Abstract
The formation of the radial electric field,
E
r
in the scrape-off layer (SOL) has been experimentally studied for attached divertor conditions in stellarator W7-X. The main objective of this ...study is to test the validity in a complex three-dimensional (3D) island divertor of simple models, typically developed in tokamaks, relating
E
r
in the SOL to the sheath potential drop gradient at the target. Additionally, we investigate the effect of the edge
E
r
shear on the reduction of density fluctuation amplitude, a well-established phenomenon according to the existing bibliography. The main diagnostic for measurements in the SOL is a V-band Doppler reflectometer that can provide the measurement of the
E
r
and density fluctuations with good spatial resolution. 3D measurements of divertor parameters have been carried out using infrared cameras, with the exponential decay length of the divertor heat flux (
λ
q
) resulting in a suitable proxy for the model-relevant
λ
T
, the exponential decay length of the temperature at the divertor. In the investigated attached regimes, it is shown for the first time that the formation of the
E
r
in the SOL depends on parameters at the divertor, following a
E
r
∝
T
e
/
λ
q
qualitatively similar to that found in a tokamak. Then, from the analyzed plasmas, the observed
E
r
shear at the edge is linked to a moderate local reduction of the amplitude of density fluctuations.
Abstract
In the present Letter, first-of-its-kind computer simulations predicting plasma profiles for modern optimized stellarators—while self-consistently retaining neoclassical transport, turbulent ...transport with 3D effects, and external physical sources—are presented. These simulations exploit a newly developed coupling framework involving the global gyrokinetic turbulence code GENE-3D, the neoclassical transport code KNOSOS, and the 1D transport solver TANGO. This framework is used to analyze the recently observed degradation of energy confinement in electron-heated plasmas in the Wendelstein 7-X stellarator, where the central ion temperature was ‘clamped’ to
T
i
≈
1.5
keV regardless of the external heating power. By performing first-principles based simulations, we provide key evidence to understand this effect, namely the inefficient thermal coupling between electrons and ions in a turbulence-dominated regime, which is exacerbated by the large
T
e
/
T
i
ratios, and show that a more efficient ion heat source, such as direct ion heating, will increase the on-axis ion temperature. This work paves the way towards the use of high-fidelity models for the development of the next generation of stellarators, in which neoclassical and turbulent transport are optimized simultaneously.
This paper presents results of experimental investigations about the spatiotemporal evolution of turbulent structures in weakly developed turbulence in the linearly magnetized helicon discharge ...VINETA. On the basis of the dispersion behavior the governing instability has been identified as the drift wave instability. A key quantity of drift waves are the associated parallel currents, which are measured with high sensitive B-probes. The fluctuating parallel currents are correlated with the drift wave density fluctuations. Drift waves are driven into a weakly developed turbulent state, which is characterized by strong intermittency of plasma density fluctuations in the far plasma edge. Spatiotemporal measurements revealed that this intermittency can be ascribed to radially propagating turbulent structures, which form in the region of large radial plasma pressure gradient and propagate radially outwards. The radial propagation speeds are typically 10% of the local ion sound speed. Measurements of the parallel electron dynamics associated with the radially propagating structures strongly suggest that the parallel electron flux contributes significantly to the polarization of turbulent structures.
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