Imaging and emission spectroscopy of the submicrosecond plasma generated from copper substrate with nanosecond laser pulses.

In this paper, an experimental investigation of the ablation plasma generated from the copper substrate by means of nanosecond laser pulses is presented. We studied the evolution of the ablation plasma in the first 1000 ns after plasma onset using the fast-gated imaging and optical emission spectroscopy methods. Plasma imaging showed that the expansion of the plume front can be described using a so-called drag model, with the expansion limit increasing with laser fluence from 254 µm for 30/ to 375 µm for 67/. By using the Boltzmann plot and Stark broadening methods, it was found that within the first microsecond after onset, the electron excitation temperature and electron number density decrease from 1.2 eV to 0.8 eV and from 4×10 to 5×10, respectively. Using the McWhirter criterion, we confirmed that in the considered time range the plasma remains in a state of local thermodynamic equilibrium.