New gyrokinetic simulations of plasmas from the National Spherical Tokamak Experiment (NSTX) have identified a previously uncharacterized form of electron-scale turbulence. The study, detailed in a preprint, reveals that drift waves driven by a negative density gradient (NDG) can cause electron thermal transport on the order of megawatts, a level consistent with experimental power flows. This finding provides a potential explanation for a portion of the anomalous electron heat loss observed in spherical tokamaks, a key challenge for optimizing the performance of these compact, high-beta devices. Source: arXiv plasm-ph

The dominant instability is characterized as a trapped-electron electron-scale tearing-parity mode. Its primary driver is a negative electron density gradient, where the normalized gradient $a/L_{ne}$ is less than zero. While the resulting thermal transport is mainly mediated by transverse magnetic fluctuations (δA∥), the mode's growth rate shows a distinct sensitivity to compressional magnetic fluctuations (δB∥). This characteristic differentiates it from conventional microtearing modes (MTMs), which are another known source of electron heat transport in high-beta plasmas. The analysis also confirms the trapped-electron nature of the mode, as it is most unstable at lower collisionality. Source: arXiv plasm-ph

The dominant instability is characterized as a trapped-electron electron-scale tearing-parity mode.

Nonlinear simulations, both at the electron-scale and across multiple scales, quantify the impact of this NDG-driven turbulence. The results show that these modes alone can account for several megawatts of experimental power flow. This is a significant fraction of the heating power in many NSTX discharges and suggests the mechanism is not a minor contributor but a primary channel for energy loss under these specific plasma conditions. The multi-scale simulations further indicate that the experimental plasma gradients in the analyzed NSTX shots may exist near a bifurcation point, where the plasma could transition between distinct turbulence regimes, potentially leading to rapid changes in transport. Source: arXiv plasm-ph

This work has direct implications for understanding and predicting plasma performance in current and future spherical tokamaks, including the upgraded NSTX-U and devices like STEP in the UK. Accurately modeling electron heat transport is critical for achieving high fusion gain, as this loss channel can significantly degrade energy confinement time. By identifying a specific, physically-grounded mechanism tied to the density profile, this research offers a new target for predictive transport models and potential control strategies. Future experimental work will be needed to validate these simulation results and search for the predicted turbulence signatures in high-performance spherical tokamak plasmas. Source: arXiv plasm-ph