Increasing lifetime of the plasma channel formed in air using picosecond and nanosecond laser pulses

We report experiments on a pump-probe configuration to elucidate the formation of a plasma channel by the hydrodynamic evolution of air breakdown in laser focus. A stable air breakdown was produced by focusing a picosecond laser pulse to create a shock driven plasma channel in the laser focus for propagating a nanosecond pulse. A four fold increase in the lifetime of the channel estimated by monitoring the temporal evolution of the fluorescence of a spectral line at 504.5nm of N+ transition 3pS3−3sP03 is reported. Assuming plasma in local thermal equilibrium plasma temperature of ∼8.2eV and an electron density of ∼1.4×1018cm−3 were determined using a Stark broadening of 649.2nm line of NII transition 3dD03−4pD3 in the channel. An enhancement in the electron density of the plasma channel was observed at the 7ns delay of the nanosecond laser pulse relative to the picosecond laser pulse.