Effect of ambient gas pressure on nanosecond laser produced plasma on nickel thin film in a forward ablation geometry

Abstract

One of the striking observations in this research is the enhancement of neutral velocity with the increase in background pressure. The earlier observations were mostly looking the effect of background gas as a medium to drag the motion of plasma species and the shock wave and drag models were very effectively used to describe the dynamics of plasma plume constituents. However, we observed that the neutrals closer to the sample(up to 5 mm), gain significant acceleration. In this chapter, a detailed study is performed with varying pressure and slightly higher laser energy so as to record the TOF spectra of ionic species and other neutral transitions from plasma. The results show that TOF spectra recorded for different neutral transitions show different dynamics and the slower peak of the neutral emission becomes faster with background pressure. A similar observation is seen for the ionic species as well. The TLP records significant electron current towards the very early stage of evolution suggesting the plasma plume losses fast electrons and due to this the charge neutrality within plasma is disturbed. This makes a double-layer like situation as reported elsewhere. The acceleration of ionic peak even at 20 mbar background pressure is very large and the calculations show an extremely high accelerating field in the plasma generated in rear ablation geometry in a lower fluence range than the reported DL observations. One of the lines (712.2 nm) of neutral nickel shows an extremely large asymmetry in spectral broadening with the increase in background pressure. This large asymmetry may be due to the presence of micro electric fields present inside the laser produced plasma plume. Interestingly, this asymmetry in spectral broadening exhibits temporal and spatial dependence which suggests that electric field is present in the plasma plume for much longer duration and also over a significant distance. Hence, it can be anticipated that the acceleration of ions continuously takes place for significantly longer distances. Because of this continuous acceleration, the estimation of electric field considering double layer separation a few Debye lengths probably gets overestimated.

On the delayed emission from a laser-produced aluminum plasma under an argon environment

“…This indicates that the connement of neutrals with increase in ambient pressure is not as signicant as reported for initial times. 41 Fig. 3 shows the snapshot of Al I emission at 200 ms recorded from ICCD images for two background gases viz nitrogen and argon for the same pressure of 20 mbar.…”

“…This indicates that the confinement of neutrals with increase in ambient pressure is not as significant as reported for initial times. 41…”

“…Recently, various groups have studied the dynamics of laserproduced plasmas in background gas [29][30][31][32]41,42 using multiple diagnostics like optical emission spectroscopy (OES), 43 imaging, 44 Langmuir probes, 45,46 etc. to study the connement and plume parameters.…”

On the delayed emission from laser produced aluminum plasma under argon environment

“…Recently, various groups have studied the dynamics of laser-produced plasma in background gas [29][30][31][32]41,42 using multiple diagnostics like Optical Emission Spectroscopy (OES) 43 , imaging 44 , Langmuir Probes 45,46 , etc. to study the confinement and plume parameters.…”

“…In vacuum where it expands adiabatically. On the other hand the expansion dynamics of plasma plume against a background meduim depends on its atomic mass, pressure, etc.Recently, various groups have studied the dynamics of laser-produced plasma in background gas [29][30][31][32]41,42 using multiple diagnostics like Optical Emission Spectroscopy (OES) 43 , imaging 44 , Langmuir Probes 45,46 , etc. to study the confinement and plume parameters.…”