Concentration measurements in dense ejecta clouds using millimeter-wave radar tomography.*

The impingement of a high-speed jet on a granular surface leads to surface erosion and the entrainment of particles into a dense ejecta cloud. Plume surface interactions (PSI) have far-reaching implications for the safety of future manned missions to the Moon and Mars. Experimental measurements of ejecta properties are necessary to improve PSI predictive models, but are hindered by excessive attenuation at visible wavelengths.

In this work, we use a novel radar tomography technique to temporally and spatially measure the concentration of ejecta produced by an under-expanded Mach 5 jet impinging on a bed of glass microspheres. The technique is based on the simultaneous measurement of path-integrated particle concentrations along multiple lines, defined by a millimeter-wave radar system and a series of reflectors. Path integrated concentrations are proportional to the phase shift caused by the particles, which is the quantity measured by the radar. Because millimeter waves are far less attenuated by micron-sized particles than visible light, this interferometric technique allows measurements at volume loadings not at reach with visible laser-based diagnostics. The set of path-integrated concentration is deconvolved into local concentrations using an Onion-peeling method.