Conductivity of self-guided laser plasma channel produced by femtosecond laser pulses in air

The conductivity of the plasma channel are shown in detail by electrical detecting method on the conditions of different focal lengths, different laser energies and polarities of the external voltage. The experimental results show that there are long ionization channels after lens with different focal length. The equivalent resistivity of the channels change slightly, and usually the maximum value of resistivity appears near the geometric focal point. If the focal length is longer, the peak point would get closer to the geometry focal point. With voltage of different polarity, the electrostatic field force and Lorentz force as well as the ponderomotive force will together change the current of plasma channel. In front of the focal point, the variation of the current is not obvious, and the