On the interpretation of fluctuation and E×B turbulent transport measured by Langmuir probes in fusion plasmas

Abstract

Plasma fluctuations and fluctuation-induced particle fluxes have been investigated in the plasma edge of the TJ-II stellarator using Langmuir probes. Simultaneous measurements of plasma fluctuations carried out by probes located in and out of the probe body sheath show similar results in the normalized level of fluctuations in the ion saturation current. However, floating potential fluctuations measured in the co and counter direction of the magnetic field on the sheath probe body show slight but significant differences. The local radial electrostatic turbulent driven transport measured in and out of the probe body sheath shows consistent results, within the errors bars due to uncertainties in the determination of the effective probe collecting area; the normalized local radial transport to the average ion saturation current (the effective velocity which is not affected by uncertainties in the probe area) show consistent results. These results and previous findings call into question the recent interpretation of probe measurements on the basis of the influence of the probe's pre-sheath zone [B. Labombard, Phys. Plasmas. 9, 1300 (2002)].

Edge turbulence measurements in toroidal fusion devices

“…In practice, this allows plasmas with up to T e ∼ 100 eV to be measured using a fast movable drive when necessary. An uncertainty in the use of these probes is the (generally) unknown level of recycling and impurity influx at the probe surface, which can affect the local plasma parameters [22,23]. Nevertheless, probe measurements of density and temperature have often been successfully compared with non-perturbing measurements such as Thomson scattering.…”

RETRACTED ARTICLE: Review on Plasma Edge Analysis Using the Auto-Correlation and Probability Distributions of Fluctuations

“…One of the most common is the so-called ''de-correlation'' model of shear suppression which suggests that the sheared flow tends to break up turbulent eddies in the plasma and consequently reduce the amplitude of fluctuations. However, many other proposed mechanisms for this damping of turbulence and transport exist including coupling to fluctuating flows called zonal flows, coupling to damped eigenmodes, a shear-induced shift in wave number spectra, and changes in deterministic chaos [7][8][9][10][11][12][13][14][15]. The issue remains that though observation of suppression of turbulence and transport has been made on a myriad of plasma devices, the level of detail in these measurements has not been particularly high and there is much need for continued rotation experimentation especially for validation of the various suppression models .…”