Insights Into Sustaining a Plasma Jet: Boundary Layer Requirement

In this paper, we investigate the effects of various background gases on the formation and sustenance of a plasma jet. A helium plasma is ignited inside a dielectric tube and introduced into a vacuum chamber where the gas composition and pressure can be controlled. The gas composition inside the chamber is altered by separately injecting various background gases, such as air, nitrogen, and argon. Here, we report that, in order to sustain a plasma jet, there needs to be a physical boundary layer around the helium flow. This boundary can be in the form of a solid surface that encapsulates the flow, or a background gas other than helium. If only helium is present inside the chamber, no propagating jet is created; rather a diffuse plasma fills the chamber. If another gas is introduced, then the diffuse plasma reverts into a jet. Additionally, we discovered what can be considered as the signature emission line from a plasma jet operating with helium. This emission line is at 587.4 nm (He). This line appears in the emission profile only in the presence of a plasma jet, and disappears as the plasma transitions to a diffuse state at the same applied power parameters and pressure.