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

Abstract

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 tmission 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.

Towards Fast Measurement of the Electron Temperature in the SOL of ASDEX Upgrade Using Swept Langmuir Probes

“…Usually a factor about 3 is used for α which was also experimentally found at ASDEX Upgrade [10]. Nevertheless strong deviations from this factor are possible [11]. For a typical probe tip distance of a few mm a T e variation of 1 eV between the pins already causes ΔE ∼ 200 − 300 V/m.…”

“…In fusion devices swept single probes are usually used at rather low sweeping frequencies (kHz range). About a decade ago theoretical and experimental investigations on fast swept single probes were made up to the MHz range [11,[21][22][23] showing at high frequencies a hysteresis in the electron saturation branch. This is caused by a polarization current due to the potential difference of the flux tube intersected by the probe and the surrounding plasma.…”

Blob velocity scaling in diverted tokamaks: A comparison between theory and simulation

“…Blobs detach from the main plasma and move radially outwards, making turbulence at the edge of fusion devices intermittent and significantly contributing to the radial transport mechanisms in the SOL. Blobs have been observed in tokamaks (e.g., in the Caltech Research Tokamak, 2 Alcator C-Mod, 3 JET, 4 JT-60U, 5 Tore Supra, 6 and TCV 7 ), stellarators (e.g., in the W7-AS stellarator 8 and in TJ-K 9 ) reversed field pinches, 10 and basic plasma devices (e.g., in LAPD 11 and in TORPEX 12 ). They can lead to enhanced intermittent heat flux on the main vessel wall, possibly damaging radio frequency antennas and wall tiles and causing sputtering of impurities.…”

Experimental evaluation of Langmuir probe sheath potential coefficient on the HL-2A tokamak

“…And the electron energy distrubution function (EEDF) is not represented by the second derivative of the V-I characteristic (Druyvesteyn formula [11]) but rather by its first derivative [12,13]. Meamwhile, some experiments were targeted on this issue with the comparison to other diagnostics, such as emissive probe, et al [18][19][20][21][22][23][24][25][26]. Most of them showed that many statistical properties of fluctuations inferred by Langmuir probes and other diagnostics, such as standard deviation, auto and cross correlation, auto and cross power, cross phase and cross coherency spectra, were similar.…”