On Increasing the Efficiency of Penetration of an Electromagnetic Wave into a Dense Plasma Layer

Abstract

UDC 533.9+537.8 We study a method for increasing the efficiency of penetration of an intense electromagnetic wave into a layer of dense plasma. The method is based on the phenomenon of spatio-temporal synchronism of the nonlinear dynamic structures, which are excited in the layer, with the modulated wave incident on the layer. It has been established that in the case of a relatively weakly overdense plasma, the efficiency of constant-amplitude wave penetration into the plasma layer is determined by a single nonlinear parameter, namely, the ratio of the intensity of the incident radiation to the degree of plasma super-criticality, and increases with an increase in this parameter. It is found that a modulation of the amplitude of an incident wave, whose period is a multiple of the soliton generation period, can lead to a significant increase in the penetration efficiency. It is well known that the interface between two transparent media can efficiently reflect the radiation which is incident on it. An example of this phenomenon in optics can be the effect of total internal reflection, on which the operation of many optical devices is based. When the ionosphere is studied, researchers make extensive use of ionosphere sounding with radio waves of the radio-frequency (RF) band and analysis of the parameters of the signal reflected from the dense plasma of the F layer [1–3]. However, in the case of strong radiation, one should allow for changes in the properties of the matter in the presence of the electromagnetic field in it [4–7]. For example, due to the striction decrease in the plasma density in a strong electromagnetic field, an incident wave can penetrate partially into the depth of the dense plasma layer under certain conditions [8–10]. In the ionosphere plasma, which is characterized by the density that increases smoothly on the scale of the radio wavelength, the striction self-action consists in the deformation of the spatial distribution of the plasma density and a shift of the wave reflection point [11, 12], formation of field solitons and cavitons (holes) of plasma density and nonstationarity of the reflection, which manifests itself, in particular, in the effects of “superreflection” and “afteraction” [2, 13–19]. A homogeneous layer of overdense plasma with a finite thickness can become transparent for intense radiation due to soliton excitation by the incident wave at the front “illuminated” boundary of the layer and radiation of energy of these solitons at the rear “dark” boundary of the layer [20, 21]. Similar effects of light penetration in the form of jets excited by light beams, which are incident on the interface boundary at angles exceeding the angle of total internal reflection, are observed in nonlinear optical media [22, 23]. However, the problem about the methods of increasing the efficiency of penetration of intense radiation beams into a medium being opaque for low-amplitude waves and their passage through finite-width layers has not been properly examined adequately so far. In this work, we use the example of a layer with homogeneous dense nonlinear plasma irradiated by a TE mode to find the coefficient of penetration of the incident wave into the layer as a function of the plasma density and amplitude and demonstrate the methods of increasing the efficiency of this penetration.