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

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.

Tokamak Edge Plasma Turbulence Interaction with Magnetic X-Point in 3D Global Simulations

Davide Galassi, Eric Serre

Turbulence in the edge plasma of a tokamak is a key actor in the determination of the confinement properties. The divertor configuration seems to be beneficial for confinement, suggesting an effect on turbulence of the particular magnetic geometry introduced by the X-point. Simulations with the 3D fluid turbulence code TOKAM3X are performed here to evaluate the impact of a diverted configuration on turbulence in the edge plasma, in an isothermal framework. The presence of the X-point is found, locally, to affect both the shape of turbulent structures and the amplitude of fluctuations, in qualitative agreement with recent experimental observations. In particular, a quiescent region is found in the divertor scrape-off layer (SOL), close to the separatrix. Globally, a mild transport barrier spontaneously forms in the closed flux surfaces region near the separatrix, differently from simulations in limiter configuration. The effect of turbulence-driven Reynolds stress on the formation of the barrier is found to be weak by dedicated simulations, while turbulence damping around the X-point seems to globally reduce turbulent transport on the whole flux surface. The magnetic shear is thus pointed out as a possible element that contributes to the formation of edge transport barriers.

2019

A new generation plasma turbulence fluid code for tokamak scrape-off layer dynamics

Federico David Halpern, Sébastien Jolliet, Joaquim Loizu Cisquella, Annamaria Mosetto, Félix Benedito Clément Musil, Paolo Ricci, Fabio Riva, Minh Quang Tran, Christoph Wersal

Understanding the turbulent dynamics of the tokamak scrape-off layer (SOL) is critical for the safe operation of future fusion reactors. In this region of the tokamak device, the plasma exhausts particles and heat onto plasma facing hardware components, which may limit their lifetime if material constraints are not met. Recently, we have gained deep insights into SOL dynamics by means of electromagnetic fluid turbulence simulations carried out with the Global Braginskii Solver (GBS). GBS is currently capable of carrying out simulations of the SOL of medium-size tokamaks such as TCV or Alcator C-Mod. We present the new version of the GBS code, which is intended to address larger SOL plasmas using advanced two-fluid models. GBS has undergone major transformations, including (a) porting of the code kernel to F2008, (b) upgrading pure MPI parallelism to employ a 3D cartesian communicator, and (c) including a matrix-free parallel multigrid solver to address generalized Poisson and Ampere operators. There is also an ongoing effort to port this code to MIC/GPU architecture. The new GBS code achieves excellent parallel scalability up to 8192 cores for a C-Mod size plasma (e.g. weak/strong scalings), while at the same time it is possible to achieve a fast and scalable solution of time dependent operators. As an example, we discuss two new physics capabilities of GBS allowed by numerical advances. The first is the inclusion of electromagnetic fluctuations at realistic plasma sizes, which is now possible due to the reduced numerical cost and improved parallel scalability. Second, the Boussinesq approximation in the evaluation of the divergence of the polarization drift is not justified in the tokamak scrape-off layer, where dn/n ~ 1. The drift-reduced equations have been rederived, and we have implemented and benchmarked a non-Boussinesq Poisson operator in GBS. Finally, we present the latest GBS simulation results using realistic size and parameters from inner-wall limited discharges, which is the chosen scenario for ITER start-up. Our simulations reveal the presence of steep plasma gradients near the last closed flux surface, which implies that a large portion of the exhaust heat can be deposited in a very small area. The turbulent dynamics recovered show low frequency, large amplitude drift and ballooning modes with a 1cm radial and poloidal correlation lengths. Intermittent transport events are present, leading to strongly skewed fluctuation probability distribution functions. The simulations have been benchmarked against measurements from the same experiments, showing very good agreement in many observables.

2015

Numerical simulations of gas puff imaging using a multi-component model of the neutral-plasma interaction in the tokamak boundary

Paolo Ricci

A three-dimensional simulation of gas puff imaging (GPI) diagnostics is carried out by using a self-consistent multi-component model of the neutral-plasma interaction. The simulation, based on the drift-reduced Braginskii model for the plasma and a kinetic model for the neutrals, is performed in a toroidally limited plasma with gas puff sources located at the low field side equatorial midplane. In addition to electrons, the simulation evolves the turbulent dynamics of D+ and D-2(+) ions as well as D and D-2 neutral species. The D-alpha emission arising from the excitation of D atoms and the contributions from dissociation of D-2 molecules and D-2+ ions are considered. The simulation points out the importance of considering D-alpha emission due to molecular dissociation in the far scrape-off layer (SOL), since it is the dominant source of D-alpha emission at distances from the gas puff considerably smaller than the mean free path of D-2 molecules. The correlation functions between the D-alpha emission rate and the plasma and neutral quantities, namely, the electron density, n(e), electron temperature, T-e, and density of neutral atoms, n(D), are evaluated considering each contribution to D alpha emission and analyzing the correlation functions between these quantities. The correlation functions strongly depend on the location considered within the edge and SOL with an important impact on the interpretation of GPI measurements. The statistical moments and the turbulence properties computed for different components of the D-alpha emission as well as for the relevant plasma and neutral quantities are also investigated. While neglecting neutral density fluctuations is a reasonable approximation that is widely used in the analysis of GPI measurements, this work reveals a 20%-30% influence of neutral fluctuations on most of the quantities measured through the GPI diagnostics with a possibly larger impact for some quantities in specific regions. These results, therefore, suggest the importance of considering neutral fluctuations for the accurate quantitative interpretations of GPI measurements. (C )2022 Author(s)