arXiv subjects

A. Stegmeir

Publications and source records attributed to A. Stegmeir.

4 recordsLinked to original sources

arXiv2025

Simulating X-point radiator turbulence

Coupling a high-performance burning plasma core to a detached boundary solution is critical for realizing magnetic confinement fusion power. Predictive simulations of the edge and scrape-off layer are therefore essential and must self-consistently account for turbulence and the interplay between the plasma, neutral gas, and impurities. We present results on controlled full detachment in ASDEX Upgrade with an X-point radiator (XPR), obtained with the edge turbulence code GRILLIX. Assuming a fixed nitrogen concentration (in terms of the electron density) in coronal equilibrium, two simulations are discussed: they exhibit dense nitrogen radiation fronts, located 5 and 12 cm above the X-point, accounting for 80 % of the input heating power. In validations against density, temperature, and bolometry measurements, the simulations show good agreement and reproduce the detached divertor conditions observed in the experiment. Neutral gas is critical for achieving detachment and modulating the height of the XPR front, in agreement with previous SOLPS-ITER transport modeling and analytical power balance studies. In addition, the front structure is highly dynamic due to turbulence, consisting of ionizing and radiative mantles surrounding intermittent cold spots of recombining plasma. Near the detachment front, density and temperature fluctuation amplitudes exceed the background by more than 400 %, compared to 40 % in an attached reference case. The radial electric field shifts inward, poloidal symmetry of the electrostatic potential is broken (inducing strong radial flows around the XPR), and radial particle and heat transport into the low-field side scrape-off layer increases. These effects may explain the ELM suppression observed in the H-mode XPR regime.

physics.plasm-ph↗

arXiv2025

Simulations of edge and SOL turbulence in diverted negative and positive triangularity plasmas

Optimizing the performance of magnetic confinement fusion devices is critical to achieving an attractive fusion reactor design. Negative triangularity (NT) scenarios have been shown to achieve excellent levels of energy confinement, while avoiding edge localized modes (ELMs). Modeling turbulent transport in the edge and SOL is key in understanding the impact of NT on turbulence and extrapolating the results to future devices and regimes. Previous gyrokinetic turbulence studies have reported beneficial effects of NT across a broad range of parameters. However, most simulations have focused on the inner plasma region, neglecting the impact of NT on the outermost edge. In this work, we investigate the effect of NT in edge and scrape-off layer (SOL) simulations, including the magnetic X-point and separatrix. For the first time, we employ a multi-fidelity approach, combining global, non-linear gyrokinetic simulations with drift-reduced fluid simulations, to gain a deeper understanding of the underlying physics at play. First-principles simulations using the GENE-X code demonstrate that in comparable NT and PT geometries, similar profiles are achieved, while the turbulent heat flux is reduced by more than 50% in NT. Comparisons with results from the drift-reduced fluid turbulence code GRILLIX suggest that the turbulence is driven by trapped electron modes (TEMs). The parallel heat flux width on the divertor targets is reduced in NT, primarily due to a lower spreading factor $S$.

physics.plasm-ph↗

arXiv2021

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 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 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 -- giving a total of 45 1- and 2-D comparison observables in two toroidal magnetic field directions. The 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, 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 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, this work should help to guide and accelerate the development of predictive turbulence simulations of the edge and SOL.

physics.plasm-ph↗

arXiv2019

Global turbulence simulations of the tokamak edge region with GRILLIX

Turbulent dynamics in the scrape-off layer (SOL) of magnetic fusion devices is intermittent with large fluctuations in density and pressure. Therefore, a model is required that allows perturbations of similar or even larger magnitude to the time-averaged background value. The fluid-turbulence code GRILLIX is extended to such a global model, which consistently accounts for large variation in plasma parameters. Derived from the drift reduced Braginskii equations, the new GRILLIX model includes electromagnetic and electron-thermal dynamics, retains global parametric dependencies and the Boussinesq approximation is not applied. The penalisation technique is combined with the flux-coordinate independent (FCI) approach [F. Hariri and M. Ottaviani, Comput.Phys.Commun. 184:2419, (2013); A. Stegmeir et al., Comput.Phys.Commun. 198:139, (2016)], which allows to study realistic diverted geometries with X-point(s) and general boundary contours. We characterise results from turbulence simulations and investigate the effect of geometry by comparing simulations in circular geometry with toroidal limiter against realistic diverted geometry at otherwise comparable parameters. Turbulence is found to be intermittent with relative fluctuation levels of up to 40% showing that a global description is indeed important. At the same time via direct comparison, we find that the Boussinesq approximation has only a small quantitative impact in a turbulent environment. In comparison to circular geometry the fluctuations are reduced in diverted geometry, which is related to a different zonal flow structure. Moreover, the fluctuation level has a more complex spatial distribution in diverted geometry. Due to local magnetic shear, which differs fundamentally in circular and diverted geometry, turbulent structures become strongly distorted in the perpendicular direction and are eventually damped away towards the X-point.

physics.plasm-ph↗