S. Mahmood

On the plume splitting of pulsed laser ablated Fe and Al plasmas

A time resolved imaging study of pulsed laser ablated Fe and Al plasma plumes with specific interest in the splitting of plumes into the slow and fast moving components as they expand through the background argon gas at different pressures is reported. The material ablation was achieved using a Q-switched Nd:YAG (yttrium aluminum garnet) laser operating at 532 nm with a pulse duration of ∼8 ns full width at half maximum and a fluence of 30 Jcm−2 at the target surface. Typical time resolved images with low magnification show that the splitting occurs at moderate background gas pressures (0.5 and 1.0 mbar for Fe, and 0.2 mbar for Al plasma plumes). The plume splitting did not occur for higher background gas pressures.

Investigation of plume expansion dynamics and estimation of ablation parameters of laser ablated Fe plasma

A systematic investigation of expansion dynamics of plasma plume, generated by a Q-switched Nd–YAG laser at 532 nm with a fluence of ~30 J cm−2 on Fe target, was done for three different ambient pressures of 2 × 10−4, 2 and 20 mbar of Ar using fast gated intensified charge coupled device imaging. The experimental observations are then fitted with the snow plow and shock wave models to estimate various laser ablation parameters such as laser energy deposited to the ablated material, mass ablated per laser shot, laser spot size and the background gas density. The estimated laser energy deposited to the ablated material is about 70% of the input energy. The density of ambient gas atoms, having interaction with the ablated species, is found to vary from 12% to 50% at 2 mbar and 24% to 80% at 20 mbar. Scanning electron microscope images confirm the effect of ambient gas pressure on the topography of the deposited thin film and size of the nanoparticles which is qualitatively explained on the basis of the changing kinetic energy of impinging plasma plume species due to change in ambient gas pressure.

Novel fast-neutron activation counter for high repetition rate measurements

A fast-neutron beryllium activation counter has been constructed for neutron measurements on a high repetition rate deuterium plasma focus. Beryllium activation is especially suitable for measurements of DD neutron yields. The cross section for the relevant reaction, Be9(n,α)He6, results in a maximum sensitivity at the characteristic energy of the DD neutrons (∼2.5MeV) and practically no sensitivity to neutrons with energies <1MeV. The short half-life (0.8s) for the decay of He6 enabled the shot-to-shot neutron yield from the plasma focus to be measured for repetition rates from 0.2to3Hz (and for a range of deuterium gas pressures). With careful analysis, the shot-to-shot yield can be measured up to a maximum repetition rate of 3Hz, beyond which the pileup of counts from the previous shots reduces the accuracy of the measurements to an unacceptable level. This new beryllium activation counter has been cross-checked against an indium activation counter to obtain absolute neutron yields. At a charging volta...

Effects of laser energy fluence on the onset and growth of the Rayleigh-Taylor instabilities and its influence on the topography of the Fe thin film grown in pulsed laser deposition facility

The effect of laser energy fluence on the onset and growth of Rayleigh–Taylor (RT) instabilities in laser induced Fe plasma is investigated using time-resolved fast gated imaging. The snow plow and shock wave models are fitted to the experimental results and used to estimate the ablation parameters and the density of gas atoms that interact with the ablated species. It is observed that RT instability develops during the interface deceleration stage and grows for a considerable time for higher laser energy fluence. The effects of RT instabilities formation on the surface topography of the Fe thin films grown in pulsed laser deposition system are investigated (i) using different laser energy fluences for the same wavelength of laser radiation and (ii) using different laser wavelengths keeping the energy fluence fixed. It is concluded that the deposition achieved under turbulent condition leads to less smooth deposition surfaces with bigger sized particle agglomerates or network.

Investigation of laser produced Fe plasma plume dynamics using time resolved imaging and snow plow model

A theoretical and experimental study on the dynamics of pulsed laser generated Fe plasma is presented. The time resolved imaging of the plasma plume was done at two different Ar gas pressures of 2 and 20 mbar. Plasma ablation from the target was done using a pulsed Nd:YAG laser at 532 nm with a fluence of ~25 J/cm2. The snow plow model is fitted to the experimental data of the plume front position, obtained by gated time resolved images, to estimate the ablation parameters. The simulation results are in good agreement with the experimental values.