Impact of magnetic islands on plasma flow and turbulence in W7-X

TL;DR

This study investigates how a 5/5 magnetic island at the edge of W7-X affects perpendicular plasma flow and turbulence using high-resolution Doppler reflectometry. The island-induced flow is localized, strongest at island boundaries, and nearly vanishes at the O-point, while density fluctuations are reduced inside the island, with flow-shear correlating with suppressed turbulence spreading. Comparisons with neoclassical $E\times B$ flow show good agreement outside the island but reveal island-driven contributions within the island, whose symmetry depends on the heating power. The findings corroborate gyrokinetic simulations and prior experiments, suggesting the island width remains below the threshold for vortex-like flows and highlighting flow shear as a key mechanism controlling turbulence and cross-field transport near islands.

Abstract

The effect of magnetic islands on plasma flow and turbulence has been experimentally investigated in the stellarator W7-X. Magnetic configurations with the 5/5 magnetic island positioned at the plasma edge, inside the last closed flux surface, are studied. The main diagnostic used in the present work is a V-band Doppler reflectometer that allows the measurement of the perpendicular plasma flow and density fluctuations with good spatial resolution. A characteristic signature of the 5/5 magnetic island is clearly detected in the perpendicular flow profile. The comparison of the experimental flow and the neoclassically driven $E\times B$ flow indicates that the island contribution to the flow is maximum at the island boundaries and close to zero at the island O-point. Besides, a reduction in the density fluctuation level is found nearby the island O-point. The similarities between these observations and those found in other devices and in gyrokinetic simulations are discussed.

Figures (6)

• Figure 1: Vacuum rotational transform profiles of three magnetic configurations: FQM, FOM and FMM (a), and the corresponding electron density and temperature profiles (b & c) measured by the Thomson scattering diagnostic; these profiles result from the fit to the experimental values whose dispersion is indicated by some representative error bars.
• Figure 2: Radial profiles of perpendicular plasma flow (in reddish colors) and density fluctuations (in blueish colors) measured by DR in three magnetic configurations: FQM (a), FOM (b) and FMM (c). Different symbols and color variations are used for the different time intervals within each experimental program. Vertical green lines indicate the DR measurement points closer to the island boundaries (dashed lines) and to the island O-point (dashed and dotted line) as obtained from the Poincaré plots. Refer to the text for more details.
• Figure 3: Poincaré plots calculated for the experimental programs shown in figure \ref{['f:fig_2']}, in the magnetic configurations FQM (left), FOM (center) and FMM (right). The 5/5 island chain is visible in the three configurations inside the LCFS (shown in green). The red and blue dots represent the DR measurements points.
• Figure 4: Radial profiles of density fluctuations (a) and flow-shearing (b) measured by DR in the three magnetic configurations; same experimental programs as shown in figure \ref{['f:fig_2']}.
• Figure 5: Radial profiles of perpendicular plasma flow (in reddish colors) and density fluctuations (in blueish colors) measured by DR in the FMM magnetic configuration with $P_{ECH} = 2$ MW. Different symbols and color variations are used for the different time intervals. Vertical green lines indicate the DR measurement points closer to the island boundaries (dashed lines) and to the island O-point (dashed and dotted line) as obtained from the Poincaré plots.