摘要： Increasing the output power of the microwave is one of the most important development directions for high-power microwave (HPM) technology. However, when the output power is increased, the radio frequency breakdown would occur owing to superhigh electric field strength. This problem limits the power handling capability of a single-HPM generator, and this problem becomes more serious for high-frequency bands and long pulse. One solution to overcome this problem is coherent power combining, and coherent power combining can achieve an equivalent power of the order of TW. The relativistic klystron amplifier (RKA) is a promising candidate for coherent power combing owing to its character of controllable frequency and steady phase. However, the radiation power of RKA quickly decreases when the source is scaled up to higher frequencies such as X-band, and moreover, the multigigawatts long pulse radiation from RKA at the X-band has not yet been obtained experimentally. In this paper, we present investigations into an X-band coaxial multibeam RKA, The power handling capability was increased by introducing 20 electron drift tubes and a four gaps extended interaction output cavity. The amplitude uniformity of the multiple electron beams was improved by optimized the anode and cathode configuration. Then, a radiation power of 2.2 GW with pulsewidth of 120 ns is generated experimentally; the working frequency and amplifier gain are 9.4 GHz and 50 dB, respectively.