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 = 2.4λ, where λ is the wavelength. The operating point on TM
₀₁
is set close to the π-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 7-10ns 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.