Probing the impact of rough vacuum on the early-stage evolution of ns laser-produced tungsten plasma

Abstract

A detailed time-resolved analysis of laser-produced plasmas (LPPs) is essential for understanding the effects of ambient gas on plasma characteristics. In this work, a nanosecond pulsed laser (1064 nm, 7 ns) generates LPPs from a tungsten (W) target, which is comprehensively analyzed using optical emission spectroscopy, laser plasma interferometry, shadowgraphy, and imaging. Measurements reveal that varying the ambient pressure in the 1.33 103 Pa - 5.33 104 Pa range significantly affects the plasma temperature, number density and plume morphology. Spectral analysis reveals a strong correlation between number densities obtained from Stark broadening measurements of the H-alpha and W I lines. The interferometric method enables the study of plasma plume morphology and retrieval of number densities even in the presence of strong continuum emission. Density maps obtained from interferometry show inhomogeneities. Shadowgraphy images show the expansion of shockwaves at different pressures, and they also reveal that the transition of plasma from non-collective to collective behavior occurs around 1 104 Pa. Despite inhomogeneities, plasma remains in Local Thermodynamic Equilibrium during the observed time window of up to 1 μs. Due to well-established Stark parameters and strong linear correlation with the W I line, the H-alpha line provides a more straightforward estimation of number densities in tungsten LPPs.