Influence of temperature fluctuations on plasma turbulence investigations with Langmuir probes

Abstract

The reliability of Langmuir probe measurements for plasmaturbulence investigations is studied on GEMR gyro-fluid simulations and compared with the results from conditionally sampled I -V characteristics as well as electron-emitting probe measurements close to the last closed flux surface of the tokamak ASDEX Upgrade. In this region, simulation and experiment consistently show coherent in-phase fluctuations in density, plasma potential and also electron temperature. Ion-saturation current measurements turn out to reproduce density fluctuations quite well. Fluctuations in the floating potential, however, are strongly influenced by temperature fluctuations and, hence, are strongly distorted compared to the actual plasma potential. These results suggest that interpreting floating as plasma-potential fluctuations while disregarding temperature effects is not justified near the separatrix of hot fusion plasmas. Here, floating potential measurements led to corrupted results on the E × B dynamics of turbulent structures in the context of, e.g., turbulent particle and momentum transport or turbulence characterization on the basis of density-potential phase relations.

Turbulent transport across shear layers in magnetically confined plasmas

“…The typical edge/SOL transition is found to occur over 10-20 ρ s corresponding to roughly 1 cm into the SOL which is in accordance to the shear layer width [32]. In the present simulation as in recent observations in ASDEX Upgrade the region around the LCFS is drift-wave dominated [30]. This region is investigated in detail in the present contribution.…”

“…However, the deviation from the preset background values has to be small. Within this framework, experimental and modeling results have already been compared with reasonably good agreement [30,[33][34][35].…”

“…Simulations at ASDEX Upgrade relevant plasma parameters [30] have been carried out to show relevance of the observation for fusion devices. Simulations provide the unique possibility to observe all relevant quantities simultaneously.…”

“…More details on the present simulation parameters can be found in Ref. [30], because the same data is analyzed in the following. Also for the numerical data conditional average is used for the analysis, therefore blobs refer to high coherent density perturbations in respect to the surrounding fluctuation level.…”

Outer divertor of ASDEX Upgrade in low-density L-mode discharges in forward and reversed magnetic field: I. Comparison between measured plasma conditions and SOLPS5.0 code calculations

“…For instance, kinetic effects or secondary electron emission could lead to deviations from the assumed values of the sheath potential (V sh ∼ −3T e /e) [33]. Furthermore, temperature fluctuations, which have been measured close to the outer midplane separatrix in ASDEX Upgrade, could influence the measurements [34]. Both diagnostics and the code solution agree, however, with the shape of the E r profile, taking into account the radial uncertainty in the measurements, particularly regarding the probe E r .…”