“…The propagation of electromagnetic radiation in nonequilib rium gaseous or plasma media is one of the most difficult problems in modern electromagnetic theory. Such a problem is of importance in the analysis of the generation of electro magnetic radiation from THz to EUV and soft xray frequen cies in plasma channels formed in gases by powerful laser pulses [1][2][3][4][5], the filamentation of laser radiation and lasing in the atmosphere [6][7][8][9], remote controls [10], long distance transportation of microwave radiation [11,12], communica tion through the atmosphere or ionosphere [13][14][15][16] and many other practical applications. Typically, all these physical pro cesses can be studied in frames of the slowvarying ampl itude (SVA) approximation [17]: the electric field strength in an electromagnetic linearly polarized wave of frequency ω propagating in an inhomogeneous medium with spatially nonuniform susceptibility χ( r) along the z-direction can be represented in the form E( r, t) = E 0 ( r, t) exp(i(kz − ωt)), (1) where k = ω/c is the wave vector and E 0 ( r) is the slowly varying amplitude (|∇E 0 | kE 0 , |∂E 0 /∂t| ωE 0 ).…”