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

With ultrashort laser pulses (25 fs) and low energy $(E=15\mathrm{}\mathrm{mJ}),$ we observe laser plasma channels with a length over 5 m in air. The diameter of the plasma channel is measured to be $120\mu \mathrm{m}.$ The average electron density in the channel is inferred to be about ${10}^{18}{\mathrm{W}/\mathrm{c}\mathrm{m}}^3$ 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 $1\Omega \mathrm{cm}.$ It is suggested that the lifetime of the plasma channel can be greatly prolonged by launching another long laser pulse along the plasma channel.