Investigation of Soft X-Ray Laser with Plasma Active Medium and Self-Filtering Unstable Resonator

This paper proposes that self-filtering unstable resonator (SFUR) is suitable to be used in soft x-ray lasers possessing an active medium with a short-gain-lifetime. For this purpose, a self-filtering unstable resonator (SFUR), in one and two dimensions with Ne-like Fe soft x-ray active medium at 25.5 nm and maximum gain 124 cm-1, has been theoretically investigated to obtain a low beam divergence of ~ 0.5 mrad. The role of the field-limiting aperture in mode propagation has also been shown. In addition, the beam quality factor M2 has been calculated and the output mode behavior has been investigated and compared with a plane-parallel (PP) resonator of equal length. The calculations indicate that the M2 factor in SFUR resonator is smaller than the PP resonator and therefore the output beam divergence is lower and the mode quality is much better resulting in a higher beam spatial coherency.