Propagation velocities of laser-produced plasmas from copper wire targets and water droplets

Abstract

Experiments were performed to determine the plasma propagation velocities resulting from KrF laser irradiation of copper wire targets (75 μm diameter) and water droplets (75 μm diameter) at irradiance levels ranging from 25 to 150 GW/cm2. Plasma propagation velocities were measured using a streak camera system oriented orthogonally to the high-energy laser propagation axis. Plasma velocities were studied as a function of position in the focused beam. Results show that both the shape of the plasma formation and material removal from the copper wire are different and depend on whether the targets are focused or slightly defocused (≊0.5 mm movement in the beam axis). Plasma formation and its position relative to the target is an important factor in determining the practical focal point during high-energy laser interaction with materials. At irradiance of 100 GW/cm2, the air plasma has two weak-velocity components which propagate toward and away from the incident laser while a strong-velocity component propagates away from the laser beam as a detonation wave. Comparison of the measured breakdown velocities (in the range of 2.22–2.27 ×105 m/s) for air and the value calculated by the nonlinear breakdown wave theory at irradiance of 100 GW/cm2 showed a quantitative agreement within approximately 50 % while the linear theory and Gaussian pulse theory failed. The detonation wave velocities of plasma generated from water droplets and copper wire targets for different focused cases were measured and analyzed theoretically. The propagation velocities of laser-induced plasma for liquid droplets obtained by previous research are compared with current work.