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Title: Spatiotemporal temperature fluctuation measurements by means of a fast swept Langmuir probe array

Abstract

Stationary Langmuir probe measurements of ion saturation current and floating potential in a plasma cannot give direct information on density and plasma potential fluctuations in the presence of temperature fluctuations. This problem can be avoided if the probe bias voltage is continuously swept faster than the fluctuation time scale, recording the current-voltage characteristic. This article reports the development of a spatiotemporal highly resolving Langmuir probe array with 15 fast swept tips, operating in the strongly magnetized, collisionless edge plasma of the Wendelstein 7-AS stellarator [Plasma Phys. Controlled Fusion 31, 1579 (1989)]. The probe tips are aligned in the poloidal direction, the tip spacing is 2 mm, and the sweeping frequency is 1.4 MHz. Current and voltage data are sampled with 50 MHz. The high bandwidth of the measurement is achieved by placing miniaturized differential amplifiers close to the probe tips in order to do an impedance transform. The surface-mounting technology and an additional inverse feedback module are utilized, allowing for an input voltage range of {+-}100 V, and a common mode rejection rate of 55 dB at 4 MHz, which is sufficient to resolve the nonlinear probe characteristic. For the evaluation of the data, a fit model for stationary probesmore » is employed and found adequate. Changes of the plasma parameters during one voltage sweep are taken into account by a linear interpolation of the fit parameters. Spatio-temporal fluctuation data gained by a fast swept Langmuir probe array, which can be relevant for the turbulent radial transport of particles and energy, are presented.« less

Authors:
; ;  [1]
  1. Max-Planck-Institut fuer Plasmaphysik, Wendelsteinstrasse 1, Greifswald 17491 (Germany)
Publication Date:
OSTI Identifier:
20953443
Resource Type:
Journal Article
Resource Relation:
Journal Name: Review of Scientific Instruments; Journal Volume: 78; Journal Issue: 5; Other Information: DOI: 10.1063/1.2740785; (c) 2007 American Institute of Physics; Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
70 PLASMA PHYSICS AND FUSION TECHNOLOGY; AMPLIFIERS; BOUNDARY LAYERS; ELECTRIC CURRENTS; FLUCTUATIONS; INTERPOLATION; LANGMUIR PROBE; MHZ RANGE 01-100; NONLINEAR PROBLEMS; PLASMA; PLASMA DENSITY; PLASMA POTENTIAL; STELLARATORS

Citation Formats

Schubert, M., Endler, M., and Thomsen, H. Spatiotemporal temperature fluctuation measurements by means of a fast swept Langmuir probe array. United States: N. p., 2007. Web. doi:10.1063/1.2740785.
Schubert, M., Endler, M., & Thomsen, H. Spatiotemporal temperature fluctuation measurements by means of a fast swept Langmuir probe array. United States. doi:10.1063/1.2740785.
Schubert, M., Endler, M., and Thomsen, H. Tue . "Spatiotemporal temperature fluctuation measurements by means of a fast swept Langmuir probe array". United States. doi:10.1063/1.2740785.
@article{osti_20953443,
title = {Spatiotemporal temperature fluctuation measurements by means of a fast swept Langmuir probe array},
author = {Schubert, M. and Endler, M. and Thomsen, H.},
abstractNote = {Stationary Langmuir probe measurements of ion saturation current and floating potential in a plasma cannot give direct information on density and plasma potential fluctuations in the presence of temperature fluctuations. This problem can be avoided if the probe bias voltage is continuously swept faster than the fluctuation time scale, recording the current-voltage characteristic. This article reports the development of a spatiotemporal highly resolving Langmuir probe array with 15 fast swept tips, operating in the strongly magnetized, collisionless edge plasma of the Wendelstein 7-AS stellarator [Plasma Phys. Controlled Fusion 31, 1579 (1989)]. The probe tips are aligned in the poloidal direction, the tip spacing is 2 mm, and the sweeping frequency is 1.4 MHz. Current and voltage data are sampled with 50 MHz. The high bandwidth of the measurement is achieved by placing miniaturized differential amplifiers close to the probe tips in order to do an impedance transform. The surface-mounting technology and an additional inverse feedback module are utilized, allowing for an input voltage range of {+-}100 V, and a common mode rejection rate of 55 dB at 4 MHz, which is sufficient to resolve the nonlinear probe characteristic. For the evaluation of the data, a fit model for stationary probes is employed and found adequate. Changes of the plasma parameters during one voltage sweep are taken into account by a linear interpolation of the fit parameters. Spatio-temporal fluctuation data gained by a fast swept Langmuir probe array, which can be relevant for the turbulent radial transport of particles and energy, are presented.},
doi = {10.1063/1.2740785},
journal = {Review of Scientific Instruments},
number = 5,
volume = 78,
place = {United States},
year = {Tue May 15 00:00:00 EDT 2007},
month = {Tue May 15 00:00:00 EDT 2007}
}
  • In the Wendelstein 7-AS stellarator (W7-AS) [Plasma Phys. Controlled Fusion [bold 33], 1591 (1991)], current-voltage characteristics of the Langmuir probe at sweep frequencies in the range 400 kHz to 1 MHz were measured and it was found that the mean and fluctuation values of the ion saturation current, floating potential, and electron temperature were independent of the sweep frequency. A radial scan in the vicinity of the velocity shear layer was performed. The simultaneous sweeping of 3 probe tips showed a statistically significant spatial coherence of the fluctuations in the poloidal direction and a decrease in spatial coherence of themore » fluctuations with increasing tip separation could be demonstrated. The observation of a change in the propagation direction of fluctuations as the shear layer was crossed and a calculation of the transport spectrum show that the swept probe method is capable of reproducing known results. Apparent temperature fluctuations, due to variations of density and potential during a sweep, are shown by simulations to be only of importance at frequencies above half the Nyquist frequency.« less
  • Swept Langmuir probes are widely used to measure electron temperature (T{sub e}) in laboratory plasmas by performing an exponential fit to the measured volt-ampere (I-V) characteristic. Often the probe voltage sweep frequency is much lower than the characteristic frequencies of the plasma fluctuations and a time-averaged I-V characteristic is used for the fit. We show by numerical modeling that in the presence of T{sub e} fluctuations with frequencies well above the voltage sweep frequency this standard technique applied to a swept single probe tends to read higher than the actual time-averaged T{sub e} provided no correlated plasma potential (V{sub p})more » fluctuations are present. In the presence of coupled T{sub e} and V{sub p} fluctuations a slowly swept single probe may read either higher or lower than the average T{sub e}, depending on the relative amplitude and phase of the temperature and potential fluctuations. In contrast, swept double probe measurements of T{sub e} are virtually unaffected by either T{sub e} or V{sub p} fluctuations.« less
  • We present a fast technique for determining the plasma electron temperature T/sub e/ automatically from the small signal application of the asymmetric double Langmuir probe when it is operated in the region where -1
  • A high density Langmuir probe array has been developed for measurements of scrape-off layer parameters in NSTX. Relevant scale lengths for heat and particle fluxes are 1-5 cm. Transient edge plasma events can occur on a time scale of several milliseconds, and the duration of a typical plasma discharge is similar to 1 s. The array consists of 99 individual electrodes arranged in three parallel radial rows to allow both swept and triple-probe operation and is mounted in a carbon tile located in the lower outer divertor of NSTX between two segments of the newly installed liquid lithium divertor. Initialmore » swept probe results tracking the outer strike point through probe flux measurements are presented.« less
  • A high density Langmuir probe array has been developed for measurements of scrape-off layer parameters in NSTX. Relevant scale lengths for heat and particle fluxes are 1-5 cm. Transient edge plasma events can occur on a time scale of several milliseconds, and the duration of a typical plasma discharge is {approx}1 s. The array consists of 99 individual electrodes arranged in three parallel radial rows to allow both swept and triple-probe operation and is mounted in a carbon tile located in the lower outer divertor of NSTX between two segments of the newly installed liquid lithium divertor. Initial swept probemore » results tracking the outer strike point through probe flux measurements are presented.« less