Study on the Increased Threshold Current in the Development of 220-GHz Folded Waveguide Backward-Wave Oscillator

Backward-wave oscillator (BWO) is a promising candidate as a millimeter-wave and submillimeter-wave (MMW/SMMW) power source and has a compact configuration, moderate output power, and frequency-tuning abilities. The physical conditions for increasing the threshold current of a typical MMW/SMMW-BWO are theoretically studied in this paper. We demonstrate the greater effect of axial phase velocity variation of deformed slow-wave structures on the threshold current in a 220-GHz folded waveguide (FW) BWO in comparison with other insensitive factors. For further verification, a prototype tube of FW BWO is developed. This tube exhibited increased threshold current during experiments. This finding is consistent with the error analysis proposed by our theory.