Influence of the pedestal on the interaction of a high-intensity, ultrashort laser pulse with a gas target

We report a study on the influence of the pedestal in laser–gas jet interaction experiments at very high intensity. The dynamic of the plasma resulting from the interaction is probed on a 3.8-ns time scale with a 60-fs time and a 5-µm space resolution using a Mach–Zehnder interferometer. It is observed that a plasma is created by the amplified spontaneous emission pedestal before the ultrashort pulse reaches the jet. We show that this preplasma modifies dramatically the propagation of the laser beam, which splits into two parts when it reaches the preplasma. The major part of the beam is strongly refracted by the preplasma while a small fraction is guided in the channel resulting from the hydrodynamic expansion of the preplasma.