Physical Design And Experimental Study On Compact Medical Heavy-Ion RFQ

The high biological killing effect of heavy ions on cancer cells and Bragg peak effect make medical heavy ion accelerator become one of the most effective methods to treat cancer at present.However,the low efficiency of heavy ion acceleration and high pre-construction cost are the main reasons that restrict the development and popularization of medical heavy ion accelerators.In order to improve the international competitiveness of domestic medical heavy-ion accelerator(HIMM)and reduce the per capita treatment cost of cancer,the Institute of Modern Physics plans to use a compact linac(HIMM-LINAC)to upgrade HIMM.In this paper,the Radio Frequency Quadrupole Linac(RFQ)in HIMM-LINAC is studied.Based on the established 162.5MHz HIMM-RFQ experimental study,a new design of 325 MHz HIMM-RFQ is proposed.The experimental study of 162.5 MHz HIMM-RFQ mainly includes electric field flatness,machining and measurement,low power test,high power test and beam experiment.In the simulation stage,the flatness of inter-vane field is optimized to ± 4%by the method of block tuning.Before the low-power test,the position deviation of the electrode was measured by laser tracker,and the measurement results can be used as important data for subsequent analysis.In low power test,the error factors of field flatness measurement are analyzed,and the method to eliminate the error caused by the gravity effect of perturbation is discussed.The measurement results of field flatness are in the range of-5% ? 3%,which agree well with the simulation calculation results and mechanical measurement results.In the high power test,the high frequency system,RF cavity conditioning and sampling signal are discussed and analyzed,which can provide reference for the operation of related accelerators.In the beam experiment,the transmission efficiency and output energy of RFQ were measured,and the measurement results all met the design requirements.The experimental results of 162.5 MHz HIMMRFQ prove the reliability of theoretical and simulation calculation,and provide experimental basis for the design of 325 MHz HIMM-RFQ.The main tasks of 325 MHz HIMM-RFQ research are beam dynamics and highfrequency structure design.In terms of beam dynamics,the frequency of 325 MHz is beneficial to the establishment of a larger acceleration gradient,but it also leads to the decrease of transverse acceptability the increase of energy requirements at the RFQ outlet.Based on the above characteristics,this paper improved the transmission acceptability of RFQ and improved the transmission efficiency of RFQ to 97.1 % by optimizing the focusing intensity,defocus factor and minimum aperture at the end of the bunching section.In the bunching section,the fast bunching design is adopted to improve the change speed of the synchronous phase and shorten the bunching section length by keeping the S factor representing the longitudinal stability area unchanged under the adiabatic approximation.In the fast bunching design,the maximum synchronous phase is increased to-16 °.Although the phase increase increases the length of the bunching section,it also increases the acceleration gradient,resulting in the overall length of the RFQ being shortened to 153.33 cm,which realizes the compactness requirements of the medical accelerator.In the follow-up error analysis,the tolerance of the dynamic design to various errors was studied.The error analysis results can be used as a theoretical guide for quality evaluation and conditioning stage.In high-frequency structure design,the preliminary RF structure of RFQ is designed through CST modeling and simulation.After the bottom cut optimization,the flatness of the field is ± 0.8%,which meets the design requirements of dynamics.After the study of the dipole mode stabilizer bar,the interval between the frequency adjacent dipole mode and the working frequency is increased to 7 MHz,which is enough to ensure that the working mode will not be significantly affected by the dipole mode.Finally,in order to compensate for the deviation of the cavity resonance frequency caused by various errors,a total of 24 tuners were designed.During the tuning process,the tuner has almost no effect on the field distribution,enabling the cavity resonance frequency to be adjusted to an operating frequency of 325 MHz.