Influence of ambient pressure and laser energy on the plasma dynamics and parameters of laser-irradiated polyvinyl chloride

Abstract

The dependence of propulsion performance generated by laser ablation of polyvinyl chloride on laser energy and pressure is investigated using Q-switched Nd: YAG laser with the wavelength of 1064 nm. When the pressure is decreased, the impulse and coupling coefficient rise first and then decline. Such a trend is also reflected in the variation of coupling coefficient with laser energy in the whole pressure range. However, the change in impulse with laser energy at atmospheric pressure is not completely consistent with that at low pressure levels. The dynamic behavior and duration of plasma plume are considered to be the factors for the difference in propulsion performance. By capturing the fast exposure images of plume, the separation at atmospheric pressure and severe expansion accompanied by rapid quenching in near vacuum are observed. Moreover, the plasma plume lasts longer time at high pressures. It is ascribed to the higher electron temperature, which promotes background gas to excite and ionize. Since the electron density increases with the improvement of laser energy and pressure, the absorption of electrons to laser energy becomes stronger through the inverse bremsstrahlung mechanism. Accordingly, the shielding effect of plasma is enhanced, causing the weak laser-target interaction. The result is that the crater depth and ablative mass increase with decreasing pressure. This work is important for understanding the energy conversion mechanism and optimizing the laser propulsion performance.