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.

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

An experimental investigation of laser-induced breakdown using Nd:YAG laser harmonics for argon, nitrogen and oxygen gases is reported. Pressure dependence as well as wavelength dependence of the breakdown threshold irradianceIth is investigated. The experimental observations for 1.064 and 0.532 μm laser wavelengths are in agreement with theoretical calculations which include the effects of multiphoton ionization and cascade ionization.

Laser induced breakdown of Ar, N2 and O2 gases using 1.064, 0.532, 0.355 and 0.266 μm radiation

Miniature neon indicator lamps acting as glow discharge detectors (GDD) are candidates to serve as very inexpensive room-temperature Terahertz radiation detectors and as pixels in THz imaging systems. Previous experiments with GDD devices with THz waves showed good responsivity and noise equivalent power using direct detection. Significant improvement of detection performance is expected using heterodyne detection. Since THz sources are expensive and heterodyne detection requires two sources, we show here a proof of concept at low frequencies. In this paper, we compare the performance of GDDs in direct detection to the performance of GDDs in heterodyne detection at 10 GHz. The experimental results show that heterodyne detection is almost two orders of magnitude more sensitive than direct detection, and that in general sensitivity is inversely proportional to increasing local oscillator power. Heterodyne detection at 300 GHz is also demonstrated.

Heterodyne Detection by Miniature Neon Indicator Lamp Glow Discharge Detectors

Nitrogen atoms, in a flame or produced by photodissociation of nitrogen-containing molecules, were excited through a two-photon absorption process at 211 nm, and the near IR fluorescence was detected. Both in flame and in photodissociation experiments of cold gases, fluorescence, which was resonantly enhanced at the atomic nitrogen wavelength but which originated from excited oxygen atoms, was also identified. This phenomenon, the photochemical effects and those of molecular absorption, stimulated emission, and the mixing of gases are discussed in connection with the detection of atomic nitrogen in flames.

Detection of nitrogen atoms in flames using two-photon laser-induced fluorescence and investigations of photochemical effects.

Dynamic breakdown of Ne and Ar gases biased to the prebreakdown stage, with and without absorption of very short duration incident N 2 laser pulses, is studied. Effects of bias and incident laser intensity are seen to be complementary. Laser illumination of the interelectrode gap causes gas breakdown at the cathode to take place at a faster rate and at lower breakdown threshold bias than without the illumination. Breakdown pulse shape varies according to gas composition and bias, and is much different from simple nonbreakdown "prebreakdown" responses to the laser pulses. The prebreakdown signals are attributed to photon-enhanced ionization in the focal volume between the electrodes, while the laser-triggered breakdown pulses are attributed to photon-enhanced excitation and diffusion of such neutral atoms to the high field gradient region near the cathode, where cascade ionization collisional effects are amplified.

Laser-triggered dynamic breakdown of gases and laser-induced prebreakdown signals

Nitrogen laser pulse irradiation of prebreakdown discharges in Ne and Ar result in pulse responses strikingly similar to those reported for dynamic optogalvanic signals. For the latter, response polarity depends primarily upon atomic transition. Here, it depends primarily upon bias. Nevertheless, analysis of the results points to similar internal processes within the gas concerning metastable generation and destruction. Photoionization-assisted electron heating is an additional photon process relevant here which changes relative populations of excited states and plays a key role in reversing prebreakdown signal response polarity. It can also explain the mechanism of reported electron temperature increase in optogalvanic experiments which cannot be explained on the basis of atomic transitions.

Dynamic nonoptogalvanic signal polarity and magnitude in prebreakdown gas discharges

The prebreakdown regime, which exists in a gas cell during the time interval of free charge density growth from first electron-ion pair generation until the onset of breakdown, is characterized by low bias and incident field intensities In our experiment, dc biased Ne and Ar gas cells at 90 torr each were illuminated with laser pulses Both the incident laser field and dc bias field were smaller, sometimes much smaller, than those required for gas breakdown by each alone Under these conditions, clear-cut electrical signals of amplitude on the order of tens of millivolts are observed as a response to laser pulse irradiation at wavelengths non resonant to any gas atomic transitions We called these non-breakdown responses “prebreakdown or precursor signals” In 1,2 such prebreakdown signal magnitude, polarity and occurrence probability as a function of incident field intensities were discussed

Non-Optogalvanic Signal Characteristic Times in Prebreakdown Discharges

Electrical responses of low-biased noble gases irradiated by medium-intensity, nonresonant, short laser pulses can be described by a limited number of internal processes. Simplified rate equations for the main processes are solved analytically. It is assumed that (1) a four-level model describes the gas energy states and (2) metastable levels (the most populated excited levels) are the principal source for free-electron concentration change. The solution in the form of a series of time-varying exponentials represents the behavior of the metastable atom density perturbed by laser pulse illumination. Although this polynomial type of representation of discharge behavior is typically assumed empirically, here it is derived analytically, and coefficients in such polynomials are related to actual quantum processes within the discharge and initial laser-induced perturbation. >

Nonresonant optogalvanic laser-induced signals in prebreakdown gas discharges. I. Simplified model for time dependence of metastable atom density changes

For pt.I see ibid., vol.26, no.3, p.597-604 (1990). The optogalvanic laser-induced signals from gas discharges reflect the changes in excited and ionized particle balance. In prebreakdown stages of low-biased noble gases the lowest metastable levels are the most populated. Using the polynomial expression derived in Part I to describe metastable population change, an analytic solution for the laser-induced voltage across the gas cell is derived form the simple relationship between metastable atom density and such signal values. The solution has the form of a time-varying exponential polynomial series. Zero, positive, and negative degrees of these exponentials indicate three different processes: steady-state and generation and destruction of the metastable population, respectively. Various coefficients and signal peak values are related to discharge and irradiation conditions. This analysis of prebreakdown gas response to incident laser pulses, based on prior experiments, can be used to predict gas time response in various applications such as nonresonant optical detection, switching, spectroscopy, etc. >

N. Yackerson

Papers

Laser-triggered dynamic breakdown of gases and laser-induced prebreakdown signals

Dynamic nonoptogalvanic signal polarity and magnitude in prebreakdown gas discharges

Non-Optogalvanic Signal Characteristic Times in Prebreakdown Discharges

Nonresonant optogalvanic laser-induced signals in prebreakdown gas discharges. I. Simplified model for time dependence of metastable atom density changes

Nonresonant optogalvanic laser-induced signals in prebreakdown gas discharges. II. Time dependence of voltage response instantaneous value