Investigation of Zn and Cu prepulse plasmas relevant to collisional excitation x-ray lasers

Abstract

This paper presents the results of a comparative experimental study of low-temperature Zn and Cu line plasmas created on slab targets by 400-ps laser pulse producing irradiance from $4\ifmmode\times\else\texttimes\fi{}{10}^{9}$ to ${10}^{11}{\mathrm{W}\mathrm{}\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}.$ The aim was to examine the nanosecond-scale postpulse evolution of plasmas created in conditions equivalent to those produced by prepulses in collisional x-ray lasers, of elements that have neighboring atomic numbers but very different material properties. The plasmas were interferometrically probed at 4 and 10 ns next to the driving pulse, using geometry that made it possible to obtain an authentic two-dimensional (2D) electron density pattern in the plane perpendicular to the plasma axis. VIS-IR spectroscopy and imaging were used to provide an indication of the electron temperature and volume of the plasma layer near the target. We observe that over the whole range of the applied irradiances the characteristics and/or the expansion history of the Zn and Cu plasmas are very different. For irradiance exceeding a threshold specific to each element the density patterns exhibit an unexpected structure characterized by symmetrical flanks strongly localized in space, suggesting plasma is generated in addition to that produced within the laser pulse duration. The results imply that during the postpulse time the energy coupling between the plasma and the target is substantial for the plasma flow that exhibits a complex 2D character. A comparison of the data and results of a 1.5D hydrodynamic simulation for ${10}^{11}{\mathrm{W}\mathrm{}\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}$ is made, indicating reasons for problems of such models in the treatment of the plasmas in question, and thus in the treatment of small-prepulse action in some x-ray laser systems.