arXiv subjects

W. Zholobenko

Publications and source records attributed to W. Zholobenko.

8 recordsLinked to original sources

arXiv2026

First-Principles Explanation of the Drift Configuration Dependence of the Radial Electric Field and High-Confinement Access in Tokamaks

The origin of the difference in the high-confinement (H-mode) power threshold between favorable and unfavorable drift configurations in tokamaks, experimentally linked to a deeper radial electric field (Er) well in the former, remains unresolved. Using first-principles gyrokinetic simulations of edge and scrape-off-layer turbulence in ASDEX Upgrade, we show that turbulence-driven poloidal flows generate this deeper Er well in the favorable configuration through enhanced nonlinear turbulence-mean flow energy transfer. This transfer is significantly weaker in the unfavorable case, yielding a shallower Er well, while turbulence intensity is simultaneously higher. Within the turbulence-flow shear suppression paradigm, the combination of stronger shear and reduced turbulence facilitates H-mode access in the favorable configuration. These results provide the first validated, self-consistent full-f gyrokinetic explanation of how drift configuration controls the nonlinear dynamics of profiles, Er, flows, and turbulence, thereby setting the H-mode power threshold.

physics.plasm-ph↗

arXiv2026

Pre L-H Transition Radial Electric Field and Transport Validations of Edge and Scrape-off Layer Gyrokinetic Simulations at ASDEX Upgrade

This work presents a stepwise validation of the evolution of the radial electric field (Er) and heat transport during the pre L-H transition phase using full-f gyrokinetic simulations of the edge and scrape-off layer in the ASDEX Upgrade (AUG) tokamak, including X-point geometry. Several L-mode time slices up to the L-H transition from a dedicated hydrogen discharge, featuring stepwise increases in ECRH input power, are selected [N. Bonanomi \textit{et al.}, Phys. Plasmas 31, 072302 (2024)] and simulated with the \texttt{GENE-X} code. As the edge boundary conditions are progressively increased between the time slices, particle and heat fluxes rise, and the radial electric field Er well deepens. A detailed validation of the Er profiles and of the Er well depth shows excellent agreement with experimental measurements at the successive time slices approaching the L-H transition. A force balance decomposition identifies turbulence-driven poloidal flows as the dominant contribution within the Er well. Edge turbulence is governed by a competition between electron drift waves and trapped-electron modes. The introduction of an edge density source, modeling neutral gas ionization, is shown to be essential to reproduce experimentally relevant density profiles, Er, and edge ion heat fluxes, which are dominated by both turbulent and diamagnetic contributions. This stepwise validation constitutes an important milestone toward predictive, first-principles gyrokinetic simulations of the L-H transition power threshold.

physics.plasm-ph↗

arXiv2025

Turbulence and Transport in Spectrally Accelerated full-f Gyrokinetic Simulations

We investigate edge and scrape-off layer (SOL) turbulence and transport using the spectrally accelerated full-f gyrokinetic (GK) code GENE-X, recently introduced in [B. J. Frei et al., arXiv:2411.09232 (2024)]. Extending previous work on the TCV-X21 scenario, we show that the velocity-space spectral approach not only reproduces outboard midplane profiles but also captures key features of trapped electron mode (TEM)-driven turbulence and transport, including fluctuation spectra, turbulent fluxes, phase shifts, and power crossing the separatrix, in close agreement with grid-based results. This agreement remains robust when increasing spectral resolutions. We further analyze the radial force balance (accurately satisfied) and the structure of the radial electric fields and poloidal flows in the edge and SOL. Finally, we contrast our results with Braginskii-like fluid models, which inherently neglect TEMs. These results confirm the spectral full-f GENE-X approach as an efficient and first-principles tool for predicting edge and SOL turbulence.

physics.plasm-ph↗

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↗

arXiv2023

Reduced model for H-mode sustainment in unfavorable $\mathbf{ \nabla B}$ drift configuration in ASDEX Upgrade

A recently developed reduced model of H-mode sustainment based on interchange-drift-Alfv\'en turbulence description in the vicinity of the separatrix matching experimental observations in ASDEX Upgrade has been extended to experiments with the unfavorable $\nabla B$ drift. The combination with the theory of the magnetic-shear-induced Reynolds stress offers a possibility to quantitatively explain the phenomena. The extension of the Reynolds stress estimate in the reduced model via the magnetic shear contribution is able to reproduce the strong asymmetry in the access conditions depending on the ion $\nabla B$ drift orientation in agreement with experimental observations. The Reynolds stress profile asymmetry predicted by the magnetic shear model is further extended by comparison with GRILLIX and GENE-X simulations matched with comparable experiments in realistic X-point geometry. The predictions of the radial electric field well depth and its difference between the favorable and unfavorable configurations at the same heating power from the extended model also show consistency with experimental measurements.

physics.plasm-ph↗

arXiv2022

Deep modelling of plasma and neutral fluctuations from gas puff turbulence imaging

The role of turbulence in setting boundary plasma conditions is presently a key uncertainty in projecting to fusion energy reactors. To robustly diagnose edge turbulence, we develop and demonstrate a technique to translate brightness measurements of HeI line radiation into local plasma fluctuations via a novel integrated deep learning framework that combines neutral transport physics and collisional radiative theory for the $3^3 D - 2^3 P$ transition in atomic helium. The tenets for experimental validity are reviewed, illustrating that this turbulence analysis for ionized gases is transferable to both magnetized and unmagnetized environments with arbitrary geometries. Based upon fast camera data on the Alcator C-Mod tokamak, we present the first 2-dimensional time-dependent experimental measurements of the turbulent electron density, electron temperature, and neutral density revealing shadowing effects in a fusion plasma using a single spectral line.

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↗