Optical measurements of plasma velocity and temperature in a low-rate, low-power LPPS system

Low-pressure plasma spraying (LPPS) is a promising method of applying coatings to surfaces for thermal, chemical, and mechanical protection. The deposition of coatings with superior properties requires an improved understanding of this complex spray process, including the detailed understanding of the time–temperature and chemical history to which an injected particle is exposed during its travel through the high-temperature and high-velocity plasma jet. In this paper, we report on measurements of the temperatures and velocities within the plasma in a low-pressure argon–hydrogen arcjet spray plasma being developed for mesoscale manufacturing of mechanical components. The measurements reported here make use of laser absorption, laser-induced fluorescence, and optical emission spectroscopy of the Hα Balmer transition in atomic hydrogen. Both the absorption and fluorescence methods take advantage of the well-known spectral line broadening of atomic hydrogen, to extract the arcjet temperature and electron density. Jet axial velocities are measured via the time of flight, as detected by optical emission, of convected disturbances caused by intrinsic fluctuations in the arc voltage. Measurements made for various anode (nozzle) geometries indicate that relatively low (∼1 km s−1 mm−1) velocity gradients in the axial direction could be achieved with nozzles that include an expanding section of 10° half-angles. The measured temperatures and velocities suggest the presence of complex gas-dynamic shock structures in the arcjet plume, depending on the particular nozzle length and expansion section.