Looking inside the Early-time Radiation Plume for Hypervelocity Impacts

Introduction: Previous studies describe the evolution of impact generated vapor plumes from above [1]. Here, a new approach is tested whereby early-time self-luminous ejecta products are examined using quarter-space experiments with spectral measurements. At early times, the forming cavity is on the order of centimeters. Hence, the pointing accuracy and small detection regions are necessary. This method allows a view exclusively inside the forming crater. To prove the usefulness of this method we impacted into several materials. The radiating gas plume due to impact has four components: the jetting phase, a downrange-moving vapor cloud, a slower expanding vapor cloud and a vapor plume that grows from containment in the cavity [2]. In addition, significant thermal debris evolves inside the growing transient crater. This study probes the fourth component as well as the thermal component and compares it with the downrange jetting phase and slower moving expanding phases. Experimental Method: Quarter-space experiments reveal a cross-sectional view of the forming crater. Natual cratering and most studies occur in half-spaces, i.e., a flat surface with the impact occurring near the center. The quarter space is formed by impacting near the edge of a clear acrylic window parallel to the projectile's trajectory so the window protrudes both above and below the surface. This method does not significantly impede or affect the crater formation. The final crater appears as half of a regular crater. Target materials included: silicates (powdered pumice and quartz sand) in order to assess the thermal characteristics during formation; a fine and a coarse dolomite powder to look at the vaporization during formation ; and quartz sand with a projectile diameter layer of dolomite (.0635 cm) on top to explore the depth of the interactions. Pyrex spheres (.0635 cm diameter) were used to minimize the projectile contribution to the spectra. The foreoptics collect light from small 2.5 cm diameter regions on the crossectional plane. Three detectors simultaneously collect light. One unit is centered above the surface 10 cm downrange to look at the jetting phase. The second is centered just above the point of impact. The third is centered just below the point of impact to look inside the transient crater.