H. Sobral

Temporal evolution of the shock wave and hot core air in laser induced plasma

The temporal evolution of electric breakdown in air at atmospheric pressure by Nd:yttrium–aluminum–garnet Q-switched nanosecond laser pulses was studied from the nanosecond to the millisecond time scale by shadowgraphy and interferometry techniques. The results were modeled with a gasdynamic code with good agreement. It was possible to simultaneously model the whole evolution of the plasma, the shock wave, and the hot core air. The shock wave velocity was determined to be ⩾60 km s−1 at 20 ns. The plasma temperature was found to reach about 1.7×104 K at 1 μs and the hot core air temperature was determined to be <103 K at 100 μs. This letter presents an experimental work that extends the study of laser induced plasmas to millisecond time scales.

Shadowgraphy and interferometry using a CW laser and a CCD of a laser-induced plasma in atmospheric air

We present a new application of a charged coupled device (CCD), where a continuous-wave (CW) laser is used as a probe beam in shadowgraphy and interferometric diagnosis of transparent media where the refractive index varies in time. The time resolution is obtained by gating the CCD. This configuration has the advantage that the temporal resolution of both techniques can be changed according to the evolution of the process under study and no stability setup is required, as usual, in CW interferometric techniques. We applied this method to the diagnosis of a laser induced plasma (LIP) in air measuring the evolution of plasma, shock wave, electron density, and hot core air expansion from the ns to the ms time scale.

Shock and thermal wave study of laser-induced plasmas in air by the probe beam deflection technique

A plasma induced by focusing a 7 ns Nd:YAG laser at 1.06 µm in air at atmospheric pressure was studied by the probe beam deflection method. The evolution of the shock wave and the thermal expansion of the hot air in the core of the decaying plasma were monitored by using this technique. The shapes of the shock and thermal waves were mapped and the velocities were determined as being as high as 528 and 40 m s−1 at 1 µs. In addition, it was also possible to measure the relaxation time required to bring the hot air to room temperature: it was measured to be 5 ms. This technique is versatile, economic, and simple to implement, giving essential information from processes that occur from the microsecond to the millisecond timescale.

The physical mechanism of nitric oxide formation in simulated lightning

We report an experimental assessment of the contributions of the shockwave and the hot channel to the production of nitric oxide by simulated lightning. Lightning in the laboratory was simulated by a hot plasma generated with a pulsed Nd-YAG laser. The temporal evolution of electric breakdown in air at atmospheric pressure was studied from the nanosecond to the millisecond time scale by shadowgraphy and interferometry techniques. The shockwave front velocity was determined to be about 60 km s−1 at 20 ns and the temperature behind the shock front was estimated to be about 105 K. The production yield of nitric oxide by shock heating is estimated to be: P(NO) (3±2) × 1014 molecule J−1. In contrast it was calculated that the production yield of NO by the hot channel is as much as P(NO)=(1.5±0.5) × 1017 molecule J−1. To the extent our simulation is an accurate representation of natural lightning, the hot channel is the dominant region for nitrogen fixation.

Characterization of pulsed laser generated plasma through its perturbation in an electric field

The perturbation produced by pulsed laser generated discharge in an electric field is studied as a tool for breakdown characterization. A focused high-power pulsed laser induces a discharge in air or in a solid target, that is placed between the plates of a planar charged capacitor. The induced discharge generates a temporal redistribution of the electrical charges on the plates that can be easily measured by a resistor connected to the ground plate. This signal depends on the energy used to generate the breakdown, the capacitor applied voltage, and the distance between the plates. In this work, we show that this signal can be readily used to optimize the relevant parameters involved in laser-induced breakdown spectroscopy in gases and in solid targets.

Plume dynamics of cross-beam pulsed-laser ablation of graphite

The dynamics of the interaction between two plasmas induced by cross-beam pulsed-laser ablation was analyzed by time resolved optical emission spectroscopy and fast photography. The plasmas were created in vacuum by irradiating two perpendicular graphite targets with an excimer (248nm) and a Nd:yttrium-aluminum-garnet (1064nm) laser. In this configuration, a laser is focused onto a target generating a highly directed plume; subsequently, an additional laser produces a second plasma from the perpendicular target which expands through the first plume. Collisional processes cause a reduction of the kinetic energy of the second plume species as compared to the single pulse experiment. For a fixed delay between lasers of 2μs, the second plume was divided in two perpendicular directions. The dynamics of this plasma has been compared with laser-induced plume propagation through a background gas in terms of the drag model.

A New Method for Determination of Self-Absorption Coefficients of Emission Lines in Laser-Induced Breakdown Spectroscopy Experiments

In this paper we present a new method for determining the self-absorption coefficients of emission lines in laser-induced breakdown spectroscopy (LIBS) experiments. With respect to other methods already present in the literature, the proposed approach has the advantage of not requiring, providing some conditions are fulfilled, any knowledge of the plasma parameters such as temperature and electron number density and of the emission line spectral coefficients such as transition probability. An example of the application of the approach is given for emission lines measured at different delay times after laser ablation of a silver target.

Experimental simulation of lightning, interacting explosions and astrophysical jets with pulsed lasers

Tabletop laboratory experiments have been used to simulate natural lightning, interacting explosions and astrophysical jets. When a high-energy laser pulse is focused in air, a laser-induced plasma (LIP) is produced, that generates a shock wave and an adiabatic expansion of the gas. In our work we have used LIPs in order to simulate lightning, for the study of chemical reactions relevant to atmospheric science. Several diagnostics have been applied to our LIPs, such as deflectometry, shadowgraphy and interferometry, which yield full spatial information of the process (electron density and temperature, the position of the shock wave fronts and the expansion of the hot gas), with a time resolution that ranges from nanoseconds to milliseconds. A new diagnostic alternative was implemented for shadowgraphy, which uses either continuous lasers or conventional light sources. The experimental results have been reproduced by hydrodynamic codes that we have developed. With astrophysical applications in mind, we have simulated and diagnosed the interaction of two explosions, with the aforementioned techniques. For this purpose, two LIPs are synchronized and diagnosed spatially and temporarily. Also, by producing the LIP in a glass sphere with a nozzle that ejects a shock wave and hot gas, we are able to simulate astrophysical jets. With such experiments, astrophysical models developed by us have been validated, showing excellent agreement between experiments and numerical simulations.

Photoacoustic and spectroscopic characterization of the ablation process in orthogonal double-pulse configuration

A photoacoustic technique was used as an alternative method to monitor the crater volume and its role in the emission line intensification in double-pulse pre-ablation configuration. The crater volume was measured using confocal microscopy and correlated with the changes in the photoacoustic signal. Laser emission spectroscopy was used to characterize the emission enhancement as a function of the delay between lasers and the first pulse energy. Optimum delay was found to be in the microsecond timescale corresponding to the maximum of the crater volume and the largest change between the single- and the double-pulse photoacoustic signals. Only a slight intensification was detected with increasing first pulse energy above the first pulse ablation threshold; however, the crater volume did not significantly change and the possible involved mechanisms are discussed.

Dipolar field and plasma expansion at the onset of laser-induced breakdown in a uniform dc field

An experimental investigation into the initial phases of laser-induced breakdown in air, with and without an external field, is presented. The plasma is produced by focusing the light of a Nd : YAG laser between two parallel plates connected to a dc bias supply. The diagnostics employed included fast photography and electric field measurements with a D-dot field probe. It is found that the laser power threshold required to initiate breakdown increases when an external field transverse to the laser beam is applied; the effect is not observed when the field is parallel to the beam. Measurements performed with a field probe show that the plasma produces a dipolar electric field that is proportional to the strength of the bias. The dipole is caused by charge redistribution over the plasma surface rather than by charge creation at separate points. The estimated plasma-polarizability coefficient associated with the dipole is a function of the beam energy and focal lengths employed only.