arXiv subjects

G. McKee

Publications and source records attributed to G. McKee.

3 recordsLinked to original sources

arXiv2026

Predicting Negative Triangularity Edge Confinement with a Neutral-Coupled Full-f Gyrokinetic Model

Global full-f gyrokinetic simulations coupled to a reduced neutral-fluid model reproduce key DIII-D experimental trends and identify mechanisms underlying improved confinement in negative triangularity (NT) configurations relative to positive triangularity (PT). Using Gkeyll, we simulate DIII-D inner-wall-limited discharges across the confined edge, scrape-off layer (SOL), and sheath boundaries, allowing the edge electric field to evolve self-consistently. Compared with an adaptive recycling-source model, the reduced neutral fluid model improves particle and energy balances and brings the simulated density and temperature profiles into closer agreement with measurements, with minimal additional computational cost. In NT, suppressed turbulent fluxes coincide with a broad, stationary zonal flow inside the last closed flux surface, whereas PT develops transient, fragmented shear layers. Spectral analysis indicates that modified fluctuation phase relationships and enhanced zonal shear both contribute to transport suppression despite broadly comparable fluctuation amplitudes. The larger co-current departure from a locally matched bi-Maxwellian in NT suggests a velocity-selective kinetic process that may help establish the edge radial electric field by modifying flows or current balance. These results identify kinetic and turbulent mechanisms associated with improved NT confinement within an experimentally established ELM-free regime and demonstrate the importance of coupling gyrokinetic plasma dynamics with neutral interactions across the confined plasma and SOL.

physics.plasm-ph↗

arXiv2026

Dependence of Momentum Transport on the Dominant Turbulence Regime in the DIII-D Tokamak

Accurate prediction of toroidal plasma rotation is essential for optimizing confinement and stability in future fusion devices. This work investigates turbulent core momentum transport in the DIII-D tokamak across a transition from ion-temperature-gradient (ITG)- to trapped-electron-mode (TEM)-dominated turbulence. A momentum transport framework previously developed for ASDEX Upgrade is applied to modulated neutral beam injection experiments, separating diffusive, convective, and residual-stress contributions via Fourier analysis of the rotation response. The dataset spans low-rotation conditions, dominant electron heating, and background ExB shearing rates below turbulence growth rates, accessing more reactor-relevant conditions. Gyrokinetic CGYRO and gyrofluid TGLF calculations confirm the scan covers an ITG-to-TEM transition. The analysis yields Prandtl numbers near unity. The pinch number shows no explicit dependence on the transition, instead ordering roughly with the logarithmic density gradient. The normalized residual stress, in contrast, exhibits a non-monotonic, V-shaped dependence across the transition: co-current in deep ITG and deep TEM regimes, near-zero or counter-current in the intermediate mixed-mode regime. This trend collapses onto an approximately linear dependence against electron kinetic profile gradients, suggesting residual stress generation by profile-shearing effects. Weaker background ExB shearing further shifts residual stress toward counter-current values. Linear CGYRO simulations for representative ITG and TEM discharges yield Prandtl and pinch numbers in good agreement with experiment, supporting gyrokinetic momentum-transport predictions in TEM-dominated regimes. These results indicate residual stress plays an important role in core rotation prediction for low-torque plasmas and should be included in predictive models of future reactor scenarios.

physics.plasm-ph↗

arXiv2026

Effects of Tungsten Radiative Cooling on Impurity, Heat and Momentum Transport in DIII-D Plasmas

A first-of-its-kind experiment was conducted in the DIII-D tokamak under WEST similarity constraints on plasma shape and core parameters. This work presents a detailed transport study comparing a reference regime dominated by intrinsic carbon radiation and a high-radiation regime resulting from controlled tungsten (W) injection using the Laser Blow-Off system, with a core tungsten concentration $n_{\mathrm{W}}/n_e \sim 3\times 10^{-4}$ and a radiated-power fraction $f_\mathrm{rad}>0.5$. The W-induced radiative cooling lowered the electron temperature, thereby decreasing $T_e/T_i$ and stabilizing trapped-electron-mode (TEM) turbulence. This transition in turbulence regime reduced momentum and ion thermal diffusivities, yielding ion temperature peaking and a factor-of-two increase in toroidal rotation. At the outer plasma region, enhanced $E\timesB$ shear and increased collisionality further suppressed ion-scale turbulence, causing a sharp drop in ion heat flux. Consequently, impurity transport, predominantly turbulent in the low-radiation regime, acquired a strong neoclassical inward W convection during radiative cooling, bootstrapping the cooling cycle. Despite $f_\mathrm{rad}>0.5$, radiative collapse was not observed, likely owing to collisional ion-to-electron energy exchange acting as an electron-energy reservoir, together with $1/1$ MHD activity modulating the radiated power through core impurity neoclassical $T_i$-screening. These results support preparation for a tungsten wall change in DIII-D by elucidating tungsten-induced turbulence stabilization. They also provide key insights for interpreting plasma performance in WEST and are relevant to future reactors expected to operate with radiating tungsten-walled plasmas.

physics.plasm-ph↗