高频段相对论速调管放大器(RKA)是近年来高功率微波领域的研究热点之一，其发展主要受限于模式竞争、相位抖动和效率偏低等问题。设计了一种径向线RKA，主要由输入腔、两组非均匀双间隙群聚腔和三间隙提取腔等四部分构成。通过比较单双间隙群聚腔与电子束互作用的耦合系数，说明了非均匀双间隙群聚腔具备对电子束较强的调制能力。前端加载TEM模式反射器的非均匀双间隙群聚腔的工作在TM₀₁-π模式，Q值较大，有利于谐振腔之间的能量隔离。采用两组非均匀双间隙群聚腔级联的方式，在注入功率仅10 kW情况下，实现短漂移管长度下电子束深度群聚达110%。粒子模拟结果表明，该器件具有效率高的优点，在电子束电压400 kV，电流5 kA，磁场强度0.4 T条件下，得到功率825 MW，频率14.25 GHz，效率41%的微波输出。

High-frequency relativistic klystron amplifier is one of the research hotspots in the field of high power microwave in recent years, and its development is mainly limited by mode competition, phase jitter and low efficiency. This paper presents the design of a novel Ku-band radial-line klystron amplifier, which consists of an input cavity, two groups of double-gap bunching cavities and a three-gap extraction cavity. By comparing the coupling coefficient of single-gap bunching cavity with that of non-uniform double-gap bunching cavities, it is proved that non-uniform double-gap bunching cavity has stronger modulation ability to the electron beam. The working mode of non-uniform double-gap bunching cavity with a TEM reflector is TM₀₁-π mode, which has a large Q value and benefit from energy isolation between resonant cavities. When the injection power is only 10 kW, the modulation depth of fundamental current is about 110% by cascading two groups of double-gap cavities. PIC simulationshows that this device has high efficiency. When electron beam voltage is 400 kV, beam current is 5 kA and magnetic field is only 0.4 T, high power microwaves with frequency of 14.25 GHz and output power of 825 MW are obtained.