Schlieren imaging investigation of the hydrodynamics of atmospheric helium plasma jets

This work investigates the hydrodynamic characteristics of a coaxial double-ring electrode helium plasma jet by means of a “Z-type” Schlieren imaging system. The Schlieren images and visual optical photographs made show that a transition point from a laminar region to a turbulent region exists for gas flow without plasma when the helium flow rate exceeds a certain value. After plasma ignition, the laminar region shrinks with voltage increases, and the maximum length of the plasma plume is confined to the laminar region. The heat transfer equation and the spectral broadening of the He I 667.8 nm were used to estimate the increased gas temperature in the plasma jet, and the change in gas velocity by ionic momentum transfer was found by application of a double sphere collision model. As a result, gas heating is considered to be the dominant factor for the earlier onset of turbulence after plasma ignition, whereas the role of ion momentum transfer to neutral gas molecules is comparatively weak. The hydrodynamic behaviors of the plasma jet at the impact region for organic glass and silicon substrates are also researched. The ionization front propagates along the organic glass surface and contracts at the impact point on the silicon surface. More visible vortices are observed from Schlieren images with silicon substrates than with organic glass substrates. Possible mechanisms related to the different treatment effects are discussed.