Observation of quasi-coherent density fluctuation in scrape-off layer enhancing boundary transport in high-betaN hybrid plasmas on DIII-D

TL;DR

This paper reports the detection of a quasi-coherent density fluctuation in the scrape-off layer of DIII-D tokamak plasmas, which enhances boundary transport during high-performance hybrid discharges, supported by experimental and simulation data.

Contribution

It provides the first observation of a long-wavelength ion-scale fluctuation in the SOL linked to increased transport, with linear modeling supporting the experimental findings.

Findings

QCF observed in high-power hybrid plasmas with double-null divertor.

QCF propagates in the ion diamagnetic direction with a radial extent of 1.5-2 cm.

Simulation reproduces the density perturbation and its characteristics.

Abstract

We report the observation of a quasi-coherent density fluctuation (QCF) by the Doppler backscattering system in the scrape-off layer (SOL) region of the DIII-D tokamak. This QCF is observed in high-power, high-performance hybrid plasmas with near double-null divertor (DND) shape during the electron cyclotron heating period. This mode is correlated with a steepened SOL density profile and leads to significantly elevated particle and heat fluxes between ELMs. The SOL QCF is a long-wavelength ion-scale fluctuation and propagates in the ion diamagnetic direction in the plasma frame. Its radial expanse is about 1.5-2 cm, well beyond the typical width of heat flux on DIII-D. Also, the SOL QCF does not show any clear dependence on the effective SOL collisionality and thus may raise issues on the control of plasma-material interactions in low collisionality plasmas in which the blob-induced transport is reduced. A linear simulation using BOUT++ with a 5-field reduced model is performed and compared with experimental observations. In simulation results, an interchange-like density perturbation can be driven by the SOL density gradient, and its peak location and the radial width of the density perturbation are in agreement with the experimental observations.