A 57GHz overmoded relativistic backward wave oscillator (RBWO) operating on the quasi-TEM mode with pure TM01 mode output is presented in this paper, by using outer trapezoidal slow wave structure (SWS) with large distance between inner and outer conductors. The large overmoded ratio can be obtained in coaxial devices to improve power handling capacity, while the large distance between inner and outer conductors can guarantee the electron beam transmit effectively. The 8π/9 mode of quasi-TEM synchronously interacts with the electron beam, while the TM01 mode diffracted by the quasi-TEM mode outputs. The existence of TM01 6π/9 mode in SWS can extract energy from the quasi-TEM mode (which has a high value of Qe) thus increasing the power handling capacity. Particle-in-cell simulation shows that generation with high power 560 MW and efficiency 43.5% is obtained under the diode voltage 520 kV and current 2.47 kA. And the microwave has the pure frequency spectrum of 56.8 GHz radiates in the pure TM01 mode (about 98%).
The radial transit time oscillator is promising to realize high power output of millimeter-waves. However, the enhancement of the electric field in extractor and the microwave extraction are two main problems to be overcome. A high power radial Ka-band transit time oscillator is proposed in the paper. Increasing the third gap by integer multiples of half the wavelength, the nonuniform extractor will be introduced. In the extractor, the radial mode of the electric field in the modified extractor is the same as that in the uniform extractor. Due to volume increase of the beam-wave interaction region, this structure can improve power capacity to a certain degree. In particle-incell simulation, the proposed radial Ka-band oscillator can output high power microwave with the power of 1.0 GW, the frequency of 31.3 GHz and the efficiency of 33%. Besides, the modified structure can decrease the maximum radial electric field in extractor to 0.9 MV cm −1 . As the diode voltage ranges from 350 to 450 kV, efficiency of the oscillator is larger than 20% and the center frequency will not change, which proves the steady operation of this device. To realize high output power, the materials with high conductivity need to be employed, such as copper and aluminum. Besides, the microwave channel goes through the inner space of the high current coils, this design can decrease the number of the support posts to 50 and increase the bandwidth where the transmission efficiency is larger than 99% to be over 1.8 GHz.
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