The possibility of formation of a virtual guiding system from a bunch of plasma channels of the filaments of femtosecond laser pulses for transmission of microwave radiation in air is considered. The effective conductivity and the skin depth of the laser plasma of such filaments are calculated for the microwave band. Optimum spatial configurations of plasma channels corresponding to a single-wire transmission line and a hollow cylindrical waveguide are proposed. Estimates of the energy loss of microwave radiation in plasma waveguides of various configurations are obtained.

Summary form only given. Ultrashort relativistic electron bunches have a large variety of applications in modern physics. For example, such bunches can be used for the injection in conventional accelerators, for ultrafast spectroscopy of atoms, for generation of coherent X-ray radiation and so on. The most important parameters in these cases are the thickness of the bunch, the density of the electrons inside the bunch and the homoenergeticity of the bunch electrons. The generation of the dense relativistic electron bunches by a high-power electromagnetic pulse with a sharp rising edge during its interaction with a thin plasma layer (foil) in vacuum is considered in this paper. If the super-intense laser pulse has a dimensionless amplitude of the field a/sub 0//spl Gt/1 and a sharp rising edge then the electrons of the plasma layer can synchronously achieve the ultrarelativistic longitudinal velocities just during the first half-cycle of the external field. In this case, the produced relativistic electron bunch can be stable for some time until the Coulomb forces destroy it. The two-dimensional PIC simulation of this process was made for investigation of the parameters of the generated electron bunches. It is shown that at the initial stage of acceleration the effective compression of the bunch takes place in longitudinal direction therefore the thickness of the relativistic bunch is considerably reduced and becomes significantly smaller than the bunch transverse dimensions (such bunches can be called the electron mirrors). The dependence of the bunch lifetime on the acceleration parameter a/sub 0/, initial thickness of the plasma layer and the electron density inside the bunch is investigated. It is shown that for the sharp enough rising edge of the laser pulse the homoenergeticity of the bunch electrons can be very high.