A new preprint details a gyrokinetic investigation into electron-scale core transport within the Mega Ampere Spherical Tokamak Upgrade (MAST-U), identifying electron temperature gradient (ETG) driven modes as a primary cause of turbulent electron heat loss. The study, based on integrated modeling and local gyrokinetic analysis, examined two L-mode discharges and one H-mode plasma. Researchers found that in certain operating regimes, particularly in the outer core of L-mode plasmas, ETG-driven turbulence can account for experimentally observed levels of heat transport, a critical factor for plasma confinement and performance in compact spherical tokamaks. This work provides a quantitative basis for understanding a transport channel that can dominate over ion-scale turbulence in these devices. Source: arXiv
The analysis focused on distinguishing between two candidate instabilities responsible for high electron heat transport: microtearing modes (MTMs) and ETG modes. While both can be active, the study found ETG modes to be linearly unstable across a broad radial region in the L-mode cases. These small-scale, high-frequency instabilities are driven by a steep gradient in the electron temperature profile. In spherical tokamaks like MAST Upgrade, which operate at lower magnetic fields and have distinct plasma geometry compared to conventional tokamaks, understanding the dominant microinstabilities is essential for developing predictive models and optimizing confinement scenarios for future power-plant concepts.
The analysis focused on distinguishing between two candidate instabilities responsible for high electron heat transport: microtearing modes (MTMs) and ETG modes.
Nonlinear gyrokinetic simulations confirmed the sensitivity of ETG-driven transport to key plasma parameters. The electron heat flux showed a strong, 'stiff' response to increases in the electron temperature gradient, meaning transport rapidly increases above a critical threshold. This behavior aligns with experimental observations. Furthermore, the transport was significantly suppressed by the E×B shearing rate, a velocity shear in the plasma that can tear apart turbulent eddies. The model's predictions for heat transport levels showed good agreement with experimental power balance analysis within the associated uncertainties, validating the physical model for the outer core region of L-mode plasmas. Source: arXiv
Despite the model's success in the outer core, the preprint notes that ETG-driven transport alone appears insufficient to explain the full extent of electron heat loss observed closer to the plasma's magnetic axis. This discrepancy suggests that other transport mechanisms, potentially including MTMs or other instabilities not fully captured in this local analysis, may contribute significantly to transport in the deep core. The findings highlight the complex, multi-scale nature of plasma turbulence and the need for comprehensive models that can simultaneously treat different instabilities across various regions of the plasma. This is a common challenge in fusion science and an active area of research.
This detailed characterization of electron-scale turbulence provides crucial inputs for integrated modeling of MAST-U and informs strategies for future experiments. By understanding the specific gradients and shearing rates that trigger or suppress ETG modes, operators can develop scenarios to mitigate this significant heat loss channel. The results contribute to a more fundamental understanding of transport physics in the spherical tokamak configuration, which is being pursued by several private companies. Validated models of core turbulence are a prerequisite for confidently extrapolating performance from current experiments to future, higher-power devices aiming for net energy gain. Source: arXiv