Time-resolved imaging and spatially-resolved spectroscopy of electrical discharge machining plasma

The plasma created during electrical discharge machining is investigated with imaging and spatially-resolved optical emission spectroscopy. Analysis of the pre-breakdown duration shows that the breakdown is of stochastic nature. Due to the presence of gas bubbles created by electrolysis, the breakdown mechanism in water could be different from the one in oil. After the breakdown, the plasma develops very fast (< 50 ns) and then remains stable. The plasma excites a broad volume around the electrode gap, between 50 and 400 mu m in diameter depending on the discharge current. The H,, line is emitted from the whole plasma, since the hydrogen atoms originate from the dielectric. On the other hand, spatially-resolved spectroscopy shows that plasma contamination from the electrodes is concentrated in their vicinity. After the discharge, light is still emitted by incandescent metallic particles coming from erosion of the workpiece. Profiles between the electrodes of electron density and of electron temperature have been estimated from spatially-resolved emission spectra. The density is found to slightly increase towards the plasma centre, whereas the temperature is quite constant across the plasma (-0.7 eV).