The University of Rochester's Laboratory for Laser Energetics (LLE) has recently completed the construction of the OMEGA EP short-pulse, petawatt laser system. A major structure for OMEGA EP is the grating compressor chamber (GCC). This large (15,750-ft3) vacuum chamber contains numerous optics used in laser-pulse compression of two 40-cm-sq-aperture, IR (1054-nm) laser beams. Critical to this compression, within the GCC, are eight sets (four per beamline) of tiled (e.g., three optical elements precisely held side by side to act as one element) multilayer-dielectric (MLD)-diffraction-grating assemblies (three gratings per assembly) that provide the capability for producing 2.6-kJ output IR energy per beam at 10 ps. The primary requirements for each of the 24 large-aperture (43-cm × 47-cm) gratings are a high diffraction efficiency greater than 95%, a peak-to-valley wavefront quality of less than &lgr;/4 waves at 1054 nm, and a laser-induced-damage threshold greater than 2.7 J/cm2 at 10-ps pulse width (measured at normal beam incidence). Degradation of grating laser-damage thresholds due to adsorption of contaminants must be prevented to maintain system performance.

Previously, significant laboratory work has been performed on the photochemical deposition and darkening of molecular contaminant films. Much of this work addresses single, purified molecular species to understand fundamental photochemical processes. However, some of this work disagrees with other studies involving mixed, real spacecraft materials. There are also points of disagreement with contaminated returned optics from the Hubble Space Telescope where mixed contaminants were found. In this paper, we describe a method for vacuum depositing a controlled, reproducible contaminant film containing two molecular species: tetramethyl-tetraphenyl trisiloxane (DC 704) and dioctyl phthalate (DOP). We use this film to show differences in photochemical processes compared to a pure film of DC 704. We show that some photopolymerization processes occur more slowly in a two-component, mixed film during accelerated exposure to vacuum ultraviolet (VUV) radiation.

Room Temperature Vulcanized (RTV) materials, such as silicone adhesives, are commonly used to bond components of communication satellites and other types of spacecraft. The elevated satellite operating temperature causes the unused catalyst material in the RTV to volatize, which can then re-deposit or condense onto other spacecraft surfaces. In the presence of sunlight, this Volatile Condensable Material (VCM) can photo-chemically deposit onto optically-sensitive spacecraft surfaces and significantly alter their original, beginning-of-life (BOL) optical properties, such as solar absorptance and emittance, causing unintended performance loss of the spacecraft. Knowledge of the optical impact of photo-chemically-deposited VCM's is therefore a major concern of spacecraft designers and spacecraft-contamination engineers. In view of this we have employed in-situ spectroscopic ellipsometry to monitor in real time the optical constants of the condensed effluent of RTV CV-566 as well as its photofixed effluent. This technique is sensitive to nm thick layers and can be used to extract n and k as a function of wavelength. We will present the optical constants, n and k, for both condensed unexposed and the photofixed film.

Molecular contamination degrades sensitive spacecraft surfaces and can adversely affect the useful life of a spacecraft. In order to accurately predict spacecraft performance and end of life, an understanding of the primary mechanisms and processes involved in the deposition and "fixing" of molecular contaminants is necessary. The objective for this research effort has been to investigate how solar vacuum ultraviolet (VUV) radiation and surface temperature influence photochemical reactions of molecular contaminants. This report presents the effects of VUV intensity and surface temperature on photo-deposition and "photo-fixing" of dioctyl phthalate (DOP) films.

Silicone materials pose a contamination control challenge because of their ubiquity in satellite hardware, the tendency of the material and its outgassed contaminants to migrate along surfaces, and the difficulty in cleaning away the residue. To devise effective mitigation strategies, accurate knowledge of the chemical identity and properties of the outgassed species is needed. This information is critical for modeling silicone outgassing deposition processes and for developing effective cleaning methods. To this end, a chemical analysis study of several common silicone materials was conducted to identify and characterize the outgassed contaminants. Gas Chromatography-Mass Spectrometry (GC-MS) and other laboratory techniques were used to identify and characterize the outgassed species. In this report, the results of this study will be discussed with a particular emphasis on comparing the outgassing properties of the species collected from these materials to DC704, which is typically used to model silicone outgassing.

To understand the dynamics of airborne particulate intrusion into a space telescope, a mechanistic model based on mass balance was developed to predict the extent to which ambient particles penetrate through vent holes and enter the interiors after the purge is off. This work describes the mathematical modeling analysis, supplementing with results from laboratory measurements using a cylindrical chamber as a simulated space telescope. It was found that the characteristic time for airborne particles to reach a saturation level, after the purge is off, can be characterized by the air exchange rate and particle deposition rate inside the confined space volume. The air exchange rate, measured using a tracer gas technique, is associated with the natural convection and air flow turbulence intensity adjacent to the chamber. During the purge outage, the steady-state airborne particle concentration inside the space telescope is governed by the ambient particle concentration, air exchange rate, and particle deposition rate.

Contamination-enhanced Laser Induced Damage (CLID) occurs when molecular or particulate contamination, present on or in the vicinity of an optical material, leads to accelerated laser power degradation and premature failure. The physical mechanisms that cause CLID are not sufficiently understood to predict the extent to which a contaminant will cause damage. Although standard computational methods can be used to predict the amount of contamination on an optic, the effects of those molecules or particles on laser performance has not been sufficiently quantified. This paper will describe an approach for managing CLID that relies on laboratory studies to understand the relationship between contaminant type or quantity and CLID thresholds. That insight can then be used to guide the definition of cleanliness requirements and the design of material screening tests. Initial efforts to study how mass transport, the movement of contaminants in and out of the laser beam, affects damage rates will be discussed as well.

Amorphous fluorocarbon (a-C:F) thin films have been developed that protect surfaces from molecular and particulate contamination. The surface energies of the thin films are low and primarily dispersive in origin with values of energies measured to be as low as 18 mJ/m2 (17.5 dispersive, 0.5 polar). The films are transparent to visible light and have a refractive index of ~1.4. The a-C:F surface energy was found to be thermally stable when exposed to temperatures that range from 77°K to 400°C. Molecular absorption rates are significantly reduced on gold surfaces when over-coated with an a-C:F thin film. The adhesion force of particles to the a-C:F surface is low and can dramatically decrease the susceptibility of particles to adhere to surfaces over-coated with the thin film. The robust nature of the diamond-like thin films make them candidates for protecting aerospace surfaces, such as optical surfaces, from contamination.

AZ93 with a fluoropolymer overcoat is an option to simplify ground handling of space hardware. The overcoat applied on some on-orbit International Space Station (ISS) hardware provides contamination protection for optically sensitive ceramic thermal control coatings. However, if the fluoropolymer is not eroded on-orbit by atomic oxygen (AO), then it will darken. This will increase the solar absorptance resulting in possible thermal performance degradation. If the fluoropolymer overcoat was not present, optical performance would be significantly improved. To characterize the optical performance of the AZ93 with the fluoropolymer overcoat for modeling the UV degradation, laboratory testing of the coating was performed at Marshall Space Flight Center (MSFC). Sample coupons prepared by AZ Technology were exposed under vacuum to ultraviolet radiation. At periodic intervals, the samples were removed from the testing chamber to acquire images and to measure the solar absorptance. The images showed visible differences between AZ93 with the overcoat and without the overcoat as vacuum ultraviolet (VUV) exposure increased. Darkening is more pronounced in the samples with the fluoropolymer overcoat. This was also evident in the solar absorptance measurements. Optical properties of AZ93 with the fluoropolymer overcoat significantly degraded in comparison to those without the overcoat. A short period of little change followed by an exponential rise in solar absorptance was observed. The optical degradation of the fluoropolymer overcoat is described in terms of surface reaction chemistry and kinetics and is found to follow a pseudo first order reaction rate.

Dust contamination is a serious problem for equipment and vehicles for space mission applications. Lunar "weathering" has left the lunar soil with a relatively fine texture, a small count median diameter and an unusually large geometrical standard deviation compared to terrestrial dust particle size distributions. Accumulated lunar dust regolith is estimated to reduce solar power system efficiencies by as much as 50 percent. Lunar dust is electrostatically charged, difficult to remove, and appears to get everywhere. Astronaut exposure to lunar dust and its risks to health and operations are also an important design consideration for long-duration lunar missions. We are attempting to design an integrated approach to solving the dust problems associated with its many elements (life support systems, EVA, docking and berthing, surface mobility, in situ resource utilization, and power system components), as opposed to leaving it to each individual element developer. Other potential applications include mitigation of unintentional capture of extraterrestrial bacteria or spores on the surfaces of the equipment. This presentation will present an overview of the lunar regolith particle size and shape distribution properties, hydrophilic and hydrophobic coating self-cleaning approaches and a new approach which incorporates various catalytic mechanisms (stoichiometric, photocatalytic and electrocatalytic) for decontamination.

The ability of a space-grade material to both sense and absorb molecular contamination is of great use. A series of polymer materials that both sense and absorb moisture, a contaminant detrimental to optical and space systems, has been created. The materials were prepared by introducing additives into polymers while still retaining the original properties of both the polymers and the additives. The additive acts as the contaminant absorber and sensor. Measurements to determine the amount of moisture absorption compared to other materials have been executed. The final materials were easy to fabricate and could be produced on a large scale. The materials also could easily be regenerated again for multiple uses. Development for practical applications such as a desiccant material has been carried out.

The International Space Station (ISS) solar arrays provide power that is needed for on-orbit experiments and operations. The ISS solar arrays are exposed to space environment effects that include contamination, atomic oxygen, ultraviolet radiation and thermal cycling. The contamination effects include exposure to thruster plume contamination and erosion. This study was performed to better understand potential solar cell optical performance degradation due to increased scatter caused by plume particle pitting. A ground test was performed using a light gas gun to shoot glass beads at a solar cell with a shotgun approach. The increase in scatter was then measured and correlated with the surface damage.

The Direct Simulation Monte Carlo (DSMC) Analysis Code (DAC), as released by NASA, is a general purpose, gas dynamic, transport analysis suite of codes. These codes have been acquired by Ball Aerospace & Technologies Corp. (BATC) through a software usage agreement and have been modified to do a more detailed analysis of contaminant molecular transport of spacecraft and spacecraft instruments over mission lifetimes, typically 5 to 7 years. This transport model takes advantage of the proven algorithms within DAC to handle complex surface geometries and time-dependant gas dynamics. Additions to the code include diffusion of contaminants through solid surfaces, temperature and coverage-dependant adsorption/desorption for the contaminants being modeled, and input data for molecular diameters, molecular weights, and diffusion parameters for the common contaminants found in spacecraft materials and coatings.

Gas-phase contamination modeling for space systems typically looks at the free molecular flow regime, Knudsen number » 1, wherein transport is characterized by collisionless motion of contaminant molecules and deposition proportional to grey- or black-body view factors. Such an approach, however, was not applicable to the contamination transport environment [to be] encountered by the NASA Mars Science Laboratory (MSL) during surface operations on the Red Planet. For MSL, we required an understanding of contaminant transport under the Mars-ambient conditions of an approximately 8 Torr CO2 atmosphere in order to provide traceability between hardware outgassing limits and the allowable vapor-phase contaminant concentrations in the vicinity of atmospheric sampling sensors and deposition to prospective solid sample sites on the Martian surface. In setting outgassing requirements for the MSL surface system, an engineering upper-bound estimate--rather than a precise result based on an all-inclusive simulation of the dynamic flow field--of the local contamination density was needed. Here we describe a 3-D, low-speed computational fluid dynamics approach, including molecular diffusion, to determine mixing ratios of contaminants at the atmospheric sample inlets and solid sample inlet deposition rates. Turbulence enhances the effective diffusion, leading to the dilution of the volatile contaminants, which reduces contamination concentration at a distance far from the source in comparison to inviscid or laminar flow fields: Therefore, the approach employed here results in a conservative upper bound compared to one in which turbulence is explicitly addressed. Because contaminant transport in this environment (Peclet number in the range of 50-1000) is advection dominated, spatial contamination concentration is a strongly-peaked function of the wind direction. Results of sample calculations for expected Mar wind speeds (u = 1-20 m/s) and several wind directions are presented.

This paper describes two mathematical purge models, transient and steady-state, developed for investigation of purging critical space systems with stringent humidity requirements. The developed single-cell purge model correlates well with measured data from a purge-test engineering model. The validated purge models are being used to support various purging activities/plans associated with spacecraft/payload integration and test and spacecraft/launch vehicle integration. This paper also includes a dew-point analysis to address water-vapor condensation concern for purging critical space systems.

Two Surface Acoustic Wave (SAW) quartz crystal microbalances were used to determine nonvolatile mass deposition from a nitrogen purge gas. The units were connected serially and both were evaluated in the first and second position. Similarities and differences will be discussed.

The James Webb Space Telescope (JWST) will carry on exploration of the early universe with a 6-m exposed primary mirror and cryogenically cooled instruments. The mirror and its instruments will perform extremely deep exposures at near infra-red wavelengths (0.6-30 microns), and will operate for 5-10 years. The contamination effects of foremost concern on JWST are those of scatter due to particulate contamination on the primary mirror, loss of transmission from particulate, molecular and ice contamination, and loss of detector operation due to ice forming during cool-down of the observatory. The effects on JWST science of these contamination sources will be described together with how requirements for cleanliness levels were subsequently established.

When measuring the BSDF of a surface, it is necessary to determine the instrument signature of the measurement device, which quantifies its intrinsic scatter with no sample present. For scatter angles near the specular beam, the equivalent BSDF of this signature is greater than the BSDF of the sample, and therefore the signature defines the minimum measurable scatter angle. For flat samples, the signature can be measured directly, but this is not possible for curved samples because the optical power of the sample changes the angular distribution of the signature. A method is described in K. A. Klicker et. al. [1] in which the signature of the device with a curved sample is determined using a raytracing simulation, however, few details of this simulation are given. Here we give the details of a simulation in which the instrument signature of a commercial scatterometer was modeled. We will show that this method is effective by comparing the modeled signature of a flat sample to the measured.

Discrepancies between recent global earth albedo anomaly data obtained from the climate models, space and ground observations call for a new and better earth reflectance measurement technique. The SALEX (Space Ashen Light Explorer) instrument is a space-based visible and IR instrument for precise estimation of the global earth albedo by measuring the ashen light reflected off the shadowy side of the Moon from the low earth orbit. The instrument consists of a conventional 2-mirror telescope, a pair of a 3-mirror visible imager and an IR bolometer. The performance of this unique multi-channel optical system is sensitive to the stray light contamination due to the complex optical train incorporating several reflecting and refracting elements, associated mounts and the payload mechanical enclosure. This could be further aggravated by the very bright and extended observation target (i.e. the Moon). In this paper, we report the details of extensive stray light analysis including ghosts and cross-talks, leading to the optimum set of stray light precautions for the highest signal-to-noise ratio attainable.

Breault Research Organization has designed and built a stray light test station. The station measures the point source transmission and background thermal irradiance of visible and infrared sensors. Two beam expanders, including a large 0.89 meter spherical mirror, expand and collimate light from laser sources at 0.658 and 10.6 µm. The large mirror is mounted on a gimbal to illuminate sensors at off-axis angles from 0° to 10°, and azimuths from 0° to 180°. Sensors with apertures as large as 0.3 meters can be tested with the existing facility. The large mirror is placed within a vacuum chamber so cryogenic infrared sensors can be tested in a vacuum environment. A dark cryogenic cold plate can be translated into the field of view of a sensor to measure its background thermal irradiance.

BATC has developed a new stray light test facility (SLTF) and performed initial tests demonstrating its capabilities. The facility interior is nearly all black and is a Class 5 cleanroom. Coupled with a double cylindrical chamber that reflects the specular light away from the instrument under test, the stray light control in the facility is excellent. The facility was designed to be able to test a wide variety of instruments at a range of source angles from in-field to large off-axis angles. Test results have demonstrated PST performance below 1E-9.