Laser-induced Sparks Research Articles

Measurements of sound propagation in Mars' lower atmosphere

Acoustics has become extraterrestrial and Mars provides a new natural laboratory for testing sound propagation models compared to those ones on Earth. Owing to the unique combination of a microphone and two sound sources, the Ingenuity helicopter and the SuperCam laser-induced sparks, the Mars 2020 Perseverance rover payload enables the in situ characterization of unique sound propagation properties of the low-pressure CO2-dominated Mars atmosphere. In this study, we show that atmospheric turbulence is responsible for a large variability in the sound amplitudes from laser-induced sparks. This variability follows the diurnal pattern of turbulence. In addition, acoustic measurements acquired over one Martian year reveal a variation of the sound intensity by a factor of 1.8 from a constant source due to the seasonal cycle of pressure and temperature that significantly modifies the acoustic impedance and shock-wave formation. Finally, we show that the evolution of the Ingenuity tones and laser spark amplitudes with distance is consistent with one of the existing sound absorption models, which is a key parameter for numerical simulations applied to geophysical experiments on CO2-rich atmospheres. Overall, these results demonstrate the potential of sound propagation to interrogate the Mars environment and will therefore help in the design of future acoustic-based experiments for Mars or other planetary atmospheres such as Venus and Titan.

The sound of geological targets on Mars from the absolute intensity of laser-induced sparks shock waves

Inspection of geological material is one of the main goals of the Perseverance rover during its journey across the landscape of the Jezero crater in Mars. NASA's rover integrates SuperCam, an instrument capable of performing standoff characterization of samples using a variety of techniques. Among those tools, SuperCam can perform laser-induced breakdown spectroscopy (LIBS) studies to elucidate the chemical composition of the targets of interest. Data from optical spectroscopy can be supplemented by simultaneously-produced laser-produced plasma acoustics in order to expand the information acquired from the probed rocks thanks to the SuperCam's microphone (MIC) as it can be synchronized with the LIBS laser. Herein, we report cover results from LIBS and MIC during Perseverance's first 380 sols on the Martian surface. We study the correlation between both recorded signals, considering the main intrasample and environmental sources of variation for each technique, to understand their behavior and how they can be interpreted together towards complimenting LIBS with acoustics. We find that louder and more stable acoustic signals are recorded from rock with compact surfaces, i.e., low presence loose particulate material, and harder mineral phases in their composition. Reported results constitute the first description of the evolution of the intensity in the time domain of shockwaves from laser-produced plasmas on geological targets recorded in Mars. These signals are expected contain physicochemical signatures pertaining to the inspected sampling positions. As the dependence of the acoustic signal recorded on the sample composition, provided by LIBS, is unveiled, the sound from sparks become a powerful tool for the identification of mineral phases with similar optical emission spectra.

In situ recording of Mars soundscape

Before the Perseverance rover landing, the acoustic environment of Mars was unknown. Models predicted that: (1) atmospheric turbulence changes at centimetre scales or smaller at the point where molecular viscosity converts kinetic energy into heat1, (2) the speed of sound varies at the surface with frequency2,3 and (3) high-frequency waves are strongly attenuated with distance in CO2 (refs. 2–4). However, theoretical models were uncertain because of a lack of experimental data at low pressure and the difficulty to characterize turbulence or attenuation in a closed environment. Here, using Perseverance microphone recordings, we present the first characterization of the acoustic environment on Mars and pressure fluctuations in the audible range and beyond, from 20 Hz to 50 kHz. We find that atmospheric sounds extend measurements of pressure variations down to 1,000 times smaller scales than ever observed before, showing a dissipative regime extending over five orders of magnitude in energy. Using point sources of sound (Ingenuity rotorcraft, laser-induced sparks), we highlight two distinct values for the speed of sound that are about 10 m s−1 apart below and above 240 Hz, a unique characteristic of low-pressure CO2-dominated atmosphere. We also provide the acoustic attenuation with distance above 2 kHz, allowing us to explain the large contribution of the CO2 vibrational relaxation in the audible range. These results establish a ground truth for the modelling of acoustic processes, which is critical for studies in atmospheres such as those of Mars and Venus.

Recording laser-induced sparks on Mars with the SuperCam microphone

The SuperCam instrument suite onboard the Mars 2020 Perseverance rover includes a microphone used to complement Laser-Induced Breakdown Spectroscopy investigations of the surface of Mars. The potential of the SuperCam microphone has already been demonstrated for laser ablation under Earth atmosphere in our preliminary study with a small set of samples and fixed experimental conditions. This new experimental study, conducted under Mars atmosphere, explores all the main environmental, instrumental and target dependent parameters that likely govern the laser-induced acoustic signal that will be generated on Mars. As SuperCam will observe targets at various distances from the rover, under an atmospheric pressure that follows diurnal and seasonal cycles, this study proposes a sequence of corrections to apply to Mars data in order to compare acoustic signal from targets sampled under different configurations.In addition, 17 samples, including pure metals but also rocks and minerals relevant to Mars' surface were tested to study the influence of target properties and laser-matter interactions on the acoustic signal and the ablated volume. A specific behavior is reported for metals and graphite, which rapidly disperse the incoming laser energy through heat diffusion. However, for other minerals and rocks, the growth of the crater is seen to be responsible for the shot-to-shot decrease in acoustic energy. As a consequence, it is confirmed that monitoring the acoustic energy during a burst of laser shots could be used to estimate the laser-induced cavity volume. Moreover, the amount of matter removed by the laser is all the more important when the target is soft. Hence, the decreasing rate of the acoustic energy is correlated with the target hardness. These complementary information will help to better document SuperCam targets.

Thermometry of combustion gases using light emission and acoustic wave from laser-induced sparks

A new method to measure local combustion gas temperatures is proposed using light emission and acoustic wave from laser-induced sparks. Because the speed of sound is a function of temperature as well as species composition, the time-of-arrival measurements of the acoustic waves from each of the two laser-induced sparks separated by a certain distance were carried out using a high-frequency response microphone. As the microphone was installed in the same plane and line as the two laser sparks, the acoustic wave from the laser spark farther from the microphone traveled through the same region as the acoustic wave from the laser spark nearer to the microphone, and thus, the speed of sound for the region between the sparks can be estimated from the difference in the arrival times. Optical emission spectroscopy was also carried out on one of the laser sparks to estimate the species composition, and the linear relationship between the intensity ratio of atomic hydrogen to oxygen and fuel–air equivalence ratio was confirmed. Assuming that the gas reached its equilibrium state, the temperatures were obtained from the measured speed of sound and found to be systematically higher than the adiabatic flame temperatures by approximately 77 K. The repeatability of the measurement was within ±38 K. The proposed method is appropriate for applications in which the temperature is high and contact measurements are difficult.

Benefits and applications of laser-induced sparks in real scale model measurements.

The characteristics of using a laser-induced spark as a monopole source in scale model measurements were assessed by comparison with an electric spark and a miniature spherical loudspeaker. Room impulse responses of first order directivity sources were synthesized off-line using six spatially distributed sparks. The source steering direction was scanned across the horizontal and vertical plane to assess the origin of early reflections. The results confirm that the characteristics of the laser-induced spark outperform those of typical sources. Its monopole characteristics enable the authors to synthesize room responses of directional sources, e.g., to obtain directional information about reflections inside scale models.

Beamforming with a volumetric array of massless laser spark sources--Application in reflection tracking.

A volumetric array of laser-induced air breakdown sparks is used to produce a directional and steerable acoustic source. The laser breakdown array element is broadband, point-like, and massless. It produces an impulse-like waveform in midair, thus generating accurate spatio-temporal information for acoustic beamforming. A laser-spark scanning setup and the concept of a massless steerable source are presented and evaluated with a cubic array by using an off-line far field delay-and-sum beamforming method. This virtual acoustic array with minimal source influence can, for instance, produce narrow transmission beams to obtain localized and directional impulse response information by reflection tracking.

Properties of an air plasma generated by ultraviolet nanosecond laser pulses

Temperatures and compositions of plasma environments created by laser pulses are wavelength-dependent, and they have not been determined as yet for air plasmas produced by ultraviolet radiation. This paper fills this gap and reports time-resolved spectral measurements of laser-induced sparks created in air using a Q-switched Nd : YAG operating at 355 nm with a pulse width of 7 ns. Temperatures are determined from Boltzmann analysis of emission from atomic oxygen lines at 715 and 777 nm. Electron number densities are inferred from Stark broadening of the H-alpha line (656 nm). The measurements have been carried out over a time interval of 90 ns–3 µs after the plasma formation that corresponds to the variation of the temperature and the electron number density from 27 000 K to 12 000 K and from 1.2 × 1018 cm−3 to 1.6 × 1017 cm−3, respectively. Simple rate estimates show that the plasma is likely to be in local thermodynamic equilibrium (LTE). Using a new approach to calculate time-dependent pressures, which employs blast-wave theory, we perform thermochemical computations of the plasma thermodynamic properties and compositions. The results strongly confirm the validity of the LTE assumption for the plasma investigated and yield electron number densities in very good agreement with the measured values.

Characteristics and application of laser-generated acoustic shock waves in air

When a high-power laser beam is focused at a point, the air at the focal point is heated to temperatures of thousands of degrees within several nanoseconds and breaks down. This generates a spark that, in turn, is accompanied by an acoustic shock wave. The acoustic shock waves generated by focussing the beam from a pulsed laser with a 1064 nm wavelength and a power of 800 mJ per pulse have been measured using 1/4″ and 1/8″ B&K microphones. Nonlinear sound levels are observed up to 1.5 m from the laser-induced sparks. Beyond a certain region close to the source, levels are found to decrease in a manner consistent with spherical spreading plus nonlinear hydrodynamic losses. Analysis of the waveforms shows that the acoustic pulses associated with the laser-induced sparks are more repeatable and have higher intensity than those from an electrical spark source. Laser-generated acoustic shock waves are ideal for simulating a blast wave or a sonic boom in the laboratory and for studying the associated propagation effects. To illustrate this application, the propagation of the laser generated shock waves over a series of different hard, rough surfaces has been investigated. The results show the distinctive influences of ground roughness on the propagation of the shock wave.

Spatial characterization of laser-induced sparks in air

Images and emission spectra of sparks produced by laser-induced breakdown in air were investigated as functions of the laser energy and optical configuration. The laser-induced breakdown was generated by focusing a 532-nm nanosecond pulse from a Q-switched Nd:YAG laser. The data were collected using an intensified CCD camera and a Cassegrain optics system coupled to an ICCD spectrometer. The results provided information about the first stages of laser-induced spark breakdown. Good reproducibility of the plasma location and shape was observed; these parameters depended largely on the optical configuration and plasma energy absorption rate. The high spatial resolution of the Cassegrain optics system was used to observe different ionization levels in the plasma kernel, which confirmed the electron cascade mechanism for plasma formation. The different ionization levels partially explained the asymmetry of the ignition induced by the plasma generation in gaseous mixture. Backward propagation of the plasma along the laser path was observed using the high spatial and temporal resolution of the experimental apparatus. The propagation was largely due to the thickness of the plasma relative to the laser wavelength, which created different ionization levels and energy absorption rates throughout the plasma. This observation was correlated with images obtained using the ICCD camera.

An optical and spectroscopic study of laser-induced sparks to determine available ignition energy

This work conducts an optical and spectroscopic study of laser-induced sparks created in air. It is aimedat determining the minimum ignition energy associated with laser spark ignition. Gas breakdown was produced using a single-mode, Q-switched Nd:YAG laser. It produced a 0.6 cm diameter beam at a wavelength of 1064 nm with a 5.5 ns pulse duration. The beam deflection and the absolute two-line intensity ratio techniques were used for measuring the spark expansion and temperature. The results were used to determine the shock propagation energy, the radiation energy losses, and the energy of the hot gas that is left over after the shock has been propagated away. With spark energies ranging from 15 to 50 mJ, we found that the shock energy was from approximately 70% to 51%, the radiation energy losses were from 22% to 34%, and the energy of the remaining hot gas was about 7% to 8% of the spark total energy. As far as ignition is concerned, the shock propagation and the radiation energies are wasted. The energy of the hot gas is the energy source that causes ignition. Thus, within the limits of error of the present measurements and calculations, the ignition energy obtained by the laser spark ignition does not differ greatly from that obtained by the electric spark ignition as has been reported.

Laser spark ignition and combustion characteristics of methane-air mixtures

Ignition breakdown kernels of methane-air mixtures initiated by laser-induced sparks and by conventional electric sparks arc compared during initial stages. Experiments were conducted using a four-stroke (Otto-cycle) single-cylinder typical high-pressure combustion chamber. The piston is cycled in the cylinder by using an electric motor driven hydraulic ram. An cxcimer laser beam, either produced from krypton fluoride gas (λ = 248 nm) or argon fluoride gas (λ = 193 nm), or a Nd:YAG laser beam (λ = 1064 nm) is focused into a combustion chamber to initiate ignition. Conventional electric spark ignition is used as a basis for comparison between the two different ignition methods and the resultant early breakdown kernel characteristics. A streak camera is used to investigate and record the initial stages of kernel formation. Both a breakdown and a radial expansion wave of the ignition plasma are observed for certain laser ignition conditions of methane-air mixtures under typical internal combustion (IC) engine conditions. Results indicate that only certain wavelengths used for producing laser ignition produce a radial expansion wave. Laser ignition kernel size is calculated and laser-supported breakdown velocity is calculated by using Raizer's theory and is compared with measured results. Laser ignition results in a 4–6 ms decrease in the time for combustion to reach peak pressure than is obtained when using electric spark ignition in the same combustion chamber and under the same ignition conditions.

Application of holographic interferometry for plasma diagnostics

The principle of holographic interferometry is described and its importance for plasma diagnostics is indicated, particularly for the determination of electron concentration (Section 2). A review is given of different methods for increasing the sensitivity associated with penetration into the infrared region of the spectrum, and also with the use of multiple-pass, resonance, dispersive nonlinear and two-wavelength holographic interferometry. Section 3 discusses the experimental techniques—recording media and radiation sources—as well as the results of fringe shift measurements, stroboscopic holography and cineholography. Section 4 gives a review of work on investigation of various plasmas—laser-induced sparks, laser jets, neutral current layers, z- and θ-pinches, flash lamps, CO2 laser-induced plasmas, exploding conductors, plasmotrons, electric arcs and various other kinds of electric discharges in gases.

An experimental study of the effects of high frequency electric fields on laser-induced flame propagation

The paper describes experiments in which high frequency electric fields were applied across flames generated by the action of laser-induced sparks. The objective of the study was to establish whether such fields affect the rates of flame propagation in mixtures at one atmosphere pressure of methane/air, methane/oxygen/argon and hydrogen/air. It is shown that there is a marked increase in the propagation rates of methane/air flames, while substitution of argon for the nitrogen in such flames appears to decrease the effect significantly. In the case of hydrogen/air flames no significant increases in flame propagation rates are reported. It is concluded that recently published proposals based on an electron energy exchange mechanism does not adequately account for the observations reported in this paper.

Laser-induced multiple breakdown in gases

Experimental observations are given of the shape and structure of laser-induced sparks formed in molecular and atomic gases. Absolute measurements of the laser energy absorbed during the breakdown process are presented.

Frequency broadening in laser-induced sparks

The spectrum of forward scattered radiation resulting from laser-induced air breakdown has been investigated using a single-mode ruby laser and a mode-locked neodymium: glass laser. The broadening observed provides additional evidence for self-focusing mechanism associated with the production of laser sparks.

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

Some Characteristics of Laser-Induced Air Sparks

Some of the optical properties of laser-produced air sparks have been measured. The sparks have been photographed to determine their structure. The angular distribution of incident laser light scattered by the plasma has been found to have a strong peak in the forward direction. The spark spectrum has been recorded and used to determine the total energy in the sparks. Application as an intense light source is considered.