Study of the harmonic multiplying gyrotron traveling wave amplifier

Summary form only given. The harmonic-multiplying gyrotron traveling wave amplifier is studied with a multi-mode particle tracing code. By amplifying a drive wave at the fundamental cyclotron harmonic in the first stage, its nonlinear component at twice the input frequency is amplified at the second cyclotron harmonic in the second stage. Although the drive frequency is cut off in the second stage waveguide, its AC signal is embedded in the beam due to interaction in the first stage and grows ballistically in amplitude in the second stage. The AC beam perturbation at the fundamental frequency interferes with the growth of the harmonic wave. It is shown that the interference effect is reduced as the length of the second stage increases. Hence, the harmonic multiplying scheme works best as a high gain device, which then implies that the interaction circuit of the second stage must be designed to minimize the possibility of oscillations, We have designed a circuit with distributed losses and achieved a theoretical gain over 35 dB with output power levels /spl sim/100 kW and efficiency /spl sim/14%, assuming a beam with axial velocity spread of 5%. Details of the interference effects and approaches leading to an optimal design will be presented.