Experimental Study on a Moderately Overmoded Ka-Band Cherenkov Oscillator Operating With Low Magnetic Field

This article presents the results of the numerical and experimental study of a Ka -band Cherenkov oscillator with the average diameter of the slow wave structure (SWS) {D}= {2.4}\lambda , where \lambda is the wavelength. The operating point on TM 01 is set close to the \pi -point to enhance the electric field in SWS. Meanwhile, by optimizing the structural parameters, the influence of TM 02 on the mixed output mode is minimized. Then in the optimized structure with a straight output waveguide, the proportion of TM 01 in the mixed output mode reaches as high as 97%, achieving the target of mode selection. The experimental researches are carried out with a high-current SINUS881 accelerator. A solenoid magnet provides the guiding magnetic field of 0.9T. In the range of voltages 552-585 kV and currents of 7.3-7.85 kA, the generation of 7-10 ns pulse at the frequency 29.93-29.96 GHz and 277-312 MW output power was realized. The radiation mode is investigated by the neon lamp luminescence and the radiation pattern of power density in far-field, which both verified that the output mode is mainly composed of TM 01 , demonstrating the practicability of the proposed method in solving the mode competition problem.