“…Recently, millimetre and sub‐millimetre wave science and technology have attracted great attention, especially millimetre and sub‐millimetre wave sources [1–5]. As one of the most promising devices for developing high‐power microwave sources, the relativistic klystron amplifier (RKA) has attracted worldwide attention due to its unique characteristics such as high power, high efficiency, amplitude and phase stability [6–8]. However, the output power of the RKA will decrease significantly in the millimetre wave regime, which is mainly due to its physical restrictions such as the space charge limited current and the surface field breakdown [9].…”