The laser produced plasma channel may be a promising means to produce an efficient compact soft X ray laser. The plasma parameters in such a plasma waveguide are calculated with a self similar model. It is shown that nonequilibrium underdense plasmas and candidate ions for recombination laser schemes can be obtained. As an example, the laser gains of the transition 1s3d-1s2p of He like nitrogen ions are calculated for given pumping energies and different initial gas densities.