Expansion characteristics of atom and ion component in laser-induced aluminum plasma

Abstract

A series of experiments is designed in order to investigate the expansion and movement characteristics of atoms and ions of the plasma in the presence of ambient gas. To obtain two-dimensional spectral images of different components in the plasma, a nanosecond laser with a wavelength of 532 nm is used to ablate an aluminum sample, forming the plasma. A C-T type of tri-grating monochromator with an emICCD detector is used for diagnosing the plasma chronologically. At the same time, a 2400 gmm-1 grating is used to replace the narrowband filter for imaging diagnosis of different components in vacuum. The spectrally resolved images of Al I (396.1 nm), Al Ⅱ (466.3 nm), and Al Ⅲ (447.9 nm) in aluminum plasma are obtained. Besides, the spectral images of plasma components under different ambient pressures are collected to explore the influence of background gas on plasma evolution. The results show that in the plasma formation process, the ion component is distributed in the anterior segment of the plume relative to the atom component, and its angular distribution is smaller. The vacuum expansion rates of atoms and ions are all on the order of 104 ms-1. The movement speed of the ion component in the plasma is higher than that of atom component, and its movement speed increases with the valence of the ion increasing. In the energy density range used in this experiment, the velocity varies slightly with the laser energy. For the neutral atom, the velocity increases obviously as the energy increases. With the expansion process progressing, each component of the plume advances along the direction normal to the sample surface, and the emission intensity gradually decreases, the corresponding plume density and its temperature also decrease. With the ambient pressure increasing, the movement characteristics of each component are obviously different from those under high vacuum. At a pressure higher than 1 Pa, the plasma and the ambient gas are infiltrated with each other, vignetting appears in the front of the plume, disturbance occurs, causing the expansion speed to decrease. In addition, the plasma plume shrinks due to the increase of pressure, and the probability of collision with the background gas increases, so that the plume emission intensity is strengthened and the plasma lifetime is prolonged. The results of the new diagnosis method and the experimental results demonstrated in this study can provide a reference for the study of plasma component dynamic process.