Experimental study on an intense free electron maser using Bragg resonators

A Bragg resonator was used to increase the frequency selectivity of an intense free electron maser powered by an intense relativistic electron beam. An electron beam with energy of around 770 keV, current of 200 A and pulse duration of 150 ns was injected into the cylindrical tube immersed in a guide and a helical wiggler magnetic field. A 40 GHz Bragg resonator with two traditional Bragg mirrors was set inside the cylindrical tube. The microwave with frequency of nearly 40 GHz was observed as expected using a 100 m dispersive line and a band pass filter only when the beam propagates through the wiggler field. However, when the Bragg resonator was inserted and the wiggler field was applied, the microwave with frequency of 45 GHz was observed by a reflection grating. Unexpected coupling resulted in the higher frequency radiation. The experiment is continued in future. (author)