Density, temperature and potential fluctuations in the edge plasma of the FTU tokamak

Abstract

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.