Research On The Application Of Carbon-based Coatings In Accelerators

Electron multipacting in vacuum chamber of high-energy particle accelerators leads to the formation of an electron cloud.Adverse effects from electron cloud that have been observed in recent years include dynamic pressure rise,transverse emittance blow-up,thermal load in cryogenic vacuum systems as well as beam lose.Therefore,electron cloud is a main limitation for the achievement of high-quality beam in high-energy modern accelerators.One of the most effective methods is to coat vacuum chambers with materials of intrinsically low secondary electron yield.The author firstly tested and studied the secondary electron emission characteristics of three carbon-based materials:amorphous carbon,diamond-like films,and graphene films.The main research contents and results of the thesis are as follows:1.Amorphous carbon film was produced by a self-built DC magnetron sputtering system.The effects of different sputtering conditions on the secondary electron emission characteristics of amorphous carbon films were studied by changing the sputtering parameters.It was found that the maximum secondary electron yield(δmax)of the amorphous carbon film was less than 1.2,and it decreased with the increase of sputtering power.The characterization of materials was investigated by scanning electron microscope,atomic force microscope,X-ray diffraction,X-ray photoelectron spectroscopy and Raman spectroscopy.The main reason for the decrease of secondary electron emission of amorphous carbon film is the increase of roughness and sp2 hybridization.Besides,amorphous carbon/titanium composite film was used to decrease the secondary electron yield of the amorphous carbon film(δmax≈1);2.The secondary electron emission characteristics of diamond-like films of different thickness prepared by plasma enhanced chemical vapor deposition method were studied in detail,and it was found that the maximum secondary electron yield(δmax)was about 1.5.The secondary electron yield of the film was analyzed uder conditions of different incident electron densities.Scanning electron microscope,atomic force microscope,X-ray diffraction,X-ray photoelectron spectroscopy and Raman spectroscopy were used to analyze the surface morphology,average roughness and chemical composition of the material.The higher secondary electron emission of diamond-like films is mainly due to a smooth surface,the roughness of only a few nanometers and the dominant high sp3 hybrid bond.It provides strong support for the application of diamond-like carbon films in suppressing the electron cloud;3.The secondary electron emission characteristics of copper-based graphene films with different layers were tested.It was found that after 3-5 layers and 6-8 layers of graphene were deposited on the copper surface,the maximum secondary electron yield decreased from 1.8 to 1.39 and 1.28,respectively.It showed that multilayer graphene can effectively suppress secondary electron emission.The maximum secondary electron yield of the film decreases as the number of layers increases.And scanning electron microscope and Raman spectrometer were used to investigate the microscopic morphology,chemical composition and quality of the multilayer graphene material.It is found that the multilayer graphene has a wrinkled surface,which is beneficial to capture secondary electrons and reduce secondary electron emission.Finally,the mechanism of copper-based graphene reducing the secondary electron emission of the copper substrate is analyzed.In summary,this research has greatly improved the understanding of the technology of alleviating the electron cloud effect in the vacuum surface coating carbon-based film treatment,and provided a material plan and experimental basis for the electron cloud suppression work in the high-energy accelerators.