UDC 537.52

STUDIES OF A SHIELDED RF TORCH DISCHARGE BY MICROWAVE AND SPECTRAL METHODS

I. A. Tikhomirov, V. V. Tikhomirov, V. Ya. Fedyanin,
A. A. Solov’ev, M. M. Pugovkin, A. I. Kuz’minykh, N. M. Sinitsyn

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

A procedure is described and results are given for measurements of the gas temperature in the pressure range 100–760 mm Hg for air plasma and for plasma of a mixture of air and decomposition products of an organic dielectric in a metallic RF torch plasmatron. A description is given of an installation for determining the electron density and the effective frequency of collisions of electrons with heavy particles by microwave methods.

Illustrations 4, tables 1, bibliography 11.

In the very near future, processes occurring under the conditions of the plasma of electric discharges at gas temperatures of \((3 \div 30)\,10^3{}^\circ\mathrm{K}\) and at still higher “electron temperatures” [1] will play a special role in the development and establishment of advanced technology. In this connection, as is evident from the results of current studies [2–4], the plasma of RF discharges will best satisfy many technological processes. In our opinion, special attention should be given to the study of the RF torch discharge, which, while possessing all the advantages of RF discharges, is at the same time free of such shortcomings of the induction discharge as the impossibility of using the latter in entirely metallic structures.

However, the RF torch discharge in a metallic plasmatron has at present been insufficiently studied, and therefore the creation of such a type of metallic RF torch plasmatron is of definite interest, in which, during plasma-chemical reactions, it would be possible to determine all the principal plasma parameters of an RF torch discharge. The scheme of such a plasmatron, developed by us, is described in [11].

To determine the gas temperature in this plasmatron, the discharge was photographed with the aid of an ISP-28 quartz spectrograph on aerophotographic film with a sensitivity of 1100 GOST units. For measuring temperatures, the method of relative line intensities in the 3064 Å band of the hydroxyl OH was chosen.

The applicability of this hydroxyl band for measuring gas temperatures in RF discharges was examined experimentally in the works of M. Z. Khokhlov [5–6], where it was shown that, at gas temperatures above \(2500^\circ\mathrm{K}\), the dissociation of water molecules in the discharge is thermal in character and the “rotational temperature” of the hydroxyl coincides with the gas temperature.

The most convenient branches for measuring the temperature under our conditions proved to be \(P_1\) and \(Q_1\). The measurements carried out using them differ from one another by 1–2%, which lies within the accuracy limits of the method.

Figure 1 shows the dependences of the gas temperature on the gas pressure in the plasmatron at various powers supplied to the discharge. The graphs show that, at pressures above 300 mm Hg, the gas temperature in the torch discharge changes little both when the pressure is varied and when the power supplied to the discharge is varied.

Fig. 1. Dependence of the gas temperature of a torch discharge in air on pressure at different powers input into the discharge (\(L = 200, 400, 600, 800\) W).

The method described above for determining the gas temperature was also applied to determine the gas temperature of the plasma of a mixture of air with decomposition products of organic compounds of complex composition.

Figure 2 shows the dependence of the plasma temperature of these reacting components on pressure.

For carrying out microwave measurements we used an apparatus (Fig. 3) that combines both the scheme of the two-frequency method [7] and a phase meter [8], making it possible to measure attenuation and reflection at \(\lambda_1\), \(\lambda_2\), and the phase shift at \(\lambda_1\) (\(\lambda_1 = 8\) mm; \(\lambda_2 = 4\) mm). Signals from generators \(G_1\), \(G_2\) are directed into a common waveguide path, made of polystyrene and designed for the larger wavelength. The radiation focused by horn-lens antennas passes through the plasma and is directed to frequency divider 4, then to measuring attenuators 5, and then to crystal detectors 6, with recording on KSP-48 self-recording instruments.

In the reference arm the wave, after passing through attenuator 2 and phase shifter 3, is mixed with the signal that has passed through measuring arm II, which makes it possible to measure the phase shift of the wave that has passed through

Fig. 2. Dependence of the gas temperature of the plasma of an air mixture with polymer decomposition products on pressure.

the plasma. For matching, a three-loop transformer 7 is included in the circuit.

The VSWR of the setup at all wavelengths was 1.14–1.20 and was measured with measuring lines 1.

Table 1

Electron concentration \(n_e\) and effective collision frequency \(\nu_{\mathrm{eff}}\) for an r.f. discharge plasma at different pressures

Calculation of the values of \(n_e\) and \(\nu_{\mathrm{eff}}\) from the nomograms of work [7] is impossible, since in our case the ratio \(S = \dfrac{\nu_{\mathrm{eff}}}{\omega} > 1\) (\(\omega\) is the frequency of the probing signal).

Table 1 gives the values of \(n_e\) and \(\nu_{\mathrm{eff}}\), calculated with the aid of nomograms computed by us on a computer [9]. In this case the calculation formula was simplified in accordance with the condition \(S>1\). The experimental error, due to reflection between the antennas and refraction in the plasma column, as well as to the comparatively low electron density, is 12–10%, which is in good agreement with the data of [10].

Fig. 3. Block diagram of the setup for microwave investigations.

Thus, carrying out the set of the above-mentioned measurements in combination with probe and electrical measurements makes it possible to obtain information on the processes occurring in the plasma, including that of a torch discharge, and to optimize plasma-chemical processes with respect to particular parameters.

REFERENCES

1. V. A. Kirilin. In the collection: Low-Temperature Plasma, Moscow, “Mir,” 1967, p. 9.

2. I. A. Tikhomirov et al. Generators of Low-Temperature Plasma. Moscow, “Energiya,” 1969.

3. I. A. Tikhomirov et al. Physics, Technology, and Application of Low-Temperature Plasma. Alma-Ata, 1970.

4. I. A. Tikhomirov et al. Author’s certificate 252300, 1968.

5. M. Z. Khokhlov. “ZhETF,” 5 (11) (1955).

6. M. Z. Khokhlov. “OS,” 4 (1958).

7. A. V. Chernetskii, O. A. Zinov’ev, O. V. Kozlov, Apparatus and Methods of Plasma Investigations, ed. by V. D. Rusanov. Moscow, Atomizdat, 1968.

8. V. E. Golant. Microwave Methods of Plasma Investigation. Moscow, “Nauka,” 1968.

9. I. A. Tikhomirov et al. Izvestiya TPI, vol. 276, p. 26.

10. S. Shiobara. “J. Appl. Phys.,” 1965, 4 No. 7, 513.

11. I. A. Tikhomirov et al. Instruments and Experimental Techniques, 1974, No. 3, p. 151.