Study On Vacuum Properties And Electrical Conductivity Of NEG Films

With the rapid development of synchrotron radiation source,scientists put forward higher requirements for brightness and coherence of synchrotron radiation source,diffraction-limited storage ring has become the main development direction of the fourth-generation synchrotron radiation source.The unique characteristics of diffraction limit storage ring are small aperture magnets and compact lattice,which bring serious challenges to the design and construction of diffraction-limited storage ring vacuum system.The small aperture magnet limits the size of the beam pipe in the storage ring,resulting in small vacuum chamber conductance.The conventional lumped pumping method cannot provide provide the required vacuum pressure for machine operation.Depositing a layer of non-evaporable getter(NEG)film on the inner surface of the storage ring vacuum chamber can realize the distributed pumping.At the same time,the NEG films provide a low thermal outgassing rate,and a low photon-and electron-stimulated desorption,which make it an ideal solution for resolving the nonuniform pressure distribution in conductance-limited narrow vacuum tubes.The sensitivity of the diffraction limit storage ring to the vacuum impedance is very high,and the vacuum impedance caused by the NEG film cannot be ignored.Therefore,it is very important to explore the NEG film with low vacuum impedance.Hefei Advanced Light Source Facility(HALF)is an under-planned diffractionlimited storage ring.The design and construction of the storage ring vacuum system are also facing the challenges of limited conductance and vacuum impedance in small aperture vacuum chamber.The pre-research project of HALF storage ring vacuum system clearly indicated that NEG film should be applied to its storage ring vacuum system.At present,the research on NEG films started late in China,and there is no experience of large-scale engineering application of NEG films.Based on the requirements of HALF storage ring ultra-high vacuum system for NEG film and accelerator physics,the following research work was carried out:According to the requirements of NEG film acquisition device for HALF storage ring,a magnetron sputtering machine suitable for small aperture vacuum chambers was designed and built.In addition to depositing the NEG film on substrates,NEG films were deposited on the inner walls of pipes with different lengths(500～1500 mm)and shapes(straight and bend pipes)for the first time.The microstructure,crystal structure and chemical composition of NEG film samples were tested and analyzed by scanning electron microscopy,X-ray diffractometer and X-ray photoelectron spectroscopy,and the activation temperature,conductivity and pumping properties of NEG film were systematically studied.Activation temperature is an important indicator of NEG film engineering applications.Excessive activation temperature will not only damage the mechanical properties of commonly used materials(copper,aluminum and stainless steel)in storage ring vacuum chamber,but also affect the structure of magnet and support near the vacuum chamber.At the same time,it will consume a large amount of heating power,making it difficult to realize its engineering.In this study,the activation properties of Ti-Zr-V thin films were systematically investigated.X-ray photoelectron spectroscopy(XPS)was used to observe the changes of chemical states on the film surface at different baking temperatures to determine the activation temperature of the film.The results show that the Ti-Zr-V NEG film deposited by alloy wire starts to activate at 150℃ and can be fully activated at 180℃.This activation temperature is consistent with the relevant international test results,and well meets the requirements of the HALF storage ring vacuum system for the activation temperature of NEG film.The pumping speed and sorption capacity of NEG film are important indexes to evaluate its pumping performance,and are also important parameters for its application in HALF storage ring vacuum system.In this study,the pumping performance of TiZr-V and Ti-Zr-Hf-V NEG films was systematically measured for the first time by using a specially designed pumping test device for NEG thin film chambers.The results showed that the two NEG films began to have the ability to absorb air after activation at 160℃ for 24 h.With the increase of activation temperature,the pumping rates of CO and H2 of the two NEG films increased.The CO sticking probabilities of Ti-Zr-V and Ti-Zr-Hf-V NEG films after activation at 180℃ were 0.15 and 0.1,respectively,and the adsorption capacities were 1.2 ML and 0.9 ML,respectively.The pumping speed of the two NEG films for H2 is relatively small,basically maintained at 10-3 level,but their adsorption capacity for H2 is large.The test results meet the requirement that the sticking probability of NEG film activated at 180℃ is 0.1 in the pre-research of the vacuum system of HALF storage ring.At the same time,Ti-Zr-V film has better activation stability and pumping performance than Ti-Zr-Hf-V film.The use of NEG films will change the conductivity of the beam vacuum chamber,resulting in an increase in vacuum impedance.Improving the conductivity of NEG films can reduce its impact on vacuum impedance.In this study,the novel quaternary Ti-Zr-V-Cu and Ti-Zr-V-Ag NEG films with high conductivity were successfully prepared by magnetron sputtering,and their activation properties and conductivity were compared with those of Ti-Zr-V NEG films,aiming to explore the conductive NEG films suitable for the diffraction-limited storage ring and the future accelerator vacuum system.The results show that Ti-Zr-V-Cu film has higher conductivity and lower secondary electron yield than Ti-Zr-V film,but its activation performance is poor.TiZr-V-Ag NEG film showed better electrical conductivity and activation performance,and its activation performance was only slightly worse than that of Ti-Zr-V film.Ti-ZrV-Ag NEG film has become a potential material for diffraction-limited storage ring and future surface treatment of accelerator vacuum system,which provides a new research direction for surface treatment of accelerator vacuum chamber.The research results of Ti-Zr-V NEG film activation temperature and pumping performance meet the requirements of HALF storage ring vacuum system pre-research project,and successfully passed the acceptance of HALF storage ring vacuum system pre-research project,which laid a good foundation for the design and construction of HALF storage ring vacuum system.At the same time,the exploration of conductive NEG film provides a new choice for diffraction-limited storage ring and the surface treatment method of vacuum chamber in the future accelerator.