Design study of a W-band high power extended interaction klystron based on the coaxial high-order mode

In this paper, a W-band multi-beam extended interaction klystron operating in the coaxial high order mode (HOM) is proposed. The HOM coaxial cavity with large transverse dimensions is suitable for accommodating multiple electron beams, promising to achieve high output power in the millimeter-wave frequency band. However, the operating method also brings lower characteristic impedance and a higher risk of mode competition. Based on the angular boundary condition of the coaxial HOM electric field, we propose a coaxial multi-gap cavity with a ridge structure to solve these problems. The TM51-2π mode is selected as the operating mode, of which the characteristic impedance can overwhelm that of the competing mode. The corresponding five-beam gun with a total current of 7 A (5 × 1.4 A) at a voltage of 25 kV is focused by a 0.65 T permanent magnet system. To demonstrate the capability of the device, the 3D particle-in-cell simulation predicts an output power of up to 30 kW at 94.4 GHz. The gain and electric efficiency are 54.8 dB and 17.2%, respectively.