Optical spectroscopy of laser plasma in a deep crater

The time dynamics of plasma-emission spectra is studied experimentally at different stages of the drilling of a steel plate by 100-fs and 5-ps laser pulses: from a shallow crater to a hole. The change in the time dependence of the plasma temperature caused by variations in the irradiated surface geometry is analysed. It is found that the time interval needed to reach a particular temperature (about 8000 K) drastically increases from 40 — 50 to 150 — 200 ns when a specific crater depth is achieved. The opposite tendency is observed as the crater depth grows further and a hole is produced. Strong self-absorption in a plasma plume inside a deep crater is experimentally confirmed which results in the appearance of line absorption against a continuous emission spectrum.