R. K. Singh

Role of ambient gas and laser fluence in governing the dynamics of the plasma plumes produced by laser blow off of LiF-C thin film

The time- and space-resolved emission profiles of LiI and LiII emission lines from the laser-blow-off plumes of a multilayered LiF–C thin film have been studied using spectroscopic technique. The evolution features were analyzed in different ambient environments ranging from high vacuum to 3mbars of argon pressures and at various fluences of the ablating laser. During the evolution of the plume, a transition region was found to exist between 4 and 6mm. Here, the plume dynamics changed from free expansion to collisional regime, where the plume experienced viscous force of the medium. The enhancement observed in neutral lines, in comparison with ionic lines, is explained in terms of the yield difference in electron impact excitation and ionization processes. Substantial difference in the arrival time distribution of the plume species was observed for LiI and LiII lines at high ambient pressures. Three expansion models are invoked to explain the evolution of the plume in different ambient conditions. The laser fluence was found to control the ratio of ions and neutrals.The time- and space-resolved emission profiles of LiI and LiII emission lines from the laser-blow-off plumes of a multilayered LiF–C thin film have been studied using spectroscopic technique. The evolution features were analyzed in different ambient environments ranging from high vacuum to 3mbars of argon pressures and at various fluences of the ablating laser. During the evolution of the plume, a transition region was found to exist between 4 and 6mm. Here, the plume dynamics changed from free expansion to collisional regime, where the plume experienced viscous force of the medium. The enhancement observed in neutral lines, in comparison with ionic lines, is explained in terms of the yield difference in electron impact excitation and ionization processes. Substantial difference in the arrival time distribution of the plume species was observed for LiI and LiII lines at high ambient pressures. Three expansion models are invoked to explain the evolution of the plume in different ambient conditions. The las...

Comparative study of laser produced Li plasma plumes from thin film and solid target

The evolution features of lithium ions and neutrals generated by two different schemes viz. the laser-blow-off (LBO) of multicomponent LiF–C thin film and conventional laser ablation (referred here as LPP) from solid lithium have been studied using optical emission spectroscopic technique. The optical signal emitted by Li I (at 670.8 nm) and Li II (548.4 nm) was monitored as a function of laser fluence, ambient gas pressure, and distance “z” from the target. Apart from their similarities, some interesting differences were noticed in temporal profiles of the plumes generated by LPP and LBO both in vacuum as well as in the presence of the ambient gas. A comparative analysis of experimental results indicates that the ablation mechanism and subsequent laser-plume interaction were responsible for the observed differences in LPP and LBO plumes. Expansion features of the plume are discussed in the light of two different models.

Effects of ambient pressure and laser fluence on the temporal evolution of 426.7 nm CII line in laser-blow-off of multilayered LiF-C thin film

The time and space resolved emission profiles of the CII line from the laser-blow-off (LBO) plumes of multilayered LiF-C thin film have been investigated using a laser-induced forward transfer technique. The evolution features of the 426.7 nm line were studied in different ambient environments ranging from high vacuum to 3 mbar of argon pressures and at various fluences of the ablating laser. It was found that many features of the plasma plume generated by the LBO method resemble that of plumes created in conventional laser produced plasma; however, a few differences were observed in their behaviour with regard to plume splitting and plume compression. A sudden change in the temporal profile was observed to occur at a distance about 4–6 mm away from the target. The variation of the plume structures observed at different fluences of the ablating laser is reported. The validity of the Rayleigh–Taylor instability, shock wave and drag force model in the present experiment are also discussed.

An experimental setup to study the expansion dynamics of laser blow-off plasma plume in variable transverse magnetic field.

In the present work we report generation of uniform and variable pulsed magnetic field in synchronization with pulsed plasma for the study of the expansion dynamics of laser blow-off (LBO) plasma plume. The experimental procedure for optimizing various parameters of the setup, e.g., the production of pulsed field and its synchronization with pulsed plasma and diagnostics system, is also reported. Until now the effect of magnetic field was studied using fixed field. The present setup, however, provides variable and uniform field in synchronization with the setup. A low cost time sequencing control module has been developed for the above purpose. Although the main emphasis is on the technical aspect of the setup, salient features of the effect of transverse magnetic field on the evolution features of the neutral and ionic species are also reported briefly. It is observed that LBO generated plume have a stronger correlation with the magnetic field in comparison to the conventional laser produced plasma experiments as reported earlier.

Parametric study of expanding plasma plume formed by laser-blow-off of thin film using triple Langmuir probe

The triple probe technique has been used to study the parameters of flowing plasma plume formed by laser-blow-off (LBO) of a multicomponent LiF–C target. The advantages of triple Langmuir probe over the single probe have been discussed with regard to the present study. Our results show that triple probe is better suited for parametric measurements close to the target. The spatiotemporal evolutions of electron density and temperature were measured in different ambient environments and for various laser fluences. The angular distribution of electron density and temperature was also measured. It was found that the presence of ambient gas drastically affects the electron density and temperature of the LBO plume. Large enhancement in ne and Te at intermediate pressures (10−2u2002Torr) is expected due to increase in collisional processes. On the other hand, laser fluence does not affect the observed ne and Te significantly. In vacuum, a new phenomenon, i.e., an oscillation in the trailing portion of the plasma plum...

Generation of fast neutrals in a laser-blow-off of LiF–C film: A formation mechanism

The temporal profiles of the spectral lines LiI and LiII emitted by a laser blow off of LiF–C film were analyzed. Fast neutrals having energies of ∼310eV were observed. It was found that these fast neutrals have kinetic energies similar to that of the ablated ions. Photon production rates at 670.8nm were estimated for the recombination and charge exchange processes. We attempted to qualitatively understand the role of electron-ion recombination (two and three body recombinations) and charge exchange processes in producing observed fast neutrals. Our analysis indicates that charge exchange is the dominant process in the production of fast neutrals.

Measurements of electron temperature and density of multi-component plasma plume formed by laser-blow-off of LiF-C film

The evolution features of electrons and ions generated by laser-blow-off (LBO) of a multi-component LiF-C target have been studied using optical spectroscopic and Langmuir probe (LP) techniques. In addition to the hydrodynamic plume splitting, multi-peak structures are reported in the temporal profiles of ions measured by the LP. We propose that the observed multi-peaks arise due to the mass dependent expansion velocity of different elements present in the plasma plume. The temporal evolutions of electron density and temperature are reported in vacuum and in a 10?2?mbar argon environment. The electron temperature of the LBO plume was found to be higher than the electron temperature of laser produced plasma from solids. It was observed that the LP was perturbing the evolving plasma plume when it was placed at a short distance from the target. We have experimentally determined the distance between the target and LP beyond which it did not perturb the characteristic features of plume expansion. No previous data related to LBO plasma exist for a direct comparison with the present results.

Fast imaging of laser-blow-off plume: Lateral confinement in ambient environment

The dynamics of plasma plume, formed by the laser-blow-off of multicomponent LiF-C thin film under various ambient pressures ranging from high vacuum to argon pressure of 3 Torr, has been studied using fast imaging technique. In vacuum, the plume has ellipsoidal shape. With the increase in the ambient pressure, sharp plume boundary is developed showing a focusing-like (confinement in the lateral space) behavior in the front end, which persists for long times. At higher ambient pressure (>10−1u2002Torr), structures are developed in the plasma plume due to hydrodynamic instability/turbulences.

Influence of laser beam intensity profile on propagation dynamics of laser-blow-off plasma plume

Effect of intensity profile of the ablating laser on the dynamics of laser-blow-off (LBO) plume has been studied by fast imaging technique. This work emphasizes the geometrical aspect of the LBO plume, which is an important parameter for various applications. Visualization of the expanding plume reveals that geometrical shape and directionality (divergence) of the plume are highly dependent on the laser intensity profile. Present results demonstrate that the Gaussian profile laser produces a well-collimated, low divergence plasma plume as compared to the plume formed by a top-hat profile laser. The sequence of film removal processes is invoked to explain the role of energy density profile of the ablating laser in LBO mechanism.

Effect of magnetic field on the expansion dynamics of laser-blow-off generated plasma plume: Role of atomic processes

The effect of a variable magnetic field on Li plasma produced by laser-blow-off technique has been studied experimentally. Enhancement in the intensity of the spectral lines from neutrals was observed, which varied with the magnetic field. The enhancement in emission from Li I was found to differ for the two different transitions viz . 670.8xa0nm (2s 2 S 1/2 xa0←xa02p 2 P 3/2,1/2 ) and 610.3xa0nm (2p 2 P 1/2,3/2 xa0←xa03d 2 P 3/2,5/2 ), which is more prominent for 670.8xa0nm. Conventionally, the enhancement in emission in the presence of the magnetic field has been explained in terms of radiative recombination. However, the atomic analysis by computing photon emissivity coefficients in the present case has revealed for the first time that it is due to electron impact excitation.