Turbulent Nature of the Quasicontinuous Exhaust Regime for Fusion Plasmas

TL;DR

This paper reveals how a kinetic ballooning mode causes oscillations and blob ejections in fusion plasma simulations, offering insights into heat exhaust mechanisms that reconcile high confinement with effective heat removal.

Contribution

It introduces a simulation-based mechanism involving a quasi-coherent mode and blob ejections that explain heat exhaust in fusion plasmas, aligning with experimental observations.

Findings

QCM causes pedestal oscillations and blob ejections

Blob ejections are triggered by resistivity and X-point interactions

SOL temperature fall-off is decoupled from pedestal gradients

Abstract

We demonstrate a mechanism for reconciling high confinement with heat exhaust in fusion plasmas. Global fluid turbulence simulations of the Quasicontinuous Exhaust regime in the ASDEX Upgrade tokamak show that a quasi-coherent mode (QCM) causes the pedestal foot to oscillate across the separatrix and eject ballistic blobs into the scrape-off layer (SOL), reproducing not only mean profiles but also fluctuation spectra and mode structure seen in experiments. The QCM is a kinetic ballooning mode that develops an extended radial correlation length via electromagnetic self-organization of turbulence, thereby driving enhanced transport, with Maxwell stress and finite Larmor radius effects mediating the process. The blobs are launched when resistivity excites a secondary mode that originates from the X-point and interacts with QCM. The blob-dominated SOL temperature fall-off is then well decoupled from the pedestal-foot gradient set by the QCM.