Rh-Nb/Zr-Mo Interlayer Coating for First Mirror in Fusion Reactor

Abstract

The nuclear fusion reactor is a very hot temperature chamber where light atoms are made to collide each other forming heavier atoms by releasing energy. In this process, most of the particles are ionized and form a burning plasma. Hence, it is necessary to know the phenomena involved in the burning plasma to control and protect the devices and components for the efficient and effective performance of the fusion reactor. There are several diagnostic systems which help to understand the physics behind the plasma reaction in the reactor. One of the diagnostic systems is the optical diagnostic system. It is an essential technique to understand the plasma physics involved in burning plasma, which helps the further study of the plasma phenomena. Metallic mirrors are used in fusion reactors like the International Thermonuclear Experiment Reactor (ITER) for optical plasma diagnostics and imaging systems. Plasma diagnostic systems are necessary tools for the future success of the ITER for a better understanding of magnetically confined burning plasma and its other phenomena inside the tokamak (Toroidal Chamber with Magnetic Coil). The closest mirrors to the plasma environment for the optical diagnostic is First Mirror (FM). In the harsh environment of fusion plasma, the FMs need to survive and perform to reflect the light signals to the analyzing system through the complicated arrangement. Meanwhile, the FMs surface will gradually change their properties due to adverse effects of erosion and redeposition of impurities owing to high energetic charge exchange neutral (CXN) particles, UVs, X-rays, and gamma radiations. Moreover, the most critical problem is the reduction of optical reflectance. Any changes in the reflectivity of the FM will affect the reliability of the signals. Despite these extreme conditions, the reliability of the diagnostic system and the plasma-facing components have to maintain the required optical properties for a better understanding of hot nuclear fusion plasma. To minimize the reduction of optical properties of the FMs, choice of material plays essential roles, like having low sputter yield, resistance to a chemical reaction, high thermal conductivity & resistance, high reflectance, and surface quality sustainability for an extended period of time under the plasma environment. In search of new inventions or continuous progress of technology to have expected optical properties of the FMs around the world, different laboratories are researching on various possible FM's materials and fabrication techniques. So far, some materials show matching features to some extent, like; copper (Cu), stainless steel (SS), tungsten (W), molybdenum (Mo), and rhodium (Rh). However, Mo and Rh are the most attractive and preferred choice. Both metals can have stability and reliability to withstand the harsh condition of the plasma environment and have better reflectivity among all the metals. Among two metals, the monocrystal mirror of Rh shows better optical properties. However, due to the expensive Rh material cost as well as a technological limitation for the production (limited size), the Rh thin-film coating on the bulk Mo is considered. Still, the thin-film surface is consumed into the plasma environment in the long run as it gets eroded on the mirror's surface due to the bombardment of high energy charged particles. Thin films might get detached from the substrate surface due to the stress that occurs between the two materials (Mo-Rh) when there is a difference in the thermal coefficient of material and lattice. The main focus of this thesis work is to provide an experimental study and fabrication of the thin interlayer coating FMs for the fusion reactor, keeping an acceptable level of reflectivity and adhesion strength of the coated films. For the experiment, we choose the DC magnetron sputtering systems to deposit a thin film of Rh metal on the bulk Mo substrate. Thin interlayer materials niobium (Nb) and zirconium (Zr) are chosen as their thermal expansion coefficient falls between Mo and Rh. We expected to bridge the thermal expansion coefficient of materials that would respond to thermal expansion reasonably, and coated films will remain attached to the surface. In this study, Si (100) wafers are used extensively for checking surface morphology, deposition rates and thicknesses of the deposited films of Nb, Zr, and Rh at different parameters. The reflectivity (before and after high-temperature exposure), scratch and adhesion tests performed on commercially purchased Mo mirrors throughout the experiment. All substrates were ultrasonicated in acetone and ethanol followed by rinsing in distilled water (DI) and finally, dried with pure nitrogen gas (N2). Prior to the deposition, all substrates were heated to 100℃ for an hour to remove residual impurities and to reduce the surface stress In DCMS, all the target materials of 99.95% purity (50.8 mm diameters, and 6.35 mm thicknesses) were used. A base pressure of 10-12 μTorr, the working pressure of 5 mTorr, and process gas (Ar) 30 SCCM were kept constant for entire experimental processes. For the characterization of thin film deposition, FE-SEM, EDS, XRD, 3D nano-system, and UV-VIS spectrometer was used. Lab-made scratch tester was used for testing scratch strength of the thin films along with adhesive tape test for adhesion strength. To date, extensive fabrication methods and their results have been presented in many articles to develop FMs as per the requirement of the fusion reactor (ITER). Most of the Mo-Rh thin-film FMs fabricated for the ITER show 70-80% of reflectivity [1] in the visible wavelength. Our experimental results of the fabricated mirror (Rh-Nb/Zr-Mo) also match with the many articles published in a different journal [1]–[3]. The reflectivity of the mirrors was 70-75% (in the visible wavelength) after high-temperature exposure. Both the scratch tests on the surface of the mirrors (film thickness:190 nm thick) with a load of 0.5-2.0 N at the rate of 37 mm/sec for 10-20mm of scratch length and the pressure-sensitive tape showed great adhesion strengths. Mirror fabricated at low power showed better reflectivity as well as better adhesion strength. Thin films coated at 40 W has a better scratch and adhesion strength than films coated at 200 W. On the other hand, Ion assisted-oblique angle deposition (IA-OAD) has good film density, but reflectivity of the mirror was lower than the normal angle deposition (NAD) processed films. This thesis has six chapters; chapter 1 composed of essential background to give information about nuclear fusion, optical diagnostics, and the importance of first mirrors. This chapter also briefly explains the deposition techniques and motivation for the research. Chapter 2 discusses the experimental method and brief descriptions about the instruments and components used in the experiment, the sample preparation process, and characterization techniques implemented to find out the results of the work. In our study, we applied three different methods for the deposition of thin-layer films (Rh-Nb/Zr) on the Mo substrate. In chapter 3, the NAD technique was used, where surfaces of the target and the substrate are parallel to each other or particle flux direction are perpendicular (normal- 90°) to the substrate's surface. Nb was the material used as interlayer between the Mo substrate and Rh film. Different powers were supplied (40, 100, 200W) to deposit the films in various time to attain the same film thickness of Nb (40 nm) and Rh (140 nm) in all samples. Moreover, the comparison between the results with and without interlayer coated mirrors was made. High temperature (200℃ for 6 hrs) exposed mirror has a reduction of reflectivity by 10-15%. The low power deposition showed better adhesion than a high power deposition process. Note that, interlayer coated Rh-Nb-Mo mirror is installed in NFRI/KSTAR for testing its reliability and reflectivity. Chapter 4 explains the interlayer Zr films deposited by NAD and OAD (oblique angle deposition) techniques to fabricate Rh-Zr-Mo FMs. The thickness of the thin-films was the same, as mentioned in the previous chapter. The NAD process showed better reflectivity of surface than the OAD process. In chapter 5, the experiment uses the IA-OAD technique to fabricate FMs. The thin film deposited by the IA-OAD process displayed better adhesion strength than the previous two deposition techniques but exhibited low surface reflectivity. The analysis showed a reflectivity of 60%. However, the film thickness of 2 μm on Mo substrate was successfully deposited by this technique without any blister or peeling off signs. This advantage can be used to increase the reflectivity of the film by polishing technique. Chapter 6 is about the summary of this thesis, the importance of this research work, significant findings, scope, and future work for the application of FMs to accomplish the requirement of the fusion reactor application to realize the fusion energy in the near future.