Experimental results are presented of the first cyclotron autoresonance maser oscillator operated using the second harmonic of the cyclotron frequency. An interaction efficiency of 3.9% has been measured at a frequency of 14.27 GHz, using an 80 ns pulsed, 14 A electron beam of energy 300 keV, to produce a coherent microwave power of 170 kW. Emission occurred at 3.7 times the relativistic cyclotron frequency, due to the large Doppler frequency increase and second harmonic interaction.

• Received 29 May 1996

DOI:https://doi.org/10.1103/PhysRevLett.77.4836