| The first mirror in the experimental setup of nuclear fusion is interacting with various electromagnetic radiations and high temperature plasmas, and is easily contaminated by the impurities produced by the interaction between the plasmas and the first wall. Under such harsh service conditions, the lifetime of the first mirror that is currently used is limited to dozens of seconds, which does not meet the design requirement of one half year lifetime. Fabrication of the first mirror and clarification of the sputtering mechanism involved in the plasma-material interacting process are of great significance in the research and development activities. The lifetime of the first mirror depends greatly on the nature of fabrication materials. Traditional polycrystalline metallic materials are microstructurally inhomogeneous and highly anisotropic in many physical properties. The first mirrors made of polycrystalline metallic materials are hard to be polished to form high quality surface with good light reflection properties, and orientation selected sputtering always occurs when interacting with plasmas. Amorphous alloys, on the other hand, are structurally homogeneous and are readily polished. The orientation selected sputtering is absent and the amorphous surface layer can maintain a high light reflectivity for a long time. The first mirror can be integrated into a film structure with different functional thin films. This would offer large room for the optimization of the structure design of the first mirror, and to meet the high demands of specific functions.The present study is mainly devoted to the sputtering behaviors of the first mirror samples made of bulk metallic glasses and amorphous alloy films. The bulk metallic glass samples are fabricated using Zr65A17.5Ni10Cu17.5, Zr63.7Al9.0Ni27.3and Co61.2B26.2Si7.8Ta4.8glassy alloys, and the film samples are made of Co61.2B26.2Si7.8Ta4.8amorphous alloy, Al, and Al2O3/Al film, respectively. By using H plasma sputtering and the320keV high charge state He2+ion beam bombardment technique, the evolution of surface morphology and light reflectivity of the first mirror and the physical mechanisms thus involved have been investigated. The research aims at presenting the candidate first mirrors with a long lifetime, a high reflectivity, and the plasma sputtering materials design for the nuclear fusion experimental device as well. The main experimental results are presented in what follows:(1) The surfaces of the bulk metallic glass structures and polycrystalline Mo have been sputtered by H plasma for different time durations. The reflectivities of both the Mo and the Zr65A17.5Ni10Cu17.5bulk metallic glass mirrors are found to decrease with increasing sputtering time. The reflectivity of the Co61.2B26.2S17.8Ta4.g glass mirror, however, first decreases and then increases against sputtering time. It is speculated that in the latter case a so-called self-repairing process has occurred on the metallic glass surface after a certain amount of time due to the H plasma sputtering. (2) The320keV high charge state He2+ion beam bombardment experiment demonstrated that the reflectivities of Zr63.7Al9.0Ni27.3, Co61.2B26.2Si7.8Ta4.8metallic glasses and the polycrystalline tungsten mirrors got decreased, but the surface roughness of the mirror were increased. Sputtering craters appeared in the polycrystalline tungsten mirrors, which are absent in the metallic glass samples. This result indicates that bulk metallic glasses can avoid the orientation selected sputtering common occurred in the polycrystalline materials. It is therefore that the bulk metallic glass alloys can be superior mirror materials to meet the performance requirements of the first mirror.(3) The experimental results show that the bombardment of a320keV high charge state He2+ion-beam has caused the deposition and and implantation of carbon and oxygen particles in the surfaces of metallic glasses. In particular, oxygen was found to be implanted into the surface of the Zr63.7Al9.oNi27.3bulk metallic glass with a micrometer scaled depth; a carbon-enriched surface layer as thick as about400nm was formed in the Co61.2B26.2Si7.8Ta4.8sample. The chemical compositions of the metallic glass samples are altered due to the deposition and implantation proceses, se process, and it is very likely that these surface contamination events result in a decreased reflectivity.(4) A circular metallic glass target of100mm diameter has been successfully made by using bulk metallic glass forming alloys. By means of non-equilibrium magnetron sputtering, we have realized surface amorphization and a first mirror with surface amorphous thin film has been made. Comparison of the spectral reflectance reveals that the optical characteristics of the bulk metallic glass and the amorphous film samples are comparable. However, the film roughness was found to be rather high and the film structure was not dense. Using chromium electroplating, me plating, the spectral reflectance of the first mirror in visible spectra region has been significantly improved, and the spectral reflectance becomes more uniform. These experimental results indicate that the first mirror with amorphous thin film, which exhibits good light reflectivity, can be made using metallic glass target and by non equilibrium magnetron sputtering.(5) By using thermal evaporation, we have prepared Al (substrate) and A12O3/Al (substrate) thin films on top of the SS304steel (a metal), Si (a semiconductor) and glass (an insulator) substrates, respectively. The changes of surface characteristics after the hydrogen plasma bombardment were examined. In the Si substrate case, the Al2O3/Al film was striped after the thin films have been sputtered, and the Al thin films was blistered. As for the SS304substrate case, the Al2O3/Al film sample was striped and blistered, and Al film substrate showed a better sputtering resistance performance. Finally in the glass substrate samples, the reflectance was stable, but the structural stability was deteriorated. Some saline and alkaline substances appeared on the surface of the substrate.(6) The H plasma sputtering for Al (substrate) and Al2O3/Al (substrate) thin film substrate have also been studied.In the course of sputtering, the deposition rate was found to be greater than the sputter etching rate, and a small amount of oxide films (the oxides of Fe,Zr,Cr or Sn elements) of about10nm thick appeared on the surface of the substrate, which led to a decreased reflectivity and red shifting. The coordination atoms and atomic clusters of the metal contamination elements were identified to O2-、OH1-、(CO)1-、CO32-、HCO31-, et.al.. The sputtering products on the surface of the Al/SS304substrate was determined to be mostly metal oxides, while in the Al2O3/Al/glass substrates, the reflectivity was fairly stable, and no red shift was observed. The sample surface structure is stable without the formation of any salts. The above experimental investigations indicate the Al/SS304, Al2O3/Al/glass and the glass/Al/Al2O3samples exhibit good plasma sputtering resistance and high structure stability, which can be a promising structure design scheme of the first mirror. |
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