Increasing the Measurement Range of Plasma Electron Parameters in the Single Langmuir Probe Method
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easing the Measurement Range of Plasma Electron Parameters in the Single Langmuir Probe Method S. N. Andreeva,*, A. V. Bernatskiya,**, and V. N. Ochkina,*** a
Lebedev Physical Institute, Russian Academy of Sciences, Moscow, 119991 Russia * e-mail: [email protected] ** e-mail: [email protected] *** e-mail: [email protected] Received March 25, 2020; revised June 25, 2020; accepted June 26, 2020
Abstract—It is proposed to use a new signal type fed to the single Langmuir probe to measure current— voltage characteristics. It is shown that such an approach makes it possible to increase the range of measured electron energy distribution functions. Keywords: Low-temperature plasma, hollow cathode discharge, Langmuir probe DOI: 10.3103/S1068335620100024
The determination of particle concentrations in plasma is important to control operation of complex electrovacuum setups. As shown in [1, 2], optical emission spectroscopy methods are most promising for such diagnostics, among which optical actinometry is the best one in sensitivity [2]. In a number of cases (when the choice of actinometric pairs is limited), to improve the measurement accuracy, preliminary calibration based on the electron energy distribution function (EEDF) should be performed [3]. In this case, it is especially important to have a notion of the EEDF shape in the energy region >10 eV (in general, this region contains excitation cross sections of emitting states of actinometers [1]). Conventionally, the Langmuir probe method is used in plasma physics to determine the EEDF [4, 5]. This method developed in the first half of the last century [6] seems very simple at first sight; however, its use application under actual experimental conditions is complicated by many factors [7, 8]. In this paper, we propose a method for forming current—voltage (I–V) characteristics on the probe, which makes it possible to extend the measurement range and to reduce EEDF errors in the region >10 eV. The scheme of probe measurements evolves that proposed in [8]. An advantage of the proposed scheme is as follows. The system forms an I–V characteristic of the probe circuit as a voltage shaped as a noise signal is applied to the probe. Feeding a signal representing white noise to the probe, it becomes possible to significantly reduce errors in the EEDF determination in comparison with a conventional periodic signal (e.g., sawtooth or sinusoidal one) [8]. However, in the region >20 eV, the measurement error remains high. In this study, it is proposed to form I–V characteristics using a noise signal with an inhomogeneous distribution of the number of points of the signal forming I–V characteristics for the purpose of reducing measurement errors on the EEDF “tail.” Measurements were performed using the “Tech” setup [1–3, 8–10] consisting of a vacuum chamber (with a residual pressure of ~10–7 mbar) and an extensive gas puffing system. A discharge device consists of a hollow rectangular tungsten cathode forming a cavity of 100 × 50 × 10 mm3. On the outside, the cathode cont
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