A three-stage, mixed geometry gyrotwystron amplifier has recently been operated and characterized at 4.5 GHz. Gyrotwystrons are hybrids of the gyroklystron and gyro-traveling-wave tube (gyro-TWT) amplifiers. In the device which was studied, the first two stages were tunable rectangular resonator cavities, and the third (output) stage was a circular cylindrical waveguide traveling wave section. All three stages operate in fundamental mode, and are separated by cut-off drift regions. This experiment was conceived as an extension of a previous three-cavity gyroklystron experiment, the motivation being to improve upon the gyroklystron instantaneous bandwidth of 0.4% while not sacrificing efficiency or gain. Measurements of gyrotwystron performance showed a 1.5% instantaneous bandwidth, a fourfold increase over that of the gyroklystron. The gain-bandwidth product of the gyrotwystron exceeded that of the gyroklystron by a factor of 6.
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