The mechanism of effect of lens-to-sample distance on laser-induced plasma

The analysis of nanosecond laser-induced plasma versus the lens-to sample distance(LTSD) is investigated by 532nm laser ablated Ti-Al alloys in air and vacuum condition. The results indicate that electron temperature and density increase rapidly with the LTSD changing from short distance to the focal length (f=111 mm). Moreover, the plasma parameters by laser ablation in vacuum condition will further increase with LTSD further increasing. As for ambient medium, the plasma parameters reach their maximum values at 107 mm and then substantially decrease. On the other hand, the electron temperature and density rise again, while the intensity of Ti(II)368.52 nm decreases rapidly when the LTSD extends 112 mm.