Spectroscopic measurements on an oxygen cutting plasma

Summary form only given. In plasma cutting systems, a highly constricted arc plasma is used to locally melt the material to be cut. The workpiece serves as the anode, and a narrow nozzle leads to the arc constriction between the cathode and the anode. The quality of the cut is dependent on the characteristics of the constricted arc near the anode surface, i.e. the temperature and velocity distributions downstream of the nozzle. To determine temperature distributions in the arc, spectroscopic measurements were made on a commercial oxygen plasma cutting system operating with a water-cooled rotating anode. The cutting system was operated with a current of 200 A with oxygen as plasma gas and air as shroud gas. Line intensity measurements were made of both atomic and ionic species of oxygen and nitrogen. Temperatures were derived using several methods, including absolute line intensity, Boltzmann plot and norm temperature (off-axis-peaking) method. Good agreement was observed between the derived temperature values for the different methods using the ion line emission, the centerline values being around 25000 K. Atom line emission data were distorted because of fluctuations in the radial intensity profile. These fluctuations had a frequency in the kHz range, and resulted in a change of the radial location of the maximum emission of the atomic line. Radial temperature profiles were obtained for several axial locations between the nozzle and the anode. Nitrogen emission was used to study the mixing between the oxygen plasma gas and the surrounding air shroud, and it was found that nitrogen reaches the arc center several nozzle diameters downstream of the nozzle exit. The changes in the arc temperature and nitrogen emission distribution, as well as the intensity fluctuations and arc symmetry, were investigated for different torch design parameters. This study shows that care must be taken to choose the appropriate spectroscopic method for temperature determination in a high- y constricted arc with superimposed flow, but that for this arc any method based on ion line emission can give consistent values which may serve as design information for torch optimization.