Mirror Material Research Articles

Design update of the ITER upper wide angle viewing system

One of the diagnostic systems being provided by the US is the Upper Wide Angle Viewing System (UWAVS) which provides real-time, simultaneous visible and infrared (IR) images of the ITER divertor regions via optical systems located in five upper ports. UWAVS is designed in three main sections: in-vessel, interspace and port cell assemblies. Each assembly utilizes multiple steering and relay mirrors to direct the in-vessel light out of the tokamak to the port cell camera sensors. The primary design challenge of the in-vessel assembly is maximizing performance of the overall system while surviving the severe electromagnetic and nuclear ITER environment. A first mirror material study was conducted and determined that single crystal molybdenum was the best choice for the first two mirrors in the optical train. A fail open, bellows actuated shutter with cross pivot flexure design was determined to be the most reliable mechanism to protect the first mirror. A geometrically representative glow discharge mirror cleaning system is being designed and will be tested to maximize cleaning effectiveness while minimizing optical degradation of the molybdenum mirrors adjacent to the plasma. The shutter and first mirror assemblies were packaged and designed for replacement via remote handling methods to minimize radiation waste and cost associated with eventual component replacement. The preliminary optical and structural design provides a robust and reliable system while maximizing the field of view. The R&D efforts, the technical challenges and issues, and the design and analysis results are presented.

First direct comparative test of single crystal rhodium and molybdenum mirrors for ITER diagnostics

All optical and laser diagnostics in ITER will use mirrors to observe the plasma radiation. In the ITER environment, mirrors may become contaminated with plasma impurities hampering the performance of corresponding diagnostics. An in-situ mirror cleaning is proposed, which relies on ion sputtering of the contaminants and the affected mirror material. Previous research demonstrated the advantages of single crystal (SC) molybdenum (Mo) under sputtering conditions over polycrystalline concepts. Recently, the first single crystal rhodium (Rh) mirrors became available and tests have been started at the Forschungszentrum Jülich.In a direct test, SC Rh and SC Mo mirrors were exposed under identical conditions in steady-state helium plasmas in the linear plasma device PSI 2. The energy of impinging ions was ∼100eV matching conditions expected in the in-situ cleaning system in ITER. During exposure, molybdenum mirrors lost 420–500nm due to sputtering. Rhodium mirrors lost about 1μm. Exposure corresponded to 50–100 cleaning cycles in ITER. Nevertheless, rhodium mirrors have preserved their specular reflectivity, showing the maximum degradation of less than 7% at 250nm. Molybdenum mirrors demonstrated a moderate decrease of specular reflectivity of 12–25%. Results open new perspectives for the use of single crystals in ITER diagnostics.

Effect of ion beam etching on the surface roughness of bare and silicon covered beryllium films

Due to low density, high mechanical rigidity and thermal conductivity, ease of machining beryllium is one of the most promising materials for space mirrors, including for studies of the solar corona in the wavelength range of 13–30.4nm. An obstacle to the widespread use of this material is its large surface roughness after mechanical polishing. In this paper, using samples of 200nm thick beryllium films deposited on silicon substrates and polished bulk beryllium, we studied the main aspects of using ion-beam etching for finish polishing of beryllium. We present the results of investigation pertaining to the influence of the neon ion energy and angle of incidence on the beryllium films surface roughness. We measured the etching rates depending on the angle of incidence and energy of neon ions. We found that 400eV is the optimal energy for neon ion etching ensuring slight surface roughness smoothing in the range of incidence angles of ±40°. The deposition of 200nm amorphous silicon films onto beryllium and their subsequent etching with the 800eV argon ions improve the effective surface roughness integrated across the range of the spatial frequencies of 0.025–60μm−1, from σeff=1.37nm down to σeff=0.29nm. The effectiveness of the smoothing technology for X-ray applications has been confirmed by the results of the study of the reflective properties of the Mo/Si mirrors deposited on the substrate. The reflectivity at a wavelength of 13.5nm increased from 2% for the substrates with the surface roughness of σeff=2.3nm (the roughness value corresponds to the as-prepared bulk Be substrates and is taken from the literature) up to 67.5% after the smoothing technology.

Damage-free finishing of CVD-SiC by a combination of dry plasma etching and plasma-assisted polishing

To realize the damage-free finishing of CVD-SiC substrates, which are used as materials for space telescope mirrors and glass lens molds, plasma chemical vaporization machining (PCVM) and plasma-assisted polishing (PAP) were combined. In this study, the properties of such CVD-SiC substrates, including their surface morphology, composition and crystalline orientation, were investigated. Lapping using diamond abrasives and conventional chemical mechanical polishing (CMP) using CeO2 slurry were conducted for comparison with the proposed atmospheric-pressure-plasma-based finishing process. Many scratches and a subsurface damage (SSD) layer were formed by the diamond lapping of CVD-SiC. Conventional CMP using CeO2 slurry was conducted for the damage-free finishing of CVD-SiC. However, the polishing efficiency was very low. In the proposed process, PCVM, which is a noncontact dry etching process, was performed to remove the SSD layer while PAP, which combines plasma modification and soft abrasive polishing, was performed for damage-free surface finishing. PCVM was conducted on a diamond-lapped CVD-SiC surface. After PCVM for a short duration of 5min, the scratches and SSD layer formed by lapping were completely removed, although the surface roughness was slightly increased. PAP using a resin-bonded CeO2 grindstone was conducted to decrease the surface roughness of CVD-SiC processed by diamond lapping and PCVM for 5min, for which a loose-held-type grindstone was demonstrated to be very useful. A flat and scratch-free surface with an rms roughness of 0.6nm was obtained after PAP finishing.

Comparative analysis of soiling of CSP mirror materials in arid zones

Considerable loss of energy in the solar fields of Concentrating Solar Power (CSP) systems is caused by accumulated dust on the surface of the solar reflectors deployed in dusty arid and semi-arid zones. The monthly dynamics of changes in reflectance of the widely used second-surface silvered mirrors and innovative candidate aluminum materials under different site conditions can be useful for finding out the soiling characteristics of these mirror materials. Knowledge of the cumulative soiling of candidate mirror materials is essential to understanding their behavior at sites where cleaning is unavailable and water is scarce.The monthly soiling study revealed that glass mirrors are more susceptible to soiling than aluminum mirrors during dry seasons with no rain. However, they recover their reflectance best after a rainfall event. In Agadir and Tantan, Aluminum type I performed differently from other aluminum types in that it recovered reflectance poorly after rain. On the other hand, it showed monthly soiling behavior similar to Aluminum types II and III during the dry summer (July–August). In terms of cumulative long-term soiling, exposed mirrors were found to lose up to 73% of their initial cleanliness during the first 12 weeks of exposure. DFA modeling of the long-term soiling time series found, based on the Akaike information criterion (AIC), that the best model has two shared trend patterns.

Comparative Numerical Studies of Ion Traps with Integrated Optical Cavities

We study a range of radio-frequency ion trap geometries and investigate the effect of integrating dielectric cavity mirrors on their trapping potential. We aim to identify ion trap and cavity configurations that are best suited for achieving small cavity volumes and thus large ion-photon coupling as required for scalable quantum information networks. In particular, we investigate the trapping potential distortions caused by the dielectric material of the cavity mirrors for different mirror orientations with respect to the trapping electrodes, as well as for mirror misalignment. We also analyze the effect of the mirror material properties such as dielectric constants and surface conductivity, and study the effect of surface charges on the mirrors. The smallest trapping potential distortions are found if the cavities are aligned along the major symmetry axis of the electrode geometries. These cavity configurations also appear to be the most stable with respect to any mirror misalignment.

Mechanical loss of a hydroxide catalysis bond between sapphire substrates and its effect on the sensitivity of future gravitational wave detectors

Hydroxide catalysis bonds are low mechanical loss joints which are used in the fused silica mirror suspensions of current room temperature interferometric gravitational wave detectors, one of the techniques which was essential to allow the recent detection of gravitational radiation by LIGO. More sensitive detectors may require cryogenic techniques with sapphire as a candidate mirror and suspension material, and thus hydroxide catalysis bonds are under consideration for jointing sapphire. This paper presents the first measurements of the mechanical loss of such a bond created between sapphire substrates and measured down to cryogenic temperatures. The mechanical loss is found to be $0.03\ifmmode\pm\else\textpm\fi{}0.01$ at room temperature, decreasing to $(3\ifmmode\pm\else\textpm\fi{}1)\ifmmode\times\else\texttimes\fi{}1{0}^{\ensuremath{-}4}$ at 20 K. The resulting thermal noise of the bonds on several possible mirror suspensions is presented.

Perceptual segregation between mirror and glass material under natural and unnatural illumination

Perceptual segregation between mirror and glass material under natural and unnatural illumination

Experimental Simulation of the Behaviour of Diagnostic First Mirrors Fabricated of Different Metals for ITER Conditions

In the experimental fusion reactor ITER, the plasma-facing component of each optical and/or laser diagnostic needs to be based on reflective optics with at least one mirror (first mirror) facing the thermonuclear plasma. The different kinds of radiation emanating from the burning plasma (neutrons, neutral atoms, electromagnetic radiation) create hostile operating conditions for the first mirrors. Therefore, a special program has been set up under the ITER framework aimed at solving the first mirror problem. This paper will review the main results in this field that have been obtained in the Institute of Plasma Physics, National Science Center “Kharkov Institute of Physics and Technology” (in many cases in cooperation with groups of other countries, as indicated in corresponding parts of the manuscript) during long-term investigations directed to find a solution of this problem,i.e., to find a material and accompanying precautions in order to satisfy the requirements for first mirrors. The main efforts were devoted to finding solutions to overcome the impact of the most severe deteriorating factors resulting in degradation of the optical properties of mirrors: sputtering by charge exchange atoms and deposition of contaminants. The obtained results are focused on: the effects of long term sputtering on mirror specimens fabricated from different metals with different structures (polycrystals, single crystals, metal film on metal substrates, amorphous), the effects of contaminating film and the possible protection to avoid of its appearance, the role of chemical processes for some metal mirrors, and the choice of material of laser mirrors.

Deformation of rectangular thin glass plate coated with magnetostrictive material

As magnetic smart materials (MSMs), magnetostrictive materials have great potential to be selected as coating materials for lightweight x-ray telescope mirrors due to their capability to tune the mirror profile to the desired shape under a magnetic field. To realize this potential, it is necessary to study the deformation of the mirror substrate with the MSM coating subjected to a localized magnetic field. In this paper, an analytical model is developed to calculate the deformation of rectangular coated samples locally affected by magnetostrictive strains driven by an external magnetic field. As a specific case to validate the model, a square glass sample coated with MSMs is prepared, and its deformation is measured in a designed experimental setup by applying a magnetic field. The measured deformation of the sample is compared with the results calculated from the analytical model. The comparison results demonstrate that the analytical model is effective in calculating the deformation of a coated sample with the localized mismatch strains between the film and the substrate. In the experiments, different shape patterns of surface profile changes are achieved by varying the direction of the magnetic field. The analytical model and the experimental method proposed in this paper can be utilized to further guide the application of magnetostrictive coating to deformable lightweight x-ray mirrors in the future.

Self-detection of x-ray Fresnel transmissivity using photoelectron-induced gas ionization

Electric response of an x-ray mirror enclosed in a gas flow ionization chamber was studied under the conditions of total external reflection for hard x-rays. It is shown that the electric response of the system as a function of the incidence angle is defined by x-ray Fresnel transmissivity and photon-electron attenuation properties of the mirror material. A simple interpretation of quantum yield of the system is presented. The approach could serve as a basis for non-invasive in situ diagnostics of hard x-ray optics, easy access to complementary x-ray transmissivity data in x-ray reflectivity experiments, and might also pave the way to advanced schemes for angle and energy resolving x-ray detectors.

Ge/ZnS-Based Micromachined Fabry–Perot Filters for Optical MEMS in the Longwave Infrared

This paper reports on the successful demonstration of Ge/ZnS-based Fabry–Perot filters operating in the longwave infrared (LWIR). The suitability of thermally deposited Ge and ZnS as thin-film mirror materials for micromachined LWIR Fabry–Perot filters has been fully investigated, and it is shown that a film growth temperature higher than 150 °C is key to depositing durable ZnS films. The optical constants of Ge and ZnS films in the LWIR band reveal that the material pair possesses high refractive index contrast and excellent LWIR transparency. Fixed-cavity LWIR Fabry–Perot filters with a 150- $\mu \text{m}$ circular single-layer Ge top mirror and a four-layer Ge/ZnS/Ge/ZnS bottom mirror were fabricated. Curvature in the suspended top mirror was corrected using a thin SiNx stress-compensation layer. After curvature correction, a mirror flatness of 550 nm was achieved, and the filter demonstrated a 60% peak transmission with a full-width at half-maximum of 700 nm as well as a out-of-band rejection of 24:1. [2015-0143]

Spectral characterization of specular reflectance of solar mirrors

Solar reflectors for concentrating solar power (CSP) concentrators require a high specular reflectance over the whole solar spectrum. A preliminary procedure to measure the reflectance of solar mirror materials has been proposed in the SolarPaces reflectance measurement guideline [1]. However, the guideline clearly states that the currently available measurement instruments need further improvement in order to be able to fully characterize the reflectance properties of solar reflectors. In this work a high precision spectral reflectometer has been developed that permits the measurement of the solar weighted specular reflectance at different incidence and acceptance angles and thus provides all relevant reflectance data of solar mirrors. Four typical reflector materials have been measured and the solar weighted reflectance as a function of the acceptance angle has been modeled.

Design of a Vertical Cavity Optical Micro-Electromechanical Filter

Based on Micro-Opto-Electro-Mechanical System (MOEMS) technology, vertical cavity devices with an air-gap enable very efficient ultra-wide continuous tuning with only a single control parameter. The vertical cavity filters and Lasers consist of an air-gap cavity embedded between two highly reflective Bragg mirrors. Vertical Cavity Surface Emitting Lasers (VCSELs) have an active layer additionally integrated in the cavity. The ultra-wide continuous tuning is achieved by micro-electromechanical actuation of the mirror membranes. Our research activities concentrated on Bragg mirror material system with InP/air-gap mirror structures. The resonator quality depends on the refractive index contrast between the Bragg mirror layer materials and on the number of periods. The advantage of the semiconductor/air-gap system is the very high refractive index contrast ensuring a high reflectivity of more than 99.8% with only three layer periods. The semiconductor/air-gap structures forming the membranes of one Bragg mirror are n-doped while the other mirror membranes are p-doped forming a diode structure. Due to device miniaturization very efficient tuning is achieved by electrostatic actuation. The investigations presented focus on an InP/air-gap Fabry-Perot-filter for air =1.55μm with an air-gap cavity thickness equal to a multiple of ½ air embedded between two InP/air Bragg mirrors. The InP membranes have a thickness of of ¾ InP, the air-gaps in between are ¼air thick. The tuning behaviour is determined by the mechanical flexibility of the membranes and by the optical properties of the vertical cavity. The electrostatic deflection due to a reverse voltage is calculated with FEM simulations incorporating the effect of stress in the membranes. The results of the model calculations compared very well with measured values. This study describes the optical micro-electromechanical design of the novel low cost device structure applicable in optical filters, vertical cavity surface emitting lasers (VCSELs) and a host of other optical applications. We also present a solution to one of the most challenging problems in the design of micro-electromechanical devices consisting of thin-walled structures - that is residual stress. Residual stress alters device characteristics if not taken into consideration during design and characterisation.

Effect of heat-treatment on stress relief and dimensional stability behavior of SiCp/Al composite with high SiC content

Abstract In this paper, the effects of heat treatment processes on microstructure, residual stress and dimensional stability of 70 vol.% SiC p /Al composites were investigated by using a field emission gun scanning electron microscope (SEM), the X-ray stress analyzer and the thermal cycling test method. The results showed that the matrix of the as-cast composite is in compression. After the solution-quenching (SQ), aging and thermal-cold cycling (TCC) treatment, the residual stress was reduced due to the plastic deformation of the matrix. With the increase in the numbers of the SQ treatment, the long rod Al(MnFe) 3 Si 2 phase and the blocky CuAl 2 phase decreased while the spheroidization of these intermetallics increased. The TCC treatment refined the size of these intermetallics. As the number of the TCC treatments is increased, the resistance to micro-plastic deformation of the composite was also enhanced. The composite, which experienced the three-step SQ and aging and 12 times TCC treatment, presented the optimum thermal stability. Its hysteresis strain achieved 0.26 × 10 − 5 , and remained constant during the second and third cycling tests. The excellent dimensional stability shows a promising potential to be used as mirror materials for preparing mirror substrates in industry.

Mechanism analysis on finishing of reaction-sintered silicon carbide by combination of water vapor plasma oxidation and ceria slurry polishing

Reaction-sintered silicon carbide (RS-SiC), which is considered as a promising mirror material for space telescope systems, requires a high surface property. An ultrasmooth surface with a Ra surface roughness of 0.480 nm was obtained after water vapor plasma oxidation for 90 min followed by ceria slurry polishing for 40 min. The oxidation process of RS-SiC by water vapor plasma was analyzed based on the Deal–Grove model, and the theoretical calculation results are consistent with the measured data obtained by scanning white light interferometer (SWLI), scanning electron microscopy/energy-dispersive x-ray, and atomic force microscope. The polishing process of oxidized RS-SiC by ceria slurry was investigated according to the Preston equation, which would theoretically forecast the evolutions of RS-SiC surfaces along with the increasing of polishing time, and it was experimentally verified by comparing the surface roughnesses obtained by SWLI and the surface morphologies obtained by SEM. The mechanism analysis on the finishing of RS-SiC would be effective for the optimization of water vapor plasma oxidation parameters and ceria slurry polishing parameters, which will promote the application of RS-SiC substrates by improving the surface property obtained by the oxidation-assisted polishing method.

Investigation of Soiling Effect on Different Solar Mirror Materials under Moroccan Climate

Abstract This paper presents some first results of the effect of dust on different solar mirror materials in Morocco. For this purpose, thirteen glass and aluminum mirrors were exposed at the University of Oujda (Eastern Morocco) for a period of three months (from April to June). For each type of the mirrors, one sample mirror was installed horizontally and twelve mirrors were installed in the so called “mirrors sphere” in four different directions (North, South, East and West) with three tilt angles relative to the vertical plane: +45°(facing the sky), 0° (vertical) and -45° (facing the ground). The drops in specular reflectance were measured after each month of exposition for both mirror materials. Furthermore, X-ray diffraction analyses were performed for the dust collected from the mirrors to identify the mineralogical components of the dirt settled on the mirror surfaces. For both mirror materials, the drop on cleanliness per time interval was quite the same for all the mirrors and over all the test periods. The highest average cleanliness drop per month for the horizontal mirrors was 45 % and 33 % for the glass and aluminum mirrors respectively. The +45° mirrors come in the second position with a cleanliness drop of about 14 % for both reflectors. However, the mirrors installed on the 0° and -45° angles remained cleaner with a cleanliness average of about 97 % for both mirrors. These first results can be of high importance to motivate scientists and actors in the field of solar energy to further investigate the soiling of CSP mirrors and add it as a parameter in solar resource assessment.

Durability Testing of Silvered-Glass Mirrors

Abstract Durability analysis of solar mirrors is attracting attention from Concentrating Solar Power (CSP) developers because guaranties of a suitable optical behavior during the whole service life time are required. To give realistic life time estimations in affordable time, research institutes are investing a lot of effort in developing appropriate accelerated aging tests. In this paper, an extensive test campaign of accelerated aging tests has been performed with silvered-glass mirrors from several manufacturers. Three commercial outdoor-proven products, a not outdoor-proven mirror prototype and two mirrors which showed degradation after only three years of outdoor operation in Spain and Australia have been tested. The accelerated agingtests were conducted for an extended period of time in order to determine the testing time at which the two mirror materials that showed degradation outdoors start to fail under the accelerated conditions. The degradation of the outdoor exposed mirrors is analyzed microscopically and compared to the degradation appearing under accelerated aging. The presented data on accelerated testing represents useful information for standardization of durability testing of solar mirrors.

Microchip laser Q‐switched with GaInNAs/GaAs SESAM emitting 204 ps pulses at 1342 nm

A 1342 nm Nd:YVO4 microchip laser is reported, Q-switched with a dilute nitride GaInNAs/GaAs saturable absorber mirror. The laser produced optical pulses as short as 204 ps with 2.3 MHz repetition rate and 24 mW average output power. In comparison to conventional InP-based saturable absorber mirrors, the advantage of the proposed approach is the availability of excellent Bragg mirror materials that enable high reflectivity and more flexibility in designing the non-linear parameters owing to the use of lattice matched GaInNAs/GaAs quantum wells.

Glass and Polymeric Mirrors Ageing under different Moroccan Weathers, an Application for CSP Power Plants

Abstract CSP technology has a huge potential. However, all the components used in the energy generation process are not yet optimized. Reflectors are one of the most important devices to improve, asthe efficiencyof the power plant is directly linked to their high performance. Because reflectors are costly and cannot be changed frequently, their reflectivity should be maintained as long as possible.For this reason it is important to study their durability under real operation weather conditions. Natural ageing allows the determination of real lifetime mirrors characteristics and better understanding of their degradation mechanisms. For this investigation, polymeric and glass mirrors were exposed in two Moroccan sites with characteristic weather conditions, one close to the ocean and one in the desert, for more than one year of natural ageing. Different characterization techniques such as optical microscopy and UV-Visible-NIR spectophotometry were used to detect and analyze degradation mechanisms. The obtained results are shown in this paper and acomparison of mirrors behavior is proposed depending on the outdoor exposure sites. It can benoted that the desertic conditions are less aggressive than coastal conditions regarding tophysico-chemical degradation of both investigated mirror materials.