$T_i/T_e$ Dependence of Core Turbulence and Transport in DIII-D QH-Mode Plasmas

This study investigates the effect of the ion-to-electron temperature ratio ($T_i/T_e$) on microturbulence driven transport in Quiescent H-mode (QH-mode) plasmas in the DIII-D tokamak. Utilizing the Gyrokinetic Toroidal Code (GTC) and the QH-mode equilibrium, we perform linear and nonlinear simulations to analyze transport properties and instability dynamics under variations of $T_i$ and $T_e$. Our results demonstrate that decreasing $T_i/T_e$ leads to a relative destabilization of trapped electron modes (TEM) over ion temperature gradient (ITG) modes, with the transition between these regimes dictated by $T_i/T_e$. When the electron temperature is increased at fixed ion temperature, we observe an increase in transport saturation levels. In contrast, decreasing the ion temperature at fixed electron temperature results in more modest transport enhancement. The radial correlation length, which characterizes eddy size, increases with rising $T_e$ and decreases with falling $T_i$, consistent with the observed trends in turbulent transport. Additionally, we examine the impact of impurity addition on turbulence and growth rates, finding that impurity presence does not significantly alter transport quantities compared to the impurity-free case. Finally, investigating helium as an alternative main ion species, we find that helium plasmas exhibit higher linear growth rates but result in lower transport saturation levels than deuterium plasmas, suggesting potential confinement benefits. These findings provide quantitative insights into the temperature ratio dependence in QH-mode plasmas and highlight the role of temperature profiles and zonal flows in influencing plasma confinement.

💡 Research Summary

This paper presents a comprehensive investigation into the influence of the ion-to-electron temperature ratio (T_i/T_e) on microturbulence-driven core transport in Quiescent H-mode (QH-mode) plasmas within the DIII-D tokamak. QH-mode is a steady, Edge Localized Mode (ELM)-free regime of interest for future fusion reactors like ITER, where understanding transport physics is critical for performance projection.

The study employs the global, nonlinear gyrokinetic code GTC (Gyrokinetic Toroidal Code) to perform first-principles simulations based on experimental equilibria from a DIII-D QH-mode discharge (#157102). The GTC model utilizes a fluid-kinetic hybrid electron treatment and a delta-f method to efficiently simulate collisionless microtusturbulence, including the effects of zonal flows.

The core analysis systematically varies T_i/T_e through two primary pathways: (1) increasing the electron temperature (T_e) while holding the ion temperature (T_i) fixed, and (2) decreasing T_i while holding T_e fixed. Both approaches decrease T_i/T_e. Key findings reveal that a lower T_i/T_e ratio leads to the relative destabilization of Trapped Electron Modes (TEM) over Ion Temperature Gradient (ITG) modes, with the transition between these dominant turbulence regimes being governed by this ratio. However, the impact on saturated transport levels is asymmetric. Notably, increasing T_e at fixed T_i causes a significant enhancement in transport saturation levels, whereas decreasing T_i at fixed T_e results in a more modest increase in transport. This asymmetry is linked to changes in the turbulent eddy size, characterized by the radial correlation length, which increases with rising T_e and decreases with falling T_i.

Furthermore, the research examines the role of impurities (carbon) and finds that their presence does not significantly alter linear growth rates or nonlinear transport levels compared to impurity-free cases, even when zonal flows are included. This suggests a degree of resilience in QH-mode transport to impurity contamination.

Finally, the paper explores the consequences of changing the main ion species from deuterium to helium, relevant for non-nuclear operational phases of ITER. Simulations indicate that helium plasmas exhibit higher linear growth rates than their deuterium counterparts. Intriguingly, despite this faster linear instability growth, nonlinear simulations show that helium plasmas saturate at lower transport levels. This points to potential confinement benefits in helium QH-mode scenarios, likely due to differences in nonlinear saturation mechanisms, such as zonal flow dynamics.

In conclusion, this work provides quantitative, physics-based insights into how T_i/T_e, manipulated through different heating schemes, critically affects core turbulence and transport in ELM-free QH-mode plasmas. The results underscore the importance of temperature profile control for optimizing confinement and offer valuable guidance for the operational planning of next-step fusion devices, highlighting both challenges (e.g., strong transport increase with electron heating) and opportunities (e.g., potential benefits of helium operation).