Influence of temperature fluctuations on plasma turbulence investigations with Langmuir probes

TL;DR

This study assesses how temperature fluctuations affect the accuracy of Langmuir probe measurements in plasma turbulence, highlighting limitations in interpreting floating potential data near the tokamak separatrix.

Contribution

It compares simulation and experimental data to demonstrate the influence of temperature fluctuations on probe measurements and challenges common assumptions about floating potential interpretations.

Findings

Ion-saturation current accurately reflects density fluctuations

Floating potential is significantly distorted by temperature fluctuations

Interpreting floating potential as plasma potential can be misleading near the separatrix

Abstract

The reliability of Langmuir probe measurements for plasma-turbulence investigations is studied on GEMR gyro-fluid simulations and compared with results from conditionally sampled I-V characteristics as well as self-emitting probe measurements in the near scrape-off layer of the tokamak ASDEX Upgrade. In this region, simulation and experiment consistently show coherent in-phase fluctuations in density, plasma potential and also in 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 lead to corrupted results on the ExB dynamics of turbulent structures in the context of, e.g., turbulent particle and momentum transport or instability identification on the basis of density-potential phase relations.