Jon W. Lindsay

Spectroscopic diagnostics in a highly constricted oxygen arc

Plasma properties in a highly constricted arc were measured using spectroscopic techniques. The arc configuration studied is typical of those produced by plasma cutting equipment. Temperatures and electron densities in the arc were measured using several different emission lines and several different methods for property calculation. Factors such as fluctuations in the arc intensity, arc asymmetry and the validity of the assumptions used for property calculation were shown to affect the accuracy of the measurements. Selection of the temperature measurement method can influence the measurement results. For this arc configuration, methods which use oxygen ion emission lines provide the most accurate results. The measurement methods developed in this study were used to investigate differences in arc properties related to arc current and nozzle diameter. Large changes in the temperature and electron density due to shocks were measured in the under-expanded region near the nozzle exit.

Methods to evaluate arc stability in plasma arc cutting torches

Plasma cutting is commonly used to cut metals in a variety of applications. This research is motivated by the lack of fundamental understanding of the dynamics of the plasma flow interacting with cold gas environment. It is focused on the characterization of the arc instabilities that affect the quality and consistency of a cut. The characterization of instabilities has been performed with high-speed Schlieren imaging technology to visualize the arc width and the light intensity fluctuations in the arc boundary layer. Measurements of arc symmetry using Fourier and wavelet transforms of the light intensity fluctuations in individual pixels in the Schlieren images are able to provide a quantitative measure for the degree of instability. Ambient gas entrainment into the plasma jet is measured using a CCD camera with a narrow bandpass filter centred on the nitrogen atomic line to detect nitrogen concentrations throughout the arc, thus showing the penetration of shield and ambient gas into the pure oxygen plasma. The effects of torch design features on these characteristics are investigated by comparing two different torch designs. The results show that the developed diagnostics are useful for evaluating new torch designs.

Torch Design Modification Using Micro-jets to Suppress Fluid Dynamic Instabilities in Plasma Arc Cutting

Highly constricted plasma arcs are widely used for metal cutting. One important characteristic of the cutting process is the consistency of the cut edge around the perimeter of the workpiece. Cut edge properties, including surface roughness, edge shape and dross formation, are presumed to depend on the local temperature and chemical composition of the cutting arc adjacent to the cut edge. Fluid dynamic instabilities in the arc boundary leading to entrainment of the low temperature ambient gas can have a strong effect on cutting performance. This paper describes the use of micro-jets to suppress fluid dynamic instabilities in the boundary layer of a plasma cutting arc. Previously developed optical diagnostics and analysis methods are used to characterize the arc boundary layer. Multiple nozzle designs have been investigated to quantify the effects of utilizing micro-jet flow around the arc column, and some relationships between nozzle design and cut quality are presented.