Arcjet flow properties determined from laser-induced fluorescence of atomic nitrogen

Emission Spectroscopic Boundary Layer Investigation during Ablative Material Testing in Plasmatron

Ablative Thermal Protection Systems (TPS) allowed the first humans to safely return to Earth from the moon and are still considered as the only solution for future high-speed reentry missions. But despite the advancements made since Apollo, heat flux prediction remains an imperfect science and engineers resort to safety factors to determine the TPS thickness. This goes at the expense of embarked payload, hampering, for example, sample return missions. Ground testing in plasma wind-tunnels is currently the only affordable possibility for both material qualification and validation of material response codes. The subsonic 1.2MW Inductively Coupled Plasmatron facility at the von Karman Institute for Fluid Dynamics is able to reproduce a wide range of reentry environments. This protocol describes a procedure for the study of the gas/surface interaction on ablative materials in high enthalpy flows and presents sample results of a non-pyrolyzing, ablating carbon fiber precursor. With this publication, the authors envisage the definition of a standard procedure, facilitating comparison with other laboratories and contributing to ongoing efforts to improve heat shield reliability and reduce design uncertainties. The described core techniques are non-intrusive methods to track the material recession with a high-speed camera along with the chemistry in the reactive boundary layer, probed by emission spectroscopy. Although optical emission spectroscopy is limited to line-of-sight measurements and is further constrained to electronically excited atoms and molecules, its simplicity and broad applicability still make it the technique of choice for analysis of the reactive boundary layer. Recession of the ablating sample further requires that the distance of the measurement location with respect to the surface is known at all times during the experiment. Calibration of the optical system of the applied three spectrometers allowed quantitative comparison. At the fiber scale, results from a post-test microscopy analysis are presented.

Time-resolved fluorescence spectroscopy of two-photon laser-excited 8p, 9p, 5f, and 6f levels in neutral xenon

Laser-induced fluorescence of two-photon excited 8p, 9p, J=0, 2 and 5f, 6f, J=2 levels in neutral xenon has been investigated in the pressure regime between 4 and 120 Pa. Radiative lifetimes and collisional deactivation rates have been deduced especially for the 5f[3/2](2), 5f[5/2](2), and 8p[1/2](0) levels using the Stern-Volmer approach. The spontaneous lifetimes for 5f[3/2](2), 5f[5/2](2), and 8p[1/2](0) levels are 94, 78, and 207 ns, respectively. These lifetimes have been calculated also by applying extended multiconfiguration Dirac-Fock wave functions and are found in agreement with experiment within 10-25 %.

Two-photon laser-induced fluorescence of atomic hydrogen in a diamond-depositing dc arcjet

Atomic hydrogen in the plume of a dc-arcjet plasma is monitored by use of two-photon excited laser-induced fluorescence (LIF) during the deposition of diamond film. The effluent of a dc-arc discharge in hydrogen and argon forms a luminous plume as it flows through a converging-diverging nozzle into a reactor. When a trace of methane (< 2%) is added to the flow in the diverging part of the nozzle, diamond thin film grows on a water-cooled molybdenum substrate from the reactive mixture. LIF of atomic hydrogen in the arcjet plume is excited to the 3S and 3D levels with two photons near 205 nm, and the subsequent fluorescence is observed at Balmer-alpha near 656 nm. Spatially resolved LIF measurements of atomic hydrogen are made as a function of the ratio of hydrogen to argon feedstock gas, methane addition, and reactor pressure. At lower reactor pressures, time-resolved LIF measurements are used to verify our collisional quenching correction algorithm. The quenching rate coefficients for collisions with the major species in the arcjet (Ar, H, and H2) do not change with gas temperature variations in the plume (T < 2300 K). Corrections of the LIF intensity measurements for the spatial variation of collisional quenching are important to determine relative distributions of the atomic hydrogen concentration. The relative atomic hydrogen concentrations measured here are calibrated with an earlier calorimetric determination of the feedstock hydrogen dissociation to provide quantitative hydrogen-atom concentration distributions.

Flow Characterization of a Diamond-Depositing DC Arcjet by Laser-Induced Fluorescence

Laser-induced fluorescence (LIF) measurements of seeded nitric oxide and naturally occurring species in a diamond-depositing dc arcjet of hydrogen/argon/methane (0.8:1.0:0.005) at 25 Torr are used to determine the temperature and velocity fields in a gas jet. LIF measurements are also used to demonstrate the importance of gas recirculation on the chemical composition of the arcjet plume. The gas flow in the arcjet plume is supersonic, with a maximum axial speed of 2.6 km/s at the center of the nozzle exit. This axial velocity decreases with radius with a parabolic distribution in the plume. There is no measurable radial velocity in the free stream of the arcjet plume, and the radial expansion of the plume is consistent with diffusion. The maximum temperature at the plume center is 2400 K and varies less than 15% with chamber pressures of 10-50 Torr. The substrate is placed in the arcjet plume normally to the directed velocity, producing a stagnation point. The gas temperature above this stagnation point is observed to rise abruptly as a consequence of the supersonic shock. The radial velocity near the stagnation point becomes significant, and a maximum radial velocity of 1300 m/s is determined.