Abstract
Turbulent transport is widely considered to be the main driver for cross-field transport in the scrape-off layer (SOL) of toroidal magnetized plasmas. Here, reciprocating Langmuir probes are employed to measure both the plasma profiles and the turbulent particle transport in the SOL of the Wendelstein 7-X stellarator. The relation between turbulent radial particle flux Γr and the local pressure gradient is often approximately linear across the entire SOL width, indicating that radial turbulence spreading is absent. This observation holds across a wide range of magnetic configurations and different plasma heating and density scenarios. The magnitude of the turbulent transport for a given gradient reveals a dependence on the magnetic configuration and the position in the SOL, which we relate to the cross-spectral characteristics of multi-tip floating potential measurements. Magnetic islands can add further complexity due to non-monotonic SOL profiles and the breaking of the transport-gradient relation. Finally, anomalous diffusion coefficients are determined from the probe measurements.
Highlights
The width of the scrape-off layer (SOL) is a fundamental control parameter for the distribution of heat loads onto plasma-facing components in magnetic fusion devices [1, 2]
The magnitude of the turbulent transport for a given gradient reveals a dependence on the magnetic configuration and the position in the SOL, which we relate to the cross-spectral characteristics of multi-tip floating potential measurements
The indications for the local character of turbulence observed in the Wendelstein 7-X (W7-X) SOL [14] imply that the magnitude of turbulent transport is expected to scale with the radial plasma pressure gradient that drives the instabilities, i.e. Γr ∼ ∇p [8, 18]
Summary
The width of the scrape-off layer (SOL) is a fundamental control parameter for the distribution of heat loads onto plasma-facing components in magnetic fusion devices [1, 2]. The indications for the local character of turbulence observed in the W7-X SOL [14] imply that the magnitude of turbulent transport is expected to scale with the radial plasma pressure gradient that drives the instabilities, i.e. Γr ∼ ∇p [8, 18]. This hypothesis is tested using reciprocating Langmuir probes mounted on the multi-purpose manipulator (MPM) [19, 20] of W7-X.
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