Abstract

In the recombination x-ray laser, the ionization distribution of the plasma medium must be far from Saha-Boltzmann equilibrium. Usually, the plasma is cooled by expansion, electronic thermal conduction, and radiation. Because the cooling rate is limited, the abundance of the recombining ions is greatly reduced when the plasma is cooled. When the x-ray-laser wavelengths scale to the water window, the problem is even more serious. If the plasma is pumped by an intense x-ray beam and the energy is just above the ionization energy from ground state so that the heating produced by photoionization can be neglected, the heating produced by inverse bremsstrahlung absorption and photoionization from the excited state can also be neglected because the frequency of the incident x-ray is very large and the cross section of photoionization from the excited state is very, small. Because of the photoionization, the amount of recombining will decrease slowly, or will even increase, when the plasma is cooled. That will give us a cooled plasma with high abundance and hence high gaing. When the intensity of the pumping x-ray continually increases, the population of the ground state of He-like ions will greatly decrease, so that the inversion between Is² and ls2p slates can be obtained.

© 1994 Optical Society of America