Development of C-band 6 MeV On-axis Coupled Standing Wave Accelerating Tube

In order to develop a compact electron linear accelerator, a C-band on-axis coupled standing wave accelerating tube operating at the frequency of 5 712 MHz was developed in this paper. The designed beam energy is 6 MeV, and the designed pulse beam current intensity is greater than 80 mA. Firstly, in order to achieve the compact goal, the beam dynamics design was carried out based on the RF phase focusing theory. By designing the phase velocity distribution and electric field distribution in the whole tube reasonably, the longitudinal bunching and transverse focusing of the electron beam were realized simultaneously without external focusing magnetic field. Secondly, the structure of this C-band accelerating tube was optimized after establishing the equivalent circuit model to obtain the required electric field distribution. This accelerating tube adopted π/2 mode onaxis coupled standing wave structure and was composed of three bunching cavities and nine uniform accelerating cavities with a total length of about 284 mm. Through the analysis of the equivalent model, the relationship between the cavity parameters and the coupling coefficient between adjacent cavities were revealed. After adjusting the basic parameters of each cavity, the structure of this tube was determined and the required electric field distribution was obtained. Then the input coupler was designed to match the coupling requirement and the RF field got from the simulation of the whole tube was used in the beam dynamics validation calculation. At last, cavities were brazed together after tuned to the designed frequency and the cold test and highpower test of this tube were carried out. The reflection parameters of this tube were tested in the vacuum condition. The test results show that the operating frequency is 5 7136 MHz, the reflection coefficient at the operating frequency is -16.5 dB, and the corresponding input coupling is 1.35, which meets the designed requirements. The electric field distribution was tested according to the bead-pull measurement method and it’s consistent with the designed field distribution. In the high-power test, the tube was driven by a C-band magnetron which delivered a 2.5 MW peak power, 4 μs width pulse with a repetition rate of 250 Hz. An X-ray target was placed at the beam exit and the maximum dose rate at 1 m behind the target is 845 cGy/min, which indicates that the beam pulse current is 84.5 mA. Based on the half-value-layer measurement methods, the test result shows the electron beam energy can reach 6 MeV. These test results verify the validity of the development process of C-band 6 MeV on-axis coupled standing wave accelerating tube.