This paper describes the spectroscopic measurement of arc plasma diameter in EDM. In order to determine the diameter of the arc plasma area where discharge current flows, radial temperature distribution in plasma generated by a single pulse discharge was measured using spectroscopy, and the area with high degree of ionization was determined. It was found that arc plasma completes expanding within a few microseconds after dielectric breakdown, while the diameter of craters grows slowly in comparison with the expanding speed of the arc plasma. This new finding is different from conventional plasma expansion models established based on the observation of generated craters and unproven assumptions. Furthermore, increase in arc plasma diameter with increasing discharge current or gap width was quantified, enabling accurate simulation of discharge crater generation using the newly established heat source model. (c) 2008 CIRP.
