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The method for calculating the dispersion relations of the slow-wave structures (SWSs) with arbitrary geometrical structures is studied in detail by using the Fourier series expansion. In addition, dispersive characteristics and longitudinal resonance properties of the SWSs with the cosinusoidal, trapezoidal, and rectangular corrugations are analyzed by numerical calculation. Based on the above discussion, a comparison on an L-band coaxial relativistic backward wave oscillator (BWO) and an L-band coaxial BWO with a coaxial extractor is investigated in detail with particle-in-cell KARAT code (V. P. Tarakanov, Berkeley Research Associates, Inc., 1992). Furthermore, experiments are carried out at the TORCH-01 accelerator under the low guiding magnetic field. At diode voltage of 647 kV, beam current of 9.3 kA, and guiding magnetic field strength of 0.75 T, the microwave is generated with power of 1.07 GW, mode of TM01, and frequency of 1.61 GHz. That is the first experimental report of the L-band BWO. [ABSTRACT FROM AUTHOR]

High power coherent electromagnetic radiation of a millimeter waveband is a tough research topic of plasma physics. In axial millimeter wave electromagnetic radiation devices, low power capacity and mode competition limited the output power. The radial structure is a promising candidate to realize high power millimeter wave radiation. Structural characteristics of a radial Ka-band high power millimeter wave transit time oscillator (TTO) are studied in this paper. Analysis without an electron beam indicates that if the average radius of radial cavities is increased, the mode of the stimulated field will not change, the power capacity will be improved, and the quality factor due to ohmic loss will remain unchanged. Particle-in-cell simulation proves that, for radial millimeter wave TTO, increasing the radius of the cathode and keeping sizes of the other parts unchanged, the output power will be increased, the efficiency will remain the same, and electric field growth will be slower than that in axial devices. Besides, taking ohmic loss into consideration, the efficiency will increase to a stable value. Therefore, by this method, high power coherent electromagnetic radiation in a millimeter wave can be realized. [ABSTRACT FROM AUTHOR]

A high efficient relativistic backward wave oscillator with coaxial nonuniform slow-wave structures (SWSs) and depth-tunable extractor is presented. The physical mechanism to increase the power efficiency is investigated theoretically and experimentally. It is shown that the nonuniform SWSs, the guiding magnetic field distribution, and the coaxial extractor depth play key roles in the enhancement of the beam-wave power conversion efficiency. The experimental results show that a 1.609 GHz, 2.3 GW microwave can be generated when the diode voltage is 890 kV and the beam current is 7.7 kA. The corresponding power efficiency reaches 33.6%. [ABSTRACT FROM AUTHOR]