Laser-induced Spark Research Articles

Laser-induced spark schlieren imaging

Laser-induced spark schlieren imaging

An experiment of the combustion characteristics with laser-induced spark ignition

The combustion characteristics and minimum ignition energies using laser-induced spark ignition were demonstrated for quiescent methane-air mixtures in an optically-accessible, constant volume combustion chamber. Initial pressure and equivalence ratio as well as spark energy were varied in order to explore the flame behavior with laser-induced spark ignition. Shadowgraphs for the early stages of combustion process showed that the flame kernel becomes separated into two, one of which grows back towards the laser source. Eventually after a short period, the two flame kernels developed into two flame fronts propagating individually, which is unique in laser-induced spark ignition. For a given mixture, lower initial mixture pressure and higher spark energy resulted in shorter flame initiation period and faster flame propagation. The results of minimum ignition energies for laser ignition shows higher values than electric discharge results, however, the difference decreases toward lean and rich flammability limits.

Laser-Induced Spark Flameholding in Supercritical, Subsonic Flow

Laser-induced spark e ameholding was demonstrated, and the effect of e ow and optical parameters on the e ameholding distance was investigated. A pulsed Nd:YAG laser was used to induce electrical breakdown in a e ow and ignite a methane/air gas mixture. The laser energy, pulse frequency, and methane concentration were varied, and the e ameholding distance as a function of these parameters was determined. Results were then compared to calculated values. For a e ow speed of approximately 110 cm/s and a laser repetition rate of 15 ‐ 20 Hz, the e ameholding distance, dee ned as the downstream distance from the spark to the position where continuous combustion is achieved, was found to be on the order of a few centimeters. Experimental results were extrapolated to determine the pulse frequencies required for e ameholding at hypersonic e ow speeds. The potential application of solid-state, continuous-wave lasers to spark e ameholding is also considered.

Time-resolved imaging of flame kernels: Laser spark ignition of H 2/O 2/Ar mixtures

The shape and structure of developing flame kernels in laser-induced spark ignited hydrogen/air mixtures is investigated as a function of gas composition and time. Using planar laser-induced fluorescence (PLIF) to measure the spatial distribution of OH radicals produced inside the reaction zone, we have recorded the evolution of the nascent flame kernel in a series of images following the laser-induced spark. This series provides the rate of flame growth, the evolution of the flame shape, and the intensity of the PLIF signal as a function of time for both igniting flames and nonignition events. The reaction zones grow quickly at early times, but slowly decrease in propagation rate as the energy density within the flame kernel decreases. A distinct anisotropy is observed in the expanding spark and flame kernel. At short times ( t < 100 μs), a toroidal shape is observed similar to that seen previously for electrode-spark ignitions and for laser ignitions in methane/air. There is also a tendency for the flame to grow back toward the ignition laser. Successful ignitions appear virtually identical to failed ignitions during the first 100 μs. Significant differences, notably in intensity, appear between 100 and 500 μs following the spark. These observations imply that early flame kernel growth is dominated by gas motion induced by the short-duration spark. The ultimate fate of an ignition lies with the chemistry of the reactions which determines whether the gas undergoes a transition from hot plasma to propagating flame.

Dynamics of flame propagation using laser-induced spark initiation: Ignition energy measurements

A method is described for measuring low-energy ignition of combustable mixtures using nanosecond and picosecond laser-induced breakdown. Ignition studies are reported for homogeneous H2/air mixtures. The extent of flame inhibition in H2/air was investigated as a function of CO2 concentration. Our electrodeless spark ignition results are compared with earlier work using electric discharge ignition. A simple model for flame ignition is advanced, based on the temporal and energy density characteristics of the source energy. Implications of laser-induced spark ignition in homogeneous fuel mixtures to the molecular dynamics of flame propagation are discussed.

Measurement of the properties of a CO 2 laser induced air-plasma by double floating probe and spectroscopic techniques

The laser-induced breakdown spark has recently been advanced as a method for real-time, in-situ spectrochemical analysis of gases. Many of these analyses take place in ambient air. To better characterize this source, we have measured the temporal variation of temperature and electron density in an air plasma induced by a CO 2 laser operating at 0.5 and 0.8 J/pulse. The electron temperature was measured by the double floating-probe technique (DFP). An excitation temperature for oxygen atoms was determined spectroscopically by Boltzmann plots. Electron density in the plasma was measured from the Stark broadening of the 715.6-nm line of 01. At 0.5 J/pulse, the DFP temperature ranged from 175000 K at 5 μs to less than 10000 K at 25 μs, while the 01 excitation temperature ranged from 19000 K at 1 μs to above 11 000 K at 25 μs. The excitation temperature and electron density agree with values calculated by others from local thermodynamic equilibrium models of an air plasma. While the electron temperature from the DFP method is much higher than the excitation temperature at 5 μs, at times greater than 25 μs the two have converged, implying thermodynamic equilibration between the species.

Direct experimental evidence of multiphoton ionization of impurities as the initiation process of laser-induced gas breakdown

Using a focused Q-switched ruby laser, multiphoton ionization of the background gas of an ordinary vacuum system was observed by the time-of-flight method and an electron multiplier. The number of free electrons created at the focal region was found to be at least more than 20. Since a few free electrons would be sufficient for the initiation of the laser-induced spark in gases (Young and Hercher), it was thus concluded that multiphoton ionization of gas impurities is the initiation mechanism of the optical breakdown of gases, as is generally believed.

Laser spark ignition of chemically reactive gases.

Exploratory experiments on the ignition by laser-induced sparks of chemically reactive gaseous mixtures, both detollable and nondetonable, were performed. It was found that the minimum energy required to induce breakdown was of sufficient magnitude to generate a strong spherical blast wave. The ignition mechanisms are strongly coupled to the dynamics of the decaying blast. For nondetonable gases, it was observed that initially the blast wave is of sufficient strength to result in auto-ignition of the reactive media with the reaction front trailing immediately behind the spherical shock front. As the blast expands, the reaction front decouples rapidly from the shock and propagates as a deflagration wave dependent on the transport properties of the medium. For detonable mixtures there exists a critical energy above which direct initiation of a spherical detonation wave occurs. This critical ignition energy varies inversely with the initial pressure of the mixture showing the same qualitative behavior as the variation of the induction time with initial pressure. For ignition energy well below this critical value, similar behavior of rapid decoupling to a spherical deflagration wave as in the case of nondetonable gases was observed. For ignition energy close to the critical, decoupling followed by re-establishment to a spherical detonation wave occurs. I. Introduction W ITH the recent development of high-powered solid-state lasers, breakdown in gases can readily be achieved at the focal point when the laser beam is collimated by a simple convex lens. The basic mechanisms of electrical breakdown at optical frequencies and the subsequent phenomena associated with laser sparks are discussed in detail in a recent review article by Meyerand. 1 In the present paper some exploratory experiments on the use of laser-induced sparks for the ignition of chemically reactive gases are described. There are a number of important advantages associated with the use of laser-induced sparks for ignition studies. For example, the laser spark is almost a perfect point energy source in which the energy and the rate of its deposition can be controlled and measured accurately. Also, pure chemical ignition, entirely free of contamination by any electrode material, can be achieved. The complete absence of material surfaces in the vicinity of the ignition area eliminates the complex effects of heat transfer on the initial growth of the flame kernel. The previously mentioned properties of the laserinduced spark make it an ideal energy source for the study of the basic mechanisms of ignition and the subsequent combustion phenomena in a chemically reactive gas. It is particularly suitable for ignition experiments in supersonic streams where electrode interference effects would be significant. The present experiments are of a qualitative nature with the objective being to determine the ignition mechanisms and subsequent development of the combustion phenomena from a laser-induced spark in a combustible gaseous mixture. The particular emphasis is centered on detonable gases where it is of interest in the present program to determine the minimum energy level of a free spark that will result in the

Direct initiation of spherical detonations in gaseous explosives

This paper summarizes the salient results of a recent theoretical and experimental study of the propagation of spherical detonation waves initiated by a laser-induced spark. In this study, theoretical solutions of the blast-wave model were obtained using a novel analytical technique that yields a complete description of the spherical detonation wave from its initial overdriven state to its asymptotic C-J condition. Based on the approximation that hydrodynamic motion is not influenced by chemical reactions, solutions were also obtained for the propagation of spherical blast waves with finite kinetic rates. Three regimes of propagation were established experimentally: the subcritical energy regime, where decoupling of shock and reaction zone occurs; the supercritical energy regime, where the initially overdriven spherical detonation decays asymptotically to its C-J state; and the critical energy regime, where decoupling first occurs followed by the reestablishment of a highly asymmetrical multi-head detonation.

Laser photolysis and spectroscopy: a new technique for the study of rapid reactions in the nanosecond time range

We have developed a new, very rapid, spectroscopic recording technique with the aid of which photographic absorption spectra of transient intermediates with lifetimes in the nanosecond time range can be obtained. The technique is a hundred times faster than present flash spectrographic instrumentation and provides time-resolved absorption spectra over a wide spectral range in a single experiment. Frequency-doubling is used to obtain both a 347 nm laser pulse suitable for excitation and a 694 nm laser pulse from a single Q -switched ruby laser pulse. The 694 nm pulse is converted into a continuum for absorption spectroscopy by bringing it to a sharp focus in a suitable gas, so as to cause a laser-induced breakdown spark. The excitation pulse has a duration of 30 ns. The duration of the continuum pulse varies with the gas used. In 1 atm of oxygen it has a duration of 30 ns, is synchronized to within 10 ns with the excitation pulse and has an intensity adequate for single-shot flash spectroscopy. The laser-induced spark in 1 atm of xenon has a duration of several μs and provides an excellent background continuum, of essentially constant intensity, for kinetic spectroscopy over periods up to 1 μs, using an image-converter camera to provide time resolution. We have applied the laser photolysis technique to observing, for the first time, absorption spectra arising from the lowest excited singlet states of several aromatic hydrocarbons. The time-resolved spectra obtained show the decay of the new excited singlet absorption bands and the concomitant build-up of triplet-triplet absorption bands during the first microsecond following light absorption, thus depicting graphically the non-radiative process of intersystem crossing from the lowest excited singlet state to the triplet manifold. The new bands also serve to locate the energies of higher excited singlet levels which, in many cases, are inaccessible from the ground state. The new technique should find wide application to solid, liquid and gaseous systems and should contribute to the understanding of photochemical primary processes in the time range 10 -9 to 10 -6 s. Eventual extension of the technique to the 10 -12 s range appears possible by using mode-locked lasers.

SUBNANOSECOND SCHLIEREN PHOTOGRAPHY OF LASER-INDUCED GAS BREAKDOWN

Second harmonic radiation from a mode-locked neodymium:glass laser, providing a 400-nsec-long train of picosecond pulses, has been used as a light source for Schlieren photography of a laser-induced spark. The beam from a ruby laser, Q-switched by means of a Pockels cell, was focused in air to produce breakdown and synchronization of the two lasers was achieved by switching the Pockels cell with a spark gap illuminated by the mode-locked pulse train.

Blast waves from a laser-induced spark in air

The explanation of the peculiar shape and motion of the laser-induced air spark given by Ramsden and Savic (1964) implies that at the end of the laser pulse heated air fills a spherical sector bounded by the focused laser beam. This air volume expands and forms a blast wave with perturbed spherical symmetry. In this paper, the motion of this wave is considered theoretically and the results are compared with recent interferometric measurements by Alcock, Panarella, and Ramsden (1965).

Optical Frequency Plasma Resonance in Gases

An intense optical beam is reflected backwards from a laser-induced spark in argon and nitrogen gas at high pressures. The reflected beam is about 10% as intense, diverges with a comparable angle, and has the same frequency (to within about 0.01 cm−1) as the incident beam. By simultaneously focusing a weak second harmonic beam onto the discharge, the reflectivity at the higher frequency was measured. One percent of the incident harmonic beam was reflected in a nearly collimated beam, and with no apparent frequency shift (less than 0.05 cm−1). The proposed model is that stationary region having very high electron density (ne≈2×1021 electrons/cc) is created in the focal volume and exhibits plasma resonance at the laser frequency. The resonance would appear quite damped due to the high collision rate. Experiments have been performed which support the hypothesis and discriminate against several other possible explanations such as reflection from the mismatch of index of refraction at the shock front, stimulated Brillouin scattering, and stimulated plasma-wave scattering.

A Radiative Detonation Model for the Development of a Laser-Induced Spark in Air

Abstract : In the course of recent work on the spark produced in air by a focused ruby laser beam, the rather surprising result was obtained that, after breakdown, the spark developed asymmetrically, moving towards the lens with an initial velocity of about 10 to the 7th power cm/sec. In this article this effect is discussed in terms of a new mechanism--that of a radiationsupported shock wave. It is assumed that after breakdown a shock wave propagates into the undisturbed gas, and that further absorption of energy from the laser beam then occurs behind the shock front travelling towards the lens, in the manner of a detonation wave. After the end of the laser pulse the heated gas then expands in the form of a blast wave. (Author)