A. V. Kostrov

Whistler wave emission by a modulated electron beam through transition radiation

Measurements have been performed in a laboratory experiment modeling the interaction of a modulated electron beam with a magnetized plasma under conditions relevant to space experiments involving beam injection. Both whistler emission through Cherenkov resonance and a nonresonant mechanism of transition radiation from the point of beam injection into the plasma have been observed. Electrons injected from the gun into the plasma pass from one medium (gun chamber) into another (plasma volume) and electromagnetic fields change as charges cross the metallic interface between both media, giving rise to transition radiation. This type of beam radiation, observed separately from the resonant Cherenkov emission owing to adequate choices of the physical conditions, has been characterized as a function of the beam and plasma parameters. Moreover, in the case of beams injected from satellites in the ionospheric and magnetospheric plasmas, this nonresonant emission, mainly located in the near gun region, can be gover...

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).

Influence of nonlinear effects on whistler emission in magnetoactive plasma

The influence of thermal and strictional nonlinear effects on the whistler emission in magnetoactive plasma is studied experimentally. It is established that a nonlocal thermal nonlinearity determines the directional pattern of the antenna, while a strictional nonlinearity, which is strongest near the antenna surface, is responsible for the matching of the emitter with the surrounding plasma.

Nonlinear trapping and self-guiding of magnetized Langmuir waves due to thermal plasma filamentation

Nonlinear interaction of Langmuir waves with a laboratory magnetoplasma has been studied under the conditions relevant to the ionospheric heating experiments. Self-guiding of magnetized Langmuir waves is observed at critical plasma density (ω=ωp): Langmuir waves are trapped inside a narrow, magnetic-field-aligned plasma density depletion region, which is formed by trapped waves due to thermal plasma nonlinearity, i.e., due to local plasma heating and consequent thermodiffusion. Magnetized Langmuir waves are trapped inside the depletion region through their specific dispersion properties; this fact has been shown using the kinetically modified dispersion relation. The threshold of the nonlinear wave trapping exhibits significant growth in the vicinity of harmonics of the electron gyrofrequency.

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...

Interaction of a modulated electron beam with a magnetoactive plasma

Experimental results concerning the interaction of a modulated electron beam with a magnetoactive plasma in the whistler frequency range are reported. It was shown experimentally that when a beam is injected into the plasma, waves can be generated by two possible mechanisms: Cherenkov emission of whistlers by the modulated beam, and transition radiation from the beam injection point. In the case of weak beam currents (Nb/N0)≪−4) the Cherenkov resonance radiation is more than an order of magnitude stronger than the transition radiation; the Cherenkov emission efficiency decreases at high beam currents. The transformation of the distribution function of the beam is investigated for the case of weak beam currents. It is shown that in the case of the Cherenkov interaction with whistlers the beam is retarded and the beam distribution function becomes wider and acquires a plateau region.

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.

Two-Wire Microwave Resonator Probe

Results are presented from theoretical and experimental studies of the influence of ponderomotive effects on the operation of a two-wire plasma microwave resonator probe. It is shown that the nonlinear regime of probe operation can be used to measure not only the plasma density, but also the plasma temperature.

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...