Measurement of plasma and shock-wave dynamics and properties inside holes during laser drilling of metals

Laser material processing can be influenced by plasma formation and dynamics such as plasma shielding or plasma supported ablation. A novel method for investigating in-situ plasma and shock-wave dynamics inside a confined space like a laser drilled hole in metals is presented. A specific sample preparation with prepared mock-up holes including a transparent material as a window is used. By use of high speed imaging and spectroscopy a time and spatially resolved monitoring of the optical emissions of plasma and shock-waves is realized during drilling stainless steel and aluminum with ns-pulsed laser radiation. Spatial intensity distributions and velocities of the shock-waves and the plasma are measured inside the holes with a temporal resolution of 1 ns and a spatial resolution of less than 1 µm. The results are compared with models for shock-wave and plasma propagation in non-confined environments. Emission spectroscopy is carried out during the expansion of the plasma inside the drilled hole allowing the determination of electron density by Stark broadening.