Mikhail V. Starodubtsev

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...

Resonant Cherenkov emission of whistlers by a modulated electron beam

Emission of whistler waves through the Cherenkov resonance mechanism has been studied in a laboratory experiment modeling the interaction of a modulated electron beam with a magnetized plasma under conditions relevant to space experiments involving electron beam injection. The characteristics of the emission and the propagation of the radiated whistlers, and namely the features of their directive radiation patterns, have been controlled as a function of the beam and plasma parameters. This beam radiation process can be distinguished from another mechanism of wave emission, the nonresonant transition radiation from the beam injection point, owing to adequate choices of the physical conditions.

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.

LOW- FREQUENCY SHEATH INSTABILITY STIMULATED BY AN ENERGETIC ION COMPONENT

Spontaneous low-frequency oscillations have been observed in the circuit of a positively biased electrode immersed in a non-Maxwellian laboratory plasma containing an energetic ion component produced by the resonant absorption of a short microwave pulse in a nonuniform plasma column. The oscillations are found to be due to an instability of the electron-rich sheath. The instability with its characteristic frequency below the ion plasma frequency is driven by the energetic ion component reflected in the sheath area. A qualitative model of the instability is suggested.

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.

Whistler excitation by short current pulses in a magnetoplasma

In the frame of laboratory studies devoted to whistler excitation by a modulated electron beam injected in a magnetoplasma under conditions relevant to space experiments, fast processes have been investigated using the modulated electron beam as a train of short-time current pulses injected in the plasma. Single pulses of duration of the order of τ≃15 ns as well as unmodulated beams with sharp fronts (step functions with rise time of the order of τ≃10 ns) have been injected in the afterglow plasma with an energy of the order of 300 eV. Plasma responses to the fast perturbations are transported by whistler waves in the frequency range ωlh≪1/τ<ωc<ωp. Different types of responses have been evidenced and characterized, depending on the nature of the whistler excitation mechanism involved: Nonresonant transition radiation from the beam injection point and Cherenkov resonant emission. Both emissions could be distinguished one from each other owing to adequate choices of plasma parameters and observation conditi...

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.

Observation of ion wave streamers and low frequency sheath instability by the resonant absorption due to nonlinear interaction of microwave-plasma

Unmagnetized, inhomogeneous laboratory plasma irradiated by an oblique p-polarized microwave with pulse length 0.2–1.5 μs and power P=1–2 kW is studied. The incident electromagnetic wave is linearly converted into an electrostatic plasma wave when the incident wave frequency ω0 is equal to the local plasma frequency ωp. The localized linear enhancement of the driven oscillating field can lead to nonlinear phenomena driven by the ponderomotive force, which expels electrons from the resonance region, and the resulting ambipolar electrostatic fields also expel the ions, creating density cavities at the resonance region. Expelled ions tend to form an ion bunch and accelerate up to energies greater than 10 kTe. After all these processes are achieved, it has been observed in the experiment that the density cavity develops as ion wave streamers and propagate both up and down the density gradient from the resonant layer. It is observed that the downward streamer velocity Vdown and upward streamer velocity Vup hav...

Oblique electron-beam injection into plasma: effect of external magnetic field upon gun environment

When injecting an electron beam with a significant pitch-angle in a magnetoplasma, one has to take care that the parallel velocity of the electrons measured in the plasma does not only depend on the acceleration potential between the anode and the cathode of the electron gun, but also on the amplitude of the ambient magnetic field. The latter can play an important role when the actual acceleration distance of electrons in the gun vicinity, determined partly by collective processes that lead to potential modification in the gun environment, is large enough. Consequently, the actual beam pitch-angle can be noticeably different from the orientation of the electron gun.