Abstract. Results obtained using a waveguide model of the AKR generation in thin plasma cavities are presented. Taking into account the occurrence of low frequency plasma motion in the regions of the AKR generation, we have considered a wave escape from a thin plasma cavity with adiabatically slowly varying width, and show that there can exist localized regions of instability from which the extraordinary X-mode waves, growing in time, can be radiated outwards. It has been found that waves, propagating quasi-tangentially to the source frontiers, have the maximum growth rate and escape outward most efficiently, which is in accordance with experimental observations.