The physics of soft x-ray lasers pumped by electron collisions in laser plasmas

Soft x-ray lasers in the 3.5–50 nm wavelength range have been developed in many laboratories. The shortest wavelengths and highest output irradiances have been produced using plasmas created by optical lasers as the lasing medium. The optical laser is focused into a line on a solid target and the x-ray laser action occurs by amplification along the line with sufficiently high gain that mirrors are not needed. Population inversions are produced by free-electron collisions exciting bound electrons into metastable levels in neon- and nickel-like ions. This topical review presents a summary of the atomic, plasma and propagation physics of x-ray lasers created in this way.