A truly noninvasive measurement technique for plasma velocity has not been demonstrated. Plasma velocities have been inferred using laser Doppler anemometry or photographic analysis of the position of smoke or small particles. This paper describes an alternative method based on the refractive index change created in a plasma by a gaseous probe material injected into the plasma. This disturbance of the refractive index can be monitored using interferometry. A multipass real-time holographic interferometry system was used to follow the changes of the interferometric pattern, and the data were recorded using high speed cinematography. A transparent model of an industrial plasma torch was employed in these studies, and a number of different types of trace gas materials were used to track the plasma flow. Using a combination of multipass interferometry and a laser line absorbing gas, sufficient interferometric sensitivity was obtained to determine plasma velocities in the 100 m/s range. Based on these results, a working plasma torch was constructed. Further studies are planned using this torch and actual plasmas.
V. A. Deason,
L. D. Reynolds,
M. E. McIlwain,
"Velocities Of Gases And Plasmas From Real-Time Holographic Interferograms," Optical Engineering 24(5), 245735 (1 August 1985). https://doi.org/10.1117/12.7973568