Eric Serre, Christian Gabriel Theiler, Cedric Kar-Wai Tsui, Nicola Vianello

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.

IOP Publishing Ltd2022

Validation of edge turbulence codes against the TCV-X21 diverted L-mode reference case

Claudia Colandrea, Hugo De Oliveira, Davide Galassi, Maurizio Giacomin, Nicola Offeddu, Holger Reimerdes, Paolo Ricci, Eric Serre, Christian Gabriel Theiler, Cedric Kar-Wai Tsui, Nicola Vianello

IOP Publishing Ltd2022