Relativistic-nonlinear resonant absorption and generation of harmonics of electromagnetic radiation in an inhomogeneous plasma

Since the pioneering work of V L Ginzburg and N G Denisov, who discovered the effect of linear absorption of electromagnetic radiation in an inhomogeneous plasma due to strong plasma resonance at near-critical density, the theory of this phenomenon has not been developed steadily in application to strong electromagnetic fields of practical interest today, primarily laser ones. This review is devoted to a systematic presentation of the results of an analytic relativistic-nonlinear theory of resonant absorption and generation of laser radiation harmonics in an inhomogeneous plasma with a strongly manifested plasma electron nonlinearity up to laser light intensities that make the motion of electrons in the vicinity of the critical density relativistic. Using the methods of the modern theory of transformation groups, we describe the structure of the nonlinear electromagnetic field in a plasma resonance, nonlinear effects of the suppression of the resonant absorption coefficient and the angle shift of the resonance absorption curve with increasing laser intensity, the formation of sufficiently smooth spectra of laser light harmonics, decreasing in accordance with a power law, the spectrum of laser light harmonics emitted by plasma near the breaking threshold of a nonlinear plasma field, and the basic properties of the electrostatic field generated in the vicinity of the critical density.