On possible amplification of coherent radiation in different schemes of near-surface channelling

Two different schemes of near-surface channelling which features glancing angles of incident and channelled particle beam with input facet of the crystal, are considered. A possibility is demonstrated to amplify hard radiation emitted during induced transitions between inverted levels of transverse energy of relativistic channelled particles under a slight excess over threshold beam current. The amplification is caused by a significant increase of active section length as compared to the traditional scheme and by a simultaneous reduction of thermal effect of particle beam on the crystal. Magnitude of possible amplification and permissible values of beam divergence and energy scattering are quantified