Tomographic Measurements of Ejecta Cloud Concentrations in Plume-Surface Interactions Using Millimeter Wave Interferometry

Millimeter-wave interferometry has shown much promise to measure particle concentrations in opaque, dispersed multiphase flows such as those encountered in plume-surface interactions. This study expands on a previously developed proof-of-concept millimeter-wave interferometer, demonstrating several mission-critical capabilities and providing unique insights in the ejecta dynamics of plume-surface interactions. An innovative vacuum calibration procedure reduced measurement uncertainties by one order of magnitude compared to prior calibrations at ambient conditions thanks to the reduction of aerodynamic entrainment. Calibration experiments verified the theoretical linear relation between phase shift and path-integrated particle number density and provided the constant proportionality factor for the specific glass microspheres and radar system employed. The instrument was demonstrated on a reduced-scale plume-surface interaction experiment. Ejecta path-integrated concentrations were measured at two ambient pressure levels representative of Lunar (14.26 Pa) and Martian (800 Pa) surface conditions, confirming the trends observed during prior experiments. Further experiments were conducted at 800 Pa to demonstrate the capability of the instrument to operate in very close proximity to the ground, and to analyze the distribution of ejecta with altitude. Ejecta path-integrated concentrations measured at 9, 6, and 3 cm above the granular surface provided maximum values of 1.284E9 #.m^(-2), 2.345E9 #.m^(-2), and 6.881E9 #.m^(-2) respectively. Ongoing developments of millimeter-wave interferometry for concentration measurements aim at providing the 3D spatial distribution of ejecta cloud concentrations. To that end, a preliminary demonstration of 3D tomographic capabilities was performed using 3 reflectors, each defining a distinct measurement path. A tomographic reconstruction algorithm was implemented to convert path-integrated concentrations into standard concentrations, providing a maximum of 10.140E9 #.m^(-3) measured at a radial distance of 8.8 cm from the nozzle axis.