Characteristics of self-guided laser plasma channels generated by femtosecond laser pulses in air.

With ultrashort laser pulses (25 fs) and low energy (E=15 mJ), we observe laser plasma channels with a length over 5 m in air. The diameter of the plasma channel is measured to be 120 microm. The average electron density in the channel is inferred to be about 10(18) W/cm(3) with an interferometric method. At the same time, the temporal evolution of the electron density in the plasma channel is investigated. The resistivity in the plasma channel is measured to be less than 1 Omega cm. It is suggested that the lifetime of the plasma channel can be greatly prolonged by launching another long laser pulse along the plasma channel.