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Tunable Laser Absorption Spectroscopy for Human Spaceflight 49th International Conference on Environmental Systems ICES-2019-358 7-11 July 2019, Boston, Massachusetts Tunable Laser Absorption Spectroscopy for Human Spaceflight Christopher M Matty1 and Lance E Christensen2 National Aeronautics and Space Administration, Houston Texas 77058 and Pasadena California 91109 Human Spaceflight consistently has need for measurement of the major constituent gasses in a habitable cabin environment. Traditionally, this measurement has been performed using a mixture of mass spectrometry, electrochemical, and other traditional sensing methods. Tunable laser spectroscopy has been used in industry and research applications since the 1970s but has not been used in primary operational human spaceflight systems aside from a handful of experimental applications. Meanwhile, recent advances in the quality, size, and cost of semiconductor lasers has had a significant impact on the range of feasible applications for tunable laser spectroscopy. In 2018, work started on the Orion Laser Air Monitor (LAM), which is designed to be the primary major constituency monitoring system for the Orion Spacecraft. This paper covers the history leading up to the selection, design, and build of the Orion Laser Air Monitor as a primary major constituent analyzer for human spaceflight. Nomenclature TLDAS = Tunable Laser Diode Absorption Spectroscopy QC = Quantum Cascade IC = Interband Cascade TEC = Thermo Electric Cooler TRL = Technology Readiness Level SWAP = System Size Weight and Power IR = Infrared MCA = International Space Station Major Constituent Analyzer CAMS = Submarine Central Atmosphere Monitoring System AGA = Anomaly Gas Analyzer MGM = Multi Gas Monitor CPM = Combustion Product Monitor LAM = Laser Air Monitor SAFFIRE = Spacecraft Fire Experiement GaSb = Gallium Antimodide I. Introduction UNABLE Laser Diode Absorption Spectroscopy (TLDAS) is currently enjoying a period at the developmental Tforefront. From the first uses of laser spectrometers; the technology has largely been driven by the availability and attendant size of serviceable lasers emitting at the necessary wavelength. Original laser instruments weighed hundreds of pounds and required cryostats or liquid nitrogen for cooling, making them at best awkward and largely impractical for operational use in spacecraft. Modern electronics miniturazation and micro-manufacturing capability, combined with advancement in semiconductor laser technology has allowed tunable semiconductor lasers to shrink to postage-stamp size, along with matching thermoelectric coolers (TEC)s that allow reliable function on an easily portable scale which lends more readily to integration into system and vehicle-level mechanisms. Additionally; the 1 Advanced Environmental Control and Life Support Systems Integrator, International Space Station Program, Johnson Space Center, Houston, Texas 77058. 2 Atmospheric Observations, Jet Propulsion Laboratory, Pasadena, California 91109 Technology Readiness Level (TRL) of semiconductor lasers has significantly increased as a function of the aforementioned technological advancements and attendant field deployment of tunable lasers in unmanned probes and rovers, to the point that use in human spaceflight, with the associated strict requirements for performance and reliability, has become a viable proposition. II. History and Background of Tunable Laser Diode Adsorption Spectroscopy Tunable semiconductor laser spectrometers have been an essential part of NASA Earth Science since the 1980s1. Early high-altitude aircraft spectrometers used cryogenically-cooled lead-salt lasers to measure chemical species at the parts-per-trillion level, enabling understanding of critical Earth systems such as the dynamics and chemistry of the ozone hole2. As tunable lasers matured towards room-temperature operation, synchronous miniaturization of tunable laser spectrometers permitted their integration into NASA Planetary Science platforms such as the Tunable Laser Spectrometer on the Mars Curiosity Rover to understand geochemical processes and possible life signatures on Mars3. NASA also invests in tunable laser spectrometer demonstrations for monitoring of gases important to human spaceflight on ISS4. Much of the attraction of laser spectroscopy for science and human exploration needs arises from its sensitivity (parts-per-billion or better), System Size, Weight and Power; SWAP, (weight less than few kg, power less than a few watts), fast response (compared with solid state sensors), relative freedom from calibration, and specificity (capability to distinguish isotopes). Sensitivity is primarily derived from the emitted wavelength and optical pathlength. Mid-IR lasers access stronger rovibrational (electronic for O2) transitions than near-IR lasers (Figure 1) and the implications for spectrometer design can be dramatic. The advent of thermoelectrically-cooled lasers at 3.3 µm in the 2000s5 meant it was possible to measure methane at parts-per-trillion with a 36×13-cm dia. instrument that could fit inside a rover6 employing a traditional beam multipass technique like Herriott cells7 proven in rugged environments like open-path configurations in Earth’s stratosphere8. Without such mid-IR laser evolution, it would have been difficult to engineer the cryogenic-cooling or cavity-enhanced solutions (cavity ringdown or integrated cavity output spectroscopy) required to obtain this sensitivity while remaining robust through spaceflight shock and vibration and environmental changes on Mars surface. Many of the trace gas monitoring needs (CO2, H2O, O2, CH4, HCl, HCN, HF, NH3, N2H4) for human exploration can be done with shoebox-sized open-path Figure 1. Rovibrational Tranistion Linestrengths Rovibrational instruments without need of pumps9. transition linestrengths for carbon dioxide (black) and methane (red). Note For chemicals that are sticky (e.g. logarithmic y-axis. HCl) or decompose on surfaces (e.g. N2H4), open-path monitoring can give a clearer assessment of local chemical concentration then if gas were pumped to the analysis region. Tens of Hz or faster measurement rates combined with the high sensitivity afforded by laser spectrometers may prove useful for human breath monitoring needs such as assessing respiratory function of astronauts. Tunable laser spectrometers can achieve millisecond response times because measurement speed is dictated by laser drive electronics as opposed to chemical-physical adsorption or reaction processes. Increasingly, the Earth science community turns to laser spectrometers for fast eddy-covariance measurements of greenhouse gases10. The spectral resolution of tunable semiconductor lasers (few MHz) enables distinguishing individual molecules and isotopologues with one laser. Sub-percent level carbon dioxide and water vapor can be measured using an 8-cm optical pathlength open-pass configuration at 2.68 microns at 1 atm (Figure 2). To understand Martian geochemistry, 13 18 17 the Tunable Laser Spectrometer measures isotopologues of carbon dioxide (CO2, CO2, OCO, OCO) at 2.79 2 International Conference on Environmental Systems microns at 0.010 atm. It can be envisioned that tunable laser spectrometers can provide non-invasive, robust, and miniature health monitoring for astronaut metabolic state (breakdown of sugar vs fat), passive indications of health problems like sepsis and liver fibrosis, and active testing (inhalation of isotopically-labeled tracers) for specific diseases like tuberculous11. III. Legacy to Current Human Spaceflight Sensing Systems Sensing and monitoring of habitable cabin atmosphere and spacesuit interface volumes is largely considered to be a necessity for human spaceflight. The performance, integration, safety, and health considerations of human crew members onboard a spacecraft necessitate monitoring of the pressure, temperature, and chemical composition of the cabin and suit atmospheres. Initially, during the Mercury through Apollo era, US spacecraft used dedicated single- channel sensors for gross measurement of CO2 and O2 levels. With the advent of the Skylab program, a flight mass spectrometer sensor system (affectionately called “the pig” for its round shape) which collectively measured N2, O2, CO2, H2O, H2, and approximate Total Hydrocarbons (THC) in one unit. Longer missions, coupled with increased vehicle size and volume, coupled with high potential measurement accuracy attendant to a mass spectrometer, made mass spectrometry attractive as a means to precisely measure and monitor cabin atmospheric constitucency. The Skylab “pig” paved the way for the current Major Constituent Analyzer (MCA) currently in use onboard the International Space Station (ISS). The MCA itself is a heritage design carried from the Central Atmosphere Monitoring System (CAMS) technology originally used in military submarines12. MCA is a magnetic-sector type mass spectrometer which measures CO2, O2, N2, H2O, CH4, and H2. In contemporary human spaceflight philosophy, the primary cabin atmosphere monitoring system is typically considered as Criticality 1, or Criticality 1, meaning that function and performance of the system are immediately critical to the safe performance of mission by the human crew. Having a long track record of field use for human systems, mass spectrometers generally are held in high regard in providing robust, dependable Criticality 1 level constituent monitoring. However, while mass spectrometers are highly regarded for precision and accuracy; there are also inherent drawbacks to the technology, namely relatively
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