Using radial plasma acceleration in a laser beam, an alternative type of free electron laser has been developed by radial injection of electrons. Its properties have been elaborated for a laser amplifier of 400 μtm wavelength. To overcome the difficulties of this FEL, a basically new laser amplifier concept has been developed where a laser pulse is amplified by nearly lateral injection of solid particles (clusters, pellets etc.) whose kinetic energy (at appropriate adjustment and synchronisation) is converted into 80% optical energy of the laser pulse. The mechanism of energy transfer is the radial slowing down by nonlinear ponderomotive forces of the plasma made from the cluster reaching the beam axis, and the switching-off process of the beam with regard to the transient nonlinear force processes. It is an inversion of the plasma ejection with ponderomotive self-focussing. Apart from the unusually high efficiency, a relatively high gain is expected even for such small laser wavelengths as 10 nm. Furthermore, the difficult problem of modification of phase fronts and convergence (focussing, directing of X-ray beams) should be solved by the optical plasma properties at injection.