Transient ultrahigh gains as a diagnostic in short-pulse heated selenium plasmas

The influence of intense picosecond heating and time-dependent ionization on the population dynamics of Ne-like selenium is investigated. We find that highly nonequilibrium plasma conditions and a large gain coefficient >500 cm^-1 can be generated for the J=0--1 line at 182 {} at KrF laser intensities >3×{}10¹⁶ W/cm². Thus, the observation of x-ray lasing at or near the critical surface of a KrF short-pulse, laser heated plasma appears feasible. Its observation would provide important tests of theoretical models for laser absorption, heat conductivity, ion heating, transient ionization, and x-ray laser pumping. The calculations in this paper help to define the minimum intensities and energies of the KrF laser that would be needed to carry out these x-ray amplification experiments using 2 ps full width at 1/e maximum pulses. The emission pattern of the amplified x rays would also provide useful diagnostics for the gradient and velocity structure of the blowoff plasma near the critical surface.