Towards fully predictive gyrokinetic full-f simulations

TL;DR

This work addresses predictive modeling of edge and scrape-off layer turbulence in tokamaks by implementing full-f gyrokinetic simulations that rely only on magnetic geometry, heating power, and particle inventory. The authors introduce an adaptive sourcing scheme in the Gkeyll code to inject energy and mimic neutral recycling without net particle addition, enabling self-consistent evolution of turbulence and profiles. Applying the framework to TCV discharges PT and NT, the simulations reproduce key features such as blob transport and self-generated E×B shear, and reveal NT-related increases in edge shear and core density consistent with proposed confinement mechanisms. The results demonstrate the feasibility of GK-based predictive edge/SOL modeling for reactor-scale design studies, while outlining future improvements through enhanced neutral physics, electromagnetic effects, and broader validation across devices and regimes.

Abstract

Designing economical magnetic confinement fusion power plants motivates computational tools that can estimate plasma behavior from engineering parameters without direct reliance on experimental measurement of the plasma profiles. In this work, we present full-$f$ global gyrokinetic (GK) turbulence simulations of edge and scrape-off layer turbulence in tokamaks that use only magnetic geometry, heating power, and particle inventory as inputs. Unlike many modeling approaches that employ free parameters fitted to experimental data, raising uncertainties when extrapolating to reactor scales, his approach directly simulates turbulence and resulting profiles through GK without such empirical adjustments. This is achieved via an adaptive sourcing algorithm in Gkeyll that strictly controls energy injection and emulates particle sourcing due to neutral recycling. We show that the simulated kinetic profiles compare reasonably well with Thomson scattering and Langmuir probe data for Tokamak à Configuration Variable (TCV) discharge #65125, and that the simulations reproduce characteristic features such as blob transport and self-organized electric fields. Applying the same framework to study triangularity effects suggests mechanisms contributing to the improved confinement reported for negative triangularity (NT). Simulations of TCV discharges #65125 and #65130 indicate that NT increases the $E \times B$ flow shear (by about 20% in these cases), which correlates with reduced turbulent losses and a modest change in the distribution of power exhaust to the vessel wall. While the physical models contain approximations that can be refined in future work, the predictive capability demonstrated here, evolving multiple profile relaxation times with kinetic electron and ion models in hundreds of GPU hours, indicates the feasibility of using Gkeyll to support design studies of fusion devices.

Figures (10)

• Figure 1: TCV magnetic equilibrium for PT (a and b) and NT (c and d) discharges. Panels (a) and (c) show poloidal cross-sections of the TCV vessel with poloidal flux contours (color scale), vessel wall (solid black line), LCFS (solid white line), and magnetic axis (white "x"). Miller equilibrium parameters used in Gkeyll are overlaid in red: LCFS (dashed line), maximal and minimal radii (dotted lines), and shifted axis ("x"). Panels (b) and (d) show the reconstructed safety factor profile (solid blue line) and the cubic fit used in simulations (dashed green line). We also report the radial limits of the simulation domain (dotted red lines).
• Figure 2: Setup of the adaptive sources. The left panel illustrates the particle fluxes (solid arrows), energy fluxes (hollow arrows) and feedback loops for adjusting source parameters based on measured losses. The right panel illustrates the positions of the core and recycling sources, with the core source centered at the inner radial boundary and the recycling source near the limiter.
• Figure 3: Comparison between the Thompson scattering (TS, $\times$) and Langmuir probe (LP, $\bullet$) electron measurements of TCV #65125 discharge (PT) with the steady-state density (a), and temperature (b) for electrons (solid) and ions (dashed) obtained with Gkeyll for different resolutions. The Gkeyll profiles are averaged over the last 200 $\mu$s of the simulation and taken at the OMP ($z/\pi = 0$), averaged over the binormal direction. We also display the total source profile in panel (a) where the core source is taken at OMP and the recycling source at the limiter position for comparison purposes.
• Figure 4: Evolution of the total heat flux through the SOL in Gkeyll PT simulation (blue), and contributions from the wall (orange) and the limiter (green). The experimental SOL power is shown in gray. Gkeyll heat fluxes are here scaled by the inverse of the volume fraction of the flux tube to compare with the experimental input power (see Sec. \ref{['sec:source_setup']} for details).
• Figure 5: Space-time evolution of radial profiles cuts at the OMP ($z=0$) and $y=0$: electron density (a), electrostatic potential (b), electron temperature (c), and ion temperature (d).