Time- and spatially-resolved characterization of electrical discharge machining plasma

Plasma created during electrical discharge machining (EDM) is investigated with fast imaging and with time- and spatially-resolved optical emission spectroscopy. After the breakdown, the plasma develops very fast (< 50 ns) and then remains stable. The plasma excites a broad volume around the electrode gap. Light is still emitted after the discharge by incandescent liquid metallic particles coming from the erosion of the workpiece. Typical plasma spectra show a strong H-alpha and continuum radiation, with many lines emitted by impurities coming from electrodes materials. The plasma is thus composed of species coming from the cracking of the dielectric molecules, mainly hydrogen, with contamination from the electrodes. This contamination is mostly concentrated in the vicinity of the electrodes. The electron density reaches 2 x 10(18) cm(-3) at the beginning of the discharge. This extreme density causes merging of lines, strong Stark broadening and shift of the H-alpha line. The density decreases afterwards rapidly with time, remaining nevertheless above 10(16) cm(-3) after 50 mu s. The electron temperature remains roughly constant around 0.7 eV in the whole plasma. The low temperature, the high density measured and other spectroscopic evidence prove that the EDM plasma is weakly non-ideal (Gamma similar or equal to 0.33).