The Development Of High-performance Buncher For CIADS Front End Injector ? And The Research Of High-Q₀ For Superconducting Cavity

The high-current high-power proton linear accelerator of Chinese Initiative Accelerator Driven System?CIADS?is using superconducting radio frequency?SRF?technology,which is designed to provide 10 m A and,600 MeV continuous wave?CW?proton beam.As the pre-research prototype of CIADS,front end injector II,will operate in CW mode and provide 10 mA,10 Me V proton beam.The high-performance buncher for medium energy transport line?MEBT?is a key element in front-end of injector II,to match the high current proton beam longitudinally from the exit of RFQ to the entrance of superconducting Linac.In addition,the superconducting cavity is one of the key components in the SRF accelerator,its quality factor?0?has great influence on the performance and the construction cost of SRF accelerator.This thesis focuses on these two key issues.Firstly,the design,manufacture,testing and operation of the two high-performance bunchers are completed.Second,the effect of trapped flux on the 0 value is systematically investigated.Compared to the bunchers in the same type of accelerator facilities worldwide,the high-performance quarter wave resonator?QWR?buncher of the front-end injector II for CIADS has a shorter longitudinal length?? 280 mm?,provides a higher effective accelerating voltage?135 kV?and working in high current CW mode.The electromagnetic parameters and mechanical structure of the QWR buncher are optimized,and the key ancillary equipment coupler and tuners are also developed by the combination of RF structure design,thermal simulation and mechanical analysis,then the buncher is fabricated and tested.At present,the two QWR bunchers have been operated stably more than 2000 hours with beam loading,and the maximum CW proton current of 11 mA.The quality of QWR bunchers developed meets the design specifications.The Q₀ value of SRF cavity are mainly reduced by BCS surface resistance,residual surface resistance and trapped flux induced surface resistance?1?l),the mechanism and physical model of the last is the focus of current research.Due to the imperfection of the crystal lattice like impurities,dislocations and grain boundaries in cavity niobium material,when the cavity is cooled down through critical temperature Tc in non-zero ambient magnetic field,the imperfection may serve as pinning center to trap the magnetic field by the form of flux tubes.The trapped flux tubes will dissipate RF power under the condition of the RF field,therefore degrade the Q₀ value of the cavity.During my research,two high-purity large-grain single cell niobium elliptical cavities with different frequency,shape and surface treatment were used.the above SRF cavities were cooled down and measured its surface resistance under the condition of the different applied magnetic field?0-20?T?and the different cooling conditions,to study the magnetic flux trapping ratio and 1)l in the cavity wall.In our experiment,the magnetic flux trapping ratio was defined by the magnetic field at the equator of SRF elliptical cavity,to study the impact of cooling dynamics on magnetic flux trapping ratio.The surface resistance in the experiment of zero applied field cooling and the surface resistance in the applied field cooling experiment were used to give the experimental measurement method of 1)lIn contrast to the existing theoretical knowledge and experimental conclusion,we draw the following new conclusions: 1)The key cooling conditions to determine the magnetic flux trapping ratio is not the cooling rate,but the spatial temperature gradient????.2?the normalized R_flby the applied magnetic field is a non-linear function of ???,decreases rapidly with an increase of ?9ds,and finally saturates to a constant related to the material properties.3?The R_flof high-purity large-grain niobium elliptical cavity is lower than that of high-purity fine-grain niobium elliptical cavity under the condition of same trapped magnetic flux,which explains the reason why high-purity large-grain elliptical cavity has higher Q₀.4)After the exclusion of influence of thermal effect and measurement error on R_fl,it is found that R_flhas nonlinear relationship with accelerating field acc,the discovery of this nonlinear relationship of R_fl provides the experimental basis for the development of the new theoretical model.