Defocusing effects of a picosecond terawatt laser pulse in an underdense plasma

Abstract The defocusing of a picosecond terawatt laser pulse in a plasma generated by the leading edge of the pulse is investigated in a vacuum chamber filled with a noble gas in the 10 mbar-1 bar pressure range. Results as a function of the gas pressure, the incident laser pulse and the gas nature are reported. The electron density, produced by multiphoton ionization of the neutral gas, builds up faster on the laser beam axis than on the edge. The plasma refractive index is then minimum on the beam axis. As a result, the plasma acts as a diverging lens for the laser beam.