Using a short-bunched beam of electrons of a linear accelerator, the electromagnetic impulses of coherent transition radiation (CTR) are superposed on the subsequent impulses coherently. The output of a closed resonator is compared with spontaneous CTR to verify amplification of radiation. The main cavity mode is assigned to be TM02 from observation of the radial distribution of the intensity. Experiment on the influence of the size of the resonator shows that the output intensity increases with the volume of the resonator. (C) 2004 Elsevier B.V. All rights reserved.