Investigation of prepulse plasmas for collisional excitation X-ray lasers

摘要

We present results of a comparative experimental study of Zn (Z=30) and Cu (Z=29) line plasmas created on slab targets by irradiances ranging from approx4x10~9 Wcm~(-2) to approx10~(11) Wcm~(-2), equivalent to those produced by low-intensity prepulses in collisional excitation systems. The plasma parameters studied were the 2D electron density profiles in times 4 and 10 ns subsequent to the laser pulse, the time-integrated electron temperature, and the time-integrated plsma selfemission in the VIS spectral region. It was found that in the whole range of the applied irradiances the Zn and Cu plasmas significantly differ from each other, yet both exhibit a complex density pattern revealing a crusical role of energy coupling into the target and suggesting that plasma is also generated after the laser pulse terminates. The data indicate that stronger plasma-solid coupling exists for Cu, resulting in a plasma laterally more spread-out than in the case of Zn targets. Under a given irradiance, these provide a higher local electron density and a larger density scalelength. From the results we conclude that the effect of prepulses inducing irradiance up to 10~(11) Wcm~(-2) is prone to be critically dependent on the solid state properties of the target material.