Experimental results of the first efficient operation of a long pulse cyclotron autoresonance maser oscillator are presented. Output power of 1.9 MW for a beam energy of 450 keV and current of 80 A, corresponding to an efficiency of 5.2%, has been measured at 27.8 GHz in the ${\mathrm{TE}}_{11}$ mode of a Bragg resonator. The observed frequency of the cyclotron autoresonance maser emission corresponds to a Doppler unshifted frequency of 2.9 times the relativistic cyclotron frequency. A significant mode competition between the ${\mathrm{TE}}_{11}$ and a ${\mathrm{TM}}_{01}$ mode is present.

• Received 29 January 1993

DOI:https://doi.org/10.1103/PhysRevLett.71.2018