ABSTRACT
Since gain in optical–field–ionised (OFI) plasmas was reported by the groups at RIKEN [1] and Stanford |2], the interest in plasmas created by subpicosecond laser pulses, as a medium for X–ray lasers, has increased significantly. However, the experimental results differ strongly from the theoretical predictions within the recombination scheme. A gain coefficient of 20 cm-1 has been published by the group from RIKEN [1] at relatively low ion densities. There are indications that gain of 35 cm-1 was obtained at Berkeley/LLNL [3] and of 17 cm-1 at Princeton [4], but a strong scatter of data reported makes the quoted values uncertain. It is believed that this marked data scattering was caused by the strong refractiom of the pump radiation due to the: intensive ionization. Plasma preforming was used in all these experiments.
