Electron and ion stagnation at the collision front between two laser produced plasmas

We report results from a combined optical interferometric and spectrally resolved imaging study on colliding laser produced aluminium plasmas. A Nomarski interferometer was used to probe the spatio-temporal distribution of the electron density at the collision front. Analysis of the resulting interferograms reveals the formation and evolution of a localised electron density feature with a well defined profile reminiscent of a stagnation layer. First signs of electron stagnation are observed at a time delay of 10 ns after the peak of the plasma generating laser pulse. The peak electron density was found to exceed 1019 cm−3 and the layer remained well defined up to a time delay of ca. 100 ns. Temporally and spectrally resolved optical imaging was also undertaken to compare the Al+ ion distribution, with that of the 2D electron density profile. This revealed nascent stagnation of singly charged ions at a delay time of 20 ns. We attribute these results to the effects of space charge separation in the seed plasma plumes.