Instabilities in plasma spray jets result in coatings with inconsistent properties.  The arc root fluctuation and shear layer instability due to strong gradients are of foremost concern. The shear layer instabilities result from shear between the high velocity, low density hot core gas, the intermediate density and velocity boundary layer, and the high density quiescent environment. Through the use of photodiodes and high speed Schlieren video images, the propagation of disturbances resulting from arc root fluctuations have been determined to have a dominant effect on the jet.  A facility with density gradients similar to a plasma torch has been used for implementation of traditional fluid dynamics measurements such as hot-wire anemometry.  The cold flow facility uses a helium core jet and an argon boundary layer gas flow issuing into a sulfur-hexafluoride environment.  The influence of the arc root motion is simulated using mesh screens, which can be applied to the boundary layer gas and/or the core jet.  In this manner the boundary layer can be forced more strongly than the core jet to simulate arc root fluctuation within the boundary layer.  Methods to control these instabilities are developed and tested using both the plasma torch and the cold flow facility.  Through nozzle design modifications the instabilities resulting from arc root fluctuations and high density gradients have been reduced.  The effectiveness of the control on the plasma jet is determined using in-flight particle characterization and coating property evaluation along with high speed Schlieren video and photodiode measurements of the jet.