An experimental characterization of core turbulence regimes in Wendelstein 7-X

Abstract

First results from the optimized helias Wendelstein 7-X stellarator (W7-X) have shown that core transport is no longer mostly neoclassical, as is the case in previous kinds of stellarators. Instead, turbulent transport poses a serious limitation to the global performance of the machine. Several studies have found this particularly relevant for ion transport, with core ion temperatures becoming clamped at relatively low values of $T_i\simeq 1.7$ keV, except in the few scenarios in which turbulence can be suppressed. In order to understand turbulent mechanisms at play, it is important to have a clear understanding of the parametric dependencies of turbulent fluctuations, and the relation between them and turbulent transport. In this work we use Doppler reflectometry measurements carried out during a number of relevant operational scenarios to provide a systematic characterization of ion-scale ($k_{\perp }{\rho }_i\simeq 1$) density fluctuations in the core of W7-X. Then, we study the relation between fluctuation amplitude and plasma profiles and show how distinct regimes can be defined for the former, depending on normalized gradients $a/L_{ne}$ and $a/L_{Ti}$. Furthermore, we discuss the importance of other potentially relevant parameters such as $T_e/T_i$, $E_r$ or collisionality. Comparing the different regimes, we find that turbulence amplitude depends generally on the gradient ratio ${\eta }_i=L_{ne}/L_{Ti}$, as would be expected for ITG modes, with the exception of a range of discharges, for which turbulence suppression may be better explained by an ITG to TEM transition triggered by a drop in collisionality. Finally, we show a number of scenarios under which $T_{i,core}>1.7$ keV is achieved and how core fluctuations are suppressed in all of them, thus providing experimental evidence of microturbulence being the main responsible for the limited ion confinement in W7-X.

Manuscript submitted to Nuclear Fusion in May 2021

References in corpus (3)

• Spatial and Wavenumber Resolution of Doppler Reflectometry
• Radial electric field and density fluctuations measured by Doppler reflectometry during the post-pellet enhanced confinement phase in W7-X
• Nonlinear gyrokinetic PIC simulations in stellarators with the code EUTERPE

Cited by in corpus (11)

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• Critical gradient turbulence optimization toward a compact stellarator reactor concept
• Robust stellarator optimization via flat mirror magnetic fields
• Turbulent transport of impurities in 3D devices
• On the role of density fluctuations in the core turbulent transport of Wendelstein 7-X
• Direct optimization of neoclassical ion transport in stellarator reactors
• On the interaction between the island divertor heat fluxes, the scrape-off layer radial electric field and the edge turbulence in Wendelstein 7-X plasmas
• Zonal flow suppression of turbulent transport in the optimized stellarators W7-X and QSTK
• Fast simulations for large aspect ratio stellarators with the neoclassical code KNOSOS
• Microtearing Turbulence and Its Role in High-Density-Gradient Plasmas in Wendelstein 7-X