Sub-milliradiant divergence soft-X-ray laser by active plasma waveguiding in

We report the observation of the sub-milliradiant divergence operation of a capillary discharge soft-X-ray laser, that indicates that X-ray radiation has been amplified in a refraction-guiding plasma active medium. The lasing is produced on the 46.9$\un{nm}$ transition of the $\chem{Ne}$-like $\chem{Ar}$. The 0.7$\un{mrad}$ divergence, axially peaked-profile beam is obtained in the single-pass amplification regime. The comparison of the recorded laser beam profiles with the results of our computer simulations shows that the capillary discharge z-pinch forms an active plasma wave guide, which has the electron density profile with depression on its axis. The waveguiding of X-ray radiation and its amplification is demonstrated over a 30$\un{cm}$ long plasma column, where the laser operates in a saturated regime with energy of $\sim 50$$\un{\mu J}$