Influence of temperature fluctuations on plasma turbulence investigations with Langmuir probes

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

Fluid structure in the immediate vicinity of an equilibrium three-phase contact line and assessment of disjoining pressure models using density functional theory

We examine the nanoscale behavior of an equilibrium three-phase contact line in the presence of long-ranged intermolecular forces by employing a statistical mechanics of fluids approach, namely density functional theory (DFT) together with fundamental measure theory (FMT). This enables us to evaluate the predictive quality of effective Hamiltonian models in the vicinity of the contact line. In particular, we compare the results for mean field effective Hamiltonians with disjoining pressures defined through (I) the adsorption isotherm for a planar liquid film, and (II) the normal force balance at the contact line. We find that the height profile obtained using (I) shows good agreement with the adsorption film thickness of the DFT-FMT equilibrium density profile in terms of maximal curvature and the behavior at large film heights. In contrast, we observe that while the height profile obtained by using (II) satisfies basic sum rules, it shows little agreement with the adsorption film thickness of the DFT results. The results are verified for contact angles of 20, 40 and 60 degrees

Observation of exponential spectra and Lorentzian pulses in the TJ-K stellarator

An experimental investigation of the low-frequency density fluctuations in the plasma edge region of the TJ-K stellarator [N. Krause et al., Rev. Sci. Inst. 73, 3474 (2002)] finds that the ensemble- averaged frequency spectra exhibit a near exponential frequency dependence whose origin can be traced to individual pulses having a Lorentzian temporal shape. Similar features have been previously observed [D. C. Pace et al., Phys. Plasmas 15, 122304 (2008)] in a linear magnetized device under conditions in which cross-field pressure gradients are present. The reported observation of such features within the turbulent environment of a toroidal confinement device provides support for the conjecture that the underlying processes are a general feature of pressure gradients. Also presented is the magnetic field strength dependence of the pulse widths and the waiting time distribution between pulses