In space applications, more and more organic polymers (adhesives, resins, paints and packaging of electronic devices) are used for their performance, their cost and their flexibility for the design of future satellites. This is especially true in the New Space era, using Off-The-Shelf devices with rarely well-known materials. Outgassed products of these materials under vacuum is a major cause of dramatic flux losses for contaminated optical devices, especially in the UV range. Thus, material outgassing must be studied and better controlled, during all the phases of satellite integration. Fluorescence hyperspectral imaging is a powerful technique for both locating and analyzing materials: their fluorescence spectra can be interpreted as a signature of their physicochemical composition. However, common commercial hyperspectral instruments do not meet the specifications required for such applications: very high sensitivity (SNR < 10000), wide spectral band (ideally 250-1000 nm), integrated multi-wavelength UV excitation and spectral range resolution of about 3 nm. In addition, classical optical design with diopters has to be avoided to prevent chromaticism, which is not compatible with wide spectral bandwidth, especially in UV range. These constraints led us to develop a new dedicated optical design, with the specificity of being catoptric on axis. Therefore, we built a first transportable instrument. In this paper, we present the evaluation of the characteristics of this instrument, its real performance and examples of measurements on flight models. A new version has been designed, using laser sources to limit exposure time of examined materials as much as possible, as they may be degraded under UV light.
KEYWORDS: Diffusion, Physics, Chemical species, Data modeling, Chemical analysis, Temperature metrology, Standards development, Mass spectrometry, Epoxies, Data processing
Progress was performed recently on the separation and characterization of the chemical species outgassed by space materials, relying on the assessment of thermogravimetric analysis (TGA) peaks by mass spectrometry (MS). A companion communication reports on this experimental technique and the first level processing of these MS data, which often allows determining which are the outgassed species, and their MS spectra. This communication focusses more on the second analysis step, i.e. the study of the MS data acquired during the initial outgassing phase. Ancient simpler outgassing analyses based on total mass measurements only, most of the time on quartz crystal microbalances (QCMs), cannot realistically determine the separate contribution of different species, even though some models consider the contribution of several species, which are indeed more “mathematical species” than physical ones. In contrast, this new approach, also taking into account the MS measurements during the outgassing, and known species spectra (from the TGA/MS analysis done previously), allows a more realistic determination of the contribution of each real chemical species to the total outgassing. Even though results are not yet final and perfect, measured outgassing fluxes from several species and materials are presented. Their physical analysis, through comparison and fit by diffusion or other possible outgassing laws are also presented. At this level, they clearly point to diffusion laws, rather than to any other outgassing law, although not necessarily always Fickian diffusion. This method was applied to typical US or European outgassing approaches, with either isothermal ASTM-1559 outgassing tests or multi-temperature VBQC-type tests.
Well-established procedures for the characterization of contamination during outgassing usually involve total mass measurements through quartz crystal microbalance (QCM). Recently, the addition of mass spectrometry (MS) measurements to these data has become more common. The combination of both high sensitivity QCM and MS data may lead to a better understanding of the physics taking place during outgassing contamination processes. The way to do so is to complement the basic measurements of total mass loss on QCMs by the identification of each species and the quantitative determination of each species contribution. In a first characterization step, the thermogravimetric analysis of contaminants deposited on QCMs allows a partial species separation that helps exploiting mass spectrometry data. In return, these data permit a finer species separation. The key to these measurements is to obtain sufficient signal to noise ratio in the mass spectrometer. Though outgassing of space materials is not done the same way in Europe (multi-temperature steps, ECSS-Q-TM-70-52A) and in the US (isothermal, ASTM E-1559-09), both tests could be used to perform a first species separation, as reported here. Most species outgassed by a few common materials were identified (and quantified) through TGA and MS coupling. As reported in a companion paper, the knowledge of these species’ spectra then allows the analysis of the MS data during the initial outgassing phase, determining the quantitative outgassing of each species and leading to the improved comprehension of the physical laws ruling outgassing.
Lasers in space often suffer from light absorbing deposits forming inside the resonator. This effect is commonly called laser-induced contamination (LIC), and its mechanism can be compared with laser chemical vapor deposition. LIC experiments were carried out in a dedicated vacuum chamber using a q-switched 355-nm laser, hafnia- or silica-coated fused silica samples and contamination by epoxy adhesive outgassing, or toluene vapor in vacuum. The typical deposit formation was observed at different experimental conditions and analyzed by in situ laser-induced fluorescence imaging and ex situ white light interference microscopy. We determined the average growth rate during the first growth stage (bump-shaped growth) and analyzed it as a function of the laser fluence, sample nature, and used contamination. The data show that the band gap of the sample is important for the LIC process in the first growth stage. The light absorbed in the sample leads to a temperature rise that drives the deposit growth. This knowledge opens a new pathway to minimize LIC that is complementary to studies that aim to reduce the adsorption of contaminant molecules by making chemical surface treatments.
The numerical assessment of in-flight contamination is a global process, which needs consistent numerical processing of elementary ground tests and global modeling of in-orbit situations. If the traditional physical approach of Europe is followed, both sides must make use of consistent physical models, and upgrade them consistently. This article presents recent progress performed at ONERA, in collaboration with CNES, in this respect. For this physical approach, elementary material outgassing tests aim at characterizing each chemical species independently, based on TGA / MS coupling for in situ characterization. Processing large data sets of mass peaks versus time, and interpreting them as resulting from a few outgassed chemical species, each one with its own mass spectrum, requires heavy computations and smart algorithms. The first results shown here are very promising. QCM and mass spec data acquired during TGAs where fitted with very convincing models for deposit reemission and mass spectra for the reemitted species that were identified with database spectra. This makes us confident in the next step consisting in similarly interpreting outgassing QCM + MS measurements in term of discriminated species, although they are all outgassed simultaneously in that case.
Contamination modeling has struggled with the challenge of species separation. Without the capabiliity to physically identify the chemical nature of contaminants and their contributions, the realistic correspondence between a chosen model and its underlying physics is very difficult to demonstrate. With the development of TGA/MS coupling experimental techniques and specific data treatments, a species separation was achieved on the ScotchWeld EC2216 adhesive. After a detailed exposure of the species separation need, this paper presents the experimental facility and the numerical procedure to effectively get contaminants identification and differentiate their contribution in a mixture.
"Laser-induced contamination" is a major difficulty for high power photonics instruments in vacuum and in sealed environments. Material outgassing causes molecular contamination on the optical components where the laser irradiation causes photo-fixation and/or polymerization leading to carbonaceous deposits at the location of the laser beam. We studied the morphology of these deposits as function of several parameters of physical and chemical nature. The influence of these parameters on the crater rim height of the "donut"-type deposits are presented and lateral growth of the deposits beyond the laser beam size is observed. The observation of lateral growth beyond the laser beam size indicates an influence of thermal energy input to the deposition process. We hypothesize that this thermal energy is provided by heat conduction from the center of the crater.
In 1990, scientists discovered and started studying the luminous events which occur above storms at altitudes of 20 to 100km. These Transient Luminous Events (TLE) are associated to high energy particle emissions like X rays, gamma rays… They cannot be easily observed from earth because of clouds and atmospheric absorption. Goal of Taranis satellite which will be launch in 2019 is to collect high resolution data on these TLE.
One of Taranis satellite instruments is a photometer which goals will be to measure emitted light in 4 different wavelength ranges and to distinguish TLE emission from lightning’s emission. Bertin Technology has been assigned the task to design, develop, integrate and validate this optical sensor.
The photometer aims at measuring very low light levels with a high signal to noise ratio and with a good uniformity all over an important field of view. These requirements lead to use high sensitivity detectors, special filtering systems in UV channel (180−220nm) and specific optical system to limit the loss of energy and to flatten the optical response over the field of view.
The development of the photometer was submitted to important environmental constraints: mass, available space, vibration, shocks, temperatures… Bertin’s challenge has been to take into account these strong environmental requirements which specific damping mechanical system working at low temperature. Final performances measured on the flying model are presented.
In this paper, we studied the onset and further development phases of LIC deposits. A slight antireflective effect was consistently observed at the onset of the deposition process in our experimental conditions. The increase of the laser transmission signal could thus be used as a signature to predict the initiation of a LIC deposit in the absence of in situ fluorescence monitoring which is even more sensitive. Using a monochromatic microscope, we showed that the analyzed deposit behaved like an interference coating and, due to its nanometric porosity shown by atomic force microscope images, its effective refractive index can be lower than that of the substrate. Such a layer could thus have slightly antireflective properties as they are observed during the first phase of the LIC deposit evolution.
KEYWORDS: Diffusion, Molecules, Contamination, Physics, Data modeling, Mathematical modeling, Chemical species, Ultraviolet radiation, Chemical reactions, Mass spectrometry
Contamination modeling in Europe has long been based on physical mechanisms, such as desorption. However other physical mechanisms, such as diffusion, evaporation or mixing effects exist. These alternative mechanisms were experimentally evaluated and modelled. It was yet observed that, without an experimental capability to reliably separate the (re)emitted chemical species, it is very difficult to determine whether the modeling and its underlying physical mechanisms are representative of reality, or simply a mathematical fit of reality. This is the reason why in the last years emphasis was put on the experimental separation of species, mostly through TGA/MS coupling. This paper presents a review of these efforts and promising results on species separation to reach a really physical modeling of outgassing, deposition/reemission and UV synergy.
A vacuum chamber was designed to study the risk of laser-induced contamination (LIC) on optical payloads integrated on spaceflight missions. In this context, tests were performed with a nanosecond pulsed laser at 355 nm on fused silica substrates under toluene exposure with multiple laser irradiation. Specific experimental procedures are described in order to obtain repeatable results. Finally, series of tests were performed to investigate the onset of the LIC deposition process and its evolution over time. A slight antireflective effect is consistently observed at the onset of the deposition process. We suggest that this is an indication that the LIC deposition process in our experimental conditions starts with a nucleation layer consisting of small dense islands of deposit.
The increase of performance of new optical instruments for science and Earth observation always leads to higher requirements in terms of contamination due to particle sedimentation in cleanrooms and deposition of chemical species in vacuum environment. Specific cleanliness control procedures are implemented in order to mitigate the risks of contamination on optical sensors and sensitive diopters, especially when used for UV applications. Such procedures are commonly carried out in cleanrooms and are described in both European ECSS-Q-ST-70-50C and NASA SN-C-0005D standards. UV light at 365 nm is often used for the inspection of optical sensitive surfaces to localize and to evaluate the amount of fluorescent particles, essentially coming from textile fibers. But other groups of compounds can be observed with a different spectral response and distribution, like adhesives and resins or even organic residues. Therefore, we could take advantage of this spectral information closely linked to specific molecules for partial identification of these materials before further investigation involving wipe on flight model and measurement in a laboratory.
Observations in the UV and EUV allow many diagnostics of the outer layers of the stars and the Sun so that more and more space telescopes are developed to operate in this fundamental spectral range. However, absorption by residual contaminants coming from polymers outgassing causes critical effects such as loss of signal, spectral shifts, stray light… Thus, a cleanliness and contamination control plan has to be defined to mitigate the risk of damage of sensitive surfaces. In order to specify acceptable cleanliness levels, it is paramount to improve our knowledge and understanding of contamination effects, especially in the UV/EUV range. Therefore, an experimental study has been carried out in collaboration between CNES and IAS, in the frame of the development of the Extreme UV Imager suite for the ESA Solar Orbiter mission; this instrument consists of two High Resolution Imagers and one Full Sun Imager designed for narrow pass-band EUV imaging of the solar corona, and thus very sensitive to contamination. Here, we describe recent results of performance loss measured on representative optical samples. Six narrow pass-band filters, with a multilayer coating designed to select the solar Lyman Alpha emission ray, were contaminated with different amounts of typical chemical species. The transmittance spectra were measured between 100 and 200 nm under high vacuum on the SOLEIL synchrotron beam line. They were compared before and after contamination, and also after a long exposure of the contaminated area to EUV-visible radiations.
Optical instruments for space applications with improved performances (smaller pixels and spectral range extension) are becoming more and more sensitive to chemical contamination and particle sedimentation. Outgassing under vacuum conditions causes dramatic flux losses, especially in the UV bandwidth. Furthermore, it is difficult to perform physicochemical analyses of contaminated surfaces on flight models, in a clean room. Conventional analytical techniques such as FTIR (Fourier Transform Infrared interferometer) need the tool to be in contact with the studied area, which is forbidden when working on satellites. In addition, it does not give any information about the distribution of the contaminants in the field of view. The probed area is large, mono-pixel, and the sensitivity of the instrument is too low for hundred nanometer thin film deposits. A first study has shown that we could benefit from using the UV/visible fluorescence spectra to partially identify contaminants and polymer materials. The shape of the fluorescence spectra of adhesives, paints and varnishes have specific signatures that could be recorded into a designated reference database. The location of the presence of these contaminants on such sensitive optics is also relevant. To acquire both these parameters, we designed a specific compact hyperspectral instrument to remotely acquire cube images (500x500 pixels) in a 5 degree field of view, and on a wide range of continuous wavelengths from UV at 320 nm up to the near infrared at 1000 nm. This paper will present the chosen trade-off between different critical optics for a new portable version of this instrument. It is dedicated to space and cultural heritage applications and the first results on an engineering prototype will be shown.
KEYWORDS: Contamination, Satellites, Ultraviolet radiation, Data modeling, Adaptive optics, Oxygen, Space operations, Thermal modeling, Solar processes, Control systems
The in-orbit aging of thermo-optical properties of thermal coatings critically impacts both spacecraft thermal balance and heating power consumption. Nevertheless, in-flight thermal coating aging is generally larger than the one measured on ground and the current knowledge does not allow making reliable predictions1. As a result, a large oversizing of thermal control systems is required. To address this issue, the Centre National d’Etudes Spatiales has developed a low-cost experiment, called THERME, which enables to monitor the in-flight time-evolution of the solar absorptivity of a large variety of coatings, including commonly used coatings and new materials by measuring their temperature. This experiment has been carried out on sunsynchronous spacecrafts for more than 27 years, allowing thus the generation of a very large set of telemetry measurements. The aim of this work was to develop a model able to semi-quantitatively reproduce these data with a restraint number of parameters. The underlying objectives were to better understand the contribution of the different involved phenomena and, later on, to predict the thermal coating aging at end of life. The physical processes modeled include contamination deposition, UV aging of both contamination layers and intrinsic material and atomic oxygen erosion. Efforts were particularly focused on the satellite leading wall as this face is exposed to the highest variations in environmental conditions during the solar cycle. The non-monotonous time-evolution of the solar absorptivity of thermal coatings is shown to be due to a succession of contamination and contaminant erosion by atomic oxygen phased with the solar cycle.
Contamination control is an important driver in the success of most space missions with more and more stringent
constraints of quality and reliability : indeed, most of spacecrafts having equipments sensitive to molecular
contamination like optics or detectors, the risk of damage and performance loss of such sensitive surfaces has to be
considered as a real concern and treated in the early phases of the development of an instrument. Since molecular
contaminants result mainly from outgassing of polymers, bakeouts under vacuum are required at the lowest possible
product level in order to reduce the contamination potential of selected materials. Nevertheless, this conventional method
takes time and could be relatively expensive. Then the use of low cost porous materials has appeared as an interesting
alternative to trap organic contaminants, taking advantage of their controlled adsorption characteristics in channels of
molecular dimensions. A recent PhD study has showed that, compared to other materials, zeolites widely used in
catalysis and separation processes have great potential in such applications. Theoretical and experimental investigations
have demonstrated the feasibility with three types of highly efficient zeolites. This paper reports on further development
related to the preparation of uniform, homogeneous thin films of pure zeolitic materials deposited on different substrates
(glass, carbon fibers...). Kinetics and sorption capacities of several representative outgassed species on these films have
been investigated by thermogravimetric analyses and the results compared with the efficiency of corresponding powder
materials. A discussion on the potential locations of such molecular adsorbers inside optical instruments is proposed.
KEYWORDS: Sensors, X-rays, Analog electronics, Electronics, Monte Carlo methods, Aluminum, Silicon, Staring arrays, Field programmable gate arrays, Quantum efficiency
The High Time Resolution Spectrometer (HTRS) is one of the five focal plane instruments of the International
X-ray Observatory (IXO). The HTRS is the only instrument matching the top level mission requirement of
handling a one Crab X-ray source with an efficiency greater than 10%. It will provide IXO with the capability
of observing the brightest X-ray sources of the sky, with sub-millisecond time resolution, low deadtime, low
pile-up (less than 2% at 1 Crab), and CCD type energy resolution (goal of 150 eV FWHM at 6 keV). The HTRS
is a non-imaging instrument, based on a monolithic array of Silicon Drift Detectors (SDDs) with 31 cells in a
circular envelope and a X-ray sensitive volume of 4.5 cm2 x 450 μm. As part of the assessment study carried
out by ESA on IXO, the HTRS is currently undergoing a phase A study, led by CNES and CESR. In this
paper, we present the current mechanical, thermal and electrical design of the HTRS, and describe the expected
performance assessed through Monte Carlo simulations.
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