Optical Properties Of Mirrors Research Articles

Surface modification of ITER-like mirrors after one hundred cleaning cycles using radio-frequency plasma

In ITER, the metallic first mirrors (FMs) will undergo erosion due to their proximity to the fusion plasma and deposition of materials originated from the first walls (mainly beryllium). In-situ plasma cleaning is a promising technique to conserve the FMs optical properties by means of ion sputtering. In this work, the evolution of the optical properties of single-crystal (Sc) and nanocrystalline (Nc) molybdenum (Mo) and rhodium (Rh) mirrors were investigated up to 100 cycles of consecutive contamination and cleaning. Aluminum oxide (Al2O3) was used as contaminant to replace the toxic beryllium. The plasma cleaning was carried out using a capacitively coupled argon (Ar) plasma excited by a 60 MHz radio-frequency generator resulting in the formation of a self-bias applied on the mirrors of -280 V. The plasma potential being around 30 V, the Ar ion energy was about 310 eV. The optical properties of the mirrors were assessed using ex-situ reflectivity measurements. Moreover, the surface topography was characterized by means of scanning electron microscopy (SEM), focused ion beam (FIB) and roughness measurements using atomic force microscopy (AFM). ScMo and ScRh mirrors formerly exposed to 80 successful cleaning cycles using aluminum/tungsten (Al/W) deposits and air storage exhibit drastic changes in their optical properties after being subject to cleaning cycles using Al2O3 as contaminant. Additionally, freshly polished ScRh were exposed to identical cleaning cycles. All Sc mirrors exhibited pits induced by the polishing procedure using diamond paste in addition of mounds/wavy patterns. The carbon incorporated during the polishing process was demonstrated to be responsible for the pitting of the surface. The Nc mirrors preserved their initial reflectivities after up to 100 cycles. The surface topography was systematically characterized and an average erosion rate for NcRh mirrors of about 59 nm per cycle has been estimated from FIB cross-sections. The optical properties of the Nc mirrors showed a superiority in the present study in comparison to the Sc materials due to the influence of their polishing procedures.

A novel experimental method to find dust deposition effect on the performance of parabolic trough solar collectors

One main reason why the performance of a parabolic trough collector measured experimentally differs from the simulation results is inaccurate prediction of absorbed solar energy. The amount of absorbed energy of such systems mainly depends on optical properties of mirrors, absorber tube and transmissivity of glass cover of absorber tube. This study examines the thermal performances of three collectors during different periods regarding the weight and thickness of deposited dust. A transient computer code is prepared to estimate thermal performances of clean collectors and compare the results with dusty collectors. A special optical instrument is designed and constructed to measure the reflectivity of mirrors with respect to the thickness of dust deposited on the surface of the mirrors. Based on the measurements a new correlation is developed to find thermal performance of a parabolic collector for various dust thicknesses in comparison to a clean collector. Different cleaning schedules are proposed for the collectors’ field; and thermal performance and yearly solar energy absorption of collectors according to each cleaning schedule are also determined. The performance of a solar-fossil hybrid power plant is studied and the impact of different cleaning schedules on reducing fossil fuel consumption and CO2 emission is investigated.

Investigations of single crystal and polycrystalline metal mirrors under erosion conditions in TEXTOR

Metal mirrors are planned for optical diagnostic systems of ITER. However, erosion, deposition and particle implantation can change the performance of mirrors. Mirrors made from the single crystal (SC) materials are among the main candidates for use in ITER diagnostic systems operating under erosion-dominated conditions. Laboratory tests have confirmed good optical performance of SC mirrors under erosion, but the dedicated direct comparative test in tokamak environment was missing. Such a direct test was performed in TEXTOR. Single crystal molybdenum, tungsten and polycrystalline (PC) molybdenum mirrors were exposed under the same conditions in the SOL plasma of TEXTOR. Surface and optical properties of mirrors were characterized before and after exposure. Before exposure glow discharge cleaning in hydrogen restored the reflectivity of mirrors oxidized during storage on air. No significant changes in total reflectivity were observed for all mirrors after exposure. Drastic increase of diffuse reflectivity was measured for PC Mo mirror, no change for a SC one. Thus, specular reflectivity of single crystal is higher than of polycrystalline one. The most affected wavelength range is 250–1000 nm, no significant change of reflectivity was noticed in the range 1000–2000 nm. Negligible effect of the exposure on polarization characteristics was observed.