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Observation of high average power soft X-ray laser beam approaching full spatial coherence

We report the observation of an extraordinarily high degree of spatial coherence from a soft X-ray laser. The laser is a tabletop 45.9 nm capillary discharge-pumped Ne-like argon laser, which is capable of generating millijoule-level pulse energies that corresponds to an average power of several milliwatts. The method for measuring the spatial coherence follows the classic Thompson-Wolf two-pinhole interference experiment. We had observed a rapid increase of the spatial coherence with amplifier length, showing the effective mode-selection mechanism of strong refractive antiguiding. Full spatial coherence was approached with a 36 cm long plasma of very high axial uniformity and a length to diameter ratio exceeding 1000:1. Interference patterns with very high visibility are obtained with pinhole separations comparable to the total beam size. These results indicate a very high degree of spatial coherence throughout practically the entire laser beam.