EXPERIMENTS ON THE PROPAGATION OF RADIO WAVES IN AN ANISOTROPIC IONIZED MEDIUM

The propagation of electromagnetic waves in an ionized medium is one of the most important areas of radiophysics, invariably attracting the attention of both theorists and experimentalists. This is also greatly favored by the practical importance of the problem, which includes such particular cases as, for example, questions of the propagation of radio waves in the ionosphere, the problem of the resonance properties of the electron plasma of a gas discharge, etc. By the present time, largely through the work of Soviet scientists, a theory has been developed for the propagation of radio waves in the ionosphere (see, for example, 1). In this connection, the full significance is clear of experiments that would make it possible to study the propagation of electromagnetic waves in an ionized medium under laboratory conditions. Such an experiment has recently been carried out 2.

A gas-discharge plasma (neon) was chosen as the ionized medium. The discharge took place in a tube with a heated cathode. The discharge voltage was applied in pulses of 5 sec duration. After such a pulse, for some time atoms and a plasma remained in the discharge plasma, where a process of deionization was already taking place; at this time high-frequency radio waves were sent in (also in pulses of 10 μsec duration). The discharge tube completely filled the volume of one of the sections of a circular waveguide. A solenoid was wound on the outside of the waveguide, creating a strong magnetic field. In this way the propagation of radio waves in the presence of a magnetic field, i.e. in an anisotropic ionized medium, was studied.

Measurements were made for a wave of type \(H_{11}\) at radio frequencies of 4600, 4900, 5200, and 5500 Mc/s (the critical frequency of the waveguide was 4430 Mc/s) and in the range of neon pressures from 0.5 to 100 mm Hg. In the waveguide with plasma the distribution of the high-frequency electric field was measured, as well as the angle of rotation of the plane of polarization (the Faraday effect—magnetic rotation of the plane of polarization).

Figure 1 shows the distribution of the electric field around the circumference of the waveguide at a signal frequency of 5500 Mc/s (\(\lambda = 5.46\) cm) and a gas pressure of 1 mm Hg. The maximum value of the discharge voltage was 1050 V, and of the discharge current 135 mA. Along the abscissa axis is plotted the angular coordinate \(\theta\). For \(\theta = 0\) the direction of the vector \(\mathbf{E}\) in the absence of discharge is taken. Along the ordinate axis is plotted the amount of attenuation of the vector \(\mathbf{E}\) in the plasma in comparison with its maximum value in the absence of discharge, \(E_0\) max. For comparison (dashed curve), the distribution of the electric field in the waveguide in the absence of discharge is shown.

Figure 2 shows the dependence of the angle of rotation of the plane of polarization on the magnitude of the magnetic field. The experimental conditions are the same as for Fig. 1.

The principal results of the measurements:

1) In a magnetic field there is observed a rotation of the plane of polarization of the radio wave through considerable angles—of the order of \(90^\circ\) and more (over a distance of one wavelength in the waveguide), and the phenomenon is sharply pro-

...pronounced resonant character; the resonant frequency is the gyroscopic frequency for the electron,

$$ \omega_H=\frac{eH}{mc} $$

(Fig. 2).

2) As resonance is approached, the wave changes the character of its polarization—from plane-polarized it becomes elliptically polarized, and at the resonance frequency it becomes a wave polarized in a circle.

3) It proves possible to construct, with the aid of two solenoids with independent supply, the radio analogue of two crossed nicols.

The results of the experiments can be explained qualitatively by the usual model. A linearly polarized wave may be decomposed into two waves polarized in a circle, with right- and left-hand rotation of the vector \(E\): into an “ordinary” wave and an “extraordinary” one. Near resonance the extraordinary wave experiences strong absorption. Therefore, at resonance only the ordinary wave remains, and circular polarization is observed.

Fig. 1.
Annotations in the figure: \(H=1250\) gauss; \(H=3000\) gauss; \(H=0\) gauss; \(H=1500\) gauss. Vertical axis: \(E/E_0\), max in dB. Horizontal axis: \(\theta\), in degrees. Legend: solid line — in plasma; dashed line — in the absence of discharge.

Fig. 2.
Annotations in the figure: vertical axis: rotation of the plane of polarization, in degrees. Horizontal axis: magnetic induction, in gauss. Mark: \(\omega_H\).

When the direction of the magnetic field is changed, the ordinary and extraordinary waves change places. Therefore, at resonance, after two solenoids with different current directions, both waves experience absorption: in one of them the wave polarized in a right-hand circle, and in the other—the one polarized in a left-hand circle. Such a device is analogous to the system of crossed nicols in optics.

This explanation can be regarded only as purely qualitative. A quantitative theory of the propagation of radio waves has been developed for the case of an unbounded medium and, correspondingly, plane waves. However, in a waveguide the wave is not plane, and the propagation of electromagnetic waves in a waveguide filled with an anisotropic ionized medium still requires its own investigation, both theoretical and experimental.

The experiments described were undertaken with a view to investigating the magnetic rotation of the plane of polarization in various media, including ionized gas. Their results, however, have a very broad significance; they affect a wide range of important questions: the modeling of phenomena in the ionosphere, the study of the resonant properties of gas-discharge plasma, amplitude and phase modulation of ultrahigh-frequency radio waves, rotation of the plane of polarization, etc.

M. Ginzburg

References

1. V. L. Ginzburg, The Theory of Radio-Wave Propagation in the Ionosphere, Gostekhizdat, 1949.
2. L. Goldstein, M. Lampert and J. Heney, Phys. Rev. 82, 956 (1951).