ABSTRACT
The variation of average ionization of the molybdenum ion within the preplasma is shown in Fig.2, for (a) At = 2 ns and (b) At = 10 ns. Results show that at At=2 ns, the longitudinal pump beam is more strongly refracted due to the larger electron density gradient in the x direction. This results in a spreading of the beam and a reduction in the peak pump intensity at the exit end of the preplasma. Consequently, the ionization level is far below the Ni-like stage in these regions, which will produce a highly absorbing region and thus reducing the x-ray laser intensity. On the other hand, the average ionization distribution for At = 10 ns is similar to that calculated for the optimum At = 4 ns case. The simulations does not reveal any evident cause for the large reduction in the x-ray laser intensity for At = 10 ns. However, one possible explanation is the change in the preplasma expansion from a cylindrical to a spherical one at large delay times. Such a change will result in increased plasma cooling and lower ionization stages, thus producing regions with strong absorption.
