On How Avalanches Penetrate the SOL and Broaden Heat Loads

TL;DR

This paper develops a threshold criterion for heat avalanches penetrating the SOL in plasma, linking avalanche strength to heat load broadening and providing insights into experimental observations of heat transport phenomena.

Contribution

It introduces a reduced model that predicts avalanche penetration based on the gradient threshold, connecting avalanche strength to shock formation and heat load broadening.

Findings

Avalanches with gradients exceeding a critical value penetrate the SOL.

Penetration correlates with nonlinear steepening and shock formation.

Results align with experimental observations of heat load broadening.

Abstract

Recent experiments reported a correlation between power law core temperature spectra and $D_\alpha$ emission, suggesting that heat avalanches penetrate the SOL. This paper derives a threshold criterion for avalanche penetration using a reduced model. Avalanches with $(\nabla\tilde T)_{rms}>\nabla\tilde T_{crit}$ at the separatrix are predicted to penetrate, and so broaden the SOL and heat load distribution. $\nabla\tilde T_{crit}$ is $\sim 1/\tau_\parallel$, where $\tau_\parallel$ is the parallel heat flow time through the SOL. Penetration occurs when avalanches are strong enough to steepen sufficiently to shock at the separatrix. A positive correlation is found between the nonlinear drive for steepening and the penetration depth. In particular, penetration depth exceeds that of the heuristic drift limit when shocks form. Implications for numerical and physical experiments are also discussed.