Abstract.  Plasma tubular waveguides with relatively low electron density n_e~10¹¹–10¹⁴ cm^-3 in a wall, being produced in air via multi-photon ionization by UV laser beam of annular cross section, can effectively transport microwave (MW) radiation with laser divergence and low attenuation. As an optical density of the plasma is less than that of the air, a total internal reflection of the MW with wavelength λ at air-plasma boundary takes place for plasma waveguide radii R>>λ. Such sliding-mode regime for the MW looks like light transfer in an optical fiber, and it is quite different of a conventional approach utilizing high-density (n_e~10¹⁵-10¹⁷cm^-3) plasma waveguides of R~λ similarly to metal waveguides. In the latter case energy consumption to produce plasma is rather high and multi-terawatt peak power of sub-ps laser pulses are required, whilst low conductivity of the plasma (several orders of magnitude less than that of the metal one) severely restricts a propagation length.

Experimental and theoretical studies of the sliding MW regime inside a hollow plasma waveguide were performed. The plasma waveguide was considered as air cylinder bounded by the plasma layer which thickness is much larger than the transverse penetration depth of the MW field. The axial-symmetric transverse magnetic (TM) E_0n and transverse magnetic (TE) H_0n modes, as well as the lowest axial-symmetric EH₁₁ mode were analyzed by solving the dispersion equations. The threshold of the sliding regime was found, for which transverse wavenumbers were much smaller (in ~ λ / R times) than the longitudinal ones. Experimentally effective channeling and transfer of the 35.3-GHz (λ=8.5 cm) MW signal along 60-m distance has been demonstrated being in a good agreement with theoretical estimations. UV radiation of GARPUN KrF laser (λ=248 nm) with intensity I~10⁷ W/cm² and 100-ns pulse duration formed a hollow plasma waveguide of the internal radius R=5 cm and plasma wall thickness ΔR≥1 cm. Electron density in the wall n_e ~10¹²cm^-3 was achieved by adding volatile hydrocarbon vapor into laboratory air. A train of sub-ps KrF laser pulses with 2-ns interval and total energy ~500 J was shown to be capable formation of a virtual plasma waveguide in a pure air with MW propagation distance ~ 1 km and with divergence ~10^-5 rad. This work was supported by RFBR grant No. 09-07-13593-OFI_ts.