Gas–Surface Scattering Dynamics Applied to Concentration of Gases for Mass Spectrometry in Tenuous Atmospheres

Abstract

A concept for a passive neutral gas concentrator that facilitates the analysis of rarefied atmospheres using mass spectrometry on spacecraft has been developed. The efficiency of the concentrator depends strongly on gas–surface scattering dynamics between the incoming gas molecules and the concentrator surface. We conducted beam–surface scattering experiments using hyperthermal beams containing atomic and molecular oxygen with speeds of approximately 5500 m s–1, with angular and velocity resolution of the inelastically scattered O and O2, on gold thin film, SiO2, and highly oriented pyrolytic graphite (HOPG) surfaces, which were chosen as candidate concentrator surfaces. The results show clearly that atoms and molecules scattering from HOPG have the narrowest and most superspecular angular distributions with the least energy transferred to the surface. A test particle model, referred to as the Statistical Program for Aerodynamic and Radiation Pressure Coefficient Simulation, utilized the experimental results to model gas concentration in three-dimensional cone and annular-ring geometries constructed of the representative materials chosen for study. The modeling results indicate that a cone concentrator with a 10° half angle opening yields the highest concentration factors. In addition, a cone constructed of HOPG yields a concentration factor that is an order of magnitude higher than what can be achieved using gold or SiO2 surfaces.

A review of gas-surface interaction models for orbital aerodynamics applications

“…For out of plane scattering considerations, techniques addressing surface corrugation in more than one dimension need to be employed. While Maxwell's model fails in reproducing the petal-shaped angular distribution observed in these experiments, it appears that its theoretical apparatus and simplified assumptions are sufficient to describe some re-emission polar plots showing a small nearly specular and a large diffuse component [104,107]. Results provided by Mehta et al [108] show that when the CLL model is adopted some difficulties are encountered in the attempt of selecting the proper combination of α n and σ t to reproduce the experimental conditions.…”

“…The generally small amount of particles that undergo multiple collisions before being scattered in the gas-phase determines the width of the lobe of distribution and seems to be susceptible to the morphology of the surface considered, despite the level of roughness. Wider angular distributions were obtained for smoother surfaces even when higher incident energies were employed [107]. This seems to suggest that the interactions within atoms in the same surface layer or adjacent layers may play a role in determining the scattering characteristic.…”

Spacecraft-plasma-debris interaction in an ion beam shepherd mission

“…In the nominal case, the backsputtered atom contribution is around 3% of the total transferred force, while the effect of recombined Xe neutrals is around 8%. This last contribution is greatly affected by the accommodation coefficient α W (here 0.98), which is typically estimated experimentally and affected by large uncertainties (values between 0.8 and 1 can be found in literature [34,35]). In particular, the force transferred by recombined neutrals rapidly drops as this coefficient approaches 1, being α W = 1 the most conservative case (lowest force transmission).…”

“…When a heavy particle hits a material surface, it can undergo two different processes: (i) a quick reflection (nearly specular) within the first atomic layers, with a backscattering probability p b , or (ii) a partial/complete accommodation within the wall, in which the particle gradually loses memory of its impact direction and approaches a thermodynamic equilibrium with the lattice atoms, being finally re-emitted diffusely [32,33,34,35]. Additionally, if the impacting particle kinetic energy is large enough to overcome the binding energy of the constituent atoms of the material lattice, it can also cause sputtering, which means that it knocks out a certain number of material atoms from its surface.…”

“…The models for both the generation of sputtered atoms and the backscattering or accomodation processes of incident particles are described in detail in the Appendix. In particular, all impacting particles that are not backscattered, are re-emitted diffusely as neutrals, with a lower energy, that depends on their impacting kinetic energy, the wall temperature and an energy accommodation coefficient α W [33,34,35].…”