Laser Produced Plasma In Air Research Articles

Dynamics of angularly colliding laser-produced plasmas in air and argon ambient

The effects of ambient gas pressure on the dynamics of angularly colliding laser-produced plasmas are investigated in air and Ar ambient. Fast formation of intense stagnation region is observed even in the low-pressure regime, and the tendency of interpenetration is also greater because of the involvement of the fast forward expansion velocity of seed plasmas. An intense and spherical stagnation region is observed at an intermediate pressure regime of Ar ambient. At higher pressure regime, shock front interaction in air ambient and at 10−1 mbar of Ar ambient is due to the regular reflection of shock waves. At 1 mbar of Ar ambient, a channel is formed at the collision front, and its width increases with time which is due to the Mach reflection of shock waves. Increasing inter-plume separation or the increase of angle between the targets significantly influences the shock wave reflections. Mach reflection of shock waves is observed even at 10−1 mbar of Ar ambient in higher angular separation. A peculiar plume splitting is also observed at 750 ns in 10−1 mbar of Ar ambient. Optical emission spectroscopic analysis was also done to understand the spectral emission from interaction region and its variation with ambient gas pressure.

Ultrabroadband microwave radiation from near- and mid-infrared laser-produced plasmas in air

An ultrashort laser pulse focused in air creates a plasma that radiates broadband electromagnetic waves. We experimentally compare the generation of microwaves from plasmas produced with two different laser systems that operate in the near- and mid-infrared regimes. Changing the laser wavelength increases the microwave power by 100 times and changing the input pulse energy allows for tuning of the microwave frequency spectrum, which we absolutely calibrate over a range of 2--70 GHz. The variation of the spectrum with laser pulse energy confirms the existence of a distinct mechanism that generates microwave radiation from laser-produced plasmas in gases. We propose that a radial diffusive expansion wave of the plasma electrons drives a longitudinal current along the plasma surface whose amplitude varies with the total residual electron energy imparted by the laser field and this longitudinal current produces the detected radiation.

Influence of heating effect in Thomson scattering diagnosis of laser-produced plasmas in air

A state diagnosis of laser-produced plasma in air generated by a 1064 nm pulse laser was investigated by the Thomson scattering (TS) method. The evolutions of the electron temperature and electron density were obtained as a function of the time delay which ranged from 300–3200 ns. The heating effect produced by the 532 nm probe beam with different energies on the air plasma at different interaction times was further studied using a time-resolved optical emission spectroscopy technique. The influence of the probe beam on the electron density was found to be negligible, whereas its influence on electron temperature is evident. In addition, the heating effect of the probe beam on the plasma strongly depends on the energy of the probe beam, and gradually weakens with increasing time delay. Our results are helpful for further understanding the TS method and its application in plasma diagnostics.

Dynamics and density distribution of laser-produced Al plasmas using optical interferometry and optical emission spectroscopy

The dynamic evolution and density distribution of laser-produced plasmas (LPP) of Al in air at atmospheric pressure were investigated using optical interferometry and optical emission spectroscopy, respectively. Interferograms were obtained in Mach–Zehnder interferometry with a laser pulse energy of 35 mJ and delay times from 200 ns to 6.9 µs. From the shift in interference fringes within this time interval and Taylor–Sedow theory, the expansion profiles of the shock wave were found to be hemispherical and approached planar propagation. The expansion and evolution of the plasma plume were also studied by exploiting the phase shift and refractive index obtained from the interferograms using the fast Fourier transformation and Abel transformation. Three-dimensional spatial expansion profiles of the plasma plume and shock wave were presented. In addition, two-dimensional electron density distributions of the Al plasmas were obtained from the spatial dependence of the refractive index of plasma, and compared with the results of optical emission spectroscopy. The results from the two diagnostic methods and their advantages and disadvantages are discussed. This work exploits two diagnostic methods to study the dynamic evolution and the density distribution of the LPP in air at atmospheric pressure and provides an important base reference for developing LPP applications in many fields.

Dynamics and density distribution of laser-produced plasma using optical interferometry

Dynamic evolution and spatio-temporally resolved density profiles of laser-produced plasma in air were investigated using optical interferometry. A series of interferograms were obtained with a pulse energy of 50 mJ and delay times from 50 ns to 2650 ns. With increasing delay time, the expansion profiles of the shock wave change from a flat ellipsoid to a spheroid. The phase shift has been extracted using a two-dimensional (2D) fast Fourier transformation algorithm and the radial distribution of the refractive index is calculated using the inverse Abel transformation assuming that the plasma is axisymmetric along the direction of the incident laser beam. Interferograms of the 2D expansion and evolution of the plasma plume and the shock wave were obtained by exploiting the spatial dependence of the refractive index. The electron densities in the plasma region and the air densities in the compressed air region were calculated from the refractive-index distributions obtained. Our results provided further understanding of the expansion and the dynamic evolution of the laser-produced plasma and the shock wave and of the spatio-temporal evolution of the density of plasma in air.

Temperature and Electron Density Measurements of Laser-Induced Plasmas in Air at Elevated Pressures

ABSTRACT We present time-resolved spectroscopic measurements of 1064-nm Nd:YAG laser-produced plasmas in air at pressures from 0.85 to 48.3 bar. We report temperatures and electron number densities of the plasmas at times between 200 ns and 3 µs after the plasma onset. Neutral atomic oxygen lines at 715 nm and 777 nm are used for temperature measurement through a Boltzmann analysis. Electron number density is measured using Stark broadened atomic hydrogen (Hα) line at 656 nm. We compare experimental results with modeling results obtained from using Taylor-Sedov blast-wave theory coupled with local thermal equilibrium composition calculations.

NO x generation in laser-produced plasma in air as a function of dissipated energy

An experimental study on the production of NO x as a function of dissipated energy in laser-produced plasma in air is presented. A plasma was produced by focusing a (60– 180) mJ , 5 ns , 532 nm pulse from a Q-switched Nd:YAG laser. The results show that for laser energy in the range of 13– 99 mJ the laser plasma generates 6.7×10 16 NO x molecules per joule and 4.6×10 16 NO molecules per joule. An order of magnitude estimate of the NO and NO x production per unit volume of heated gas based on a simple model show that the NO x and NO production efficiency in air are about 3×10 22 and 2×10 22 molecules J −1 m −3.

Plasma chemistry in laser ablation processes

Based on the results of quantitative spectroscopic diagnostics (LIF in combination with time resolved emission spectroscopy) chemical dynamics in laser-produced plasmas of metallic (Ti, Al,), and graphite samples have been examined. The Nd-YAG (1064 nm, 10 ns, 100 mJ) and excimer XeCl (308 nm, 10 ns, 10 mJ) lasers were employed for ablation. The main attention was focused on the elucidation of a role of oxide and dimer formation in controlling spatio-temporal distributions of different species in the ablation plume. The results of the spatial and temporal analysis of a laser-produced plasma in air indicates the existence of diatomic oxides in the ablation plume both in the ground and excited states, which are formed from reactions between ablated metal atoms and oxygen. The efficiency of the oxidation reaction depends on the intensity and spot diameter of the ablation laser beam. The maximal concentration of TiO molecules are estimated to be of 1×10 14 cm −3 at the time of 10 μs after the start of the ablation pulse. A comparison of spatial–temporal distributions of Ti atoms and excited TiO molecules allow us to find a correlation in their change, which proves that electronically excited Ti oxides are most probably formed from oxidation of atoms in the ground and low lying metastable states. The spectroscopic characterization of pulsed laser ablation carbon plasma has also been performed. The time–space distributions as well as the high vibrational temperature of C 2 molecules indicate that the dominant mechanism for production of C 2 is the atomic carbon recombination.

Analysis of a high-concentration copper in metal alloys by emission spectroscopy of a laser-produced plasma in air at atmospheric pressure

In order to investigate the possibility of analysis of a high concentration (4.5–90%) of copper in metal alloy samples (Al alloys and brass) with a laser microprobe analyzer in air at atmospheric pressure, the emission characteristics of plasmas produced by a Q-switched Nd:YAG laser over a laser energy range of 30–90 mJ (irradiance: 9.5×10 9–2.9×10 10 W cm −2) were studied using time-resolved spectroscopy. The experimental results showed that the effect of self-absorption can be reduced by increasing the laser pulse energy. The results of time-resolved spectroscopy of laser-induced plasmas indicated that self-absorption is predominant during short time-periods ( t<0.4 μs) after the laser pulse, and a time delay is effective for the reduction of self-absorption. At a high laser energy of 90 mJ, a linear calibration curve with a slope near unity was obtained for the analysis of copper in the concentration range from 4.5 to 20% with a gate width of 0.4 μs at a delay time of 0.4 μs after the laser pulse.

Two-Dimensional Spatial Distribution of the Time-Integrated Emission from Laser-Produced Plasmas in Air at Atmospheric Pressure

Emission from Nd:YAG laser-produced plasmas generated in air at atmospheric pressure has been studied with spatial resolution. With the use of a charge-coupled device (CCD), two-dimensional spatial distributions of the time-integrated line emission from plasmas of metallic (copper and stainless steel) and ceramic (alumina) samples were measured for laser power densities in the range 80–900 GW/cm2. Geometrical parameters (distance of the maximum, width in two directions) and the total space- and time-integrated emission were obtained as a function of power density. At the range 100–700 GW/cm2, the formation of a plasma in air for the metallic samples was shown to be responsible for a decrease in the total intensity of lines emitted from the sample plasma, accompanied by effects in the spatial profiles along the laser direction. Beyond 700 GW/cm2, a sharp increase of the line intensities was observed, related to the absence of the shielding air plasma.

Picosecond framing photography of a laser-produced plasma

Sequential two-dimensional photographs of a laser-produced plasma in air with effective individual exposure times of [inverted lazy s] 5 psec have been obtained by utilizing a focal-plane optical Kerr effect shutter.