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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 (< 50ns) and then remains stable. The plasma excites a broad volume around the electrode gap. Typical spectra show a strong H α and continuum radiation, with many lines emitted by impurities coming from electrodes materials. The plasma contamination from the electrodes is mostly concentrated in their vicinity. The electron density reaches 2·10 18 cm -3 at the beginning of the discharge. This extreme density causes merging of lines, strong Stark broadening and shift of the H α line. The density decreases afterwards rapidly with time. The electron temperature remains roughly constant around 0.7eV. The low temperature, the high density measured and other spectroscopic evidences prove that the EDM plasma is non-ideal (Γ ≅ 0.45).

Plasma created during electrical discharge machining is systematically investigated using optical emission spectroscopy. Typical spectra show a strong Hα and continuum radiation, with many lines emitted by impurities coming from electrode and workpiece materials. The dielectric molecules are cracked by the discharge. Changing polarity affects the electrode wear and workpiece erosion rates, which can be qualitatively seen on the spectra. Time-resolved spectroscopy shows that the plasma density reaches 2 × 1018 cm−3 at the beginning of the discharge. This extreme density causes the merging of lines, strong Stark broadening and shift of the Hα line. Afterwards, the density decreases rapidly with time. The electron temperature remains roughly constant around 0.7 eV. The low temperature and the high density measured prove that the EDM plasma is non-ideal (Γ ≃ 0.45). Absence of the Hβ line, asymmetric shape of the Hα line and complex structures around Hα are other spectroscopic evidences of the plasma non-ideality.

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