M. E. Gushchin

Parametric transformation of the amplitude and frequency of a whistler wave in a magnetoactive plasma

The results of experiments studying the propagation of a high-frequency whistler wave in a magnetized plasma duct in the presence of an intense low-frequency wave also related to the whistler frequency range are reported. Amplitude-frequency modulation of the high-frequency whistler wave trapped in the duct was observed. A deep amplitude modulation of the signal that can lead to its splitting into separate wave packets is observed. It is shown that an increase in the wave propagation path leads to a broadening of the wave frequency spectrum and to a shift of the signal spectrum predominantly toward the red side. The transformation of the frequency of the high-frequency wave is related with the time-dependent perturbation of the external magnetic field by the field of the low-frequency whistler wave (the relative perturbation of the magnetic field δB/B≤5×10−2).

Control of whistler radiation efficiency of a loop antenna by generation of ambient magnetic field irregularities

Electrodynamic means for the control of loop antenna radiation efficiency in plasma is proposed, which can be used in the whistler frequency band. The method is based on the generation, without perturbing the plasma density, of localized ambient magnetic field irregularities in the vicinity of the antenna. In order to produce such irregularities, it is suggested to feed the antenna with additional dc current along with the rf current. Experiments performed in a large laboratory magnetoplasma showed that the generation of localized magnetic field enhancements provides the possibility of increasing the amplitude of the whistlers emitted by the loop antenna. Moreover, experiments have shown that the amplification of the whistlers’ signals from the receiving loop antenna fed with additional dc current is observed when a static magnetic field enhancement is generated in the vicinity of the receiver. The experimental data are in good agreement with the theoretical results obtained for comparatively weak ambient...

Parametric generation of whistler waves due to the interaction of high-frequency wave beams with a magnetoplasma

The parametric generation of low-frequency whistler waves by a pump wave beam formed by high-frequency whistler waves with close frequencies is studied experimentally. The electromagnetic fields excited by the beats of two co- or counterpropagating high-frequency waves, or by an amplitude-modulated pump are studied. It is shown that the nonlinear currents at the beat (modulation) frequency are generated by a transverse ponderomotive force arising due to the finite width of the high-frequency beam. In this case, the nonlinear azimuthal drift currents enclose the pump beam and can radiate low-frequency whistler waves to the surrounding plasma.

Propagation of whistlers in a plasma with a magnetic field duct

The propagation of whistlers in a homogeneous magnetized plasma in the presence of a magnetic field duct has been experimentally investigated. The possibility of efficiently trapping whistlers in a narrow (wavelength-scale) cylindrical duct with the increased field has been demonstrated. It has been shown that a comparatively slight perturbation of the external magnetic field (δB/B0 ∼ 0.1) can significantly change the spatial structure and increase the amplitude of whistlers near the duct axis.

Whistler waves in plasmas with magnetic field irregularities: Experiment and theory

The properties of whistler waves propagating in a large laboratory magnetoplasma with magnetic field irregularities have been studied. Two types of ambient magnetic field inhomogeneities have been considered: (i) a localized “lenslike” perturbation and (ii) an elongated “ductlike” irregularity. The magnetic field was perturbed by immersing into the plasma, without creating any significant plasma density disturbances, additional current-carrying coils. It has been found that the presence of magnetic field irregularities causes the whistler wave’s diffraction and affects their patterns substantially. Plasma regions with locally enhanced magnetic field strength focus oblique whistlers; oppositely, local magnetic field minima debunch the whistler waves. In case of prolonged magnetic field irregularity formation—encompassing several whistler wavelengths along its size—the diffraction effects are distinctly pronounced; even the comparatively weak magnetic field disturbances at the level of 10% lead to strong mo...

Fine structure of density ducts formed by active radiofrequency action on laboratory and space plasmas

The results of active ionospheric and model laboratory experiments on the generation of artificial irregularities of a magnetized plasma (density ducts), which can be used as waveguide channels for low-frequency waves, have been reported. It has been found that ducts formed at the localized high-frequency heating of the plasma have a fine structure under certain conditions: they include irregularities of the plasma density, which significantly affect the propagation of low-frequency waves, ensuring the deep amplitude modulation of low-frequency radiation and changing its spatial structure. A mechanism of the formation of such irregularities has been proposed.

Near field of a loop antenna operating in plasma in the whistler frequency range

The structure of the RF magnetic field in the vicinity of a loop antenna operating in the whistler frequency range has been studied experimentally and theoretically. The experiments were performed over a wide frequency range at different values of the plasma density, electron temperature, and ambient magnetic field strength. It is shown that, when a loop antenna is smaller than the wavelength of a quasi-longitudinal whistler, the structure of the magnetic field of such an antenna is nearly the same as that of the field of a current-carrying loop in vacuum; otherwise, the RF field is localized near the antenna wire. The results of numerical calculations agree with the measured field distributions. The antenna field is calculated by expanding it in the eigenmodes of a magnetized plasma with allowance for not only propagating but also nonpropagating (exponentially decaying) waves, which make the main contribution to the near field. An analytic estimate of the depth to which the RF magnetic field of a loop antenna penetrates into the plasma is obtained.

Diagnostics of the atmospheric-pressure plasma parameters using the method of near-field microwave sounding

A method of resonant near-field microwave probing is developed for contactless diagnostics of a high-pressure plasma. The efficiency of this method in measuring the parameters of the plasma of an rf capacitive discharge in argon under atmospheric pressure is demonstrated. The experimental results are compared with the data obtained using the independent method, the microwave radiation “cutoff,” and with theoretical estimates.

Numerical simulation of the electromagnetic fields excited by loop antennas in plasma in the whistler frequency range

The electromagnetic fields excited by circular loop antennas in a magnetized plasma in the whistler frequency range are simulated by the finite-difference time-domain method. The spatial structure of quasi-monochromatic fields excited in the near- and far-field zones by an antenna with a harmonic current, as well as the dynamics of the electromagnetic field excited by an antenna with a current in the form of a single video pulse, is studied. Simulations performed for a uniform plasma and uniform ambient magnetic field agree well with the results of theoretical analysis and model laboratory experiments performed on large-scale plasma devices.

Laboratory modeling of ionospheric heating experiments

Turbulent plasma processes, such as those which occur in the Earths ionosphere during ionospheric heating by powerful radio waves, were studied under laboratory conditions and new physical models of small-scale ionospheric turbulence are proposed as a result of these studies. It is shown here that the mechanism of small-scale plasma filamentation can be connected with the thermal self-channeling of Langmuir waves. During this process, Langmuir waves are guided by a plasma channel, which in turn is formed by the guided waves through a thermal plasma nonlinearity. The spectrum of the self-guided Langmuir waves exhibits sidebands whose features are similar to stimulated electromagnetic emission. We present two mechanisms of sideband generation. The first mechanism can be observed during the formation of the plasma channel and is connected with the parametric shift in the frequency of the self-channeling wave. The second mechanism is connected with the scattering of the self-channeling wave on the low-frequency eigenmodes of the plasma irregularity.