Ponderomotive force of the low-frequency field and modulational instability of drift waves

A study is made of the processes that occur in an inhomogeneous nonisothermal plasma in a strong external magnetic field and whose characteristic frequencies are lower than the ion Langmuir frequency but higher than the collision frequency. An expression for the ponderomotive force of the low-frequency field is derived. The excitation of a long-wavelength low-frequency drift wave during the development of the modulational instability of a drift pump wave is investigated. The growth rates of the instability are obtained, and the conditions for its onset are determined. The possible relation of the modulational instability to the formation of structures in the plasma is discussed.     

Suppression of the current instability by decay processes

A study is made of the instability of lower hybrid waves in an isothermal magnetized current-carrying plasma. It is assumed that the dynamic suppression of the current instability is associated with the excitation of a quasi-monochromatic wave near the instability threshold and its subsequent decay into two strongly damped lower hybrid waves. It is shown that the decay process results in the onset of either a quasi-periodic or stochastic nonlinear stabilization regime involving a small number of modes. The anomalous resistance is estimated.     

Decay instability of a lower hybrid wave

A study is made of the decay instability of a lower hybrid wave with a finite wave vector (k ₀≠0) and a large amplitude such that the oscillatory velocity of the electrons with respect to the ions cannot be neglected. It is shown that, depending on the angle between the propagation direction of the lower hybrid wave and the external magnetic field and the angle through which the wave is scattered, the decay instability is primarily governed either by the oscillatory electron motion with respect to the ions or by the nonlinear response of the plasma to the lower hybrid wave propagating in it. The role of the nonlinear frequency shift in the saturation of the lower hybrid decay instability is clarified.     

Parametric excitation of surface waves at the boundary between a magnetized plasma and a metal

A study is made of the parametric excitation of potential surface waves propagating in a planar plasma-metal waveguide structure in a magnetic field perpendicular to the plasma-metal boundary. An external, spatially uniform, alternating electric field at the second harmonic of the excited wave is used as the source of parametric excitation. A set of equations is derived that describes the excitation of surface waves due to the onset of decay instability. Expressions for the growth rates in the linear stage of instability are obtained, and the threshold amplitudes of the external electric field above which the parametric instability can occur are found. Analytic expressions for the saturation amplitudes are derived with allowance for the self-interaction of each of the excited waves and the interaction between them. The effect of the plasma parameters and the strength of the external magnetic field on the saturation amplitude, growth rates, and the threshold amplitudes of the pump electric field are analyzed.     

Excitation of electron Langmuir frequency harmonics in the solar atmosphere

An alternative mechanism for the excitation of electron Langmuir frequency harmonics as a result of the development of explosive instability in a weakly relativistic beam-plasma system in the solar atmosphere is proposed. The efficiency of the new mechanism as compared to the previously discussed ones is analyzed.     

Low-threshold parametric excitation of the upper hybrid wave in experiments on electron-cyclotron resonance heating by an ordinary wave

The possibility of the low-threshold decay of an ordinary wave into an upper hybrid wave localized in a plasma column (or in an axisymmetric plasma filament) and a low-frequency wave is analyzed. It is shown that the threshold for such a decay, accompanied by the excitation of an ion-acoustic wave, can easily be overcome for plasma parameters typical of model experiments on the Granit linear plasma facility.     

Nonlinear mechanism for the generation of electromagnetic fields in a magnetized plasma by the beatings of waves

The modulational instability in a plasma in a strong constant external magnetic field is considered. The plasmon condensate is modulated not by conventional low-frequency ion sound but by the beatings of two high-frequency transverse electromagnetic waves propagating along the magnetic field. The instability reduces the spatial scales of Langmuir turbulence along the external magnetic field and generates electromagnetic fields. It is shown that, for a pump wave with a sufficiently large amplitude, the effect described in the present paper can be a dominant nonlinear process.     

Nonresonant excitation of surface waves by a normal electric field

A study is made of nonresonant parametric excitation of surface waves by a spatially uniform, time-dependent electric pump field directed perpendicular to a plane plasma-dielectric interface. A set of equations is derived that describes the dynamics of surface wave excitation. Expression for the growth rate in the linear stage of instability is obtained, and the threshold amplitude of the external electric field above which the parametric instability can occur is found. The spectrum of the excited waves is analyzed. Published in Russian in Fizika Plazmy, 2006, Vol. 32, No. 11, pp. 994–998. The article was translated by the author.     

Spectra of langmuir waves in a magnetized plasma with low-frequency turbulence

A study is made of the formation of the spectra of Langmuir waves excited as a result of the development of beam-plasma instability in a collisionless magnetized plasma with low-frequency turbulence. Equations are derived that describe the dynamics of the formation of spectra in the quasilinear statistical approximation.The equations obtained account for small-and large-angle scattering of the electron-beam-excited waves by given background plasma density fluctuations. The scattering of Langmuir waves leads to the redistribution of their energy in phase space and, under appropriate conditions, to the appearance of a characteristic dent in the wave spectra in the frequency range where the spectral intensity is maximum. Numerical simulations carried out for plasma parameters typical of the polar cap of the Earth’s magnetosphere help to explain the shape of the spectra of Langmuir waves that were recorded by the Interball-2 satellite when it was flying through this magnetospheric region.     

Self-consistent model of an inductive RF plasma source in an external magnetic field

A theory is developed that makes it possible to calculate RF power absorption in an inductive plasma source. Conditions are determined under which most of the power is deposited in the plasma. It is shown that these conditions correspond to the excitation of spatial waves (an oblique Langmuir wave and a helicon wave). A simple self-consistent model of a plasma source is proposed that describes all of the experimentally observed distinctive properties of plasma sources well.     

Generation of ion-acoustic and magnetoacoustic waves in an RF helicon discharge

A study is made of the generation of ion-acoustic and magnetoacoustic waves in a discharge excited in an external magnetic field by an electromagnetic wave in the whistler frequency range (ω_LH ? ω ? ω_He, where ω_LH = $\sqrt {\omega _{He} \omega _{Hi} } $ and ω_He and ω_Hi are the electron and ion gyrofrequencies, respectively). The excitation of acoustic waves is attributed to the decay of a high-frequency hybrid mode forming a plasma waveguide into low-frequency acoustic waves and new high-frequency waves that satisfy both the decay conditions and the waveguide dispersion relations. The excitation of acoustic waves is resonant in character because the conditions for the generation of waveguide modes and for the occurrence of the corresponding nonlinear wave processes should be satisfied simultaneously. An unexpected effect is the generation of magnetoacoustic waves by whistlers. A diagnostic technique is proposed that allows one to determine the thermal electron velocity by analyzing decay conditions and dispersion relations for waves in the discharge channel.     

Decay instability of a laser pulse propagating in a transverse direction in a magnetized plasma

The decay instability of a lser pulse propagating across an external magnetic field in a subscritical plasma is investigated analytically and numerically. It is shown that, when the relaxation of the pulse is taken into account, the hydrodynamic growth rate of the decay instability is slower than that obtained earlier in the constant-amplitude pump wave approximation. The results of numerical simulations by a particle-in-cell method demonstrate that an increase in the amplitude of the parametrically excited waves is accompanied by a decrease in their group velocity; in this case, up to 85% of the laser energy is converted into the energy of the plasma particles. It is found that, under resonance conditions, the magnetic field acts to increase the energy of the accelerated ions that escape from the plasma slab through its front boundary.     

Nonlinear regimes of Farley-Buneman instability

The dynamic suppression of the instability of a quasi-monochromatic wave by nonlinear wave-wave interaction is considered. It is shown that, near the threshold of linear instability, the process of decay into two strongly damped waves leads to the onset of a quasi-periodic or a stochastic nonlinear stabilization regime involving a small number of modes. A case study is made of the Farley-Buneman instability in an isothermal magnetized current-carrying plasma in which particle collisions play an important role. Typical characteristic features of different stabilization regimes are analyzed as functions of current and other plasma parameters.     

Quantum theory of Cherenkov beam instabilities in plasma

A quantum theory of stimulated Cherenkov emission of longitudinal waves by an electron beam in an isotropic plasma is presented. The emitted radiation is interpreted as instability due to the decay of the de Broglie wave of a beam electron. Nonrelativistic and relativistic nonlinear quantum equations for Cherenkov beam instabilities are obtained. A linear approximation is used to derive quantum dispersion relations and to determine the instability growth rates. The mechanisms for nonlinear saturation of quantum Cherenkov beam instabilities are investigated, and the corresponding analytic solutions are found.     

Excitation of inertial Alfvén waves via modulational interaction with lower hybrid waves

The concept of modulational instability, which results from the coupling of waves modes of very different time and space scales, was introduced to plasma physics through an elegant paper by Vedenov and Rudakov in 1964 [1]. Our paper is devoted to the theory of modulational instability resulting from the interaction of lower hybrid waves and slow density perturbations associated with inertial Alfvén waves. The nonlinear set of equations describing the modulational coupling of these two types of waves is constructed. The lower hybrid wave trajectories are analyzed within predefined density structures and it is shown that these waves can be trapped in the vicinity of the density extremum. The density modulations, originally being associated with inertial Alfvén waves, deepen due to the trapping of lower hybrid waves; this leads to modulational instability. A dispersion relation describing the modulational instability is constructed and analyzed. The threshold intensity of the lower hybrid waves for the onset of instability is obtained and it is shown that instability can serve as an efficient mechanism for the excitation of inertial Alfvén waves in the auroral ionosphere.     

Theoretical characterization of excitation energy transfer in chlorosome light-harvesting antennae from green sulfur bacteria

We present a theoretical study of excitation dynamics in the chlorosome antenna complex of green photosynthetic bacteria based on a recently proposed model for the molecular assembly. Our model for the excitation energy transfer (EET) throughout the antenna combines a stochastic time propagation of the excitonic wave function with molecular dynamics simulations of the supramolecular structure and electronic structure calculations of the excited states. We characterized the optical properties of the chlorosome with absorption, circular dichroism and fluorescence polarization anisotropy decay spectra. The simulation results for the excitation dynamics reveal a detailed picture of the EET in the chlorosome. Coherent energy transfer is significant only for the first 50 fs after the initial excitation, and the wavelike motion of the exciton is completely damped at 100 fs. Characteristic time constants of incoherent energy transfer, subsequently, vary from 1 ps to several tens of ps. We assign the time scales of the EET to specific physical processes by comparing our results with the data obtained from time-resolved spectroscopy experiments.     

Effect of the spectral composition of light on the mechanism of reaction of chlorophyll oxidation]

Photochemical oxidation of chlorophyll "a" on excitation of the pigment in different spectrum region (400-800 nm) was studied by flash photolysis. The absorption spectrum of chlorophyll cation-radical Chl+ was obtained and the values of extinction coefficient found. An attempt was made of photochemical generation of dication form of chlorophyll. Thermodynamical calculation supporting the possibility of the following reaction is presented: (Chl+)*+Ae -- Chl2"Ae. The absence of Chl2+ is explained by a short life time of the excited cation-radical of chlorophyll (Chl+)*. The effect of the wave length of excited light on the kinetics of the decay of chlorophyll cation-radical is studied. It is shown that on excitation of chlorophyll "a" with white light the life time of Chl+ decreases and its death is described by an equation of first order, which is explained by the formation of ion-radical of the electron acceptor resulting from the direct excitation of Ae.     

The DNLS equation and parametric decay instability

A model that describes the interaction of nonlinear Alfvén packets propagating in opposite directions parallel to the ambient magnetic field is constructed. This model incorporates both (i) the parametric interaction of harmonics propagating in the same direction, which can be responsible for the transportation of the wave energy to the short-wavelength region of the spectrum, and (ii) the parametric interaction of Alfvén waves propagating in opposite directions, which can be responsible for the excitation of backward-propagating waves by the parametric decaylike instability of the forward-propagating fluctuations.