Abstract
The gyrotron backward-wave oscillator (gyro-BWO) is a continuously tunable source of coherent millimeter-wave radiation based on electron cyclotron maser. The fundamental mode operation is generally adopted to simplify the experiment which can avoid the mode transition problem and the mode competition behavior. As the high frequency requirement increase, high order mode operation is an effective method to solve the structure size limit. A taper structure Ka-band gyro-BWO which operated at TE01 mode in cylindrical waveguide is designed in this thesis. The mode competition behavior is discussed which major unwanted competing mode is TE31 first harmonic oscillation. A single mode stationary code is employed to simulate the start oscillation current and nonlinear behavior of each interaction mode. The relative value of start oscillation current between operating mode and the other waveguide modes are applied to judge the operating stability. The question which we must consider is the short end boundaries to the TE31 mode. As a result, the close cavity structure brings about the gyromonotron dynamic which have lower oscillation threshold. A distributed loss is applied to suppress the unwanted oscillation but operating mode. A novel design and of high spectral purity Ka-band TE01 mode converter are presented to avoid the mode transition when wave coupled out. Back-to-back transmission measurements show excellent agreement with computer simulations. The measured optimum transmissions are 97% with 1-dB bandwidth of 5.8 GHz at center frequency 34 GHz. In addition to high conversion efficiency, high mode purity, and broad bandwidth, this converter also features easy construction and compact size. The preliminary experimental results show the spurious modes can be suppressed by distributed loss. The maximum output power is 46kW with efficiency 8% and the bandwidth is 3.7%.