An active circulator-gyrotron traveling-wave amplifier

A recent experiment demonstrated that wide-band amplification of millimeter waves is feasible by using a tapered gyrotron traveling-wave amplifier (gyro-TWA) of the reflection type. In this paper, we show that the difficult problem of input/output coupling in such a reflection amplifier may be solved simply by the use of an almost square waveguide, with or without a dielectric lining. An input signal in the form of TEmin{01}max_{square} mode, say, is injected from the downstream of the electron beam of the gyro-TWA. It propagates upstream and is reflected by the constriction of the tapered waveguide. By a judicious choice of the waveguide dimensions and of the magnetic field profile, we find that it is possible for the reflected wave to be strongly amplified in the TEmin{10}max_{square} mode, which has a polarization orthogonal to the input mode, thereby permittting easy separation of the input and the output signal. This novel device thus performs the dual function of an amplifier and of a circulator with wide bandwidth and good power handling capabilities at millimeter wavelengths. This optimism is based on the numerical studies presented in this paper. Furthermore, the simultaneous presence of the TEmin{01}max_{square} and TEmin{10}max_{square} modes is found to strengthen the beam-circuit interaction. All of the known advantages of the reflection gyro-TWA, such as the lack of the 9-dB launching loss and the insensitivity to the beam velocity spread, are expected to be preserved in this device. The effects of reflection from the load is considered in some detail. A Proof-of-Principle experiment is suggested.