Study On Glow Discharge Cleaning In Synchrotron Radiation Beamline

Synchrotron radiation induced carbon contamination on optical elements is widely considered to be a major concern for a beamline.Carbon contamination not only affects the transmission efficiency of beam line,especially soft X beam line,but also reduces the service life of optical elements.The main solution to the current carbon contami-nation is to clean or replace the contaminated optical components.A large number of experimental studies have shown that the formation of carbon contamination is related to the presence of a small amount of oil on the chamber wall.Therefore,the purpose of this paper is to clean the vacuum chamber wall of the mirror box and obtain a cleaner vacuum chamber by glow discharge,striving to prevent carbon contamination formation fundamentally.In order to theoretically understand GDC,a 2D fluid model of Ar/O2 discharge was established and the discharge physical parameters(electron density,electron tem-perature,potential,metastable particles Ar^*?O₂?a¹?_g??O₂?b¹?_g⁺??O?¹D?and positive ions Ar⁺?O₂⁺mean density were obtained under different conditions?Ar/O₂mixing ratio,energy,pressure,electrode arrangement?.It is found that the average density of metastable particles increases with the increase of oxygen-to-argon ratio,energy and pressure.The arrangement of a single electrode is more reasonable for GDC compared with dual electrodes,since that the average density of metastable particles is higher.GDC system was designed and built on the basis of simulation to verify the GDC results.From the results of quadrupole mass spectrum analysis,we identify that the oil characteristic peaks?39,41,43,55,57,69,71?decreased rapidly and then increased in the GDC process,indicating that hydrocarbons were cracked into smaller molecular weight particles.After 20 hours of glow discharge cleaning,the amount of oil molecules?molecular weight 39,41,43,55,57,69,71?could be reduced significantly after being cleaned by glow discharge?GDC has a remarkable effect on removing oil molecules on the surface of the mirror box.By comparing glow discharge cleaning under normal temperature with glow dis-charge cleaning under high temperature baking,it is concluded that glow discharge cleaning under high temperature baking is more suitable for practical application,which can not only effectively remove hydrocarbons but also remove water.The results show that the glow discharge cleaning under all different conditions has damage to the sur-face roughness?R_q?of Au,Al and Si optics.After 90 minutes of GDC cleaning,the Au-coated optics'thickness decreased from 41.85 nm to 18.60 nm.Finally,the paper systematically studied the damage of glow discharge cleaning on the surface of optical components and other commonly used materials in vacuum.The effects of GDC on the surface of optical components were obtained by measuring the surface roughness,reflectivity and chemical composition of Au,Al and Si optical components before and after glow discharge cleaning.Glow discharge cleaning has no significant effect on the X-ray reflectivity of the Au-coated optical components in the range of 5?48 nm,but has an effect on the re-flectance of the Al and Si optical components.After the glow discharge cleaning,there is no change in the chemical elements on the surface of the Au-coated optical com-ponent,but the surface of the Al optical component is changed to Al₂O₃.Based on the numerical simulation results,the causes of these changes on the surface of optical elements after glow discharge cleaning are analyzed.The test of stainless steel,aluminum and oxygen-free copper before and after glow discharge cleaning by roughness profiler shows that glow discharge cleaning has little effect on the surface roughness of stainless steel and oxygen-free copper,but cleaning will increase the surface roughness of aluminum.It can be seen that there are arc marks on the surface of these parts after 20-hour glow discharge cleaning by using an optical microscope.In summary,this paper aims to introduce the glow discharge cleaning into the beam line,especially the soft X-ray beam box vacuum pretreatment process,to obtain a cleaner vacuum chamber,and strive to obtain a cleaner vacuum chamber.It funda-mentally reduces the generation of carbon contamination of optical components.The content of this paper is of great significance to reduce the carbon contamination,espe-cially in soft x-ray beamline.