Impact of edge turbulence spreading on broadening the heat flux width with plasma approaching the density limit

TL;DR

This study demonstrates that edge turbulence spreading significantly broadens the heat flux width in tokamak plasmas near the density limit, with blob-induced transport being a major contributor.

Contribution

It provides experimental evidence linking edge turbulence spreading and blob dynamics to heat flux width broadening near the density limit.

Findings

Edge turbulence spreading increases with normalized density.

Blob-induced transport accounts for about 81% of edge spreading in high-density scenarios.

Turbulence spreading at the separatrix is the main origin of SOL turbulence.

Abstract

This paper investigates the impact of edge turbulence spreading on broadening the heat flux width in Ohmic-plasma approaching the density limit of the J-TEXT tokamak. At the plasma edge, the EXB shear flow collapses while turbulence transport and spreading enhances significantly when approaching the density limit. The heat flux width increases with normalized density. An energy production ratio model is used to quantify the contribution of edge turbulence spreading to the origin of the SOL turbulence. Experimental data show that the energy production ratio is much larger than 1, indicating that turbulence spreading at separatrix is the origin of the SOL turbulence. The heat flux widths increase with edge turbulence spreading as well as the energy production ratio. The impact of blob-induced transport on the heat flux width is investigated in detail. Especially, the average blob-induced spreading is about 81% of the total edge spreading in the high-density scenario. Blobs with larger radial scales enhance edge spreading into the SOL, thus dominating the SOL turbulence and consequently broadening the heat flux width. These results suggest that edge turbulence spreading plays a crucial role in broadening the heat flux width as plasma approaches the density limit.