Density, temperature and plasma potential are measured simultaneously at the same spatial point in the SOL plasma of FTU at a rate faster than the typical turbulence frequency. This is done through a harmonic analysis of the current drawn on a single Langmuir probe swept in voltage at high rate (≈ 0.9 MHz). The current signal is detected with a down frequency conversion by means of a homodyne technique. The fluctuation levels are around 30% for all three quantities, but the autocorrelation times of temperature and potential are shorter than that of density: <3 μs against ≈ 13 μs. The magnitude and mutual coherence of the fluctuations are affected by the connection length Lc of the flux tube connected to the probe, offering a possible explanation of the variation of both the particle and the energy transport coefficient with Lc found experimentally.
Fluctuations and associated transport in the L mode in Tore Supra