Plasma diagnostics using optical emission spectroscopy in laser drilling process

An aero space material, Inconel 718, was laser drilled with a diode-pumped solid-state Nd:YAG laser. During the laser drilling process, the emission spectra of the Inconel 718 plasma have been recorded and analyzed using an optical emission spectroscopic method. Temporal histories of the plasma parameters including emission line intensity, electron temperature, and number density were examined in relation to drilling depths. The line intensity had an inverse proportional relationship with the drilling depth. The line intensity varied greatly at an earlier stage because a focused beam initially removed a large amount of material (a rate of the drilling depth increase was very high at the early stage). At a later stage, however, it varied as a function of drilling depth only to a minor degree because of the beam defocusing and further beam energy loss by the plasma. The electron temperature measured was in the range of 7700–9200 K, and it tended to decrease with drilling depth due to the downward movement of plasma core. The electron number density measured varied from ∼5 × 10¹⁵ to ∼8 × 10¹⁵cm⁻³. The electron number density also tended to decrease with increasing drilling depth mostly due to the reduction of material removal.