Development of a vapor-based method for seeding alkali metals in shock tube facilities

This note presents a vapor-based seeding apparatus, named the external alkali seeding instrument (EASI), which is designed to introduce alkali metal vapors into experimental facilities without using precursors or large auxiliary equipment. The device vaporizes small amounts of alkali metals, potassium in this work, which are then carried away by an inert gas. In a benchtop flow cell, carrier gas flow rate (6–\(200~\hbox {cm}^3/\hbox {s}\)) and device temperature (150–\(250\,^{\circ }\hbox {C}\)) most strongly affected potassium-vapor concentrations. Higher values of either quantity lead to increased potassium-vapor concentrations. When using the EASI to seed a shock tube experiment, vapor-phase potassium was detected immediately after the incident and reflected shockwaves using a laser absorption diagnostic. Mole fraction time histories stay within a factor of 2 over the test time as compared with those from a precursor-based seeding approach, which may span multiple orders of magnitude. This suggests potassium is nearly homogeneously distributed throughout the test gas. This design can be extended to other low-vapor-pressure elements, such as other alkalis or sulfur, with minimal modifications. The EASI simplifies seeding for laboratory experiments targeting potassium and other alkali metals—enabling advances in fundamental spectroscopy, diagnostic development, and chemical kinetics.