Review of soft x-ray laser researches and developments

In this paper, the author reviews the plasma-based x-ray lasers which we have already demonstrated saturated amplification whose wavelengths are between 50 and 6 nm., Section 1 describes the motivation of this review paper which includes basic ideas, developments and their applications of x-ray lasers. In section 2, the author describes the early x-ray laser researches on the recombination and the electron collisional excitation schemes including the hydrogen-like and lithium-like ion recombination schemes and the electron collisional excitation scheme. Section 3 describes the first demonstration of significant lasing at Livermore for the electron collisional excitation scheme of neon-like selenium ions at a wavelength of 20.6 and 20.9 nm and Princeton for the recombination scheme of hydrogenlike carbon ions at a wavelength of 18.2 nm. In section 4, the author describes the electron collisional excitation type soft x-ray lasers which are at present the most successful x-ray lasers. The subjects with which the author deals are saturated amplification neon-like soft x-ray lasers, improvement of neon-like soft x-ray laser performance using multi-layer mirrors, atomic physics issues of the neon-like soft x-ray lasers, gain guiding of the x-ray laser beam propagation, discharge-pumped compact repetitive neon-like ion soft x-ray lasers, the collisional excitation nickel-like ion soft x-ray lasers, high gain and saturated amplification nickel-like soft x-ray lasers at wavelengths as short as 7 nm, the short wavelength nickel-like x-ray lasers whose wavelengths are close to the longest wavelength edge the water window of 4.4 nm, transient collisional excitation scheme which is currently the most popular soft x-ray lasers pumped by short-pulse compact lasers with a laser energy of a few J to a few tens of J. In section 5, the author describes various plasma-based x-ray laser schemes other than the recombination and the collisional schemes, such as the optical field ionization schemes and inner-shell ionization schemes. Section 6 includes soft x-ray laser applications such as soft x-ray holography, soft x-ray interferometers, soft x-ray microscopy and other applications. In section 7, the author summarizes this review paper and he proposes a future direction for x-ray laser researches.