Search

In this work, the assumption of local thermodynamic equilibrium (LTE) for a laser-induced plasma in ambient air is examined experimentally using two different laser systems, namely an infrared short-pulse Ti:Sapphire laser and an ultraviolet long-pulse XeCl excimer laser. The LTE assumption is investigated by examining the plasma produced at a laser fluence of 10 J/cm2 from aluminum targets containing iron and magnesium impurities. The excitation temperature is deduced from Boltzmann diagrams built from a large number of spatially integrated neutral iron lines distributed from 3.21 to 6.56 eV. It is shown that at any time after the end of the laser pulse, the neutral excited states are in excellent Boltzmann equilibrium. Detailed investigation of Boltzmann equilibrium further validates previous temperature measurements using less accurate diagrams. However, observations of ion lines provide some evidence that the ionized species do not obey Saha equilibrium, thereby indicating departure from LTE. This could be explained by the fact that the plasma cannot be considered as stationary for these species.

In this paper, the expansion in ambient air of a plasma produced by laser ablation from an aluminum target is studied with the help of a one-dimensional fluid code that includes a consistent description of ablation and of the subsequent plasma expansion. Two limiting cases for the expansion geometry have been considered: Cartesian and hemispheric. The plasma temperature and electron density as a function of the space coordinate and time have been obtained for laser pulse durations of 100 fs, 200 ps, and 5 ns, and laser fluences up to a few tens of J/cm2. Simulation results are in good agreement with experiments. The code shows that for times typically longer than 1 μs, the plasma space-averaged temperature and electron density are nearly independent of the laser parameters and of the chosen expansion geometry. For a given pulse duration the plasma temperature first increases with the fluence and then reaches a saturation value. It is shown that most of the observed plasma behavior can be explained by radiative cooling.

In the framework of laser-induced plasma spectroscopy (LIPS) applications, time-resolved characteristics of laser-produced aluminium plasmas in air at atmospheric pressure are investigated for laser pulse durations ranging from 100 fs to 270 ps. Measurements show that for delays after the laser pulse longer than ~100 ns, the plasma temperature increases slightly with the laser pulse duration, while the electron density is independent of it. In addition, as the pulse duration increases, the plasma radiation emission lasts longer and the spectral lines arise later from the continuum emission. The time dependence of the continuum emission appears to be similar whatever the duration of the laser pulse is, while the temporal evolution of the line emission seems to be affected mainly by the plasma temperature. Finally, as far as spectrochemical applications (such as LIPS) of laser-produced plasmas are concerned, this study highlights the importance of the choice of appropriate temporal gating parameters for each laser pulse duration.

Using a self-consistent one-dimensional Cartesian Lagrangian fluid code, we modeled the ultrashort laser pulse ablation of solid aluminum and the subsequent plasma expansion in ambient air. A laser fluence of approximately 10 J/cm2 is considered. The code axial plasma temperature and density are strongly inhomogeneous and the maximum radiation emission generally occurs in the front of the plasma. The code average plasma temperature is in good agreement with the experiments for all times, while larger discrepancies with respect to the experiments are observed at late times for the plasma density. Experimental results are in reasonable agreement with the condition of thermodynamic equilibrium, which is an important assumption in the model.

This paper reports studies on time-resolved space-integrated laser induced breakdown spectroscopy (LIBS) of plasmas produced by ultrashort laser pulses at atmospheric pressure, on aluminum alloy targets. The temporal behavior of specific ion and neutral emission lines of Al, Mg and Fe has been characterized. The results show a faster decay of continuum and spectral lines, and a shorter plasma lifetime than in the case of longer laser pulses. Spectroscopic diagnostics were used to determine the time-resolved electron density, as well as the excitation and ionization temperatures. In comparison with plasmas produced by ns laser pulses, the plasma generated by ultrashort pulses exhibits a faster thermalization. Analytical performances of fs-LIBS were also evaluated. Linear calibration curves for minor elements (Mg, Fe, Si, Mn, Cu) presented in aluminum alloys were obtained. The limits of detection are in the parts per million (ppm) range and are element-dependent.

Laser ablation due to an ultrashort laser pulse on a massive aluminum target was investigated by means of a one-dimensional fluid code. Clear separation between the ablated matter and the unablated target is seen to occur through spinodal decomposition involving thermodynamic instabilities near the critical point of aluminum. The code also shows that the end of the ablation process is preceded by the ejection of droplets, which form about $15%$ of the total ejected mass.

Treatment of intracoronary thrombus is well documented. Three situations should be differentiated Primary percutaneous coronary intervention for early STEMI presenters is the most frequent one. Glycoprotein IIb/IIIa inhibitors are the gold standard antithrombotic treatment with a clear mortality benefit with abciximab. Thrombectomy with simple to use devices is another attractive option for interventionalists, although there is no clear established clinical benefit. Rescue PCI following failed thrombolysis is a more complicated situation given the underlying bleeding risk that is difficult to evaluate. The second situation is when a thrombus appears during an elective PCI. Although much less frequent than primary PCI, it is more often related to a lack of identification of the risk, to an inappropriate choice of the materials or to a non-optimal upstream antithrombotic treatment. A careful identification of all potential relevant causes is the key point of the management strategy. Post-PCI rethrombosis is the third situation and probably the less frequent. However, it is the most difficult to deal with.

Quantitative analysis of aluminum and copper alloys by means of laser-induced plasma spectroscopy (LIPS) has been investigated for three representative laser pulse durations (80 fs, 2 ps, and 270 ps). The experiments were carried out in air at atmospheric pressure with a constant energy density of 20 J/cm2. Because the decay rate of the spectral emission depends on the laser pulse duration, the optimum detection requires an optimization of the temporal gating acquisition parameters. LIPS calibration (sensitivity and nonlinearity) and the limit of detection (LOD) are discussed in detail. While the LOD of minor elements embedded in alloy samples obtained by sub-picosecond or sub-nanosecond laser pulses are both time and element dependent, provided an appropriate temporal window is chosen, the optimum LODs (several parts per million (ppm)) prove to be independent of the laser pulse duration. Finally, it is found that for elements such as those detected here, gated LIPS spectra using picosecond or sub-picosecond laser pulses provide much better LOD values than non-gated spectra.

Summary form only given, as follows. Of the few studies that have been devoted to examining departure from LTE conditions, several suggest that laser produced plasmas at low fluences could be far from LTE at early times (before typically 1 /spl mu/s). The present work addresses this issue by investigating the departure from LTE conditions in Al plasmas created by a 10 ns excimer laser pulse at a wavelength of 308 nm with fluences of about 10 J/cm/sup 2/. The Boltzmann diagram method is applied to a few lines emitted by iron impurities contained in an Al matrix. Time evolution of the LTE departure is investigated by probing the plume at various delays after the laser pulse interaction with the target.

Laser ablation of an aluminum target as a function of the pulse duration, for fluences up to 30 J/cm(2) and a wavelength of 0.8 microm, is investigated by means of a fluid code. For a given fluence, the ablation depth shows a minimum for a pulse duration of approximately 10 ps between a maximum obtained for pulses shorter than approximately 1 ps and a lower maximum obtained for pulses in the nanosecond range, in qualitative agreement with published experimental results. The decrease in ablation depth with increase in pulse duration observed between 1 and 10 ps results from the reduced temperature rise near the surface due to increased inward heat transport. The increase in the ablation depth above approximately 10 ps is due to the increase in electron density gradient length while the laser pulse intensity is close to maximum, which thus enables the plasma to absorb more of the laser pulse energy for increased ablation.

Laser-induced aluminum plasma in ambient air at a fluence of 10 J/cm/sup 2/ is observed by means of fast photography. Plasma expansion is found to be nearly hemispherical. The existence of an air plasma at the edge of the Al plasma is suggested by comparing spectrally resolved and unresolved pictures.

The ablated depth and volume per laser pulse from an aluminum target were measured for pulse durations that ranged from 80 fs to 270 ps at an average fluence of approximately 100 J/cm2 and a wavelength of 0.8 microm. The ablated volume showed a flat maximum for subpicosecond pulses and a minimum for approximately 6 ps. The crater diameters were rather constant up to pulse durations of approximately 6 ps and increased for larger pulse durations. As a result, the ablated depth also showed a plateau for subpicosecond pulses but decreased monotonically with pulse duration. A physical interpretation of these results and their consequences for laser applications are discussed.