Plasma production by multiphoton ionization: Density inhomogeneities due to ponderomotive effects

The generation of long and homogeneous plasmas by multiphoton ionization of low‐pressure gases—H2, D2, and N2 at pressures of a few Torr—with a high‐power laser at λ=0.53 μm and intensities of the order of 1015 W/cm2 has been studied. The temperature and density of the plasma are measured by Thomson thermal scattering. After the initial full ionization the hydrodynamic density evolution with time is observed. The comparison with a simple model shows that this evolution is mainly due to the ponderomotive force of the laser beam itself. Typical density variations range from 1% to 5% per 100 psec.