Nitrogen thermometry in an inductively coupled plasma torch using broadband nanosecond coherent anti-Stokes Raman scattering.

The development of atmospheric hypersonic flight and re-entry capabilities requires the characterization of the thermo-chemical state of representative test environments. This study demonstrates the usage of multiplex nanosecond N 2 coherent anti-Stokes Raman scattering (CARS) to measure temperatures in an atmospheric, high-temperature (>6000K), air plasma plume, generated by an inductively coupled plasma torch. These are some of the highest temperatures ever accessed via gas-phase CARS, to our knowledge. Temperatures of N 2 in the equilibrium plasma plume are determined via theoretical fits to measured CARS spectra. We discuss the practical implementation of CARS at very high temperatures, including the scaling of the N 2 CARS signal strength from 300 to 6700 K, where the expected peak signal from the high-temperature plasma torch gases is two orders of magnitude less than commonly encountered in combustion environments. An intensified CCD camera enables single-laser-shot detection at temperatures as high as 6200 K, by increasing sensitivity and providing a time gate against intense background luminosity. We also discuss the impacts of unwanted two-beam CARS contributions from outside the nominal three-beam measurement volume. We present mean axial and radial temperature profiles, as well as time-series data derived from both single-laser-shot and accumulated CARS spectra. The single-laser-shot precision is 1.7%-2.6% at temperatures of 3500 to 6200 K. The presented results pave the way for the use of CARS at very high temperatures and the measurement of spatially resolved interface processes in high-enthalpy flows.

The Evolution of a Spacecraft-Generated Lunar Exosphere.

Understanding how spacecraft alter planetary environments can offer important insights into key physical processes, as well as being critical to planning mission operations and observations. In this context, it is important to recognize that almost any powered lunar landing will be an active volatile release experiment, due to the release of exhaust gases during descent. This presents both an opportunity to study the interaction of volatiles with the lunar surface, and a need to predict how non-indigenous gases are dispersed, and how long they persist in the lunar environment. This work investigates these questions through numerical simulations of the transport of water vapor during a nominal lunar landing and for two lunar days afterwards. Simulation results indicate that the water vapor component of spacecraft exhaust is globally redistributed, with a significant amount reaching permanently shadowed regions (cold traps) near the closest pole, where temperatures are sufficiently low that volatiles may remain stable over geological timescales. Exospheric evolution and surface deposition patterns are highly sensitive to desorption activation energy, providing a means to constrain this critical parameter through landed or orbital measurements during future missions. Contamination of cold traps by exhaust gases is likely to scale with exhaust mass and proximity of the landing site to the poles. Exhaust propagation is perhaps the most widespread and long-lived impact of spacecraft operations on a nominally airless solar system body, and should be a key consideration in mission planning and in interpreting measurements made by landed lunar missions, particularly at near-polar regions. PLAIN LANGUAGE SUMMARY: There has been increasing interest lately in learning more about the origin and distribution of water on the Moon. However, whenever a spacecraft descends to land on the lunar surface, it releases water vapor and other gases into the lunar environment, complicating the situation. In this work, we use computer simulations to understand what happens to the water released by a spacecraft during a typical landing. The simulated landing creates a temporary, very thin atmosphere all around the Moon. The behavior of this atmosphere depends on how strongly water sticks to the lunar surface, such that comparing simulations to measurements of water in the lunar environment during and after future lunar landings could help us figure out the "stickiness" of the lunar surface - something that we don't yet accurately know, but is important to understanding the past, present and future distribution of water on the Moon. Our simulations also show that some spacecraft-delivered water travels to regions near the poles that are cold enough to trap water for very long periods of time. If the spacecraft is heavier, or lands closer to the poles, its influence on the lunar surface and atmosphere may be more significant.

Accurate temperature measurements in flames with high spatial resolution using Stokes Raman scattering from nitrogen in a multiple-pass cell.

A multiple-pass cell is aligned to focus light at two regions at the center of the cell. The two "points" are separated by 2.0 mm. Each probe region is 200 µm×300 µm. The cell is used to amplify spontaneous Raman scattering from a CH4-air laminar flame. The signal gain is 20, and the improvement in signal-to-noise ratio varies according to the number of laser pulses used for signal acquisition. The temperature is inferred by curve fitting high-resolution spectra of the Stokes signal from N2. The model accounts for details, such as the angular dependence of Raman scattering, the presence of a rare isotope of N2 in air, anharmonic oscillator terms in the vibrational polarizability matrix elements, and the dependence of Herman-Wallis factors on the vibrational level. The apparatus function is modeled using a new line shape function that is the convolution of a trapezoid function and a Lorentzian. The uncertainty in the value of temperature arising from noise, the uncertainty in the model input parameters, and various approximations in the theory have been characterized. We estimate that the uncertainty in our measurement of flame temperature in the least noisy data is ±9 K.

Improved multiple-pass Raman spectrometer.

An improved Raman gain spectrometer for flame measurements of gas temperature and species concentrations is described. This instrument uses a multiple-pass optical cell to enhance the incident light intensity in the measurement volume. The Raman signal is 83 times larger than from a single pass, and the Raman signal-to-noise ratio (SNR) in room-temperature air of 153 is an improvement over that from a single-pass cell by a factor of 9.3 when the cell is operated with 100 passes and the signal is integrated over 20 laser shots. The SNR improvement with the multipass cell is even higher for flame measurements at atmospheric pressure, because detector readout noise is more significant for single-pass measurements when the gas density is lower. Raman scattering is collected and dispersed in a spectrograph with a transmission grating and recorded with a fast gated CCD array detector to help eliminate flame interferences. The instrument is used to record spontaneous Raman spectra from N(2), CO(2), O(2), and CO in a methane-air flame. Curve fits of the recorded Raman spectra to detailed simulations of nitrogen spectra are used to determine the flame temperature from the shapes of the spectral signatures and from the ratio of the total intensities of the Stokes and anti-Stokes signals. The temperatures measured are in good agreement with radiation-corrected thermocouple measurements for a range of equivalence ratios.

Frequency-resolved absorption tomography with tunable diode lasers.

A tunable diode laser was used for absorption tomography in an axisymmetric atmospheric pressure flat-flame burner. A rapid tomographic inversion algorithm was used to facilitate the many reconstructions at a relatively sparse set of projections typical of laser absorption tomography. Profiles of temperature and CO2 mole fraction were measured simultaneously in methane-air flames. Absorption measurements were made near the R-branch bandhead at 4.17 microm to minimize interferences with other species, while providing good temperature and concentration sensitivity at flame conditions. The procedure showed the advantage of reconstructing detailed spectra at each radial node.