Experimental Study of Planar Langmuir Probe Characteristics in Electron Current‐Carrying Magnetized Plasma

Abstract

A b s t r a c tExperimental study of planar Langmuir probe characteristics in a magnetized plasma with an electron current along the direction of the magnetic field shows that the usual procedure for determination of the electron temperature and plasma density, which is applicable in a current-free magnetiied plasma, gives erroneous results for these plasma parameters. When this procedure is applied on the characteristics measured at two opposite orientations of the probe collecting surface with respect to the direction of the electron drift, different values of the electron temperature are obtained. These virtual electron temperatures and corresponding plasma densities calculated from the measured ion saturation currents are higher and/or smaller than the exact local electron temperature and plasma density. Calculation of particular averages of these quantities is proposed as a possible way to obtain correct results for the local electron temperature and plasma density. These averages are used in the approximate evaluation of the electron drift velocity from the electron saturation currents measured at the two orientations of the probe collecting surface. I n t r o d u c t i o nThe interpretation of the Langmuir probe (LP) characteristics measured in the edge plasma of magnetically confined fusion devices such as tokamaks or in other magnetized plasmas is usually based on the assumption that the electron velocity distribution is Maxwellian. However, since only a small part of the measured characteristic below the probe floating potential V, can be used for determination of the electron temperature T, and unperturbed plasma density no [l], only the high energy tail of the electron energy distribution is taken into consideration, which does not guarantee that the initial assumption about the electron velocity distribution is correct. It was also shown in the theoretical study of Stangeby [2] that when there are two electron components with different temperatures present in a magnetized plasma, the usual procedure for determination of T, from the portion of the measured characteristic below Vf yields the higher temperature even when the concentration of the warmer component is very low, that is only several percents. The distortion of the probe characteristic near the plasma potential caused by the magnetic field effects and related uncertainty in determination of the plasma potential can be considerable even in relatively weak magnetic fields (e.g. B -10-2T), such as those used in some plasma discharges for materials processing [3, 41, so that accurate determination of the electron energy distribution from the measured characteristic is not possible. In this article we present the experimental study of the LP characteristics measured in a magnetized plasma with

Experimental investigation of plasma sheaths in magnetic mirror and cusp configurations

“…In such a way, the influence of the magnetic field on the current-voltage (I-V ) characteristic could be minimized, since the probe mainly collects the current parallel to the magnetic field. It was shown by Popov et al [33] and Stanojevic et al [34] that the distortion of the I−V characteristics of the Langmuir probe occurs in strong magnetic field and the caution should be made in determining the plasma parameters. In the present experiment, the magnetic field is weak and I−V distortion can be neglected.…”

Analysis of in situ measurements of electron, ion and neutral temperatures in the lower thermosphere–ionosphere

“…With the exception of one rocket observation (Sasaki and Kawashima, 1975), the only in situ co-temporal and cospatial observations of T e , T i and T n within the LTI were obtained from satellites AE-C, AE-D and AE-E in the 1970s and early 1980s. T e measurements on board all three AE satellites have been obtained via the cylindrical electrostatic probe (CEP) instruments (Brace et al, 1973;Benson et al, 1977), which are retarding potential Langmuir probe devices, providing electron temperature measurements by the current-voltage (I-V) characteristic relationship of the Debye sheath (Chen and Sekiguchi, 1965;Block, 1978;Hopwood et al, 1993;Schwabedissen et al, 1997;Stanojević et al, 1999). T i measurements have been obtained from the retarding potential analyzer (RPA) on board the AE satellites (Hanson et al, 1973;Hanson and Heelis, 1975) and on board the DE-2 satellite (Hanson et al, 1981), which provide ion temperature by I-V characteristics delivered by the instruments (Whipple, 1961;Hanson et al, 1972Hanson et al, , 1973Nenovski et al, 1980).…”