On the role of density fluctuations in the core turbulent transport of Wendelstein 7-X

TL;DR

This study investigates how density fluctuations driven by ion temperature gradient (ITG) turbulence influence core plasma transport and confinement in the Wendelstein 7-X stellarator, confirming ITG's role in limiting ion temperature.

Contribution

It experimentally links core density fluctuation suppression to improved confinement and clarifies the role of ITG turbulence in W7-X plasma performance.

Findings

Suppression of turbulence reduces ion transport and improves confinement.

Reduction in collisionality alone does not change transport or confinement.

ITG turbulence interacts with TEM turbulence at low collisionality.

Abstract

A recent characterization of core turbulence carried out with a Doppler reflectometer in the optimized stellarator Wendelstein 7-X (W7-X) found that discharges achieving high ion temperatures at the core featured an ITG-like suppression of density fluctuations driven by a reduction of the gradient ratio $\eta_i = L_n/L_{T_i}$ [D. Carralero et al., Nucl. Fusion, 2021]. In order to confirm the role of ITG turbulence in this process, we set out to establish experimentally the relation between core density fluctuations, turbulent heat flux and global confinement. With this aim, we consider the scenarios found in the previous work and carry out power balance analysis for a number of representative ones, including some featuring high ion temperature. As well, we evaluate the global energy confinement time and discuss it in the context of the ISS04 inter-stellarator scaling. We find that, when turbulence is suppressed as a result of a reduction of $\eta_i$, there is a reduction of ion turbulent transport, and global performance is improved as a result. This is consistent with ITG turbulence limiting the ion temperature at the core of W7-X. In contrast, when turbulence is reduced following a decrease in collisionality, no changes are observed in transport or confinement. This could be explained by ITG modes being combined with TEM turbulence when the later is destabilized at low collisionalities.