Research On Laser Etching To Suppress Secondary Electron Emission From The Surface Of Vacuum Chamber Material Of Accelerator

The particle beam in the storage ring of the particle accelerator interacts with the inner wall of the vacuum chamber and the residual gas to generate secondary electrons.These electrons gather in the vacuum chamber to form the electron cloud,which seriously affects the intensity,energy,emissivity,stability and lifetime of the storage beam.The electron cloud is the most critical issue facing new generation of accelerators.The most effective way to mitigate electron cloud is to reduce secondary electron yield on the inner surface of vacuum chamber material.Research on methods of suppressing secondary electrons on the surface of materials has become the focus of research in many laboratories.Twining around the periphery of vacuum chamber with solenoid,coating a low secondary electron yield film and processing millimeter-level groove structure on the surface are commonly-used to mitigate electron cloud in accelerator.However,these conventional surface treatment methods generally can only reduce secondary electron yield to about 1.2,and there is also the risk of generating an electron cloud.With the development of accelerators,the requirements for electron clouds have become more and more stringent.Therefore,the surface of vacuum chamber structure materials with extremely low secondary electron yield has become an important goal of the new generation accelerators,and the corresponding surface treatment methods have also become the key technology of vacuum system design.In response to this problem,the laser etching method is used to process the vacuum chamber structure material for the first time in China.A three-dimensional high-precision laser material etching test platform is integrated and built to utilize laser etching method to process the vacuum material.The surface of the processed material has a very low secondary electron yield.Using this platform,a surface with a secondary electron yield of less than 1 is obtained.Compared with conventional processing methods,laser-etched surface has a lower secondary electron yield,and the surface of the material can absolutely eliminate electron cloud.By adjusting the laser parameters,a low secondary electron yield etching surface with different microstructures was prepared,and a laser process for obtaining a low secondary electron yield was determined.Relying on the original secondary electron test device of the research group,the influence of electron incident angle and electron bombardment on the secondary electron yield on the etched surface was determined.The phenomenological method and Monte Carlo simulation method are used to analyze the emission of secondary electrons on the etched surface,and the calculation formula of the secondary electron yield on the etched surface is obtained.The main results are as follows:1.For the first time in China,the author built a laser etching system specially used to process surface of commonly used materials in accelerator vacuum chamber.Surface reflectivity of metal materials is relatively high.Therefore,laser with 355 nm wavelength is chosen with high absorptivity as the light source of laser etching system.The author has designed a set of focusing system to reduce the laser spot of millimeter size to micron level.The mobile platform realizes the grating etching of the material surface,forming a periodic micron-level groove structure on the material surface.The author used the laser etching system to obtain the surface of the material with secondary electron yield less than 1.2.The author explored the effects of laser parameters and other conditions on secondary electrons yield on material surface of the accelerator vacuum chamber.The influence of various laser parameters on the surface morphology and chemical composition is reflected in the yield of secondary electrons.The greater the laser energy,the more obvious the surface morphology and chemical composition changes,and the higher the proportion of trenches produced by etching.As the proportion of trenches increases,secondary electron yield on the surface gradually decreases.The author has studied the effect of electron bombardment on secondary electron yield on the surface of etched material.Electron bombardment can remove surface contaminants and form a graphite-like C-C bond structure on the etched surface.These changes can reduce the secondary electron yield on the surface of the material.In addition,secondary electron yield of the etched surface will gradually increase as the angle of incident electrons increases.3.Laser etching can form a regular groove structure on the surface of commonly used materials in the accelerator vacuum chamber.Scanning electron microscope was used to analyze the surface morphology of the etched sample,and X-ray photoelectron spectroscopy was used to analyze the surface chemical composition and the valence state of each element.The author studied the influence of the protective gas on the surface of the copper sample during the etching process.The research results show that when the protective gas is nitrogen,the surface is still completely oxidized,which proves that nitrogen cannot play a protective role.When the protective gas is argon,metallic Cu still exists on the etched surface,which proves that argon can prevent oxidation on the surface during the etching process.4.By analyzing scanning electron microscope image of the surface of the etched sample,it can be known that the etched groove is an approximate triangular structure.The author uses the phenomenological method to study the secondary electron yield and its various components of the triangular groove,and through calculation,gives a calculation formula for the secondary electron yield with an acute angle.Comparing secondary electron yield data calculated by the phenomenological method with experimental test data of the secondary electron yield of the etched sample,the validity of the calculation formula is verified.5.CASINO is a software based on the Monte Carlo method to simulate electronic trajectories.The author uses CASINO software to analyze the secondary electron yields of flat and triangular grooves respectively.Through the combination with the phenomenological method,the result is consistent with the experimental data.The secondary electron yield is an important indicator of the material surface of the vacuum chamber of the particle accelerator in the future.As a new method of suppressing secondary electron emission,laser etching has great potential.The study on the mechanism of laser etching to suppress the secondary electron emission on the surface of accelerator vacuum chamber can provide a very important reference value for the application of laser etching methods in accelerators.