Determination of Certain Parameters of a High-Frequency Torch Discharge

I. A. Tikhomirov, V. V. Tikhomirov, V. Ya. Fedyanin

(Presented by the scientific seminar of the Physics and Technology Faculty)

For the successful application of the plasma of high-frequency discharges in science and technology, it is necessary to know such parameters of it as the electron concentration, the collision frequency of electrons with heavy particles, and the electron temperature. Determination of these parameters is possible by means of microwave and probe methods of plasma diagnostics.

In the present work, some results are set forth of microwave and probe measurements of the parameters of high-frequency electrode, nonturbulent, high-pressure plasma in air.

In carrying out the investigations, the discharge was excited on a water-cooled nickel electrode in a quartz chamber by means of an LGE-3B generator with an operating frequency of 30 MHz. The power released in the discharge reached 2 kW; the length of the torch channel was up to 18 cm at pressures from 300 mm Hg to atmospheric pressure.

The experiments showed that reflection of the microwave signal from the given plasma is practically absent; thus, it may be assumed that the decrease in the signal in comparison with the reference signal is due only to its attenuation in the plasma.

Microwave measurements were carried out according to the scheme shown in Fig. 1.

Here: 1—the discharge, 3—the measuring line, 4—the measuring attenuator, 5—the matching transformer, 6—the recording device (S1-8 oscilloscope), 2—the microwave oscillation generator on K-49, K-45, and K-46 klystrons with a range of covered wavelengths of 8–13 mm.

Signals from the klystrons were fed into one waveguide path by means of directional couplers. Focusing of the microwave radiation into a spot on the order of the wavelength was carried out with Teflon lenses.

Fig. 1. Block diagram of the experiment for determining “$\beta$”

The channel radius, determined according to [2], varied from 0.76 cm to 1.05 cm, i.e., the channel diameter always remained greater than the wavelength of the microwave radiation, which made it possible to avoid measurement errors due to—

…caused by the passage of part of the microwave radiation past the torch channel.

Figure 2 presents the dependence of the attenuation \(\beta\) on the pressure in the discharge chamber. The results were obtained for a plasma region located at a distance of \(7\ \text{cm}\) from the electrode, with power supplied to the discharge of about \(3\ \text{kW}\).

The method for processing the obtained results, as well as its applicability to a high-frequency torch discharge at high pressure, are set forth in [1].

The obtained dependences of the collision frequency \(\nu\) and the electron concentration \(N\) in the plasma on pressure are illustrated by the plot in Fig. 3. The mean values \(\nu \sim 10^{11}\ \text{s}^{-1}\), \(N \sim 10^{11} \div 10^{12}\ \text{cm}^{-3}\) agree with the data of [1], [2], [3]. From the plot in Fig. 3 one can observe a certain increase in \(\nu\) and \(N\) with increasing pressure. With increasing pressure, the density of heavy particles in the plasma increases, which leads to an increase in the probability of collisions of electrons with heavy particles and, consequently, may lead to an increase in \(\nu\) as the pressure increases, which agrees with experiment.

Fig. 2. Dependence of “\(\beta\)” on the pressure in the discharge chamber

Fig. 3. Dependence of “\(\nu\)” and “\(N\)” on the pressure in the discharge chamber

In the indicated pressure range, as it increases the temperature of the torch channel rises somewhat [4] and, consequently, so does the degree of thermal ionization. This fact, as well as a certain increase in \(\nu\) with increasing pressure, may lead to an increase in \(N\), which is also confirmed by experiment.

For carrying out probe measurements we used movable nickel and tungsten probes with \(d \leq 0.8\ \text{mm}\). The residence time of the probe in the plasma was \(\sim 0.1\ \text{s}\), in order to avoid the need to take into account electron emission from the hot probe, which is as yet difficult [5]. The measurement was carried out according to the scheme proposed in [6]. As the probe passed through the plasma, \(7 \div 8\) probe characteristics were recorded on the oscilloscope screen.

The change in electron concentration along the length of the channel, calculated from the bend of the characteristic, lies within \(N \approx 10^{11} \div 10^{12}\ \text{cm}^{-3}\), which agrees with the microwave measurements and with the data of [1], [2], [3].

Figure 4 presents the change in the electron temperature \(T_e\), determined from the slope of the characteristic, along the channel length for atmospheric pressure. Unfortunately, the results of both the microwave and

and probe measurements at distances from the electrode of less than 3 cm are ambiguous, which can apparently be explained by the influence of near-electrode effects. The values obtained for \(T_e\), of order \((4 \div 7)\, \(10^4\) K, agree with the data of [7], [8], obtained for an r.f. torch in molecular gases. It should be noted that accurate quantitative estimates of the parameters \(v\), \(N\), and \(T_e\) for the channel of a torch discharge are at present difficult. The main obstacles

Fig. 4. Change of \(T_e\) along the length of the torch channel

to carrying them out are the inhomogeneity of the channel plasma over radius and length, the absence of data on the distribution of r.f. fields in the discharge, etc.

LITERATURE

1. V. V. Marusin, I. A. Tikhomirov, V. V. Tikhomirov. Measurement of the parameters of r.f. electrode plasma. In the abstracts of the seminar “Application of Low-Temperature Plasma in the Technology of Inorganic Substances and Powder Metallurgy,” Kiev, April, 1968.

2. A. V. Kachanov et al. In: Physics of Gas-Discharge Plasma. Atomizdat, Moscow, 1968.

3. Kristecu and Grigorovich. “Optics and Spectroscopy,” 6, 129 (1959).

4. R. Dzhumakuliev, A. Devyatov. “Optics and Spectroscopy,” 13, 20 (1962).

5. V. A. Zherebtsov, M. A. Lebedev. “Thermophysics of High Temperatures,” 5, 568 (1967).

6. S. M. Levitskii, Z. A. Plyatsok. “Instruments and Experimental Techniques,” 2, 150 (1961).

7. J. Laroche, “Appl. Scient. Res.,” 5, 294 (1955).

8. I. D. Cobine, D. A. Wilbur, “J. appl. Phys.,” 22, 645 (1955).