Frequency-tunable CW gyro-BWO with a helically rippled operating waveguide

The gyrotron backward-wave oscillator (gyro-BWO) is known as a source of frequency-tunable high-power coherent radiation, which is attractive for a number of applications. This was the motivation to develop a CW tunable gyro-BWO with a power of several kilowatts at the frequency of about 24 GHz. The gyro-BWO exploits a DC oil-cooled magnet with magnetic field up to 0.5 T and utilizes a weakly-relativistic (20 keV) electron beam produced by a magnetron injection gun. A stable generation at the second cyclotron harmonic with a maximum power of 7 kW and interaction efficiency of 15% at the frequency of 24.7 GHz was achieved. Smooth oscillation frequency tuning by varying the magnetic field was measured to be as wide as 5% at half-power level. The use of a depressed collector provided an efficiency increase up to 23% and an opportunity for a reduction of the main power supply voltage down to 10 kV.