Simulation And Calculation Of A Standing-wave Side-coupled Accelerating Structure

Medical linear accelerators have been widely utilized for radiation therapy.Because of good stability and compact structure,side-coupled accelerating structure has become the most common medical accelerating structure.In this paper,the design and experimental studies of the side-coupled accelerating structure with the operating frequency of 2998 MHz were performed.First,lots of numerical simulation and calculation was conducted with CST MWS.In order to obtain the cavity with high shunt impedance and Q factor,we comprehensively optimized the key dimensions of the nose cone by ensuring the resonant frequency of 2998 MHz.The effective shunt impedance of the uniform accelerating cavities is?100M?/m,and the Q factor and R/Q are 18730 and 456 respectively.When coupling cavities are overlapped on the accelerating cavities,the side-coupled structure operating on n/2 mode will introduce two different periodic units(ACA and CAC,A represents accelerating cavity and C represents coupling cavity).After accurate frequency modulation of the two units,good accelerating electric field of the ?/2 mode was obtained.Energy switch is a necessary device in the medical accelerator.According to different adjustment principle,the paper has carried on the research to three types of energy switch.Then,for easy operation,we chose the type of energy switch in which one plunger moves transversely.The beam dynamics were carried out with the final energy of 4 and 6 MeV.As a result,the electrons can be accelerated from 10 keV to 4 or 6 MeV with the maximum electric field amplitude of 36 MV/m.The beam characteristics met the design requirements.Due to the unique structural characteristics,the electric field in the side-coupled structure is asymmetric.The symmetry of electric field in the first cavity is of great importance to beam properties.In this paper,the analysis method of asymmetric electric field in the first cavity was studied.In order to improve the symmetry electric field of the first cavity,the paper proposed two improvement schemes.After optimization,the two schemes can increase the symmetry by 5 and 10 times respectively.To evaluate the feasibility of the structure,we performed the thermo-mechanical analysis by ANSYS Workbench.In the case of an average electric field of 23.8 MV/m and a duty cycle of 2%,the highest temperature was 72.5? at the nose cone and the maximal deformation was 73?m at the outer edge of the coupling cavity.The frequency shift was-2.21MHz.The above data provide reference and basis for future design of cooling system.