Free electron maser amplifier experiments

Summary form only given. Centimetre wavelength broadband amplifiers at MW power levels are required for a number of applications including RaDAR as well as spherical tokamak fusion plasma heating and diagnostics. Recent publications on gyro-amplifiers have shown exceptional gain, power, bandwidth and efficiency performance in this wavelength range, however, when larger intrinsic and tunable bandwidths are required, gyro-amplifiers are not the best choice. We present results from a reversed guide magnetic field Free Electron Maser (FEM) amplifier experiment at Strathclyde University. FEM oscillators have also been successfully operated at Strathclyde University but the focus of this present report is the FEM amplifier experiment. The FEM amplifier has been designed for zero beam-wave slippage to achieve maximum instantaneous bandwidth. The tunability is further extended by adjustment of beam voltage and undulator field strength. A two stage depressed collector was used to improve the amplifier efficiency. A 1 kW TWT has been used as the input source for the broadband FEM amplifier measurements and two 25 kW magnetrons for saturated output power measurements at discrete frequencies in the range. Results show instantaneous 3 dB bandwidth of 30% for fixed cathode voltage and magnetic field magnitude. The device can be tuned over a range of 65% of the centre frequency by adjusting the cathode voltage. Amplifier saturated gain was measured at 30 dB giving a peak output power of approximately 1 MW with 6% interaction efficiency. Operation with the two stage depressed collector resulted in an increase in efficiency to approximately 25%.