Three-dimensional boundary turbulence simulations of a RFX-mod plasma in the presence of voltage biasing

TL;DR

This paper uses 3D turbulence simulations to show how voltage biasing suppresses turbulence and enhances the edge transport barrier in RFX-mod plasmas, potentially increasing maximum density limits.

Contribution

It extends the theoretical scaling law of edge pressure gradient length to include effects of flow shear turbulence suppression from voltage biasing.

Findings

Suppression of turbulent transport by E×B flow shear.

Formation of an edge transport barrier with pedestal-like structure.

Predicted doubling of pressure gradient at the separatrix with voltage biasing.

Abstract

Three-dimensional turbulence simulations of a RFX-mod diverted plasma are performed in the presence of a biasing electrode. The simulations show a strong suppression of turbulent transport caused by the induced $\mathbf{E}\times\mathbf{B}$ flow shear, which leads to the formation of an edge transport barrier with a pedestal-like structure, in qualitative agreement with RFX-mod experiments. The strong $\mathbf{E}\times\mathbf{B}$ flow shear turbulence suppression with edge voltage biasing is also observed in the proximity of the density limit crossing, suggesting that edge voltage biasing may allow for larger maximum achievable density values. By leveraging the simulation results, the theoretical scaling law of the edge pressure gradient length derived in Giacomin & Ricci (2020) J. Plasma Phys. 86(5) is extended here to account for the $\mathbf{E}\times\mathbf{B}$ flow shear turbulence suppression caused by voltage biasing. The improved theoretical scaling with typical RFX-mod shearing rate values predicts a factor of two increase of the pressure gradient at the separatrix, which is comparable to RFX-mod experiments in the presence of voltage biasing. The implications of the flow shear turbulence suppression due to voltage biasing on the density limit in RFX-mod are also discussed.