Density fluctuation spectrum in whistler turbulence

We develop a nonlinear two-dimensional fluid model of whistler turbulence that includes effect of electron fluid density perturbations. The latter is coupled nonlinearly with wave magnetic field. This coupling leads essentially to finite compressibility effects in whistler turbulence model. We find from our simulations that despite strong compressibility effects, the density fluctuations follow the evolution of the wave magnetic field fluctuations. In a characteristic regime where large scale whistlers are predominant, the coupled density fluctuations are found to follow a Kolmogorov-like phenomenology in the inertial range turbulence. Consequently, the turbulent energy is dominated by the large scale (compared to electron inertial length) eddies and it follows a Kolmogorov-like k−7/3 spectrum, where k is a characteristic wavenumber.     

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 (N _b/N ₀)≪⁻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. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 6, 378–382 (25 March 1998)     

Equations describing the interaction between whistlers and magnetosonic waves

Simplified equations for the nonlinear interaction between whistlers and magnetosonic waves are formulated. These equations describe all the different branches for modulational instabilities of whistler waves, and lead to dispersion relations which are the same as those found from the full set of equations. Our new equations are much more convenient than previously used equations in describing nonlinear whistler wave phenomena.     

Dispersion properties and field structures of eigenmodes of nonuniform plasma waveguides in a longitudinal magnetic field

We study the field structure and dispersion properties of a hybrid eigenmode guided by a nonuniform magnetized plasma waveguide. It is shown that the rotational and quasi-potential waves contribute to the formation of such a mode in the whistler frequency range. Depending on the plasma density, the rotational component of the hybrid mode is determined by either waves with complex transverse wave numbers or whistler waves, or by true surface waves. In the presence of an axial nonuniformity of the plasma in a channel, the transverse field structure of the propagating mode changes, which is stipulated by changes in both the values of transverse wave numbers and their dependence on the radial coordinate. It is found that the spectrum of axial wave numbers of eigenmodes of a plasma waveguide undergoes a pronounced condensation when smoothing the waveguide walls. The damping of the hybrid mode of a nonuniform waveguide due to electron collisions is found and it is shown that collisional losses determine the damping of waves trapped in the waveguide in the experiments on ionization self-channeling of whistler waves. We have found the effect of “displacing” the strong field from the inner core to the background outer region of the waveguide with increasing plasma density on its axis and broadening background region. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 49, No. 7, pp. 607–617, July 2006.     

Numerical solution of the electron distribution function for ECRheating in magnetic mirror

The Fokker-Planck equation is solved with electron cyclotron resonance heating in a magnetic mirror with a fixed electrostatic potential on a loss-cone boundary and ECR heating. A self-consistent calculation of the plasma concentration justifies the fact that the degree of ionization of the ECR plasma is about 10%. The high values of the total electron energy (15-30 eV) are achieved. The electron concentration and the EVDF anisotropy degree are the nonmonotonic functions of the applied MW electric field. The basis for such dependencies is discussed. A linear perturbation analysis of the Vlasov equation together with with Maxwell's equations is briefly reviewed. The criteria for the start of unstable whistler oscillation growth are proposed. Using the simulated distribution function, the regimes where the unstable oscillations should to be taken into account are considered     

Development of ionization in nonequilibrium inert-gas plasmas in magnetogasdynamic channels

Effects accompanying the interaction of a flow of preionized inert gas with a magnetic field are studied: selective electron heating, the development of nonequilibrium ionization, and the onset of the ionization instability. Local and average densities and temperatures of the electrons are measured and the average ionization rate is determined. It is found that the average electron density increases as the magnetic induction is raised, in both stable and ionization unstable plasmas. The difference in the rates at which ionization develops in these two states is revealed. The mechanism for the coupling between the average ionization rate in an ionization unstable plasma and the spatial-temporal characteristics of the plasma inhomogeneities is established. Zh. Tekh. Fiz. 69, 56–61 (November 1999)     

Some unusual high dispersion whistler triplets recorded at a low L-value (L=1.17)

A detailed analysis of the VLF data collected during January to December, 2003 at the low latitude station Jammu (geomag. lat., 22° 26′ N; L = 1.17) has yielded some unusual VLF events like whistler triplets of very high dispersion D ∼ 70 − 90 sec^1/2. From the dispersion analysis of the whistler triplets, it is found that the individual whistlers of the triplets are one-hop high dispersion ducted whistlers having propagation path along higher and closely spaced L-values (L∼4.0) than the L-value of the recording station. The dispersion and occurrence characteristics of these high dispersion whistlers of the triplets are compared with those of the majority of low dispersion whistlers (D ∼ 30 − 40 sec^1/2) recorded during the same period. Generation and propagation mechanisms are briefly discussed.     

Quantum regime of a plasma‐wave‐pumped free‐electron laser in the presence of an axial magnetic field

The quantum regime of a plasma‐whistler‐wave‐pumped free‐electron laser (FEL) in the presence of an axial‐guide magnetic field is presented. By quantizing both the plasma whistler field and axial magnetic field, an N‐particle three‐dimensional Hamiltonian of quantum‐FEL (QFEL) has been derived. Employing Heisenberg evolution equations and introducing a new collective operator which controls the vertical motion of electrons, a quantum dispersion relation of the plasma whistler wiggler has been obtained analytically. Numerical results indicate that, by increasing the intrinsic quantum momentum spread and/or increasing the axial magnetic field strength, the bunching and the radiation fields grow exponentially. In addition, a spiking behavior of the spectrum was observed with increasing cyclotron frequency which provides an enormous improvement in the coherence of QFEL radiation even in a limit close‐to‐classical regime, where an overlapping of these spikes is observed. Also, an upper limit of the intrinsic quantum momentum spread which depends on the value of the cyclotron frequency was found.     

Ionization formation of plasma inhomogeneity by the near-zone field of a magnetic-type source in a magnetized plasma

An investigations is made of the steady-state structure of a plasma inhomogeneity arising as a result of high-frequency heating and additional ionization of a background magnetized plasma by the near-zone field of a magnetic-type source (ring electric current). It is assumed that the source axis is parallel to an external magnetic field; the source frequency belongs in the low hybrid band. The main attention is focused on the particular case (important for possible applications) when the characteristic longitudinal and transverse scales of density distribution considerably exceed the corresponding scales of distribution of the electron temperature and of the source field. Simplified equations for the near-zone field of the source, the electron temperature, and the plasma density are written for this particular case. Based on the numerical solution of these equations, steady-state distributions of plasma parameters in the formed plasma inhomogeneity are found. It is demonstrated that a plasma inhomogeneity proves to be markedly extended along the external magnetic field. It is found that, for the values of the source current that are attainable under the conditions of active ionospheric and model laboratory experiments, the maximum plasma density in a nonuniform plasma may appreciably exceed the background value.     

Whistler waves guided by ducts with enhanced density in a collisional magnetoplasma

We study the guided propagation of whistler waves along cylindrical ducts with enhanced density in a collisional magnetoplasma. It is shown that under certain conditions, the presence of comparatively small dissipative losses due to electron collisions in a plasma medium can lead to significant changes in the dispersion characteristics and field structures of whistler modes guided by such ducts compared with the case of a collisionless plasma. We present the results of numerical calculations showing such changes in the properties of whistler modes. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 51, No. 1, pp. 31–49, January 2008.     

Self-consistent mechanism for sustaining ionization waves in a low-pressure discharge

The possibility of constructing a self-consistent model for the sustaining of ionization waves is demonstrated for a low-pressure discharge in an inert gas. The model is based on the combined solution of the kinetic equation of the electrons and the equation of motion of the ions in a spatially periodic field. The distribution function is constructed in an experimentally measured field and then used to calculate the spatial distributions of the plasma density and the ionization rate. The solution of the equation of motion of the ions makes it possible to reconstruct a field similar to the original one. One specific feature of the mechanism considered is the pronounced nonlocal character of the formation of the electron distribution function by the entire nonuniform potential profile of the ionization wave. Zh. Tekh. Fiz. 67, 24–30 (February 1997)     

Matched filtering-parameter estimation method and analysis of whistlers recorded at Varanasi

The matched filtering technique is based on the digital-construction of theoretical whistlers and their comparison with observed whistlers. The parameters estimated from the theoretical and experimental whistler curves are matched to have higher accuracy using digital filters. This yields a resolution ten times better in the time domain. We have tested the applicability of this technique for the analysis of whistlers recorded at Varanasi. It is found that the whistlers have propagated along L>2 and have wave normal angles after exiting from the ionosphere such that they propagate towards equator in the earth-ionosphere wave-guide. High-resolution analysis shows the presence of fine structures present in the dynamic spectrum. An effort is made to interpret the results.     

Compression of whistler waves in a plasma with a nonstationary magnetic field

We present the results of our experiments in which the propagation of whistler waves in a plasma with a nonstationary magnetic-field perturbation (B=B₀+δB(t), δB/B₀ ≤ 5%) was investigated. The parametric and dispersive phenomena in a variable magnetic field were studied on the unique Krot plasma bench (the plasma column was 4 m in length and 1.5 m in diameter). A periodic field perturbation is shown to lead to an amplitude-frequency modulation of the whistler wave and to fragmentation of the signal into separate frequency-modulated wavepackets followed by their compression. The formation and compression of pulses is attributable to strong whistler group-velocity dispersion near the electron cyclotron frequency (ω ≤ ω_H). The results can be used to interpret the spectral shapes of the signals received from the Earth’s magnetosphere and ionosphere in the electron and ion whistler frequency ranges.     

Self-Focusing and Channeling of Wave Beams in a Nonuniform Magnetic Field under Conditions of Ionization Nonlinearity

We study experimentally the effect of ionization self-channeling of waves at the whistler frequencies in a nonuniform magnetic field. It is shown that the formed plasma nonuniformity localizes the radiation from a short high-frequency source inside a discharge channel stretched along an external magnetic field. We found a possibility to control the parameters of the formed plasma-wave channel as well as the dispersion characteristics and structure of wave fields in wide limits by varying the magnetic field in a specified spatial region. We propose a method for the formation of a plasma resonator and test this method in the laboratory experiment. The spatial plasma and field distributions in this resonator are similar to those along a geomagnetic field tube of the magnetospheric resonator. We reveal the plasma instability in such a resonator in the vicinity of the frequency of electron bounce oscillations between magnetic mirrors.     

RF breakdown by toroidal helicons

Bounded whistlers are well-known for their efficient plasma production capabilities in thin cylindrical tubes. In this paper we shall present their radio frequency (RF) breakdown and discharge sustaining capabilities in toroidal systems. Pulsed RF power in the electronmagnetohydrodynamic (EMHD) frequency regime is fed to the neutral background medium. After the breakdown stage, discharge is sustained by toroidal bounded whistlers. In these pulsed experiments the behaviour of the time evolution of the discharge could be studied in four distinct phases of RF breakdown, steady state attainment, decay and afterglow. In the steady state average electron density of ≈10¹² per cc and average electron temperature of ≈20 eV are obtained at 10⁻³ mbar of argon filling pressure. Experimental results on toroidal mode structure, background effects and time evolution of the electron distribution function will be presented and their implications in understanding the breakdown mechanism are discussed.     

The Impact of Direct Ionization by Beam Electrons on the Electron Kinetics of the Beam Discharge Plasma in Molecular Hydrogen

In a recent paper the stationary beam plasma discharge in partially dissociated hydrogen was investigated where the electron component was described by the Boltzmann equation for a mixture of atomic and molecular hydrogen and the main heavy charged and neutral particles by balance equations. It was assumed that, via the quasilinear beam plasma interaction, the electron beam produces only the turbulent electric field whilst an additional production of plasma electrons due to direct ionization by the beam and thus a direct influence on the balances of charge carriers were neglected. Now the additional production of plasma electrons due to direct ionization by the beam is studied on the basis of a generalized Boltzmann equation but for the simpler model of a purely molecular hydrogen plasma. For experimentally obtainable values of the turbulence energy density, beam energy, beam ionization degree and electron life time the calculation of the electron energy distribution function and of the direct beam contribution to the electron particle balance shows a marked influence of the direct beam ionization with increasing degree of beam ionization.     

Anomalous kinetics of plasmas in high-power radiation fields

Transport equations are obtained in the nine-moment approximation for plasmas in intense radiation fields where the amplitude of the electron oscillations in the electromagnetic field exceeds the thermal speed. It is shown that for plasmas with a high degree of ionization, Z, the electron thermal conductivity is higher by approximately a factor of Z. The change in the frictional force on electrons colliding with ions owing to the effect of the radiation field leads to the possibility of electron acceleration and to a change in the sign of the dc and low-frequency electrical conductivities. Zh. éksp. Teor. Fiz. 111, 478–495 (February 1997)     

Nonsymmetric Whistler Waves Guided by Cylindrical Ducts with Enhanced Plasma Density

We study the guided propagation of whistler waves whose fields depend on the azimuthal angle in cylindrical plasma-waveguide channels (density ducts) aligned with an external magnetic field and surrounded by a uniform magnetoplasma. The main attention is paid to ducts with enhanced plasma density. It is shown that, under certain conditions, such ducts are capable of guiding proper (eigen) modes and improper leaky modes. We present the results of analysis of the dispersion properties and field structures of nonsymmetric modes guided by cylindrical ducts in the whistler frequency range.     

Whistler channeling in ducts with enhanced density in a magnetoactive plasma

We study the guided propagation of whistler waves along the density ducts created in a magnetoactive plasma under thermal nonlinear conditions. It is found that the thermal diffusion-driven redistribution of plasma due to electron heating in the quasistatic field of a current loop of fairly large radius leads to the formation of a duct with enhanced density. Based on experimental data and theoretical calculations, it is shown that such a duct can sustain the weakly decaying whistler modes excited by a magnetic-type antenna immersed in it. Radiophysical Research Institute, Institute of Applied Physics, Russian Academy of Sciences, Nizhny Novgorod, Russia. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 41, No. 3, pp. 384–394, March, 1998.     

Whistlers and negative ions

Using an appropriate model of the ionosphere, we find the travel time for proton whistlers to go from their source to an observer at a satellite. The results differ from earlier ones. The physical parameters obtained through whistler observations agree with these results. Damping and attenuation of whistlers are related to the ionospheric parameters.