Abstract
Plasma potential measurements using the conventional Langmuir probe may cause an error due to the space charge effect. To solve the problem, a tube probe is proposed in this study which can minimize the space charge effect by collecting electrons with an orifice instead of the solid surface of the Langmuir probe. The I-V characteristic of the tube probe exhibits a clear turning point, accurately indicating the plasma potential. Comparing with the results of the conventional Langmuir probe, it suggests that the plasma potential measured by the Langmuir probe may be underestimated by about 0.1-0.2 Te/e, which may cause underestimation of the electron density by about 10%-20%. Combination use of the tube probe and the Langmuir probe is suggested for accurate measurement of the electron density.
Highlights
Since introduced by Langmuir, the Langmuir probe has been one of the most attractive and widely used tools for plasma diagnostics due to many of its advantages such as the simpleness and the capability of local measurements.[1,2,3,4] By analyzing the I-V characteristics of a Langmuir probe in a plasma, many important plasma parameters, such as the plasma potential, the electron density, the electron temperature and the electron energy distribution function (EEDF), can be obtained
Considering that the space charge accumulation is due to the existence of the probe which hinders the transport of the charges in the plasma, we propose that using an orifice of a cavity, instead of the solid surface of the Langmuir probe, should minimize the space charge effect and sharpen the turning point indicating the accurate plasma potential
The two I-V characteristics show qualitatively similar features, including the small ion accumulative current, the large “saturation” electron current, and the transition between them. They show quite clear difference at where they change from the transition region to the electron saturation region — while the Langmuir probe I-V characteristic shows a gradual change from concave to convex, the tube probe I-V characteristic shows a clear turning point
Summary
Since introduced by Langmuir, the Langmuir probe has been one of the most attractive and widely used tools for plasma diagnostics due to many of its advantages such as the simpleness and the capability of local measurements.[1,2,3,4] By analyzing the I-V characteristics of a Langmuir probe in a plasma, many important plasma parameters, such as the plasma potential, the electron density, the electron temperature and the electron energy distribution function (EEDF), can be obtained. The plasma potential is the base point for calculation of the electron density and the EEDF,[5,6] it is critical to the accuracy of the diagnostic results. According to the basic Langmuir probe theory,[7,8,9] there should be an abrupt change of the electron collection current in the probe I-V characteristics, suggesting the accurate plasma potential. The error in the plasma potential measurement further causes inaccuracy on the results of the electron density and EEDF.[5,6] In addition, the inflection point method can yields incorrect results due to the distorted probe I-V curves in many circumstances, such as the presence of
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