Density, Temperature and Potential Fluctuation Measurements With Multiple Fast Swept Langmuir Probes on W7-AS

To measure fluctuations of plasma density, electron temperature and electric plasma potential in the edge of fusion experiments simultaneously and with high spatial resolution, the best approach is generally agreed to be the fast sweeping of the Langmuir probe characteristic, a technique which has been applied on several experiments in the past few years. “Fast” in this context means that the sweep frequency should be well above typical fluctuation frequencies which are known to range up to a few 100 kHz. A new experimental arrangement for such measurements has been used on the Wendelstein 7-AS stellarator, making use of amplifiers in the reciprocating probe head, only few cm from the probe tips. It is thus possible to sweep the probes in the MHz range and to obtain good quality probe characteristics without having to correct for cable resonances and capacitance. Four poloidally displaced probe tips were swept with 1–4 MHz and the voltage and current signals were recorded with a sampling frequency of 50–200 MHz. Ion saturation current and floating potential fluctuations obtained from the swept signals are compared with static measurements of these two quantities measured simultaneously by further tips on the same probe head. If we use “standard” probe theory to analyse these measurements, the temperature fluctuations contribute significantly when calculating the plasma potential from the floating potential. In standard probe theory some basic assumptions are made, e.g., on the ratio of electron to ion temperature or the secondary electron emission coefficient of the probe tips. The influence of changes in these assumptions on the calculated phase angle between density, electron temperature and electric potential fluctuations and on the resulting radial particle and thermal transport is discussed. The fluctuation-induced radial particle transport is compared with the value obtained from ion saturation current and floating potential neglecting temperature fluctuations.