Pyrometer and Optical Emission Investigations on a Plasma Torch

A high voltage arc discharge was produced in a fast flow of nitrogen. A pyrometer was used to measure temperature profiles along the torch's axis. As the pyrometer could not directly read the temperature of the plasma flow, a probe was placed inside the torch and its black body radiation measured in a distance of 20 to 70 mm from the nozzle. The probe was a tungsten (W) rod of 5 mm diameter. The measured temperatures were in the range of 1000 < 2300 C. These values are far below the usual temperatures of such a torch (> 25000 C). Expectedly, the W-probe was strongly heated, quickly becoming liquid and gaseous, producing a mixture of W, N and O atoms and ions, in ground or excited state. The strong temperature gradient between torch and probe was investigated. Spectroscopic measurements were carried out with an Ocean Optics spectrophotometer (range 200 < 1100 nm). An optic fiber placed in a lens's focus collected the light. Background spectra were collected and subtracted. Stray light was shielded off. Directly at the nozzle distinct peaks could be observed, at 70 mm no significant spectra were detectable. Temperatures were calculated using data in the range of 22000 and 27000 K and 0 and 35 mm from the nozzle.