Investigation of dynamics of laser-produced carbon plasma during the laser irradiation using collective Thomson scattering

Plasma temperature, density, and flow velocity are the critical physical properties of laser produced plasma (LPP) to reveal the ablation dynamics, energy transport, and hydrodynamic evolution. In the time window during and just after laser irradiation, experimental data are very scarce so that many theoretical models remain untested. Here we report a clear evolution history of LPP expansion dynamics within 0-14 ns after the laser peak and in a region very close to the target (0.13-0.6 mm). A table-top Nd:YAG laser (intensity 6 × 10⁹ W cm⁻², pulse 7 ns) was used to generate the LPP from a planar graphite target, whose width was arranged to be smaller than the laser spot diameter to produce a one-dimensional planar expansion plasma near the target. The electron density ( n e ), temperature ( T e ), and drift velocity ( V d ) in the LPPs were measured using the ion feature of collective Thomson scattering, providing a space- and time-resolved 2D profile of the LPP. The experimental observations made it possible for the expansion dynamics to be compared directly with the LPP expansion models. The results suggest that during the laser pulse, the LPP is approximately isothermal and expands predominantly one-dimensionally in the target normal direction, in which the LPP drift velocity is found to increase linearly with distance. The linear extrapolation of the velocity indicates that the LPP has a considerable velocity at the initial target surface; this velocity is approximately the speed of sound derived from the observed T e . The experimental results were found to be in moderate agreement with the 1D self-similar isothermal expansion model. The ratio of the internal to kinetic energy in the observed area was ∼0.6, as predicted by the isothermal expansion model. The experimental findings were compared with the results of the 2D hybrid code STAR, and good agreement was obtained.