Spark Formation Research Articles

Minimum ignition energy for laser spark ignition

Combustion ignition by electrical sparks and nonresonant laser sparks is preceded by breakdown, electron heating, relaxation of internal plasma energy, shock wave generation and propagation, and subsonic flows. Measurements and model calculations of these processes (which occur over times of 10 −12–10 −1 s) in laser sparks show that the minimum ignition energy (MIE) for laser sparks is higher than for electrical sparks, fundamentally because of the higher energy cost of laser spark formation (both breakdown and heating), but also because of more efficient removal of the energy absorbed in laser sparks to regions outside the nominal ignition kernel by the shock. The MIE is bounded below by the breakdown energy, which depends strongly on the focal length of the lens in laser systems; this partly explains the wide range of reported laser spark MIEs. Another possible factor leading to a lower MIE for electrical sparks is supplemental heating of the kernel region long after emission of a shock wave, an effect entirely absent in laser sparks. Energy removal by shocks is more efficient for both types of spark than convective energy transport or diffusion out of an ignition kernel, so the spark ignition MIEs inevitably exceed the thermal MIE based on chemical kinetics for local heating of the kernel region.

Spatial and temporal dependence of interspark interactions in femtosecond-nanosecond dual-pulse laser-induced breakdown spectroscopy.

A femtosecond air spark has recently been combined with a nanosecond ablative pulse in order to map the spatial and temporal interactions of the two plasmas in femtosecond-nanosecond orthogonal preablation spark dual-pulse laser-induced breakdown spectroscopy (LIBS). Good spatial and temporal correlation was found for reduced atomic emission from atmospheric species (nitrogen and oxygen) and increased atomic emission from ablated species (copper and aluminum) in the femtosecond-nanosecond plasma, suggesting a potential role for atmospheric pressure or nitrogen/oxygen concentration reduction following air spark formation in generating atomic emission enhancements in dual-pulse LIBS.

Effect of nonthermal plasma reactor for CF/sub 4/ decomposition

In this paper, the CF/sub 4/ decomposition efficiency is investigated for three simulated plasma reactors: needle-plate reactor (NPR); metal-particle reactor (MPR); and dielectric-barrier reactor with hole (DBH). The CF/sub 4/ decomposition efficiency by DBH is much better than that by NPR or MPR. When applied voltage is increased up to the critical voltage for spark formation in the all reactors, the CF/sub 4/ decomposition efficiency is increased. The CF/sub 4/ decomposition efficiency in NPR and MPR is about 12% and 22%, respectively, at an applied voltage of 23 kV (consumption power: 110 W) and CF/sub 4/ concentration of 500 p/min, however, the CF/sub 4/ decomposition efficiency is higher than 95% in the case of DBH. The DBH should be much better than two reactors investigated for CF/sub 4/ decomposition.

Structural Properties of Dielectric Barrier Reactor with Hole (DBH) for CF4Decomposition

In this paper, the <TEX>$CF_4$</TEX> decomposition efficiency is investigated for three simulated plasma reactors that are needle plate reactor, metal particle reactor, and dielectric barrier reactor with hole (DBH). The<TEX>$CF_4$</TEX> decomposition efficiency by DBH is much better than that by needle plate reactor or metal particle reactor. When applied voltage is increased up to the critical voltage for spark formation in the all reactors, the <TEX>$CF_4$</TEX> decomposition efficiency is increased. The <TEX>$CF_4$</TEX> decomposition efficiency in needle plate reactor and metal particle reactor is about <TEX>$12\%$</TEX> and <TEX>$22\%$</TEX> respectively at applied voltage of 23 kV (consumption power: 110 W) and <TEX>$CF_4$</TEX> concentration of 500 ppm, however, the <TEX>$CF_4$</TEX> decomposition efficiency is more than <TEX>$95\%$</TEX> in case of DBH. DBH should be much better than two reactors investigated for <TEX>$CF_4$</TEX> decomposition.

비열플라즈마를 이용한 CF4분해에 미치는 혼합가스의 영향

In this paper, the <TEX>$CF_4$</TEX> decomposition rate and by-product were investigated for two simulated plasma reactors which are metal particle reactor and spiral wire reactors as a function of mixed gases. The <TEX>$CF_4$</TEX> decomposition rate by plasma reactor with metal particle electrode had a gain of 20~25% over that by plasma reactor with spiral wire electrode. The <TEX>$CF_4$</TEX> decomposition efficiency increases with increasing applied voltage up to the critical voltage for spark formation. The <TEX>$CF_4$</TEX> decomposition efficiency of metal particle reactor was about 80% at AC 24kV. The <TEX>$CF_4$</TEX> decomposition rate used Ar-<TEX>$N_2$</TEX> as base gas was the highest among three base gases of <TEX>$N_2$</TEX>, <TEX>$Ar-N_2$</TEX>, air. The by-products of the <TEX>$N_2$</TEX>, <TEX>$N_2Ar$</TEX> base as were similar, but in case of air base they were different.

Characteristics of the surface oxides on turned and electrochemically oxidized pure titanium implants up to dielectric breakdown:: the oxide thickness, micropore configurations, surface roughness, crystal structure and chemical composition

Titanium implants have been used widely and successfully for various types of bone-anchored reconstructions. It is believed that properties of oxide films covering titanium implant surfaces are of crucial importance for a successful osseointegration, in particular at compromized bone sites. The aim of the present study is to investigate the surface properties of anodic oxides formed on commercially pure (c.p.) titanium screw implants as well as to study ‘native’ oxides on turned c.p. titanium implants. Anodic oxides were prepared by galvanostatic mode in CH 3COOH up to the high forming voltage of dielectric breakdown and spark formation. The oxide thicknesses, measured with Auger electron spectroscopy (AES), were in the range of about 200–1000 nm. Barrier and porous structures dominated the surface morphology of the anodic film. Quantitative morphometric analyses of the micropore structures were performed using an image analysis system on scanning electron microscopy (SEM) negatives. The pore sizes were ⩽8 μm in diameter and had 1.27–2.1 μm 2 opening area. The porosity was in the range of 12.7–24.4%. The surface roughness was in the range of 0.96–1.03 μm ( S a), measured with TopScan 3D ®. The crystal structures of the titanium oxide were amorphous, anatase, and a mixtures of anatase and rutile type, as analyzed with thin-film X-ray diffractometry (TF-XRD) and Raman spectroscopy. The chemical compositions consisted mainly of TiO 2, characterized with X-ray photoelectron spectroscopy (XPS). The native (thermal) oxide on turned implants was 17.4 nm (±6.2) thick and amorphous. Its chemical composition was TiO 2. The surface roughness had an average height deviation of 0.83 μm ( S a). The present results are needed to elucidate the influence of the oxide properties on the biological reaction. The results of animal studies using the presently characterized surface oxides on titanium implants will be published separately.

Implosion experiments of gas-filled plastic-shell targets with [ell ] = 1 drive nonuniformity at the Gekko-XII glass laser

Effects of an implosion nonuniformity with [ell ] = 1 ([ell ]: Legendre polynomial mode number) on the hot spark formation were investigated in a series of direct-drive implosion experiments at the Gekko-Xll glass laser (Yamanaka et al., 1987). The implosion dynamics and the performance from the early to final stage of the implosion were observed with a variety of X-ray imaging and neutron diagnostics. A drive nonuniformity in the implosion with [ell ] = 1 was observed in the shape of the accelerated target at the early stage of the implosion. At the final stage of the implosion, the resultant nonuniformity with [ell ] = 1 was also observed as a geometrical shift of core plasmas from the center of the chamber. The observed neutron yield and X-ray emission properties at the final stage of the implosion were significantly degraded with an increase of the implosion nonuniformity with [ell ] = 1. The experimental results were compared with one-dimensional (1-D) and two-dimensional (2-D) hydrodynamic simulations. As a result, it was found that the implosion nonuniformity with [ell ] = 1 shifts the whole implosion dynamics towards its direction and prevents the confinement of the gas fuel considerably. However, the experimentally observed degradation in the hot spark formation, such as reductions in neutron yield and features in X-ray emission, can be reproduced in 2-D simulations not with an asymmetric perturbation of [ell ] = 1 only but with multimode nonuniformities such as [ell ] = 1 coupled with some additional middle-mode ones (e.g., [ell ] = 6). Such a complex spike structure caused by the multimode nonuniformities was found to be essential for the experimentally observed rapid cooling of the hot spark.

Studies on nonconventional high-gain target design for ICF

This paper reports some of the studies on nonconventional ICF approaches performed at the ICF Physics and Technology Laboratory of the AEEF in Frascati, Italy. Having as reference potential difficulties associated to the conventional central spark ignition (fuel mixing) and to the usual approach to fast ignition by laser (transfer and coupling of the energy pulse, fast electrons energy tuning), we have made explorative work on possible alternatives. The performances of targets ignited near stagnation by pulses of heavy ion beams (HIB) or by macroparticle impact were previously studied. The needed driver energy, the power, and the beam quality requirements, as well as the level of synchronization the implosion and the igniting pulse have been found. More recently, to relax some requirements on the HIB beam parameters set by the previous approach, the injected entropy approach (IE) has been introduced. In this method, the conditions for spark formation are set in the final stages of the implosion, when the spark fuel size is a few times the final size at stagnation (volume a few tens of the final). Energy is injected at this time to set the spark fuel on a high adiabat. In this paper, for illustration and comparison purposes, some relevant results we previously obtained for near-stagnation ignition are first introduced and critically reviewed. The new IE method, after a short analytical introduction, is presented and illustrated by the results of extensive 2-D numerical simulations. The considered cases refer to imploding cylinders of finite length. As required by this approach, one or two opposing beams axially injected additional energy, whereas the acceleration stage of the cylindrical low-entropy implosion was assumed driven by a different driver. Heavy ion beams, soft X-rays (SXR), and laser generated light ion beams were considered as vectors for the entropy injection. Issues related to the feasibility of these generators are discussed. The study was made for various initial conditions leading to different ignition modes and burn propagation. The most recent results on the injected entropy method to the ignition of high gain targets are included.

Inhibition of Ca 2+ Sparks by Ruthenium Red in Permeabilized Rat Ventricular Myocytes

We have compared the effects of the sarcoplasmic reticulum (SR) Ca 2+ release inhibitor, ruthenium red (RR), on single ryanodine receptor (RyR) channels in lipid bilayers, and on Ca 2+ sparks in permeabilized rat ventricular myocytes. Ruthenium red at 5 μM inhibited the open probability ( P o) of RyRs ∼20–50-fold, without significantly affecting the conductance or mean open time of the channel. At the same concentration, RR inhibited the frequency of Ca 2+ sparks in permeabilized myocytes by ∼10-fold, and reduced the amplitude of large amplitude events (with most probable localization on the line scan) by ∼3-fold. According to our theoretical simulations, performed with a numerical model of Ca 2+ spark formation, this reduction in Ca 2+ spark amplitude corresponds to an ∼4-fold decrease in Ca 2+ release flux underlying Ca 2+ sparks. Ruthenium red (5 μM) increased the SR Ca 2+ content by ∼2-fold (from 151 to 312 μmol/l cytosol). Considering the degree of inhibition of local Ca 2+ release events, the increase in SR Ca 2+ load by RR, and the lack of effects of RR on single RyR open time and conductance, we have estimated that Ca 2+ sparks under normal conditions are generated by openings of at least 10 single RyRs.

Simulation of streamer-to-spark transition in short non-uniform air gaps

Propagation of positive streamers in a short non-uniform gap in dry air and the evolution of plasma channels after bridging of the gap have been simulated. The dependence of the streamer-to-spark transition time br on the mean electric field in the gap has been obtained for conditions when the transition occurs in a nanosecond time range. Calculated values of br are in a reasonable agreement with experimental data. According to the results of the simulation, the major cause of spark formation is an increase, with time, of the electron detachment rate due to the accumulation of oxygen atoms and other active particles.

Ca 2+ Sparks in Embryonic Mouse Skeletal Muscle Selectively Deficient in Dihydropyridine Receptor α1S or β1a Subunits

Ca 2+ sparks are miniature Ca 2+ release events from the sarcoplasmic reticulum of muscle cells. We examined the kinetics of Ca 2+ sparks in excitation-contraction uncoupled myotubes from mouse embryos lacking the β 1 subunit and mdg embryos lacking the α 1S subunit of the dihydropyridine receptor. Ca 2+ sparks occurred spontaneously without a preferential location in the myotube. Ca 2+ sparks had a broad distribution of spatial and temporal dimensions with means much larger than those reported in adult muscle. In normal myotubes ( n = 248 sparks), the peak fluorescence ratio, ΔF/Fo, was 1.6 ± 0.6 (mean ± SD), the full spatial width at half-maximal fluorescence (FWHM) was 3.6 ± 1.1 μm and the full duration of individual sparks, Δt, was 145 ± 64 ms. In β-null myotubes ( n = 284 sparks), ΔF/Fo = 1.9 ± 0.4, FWHM = 5.1 ± 1.5 μm, and Δ t = 168 ± 43 ms. In mdg myotubes ( n = 426 sparks), ΔF/Fo = 1 ± 0.5, the FWHM = 2.5 ± 1.1 μm, and Δ t = 97 ± 50 ms. Thus, Ca 2+ sparks in mdg myotubes were significantly dimmer, smaller, and briefer than Ca 2+ sparks in normal or β-deficient myotubes. In all cell types, the frequency of sparks, ΔF/Fo, and FWHM were gradually decreased by tetracaine and increased by caffeine. Both results confirmed that Ca 2+ sparks of resting embryonic muscle originated from spontaneous openings of ryanodine receptor channels. We conclude that dihydropyridine receptor α 1S and β 1 subunits participate in the control of Ca 2+ sparks in embryonic skeletal muscle. However, excitation-contraction coupling is not essential for Ca 2+ spark formation in these cells.

Ionization processes in spark discharge plasmas

The dominant mechanisms of electron generation and loss during different phases of spark discharge are considered. The focus is on spark formation in long non-uniform-field air gaps through leader and streamer processes. The streamer development in dry air and the effects of humidity, temperature and density on the properties of a long positive streamer in air are discussed. The ionization kinetics in the leader channel in long air gaps is described. The ionization mechanisms which lead to the re-breakdown within a post-arc air channel and to the streamer breakdown in undisturbed air and in Ar are presented.

A Simple Numerical Model of Calcium Spark Formation and Detection in Cardiac Myocytes

The elementary events of excitation-contraction coupling in heart muscle are Ca 2+ sparks, which arise from one or more ryanodine receptors in the sarcoplasmic reticulum (SR). Here a simple numerical model is constructed to explore Ca 2+ spark formation, detection, and interpretation in cardiac myocytes. This model includes Ca 2+ release, cytosolic diffusion, resequestration by SR Ca 2+-ATPases, and the association and dissociation of Ca 2+ with endogenous Ca 2+-binding sites and a diffusible indicator dye (fluo-3). Simulations in a homogeneous, isotropic cytosol reproduce the brightness and the time course of a typical cardiac Ca 2+ spark, but underestimate its spatial size (∼1.1 μm vs. ∼2.0 μm). Back-calculating [Ca 2+] i by assuming equilibrium with indicator fails to provide a good estimate of the free Ca 2+ concentration even when using blur-free fluorescence data. A parameter sensitivity study reveals that the mobility, kinetics, and concentration of the indicator are essential determinants of the shape of Ca 2+ sparks, whereas the stationary buffers and pumps are less influential. Using a geometrically more complex version of the model, we show that the asymmetric shape of Ca 2+ sparks is better explained by anisotropic diffusion of Ca 2+ ions and indicator dye rather than by subsarcomeric inhomogeneities of the Ca 2+ buffer and transport system. In addition, we examine the contribution of off-center confocal sampling to the variance of spark statistics.

The non-spark mode and high rate operation of resistive parallel plate chambers

The good time and position resolution of the resistive plate chamber (RPC) make it an attractive candidate for muon trigger systems at future colliders. However, this device has severe rate problems that make it unusable above 1 Hz/cm 2 in its present form. We have investigated various materials and have also discovered a new mode of operation that allowed us to operate the RPC at 150 Hz/cm 2. We discuss further improvements that may extend operation to even higher rates. We also discuss spark formation and explain the cause for the abnormally late spark signals.

Air breakdown times of two-stage spark gaps and their applications

The air breakdown times of second spark formation, geometrically in series with trigatron switching, are studied and compared to that of a triggered spark gap. Despite a large breakdown jitter of >or=1 mu s in the trigatron, the latter is not observed to affect the electrical closure times of the series-connected second spark gap, which is triggered largely as a result of their direct electrical and optical coupling. While most of the multistage spark gaps are designed for fast and precision switching of high voltages, the air breakdown times of the second spark formation find a number of useful applications, particularly in achieving uniform glow discharge in pulsed gas lasers.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Assessment of SQS charge distribution models by pulse shape analysis

Pulse shape analysis of the signals induced by SQS avalanches is used for the validation of proposed charge distribution models. A simulation program for the development of the induced charges by SQS in multicathode coaxial detectors was written and the induced pulses were computed for a few charge models recently proposed in the literature. The results were compared with the experimentally collected data and the model derived from the application of the long spark formation theory of Gallimberti stood out as a plausible charge model.

Mechanisms of high-current pulses in lightning and long-spark stepped leaders

The mechanisms of high-current transients in lightning stepped leaders and in long laboratory sparks at negative polarity are analyzed both from improved time resolution measurements and from systematic identifications of the various elements of the discharge during a pulse. A qualitative model was first constructed when the observations made were correlated with some previously known basic phenomena. Then the model was made quantitative by means of a computer simulation of spark formation, carried out from a modified program in electromagnetics. The relevance of the proposed model was checked for long sparks at negative polarity when computed and recorded current pulse wave forms were compared. Finally, the model of stepped-leader development was applied to a full scale event: an in-flight lightning strike on an instrumented aircraft. Slight discrepancies between computed and recorded current pulse wave forms may indicate possible underestimation of the electromagnetic high-frequency threat to sensitive airborne equipment due to the frequency limitations of the transient recorders used for lightning characterization on aircraft.

A simple two-stage cascading spark gap for ultraviolet preionized transversely excited atmospheric CO2 lasers

A simple trigatron spark-gap chamber is modified to accommodate two cascading high-voltage breakdowns, separated by the time lag of the second spark formation. The time lag is employed as a temporal delay for synchronizing the ultraviolet preionization and the main discharge in the transversely excited atmospheric CO2 laser. Owing to prompt triggering and in situ preionization of the second breakdown, reliable delay up to 500 ns is possible by simply varying the separation of the second spark-gap electrode to that of the first spark gap.

High-intensity CO_2 laser interactions with indium antimonide

The interaction of picosecond CO2 laser pulses with intrinsic and doped indium antimonide at high intensities is investigated both experimentally and theoretically. A consistent theory is presented that explains the phenomena of high-density carrier generation, transient reflectivity, and melting in the semiconductor. All physical parameters become highly nonlinear at high laser intensities and carrier densities. Account was taken of the dynamic energy band-gap shift, changes in the multiphoton absorption cross section and the effective mass of the electron, dynamic reflectivity, and transmission. Keldysh nonlinear carrier generation is found to be essential to explain the experimental observations. The spark formation threshold of both intrinsic and doped InSb was measured to be 4.3 GW/cm2, corresponding to an energy fluence threshold of 0.26 J/cm2, at a laser pulse duration of 45 psec. The pulse duration scaling of the damage threshold is also discussed.

Nanosecond evolution of a low-inductance air spark

The expansion of the shock front of a laser-triggered, atmospheric air spark was studied by shadow photography with a transversely excited atmospheric UV nitrogen laser. It was found that the shock front of the short spark (0.5 mm) was cylindrical. Within a few nanoseconds of spark formation the diameter of the spark channel grows linearly with time and the index of refraction is less than in the surrounding gas. The shock velocity was measured to be 7 km/s (Mach 20).