Angular divergence and spatial coherence of X-ray laser radiation

X-ray lasers have been the subject of intense research since the mid-1980s owing to continuous advances in technology and computer engineering. By the mid-1990s, extreme ultraviolet (XUV) and soft X-ray lasing has already been achieved for approximately a hundred lines, with a lower boundary within the 'water window.' At present, the generation of highly directional and highly coherent laser radiation is of great interest for a variety of scientific, technological and medical applications. The formation of X-ray laser radiation is only one aspect of the general field of X-ray lasing, where a number of interrelated atomic physics, plasma physics, and quantum electronics problems are also to be solved. In the present paper, the historical development and current state of theoretical and experimental work on the angular divergence and spatial coherence of X-ray laser radiation is reviewed. Various approaches to the dynamics of laser-amplified radiation are considered and prospects for improving laser beam quality and increasing radiation intensity discussed. A comparison is made between the experimental results and theoretical predictions.