Measurement of the properties of plasma from ETC capillary plasma generators

Essential to the optimization of plasma generators for electrothermal-chemical (ETC) guns is an understanding of the physics of their operation throughout the entire plasma cycle. Identification and measurement of the physical quantities within the plasma parameter space are central to such an understanding. Presented within this paper are results from a series of tests in which a capillary plasma generator was discharged into air at atmospheric pressure. The process is explored from the start of the electrical discharge, including details of the conditions within the capillary to the exit of the plasma from the nozzle of the plasma generator and its expansion into free space, X-radiography, fast-frame photography and time resolved spectroscopy have been used together to build-up a basic understanding of the operation of the capillary plasma generators used. The state of the plasma initiating wire, the plasma temperature, emissivity and velocity on exit from the plasma generator and at various locations from the plasma generator nozzle are examined. Estimates for the total radiative power loss from the plasma are also made. Results are compared with the predictions from a one-dimensional model of capillary plasma generators.