A preformed plasma channel of light ion species, with a small percentage of high-Z atoms, can be produced by a laser prepulse. When an intense short laser pulse propagates through the channel, high-Z atoms are stripped of most of their electrons. After the passage of the pulse, recombination of electrons with high-Z ions produces population inversion for the generation of coherent X rays. However, the gain is strongly dependent on the radial coordinate r. A paraxial-ray theory of X-ray amplification in such a situation reveals that gain focusing and refraction guiding play an important role in determining the gain and the spot size of the X-ray laser.