Threshold characteristics of ultraviolet and near infrared nanosecond laser induced plasmas

Dumitrache, Ciprian; Limbach, Christopher; Yalin, Azer P.

doi:10.1063/1.4963248

Cited by 33 publications

(22 citation statements)

References 34 publications

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“…The initial kernel shape is based on broadband optical emission images taken immediately after the pulse has ended [32]. The pressure and temperature of the kernel at the end of the discharge are determined from 1-D Rayleigh/Thomson scattering measurements [33]. A typical scattering image is show in the left of Figure 1 [33][34][35].…”

Section: A Determination Of Initial Conditionsmentioning

confidence: 99%

Gas dynamics and vorticity generation in laser-induced breakdown of air

Dumitrache¹,

Yalin²

2020

Opt. Express

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Research has shown that the ignition characteristics of laser induced plasmas in fuel-air mixtures are influenced by the gas dynamics effects induced during the gas breakdown stage. Here, we present numerical modeling of the fluid mechanics induced by breakdown (plasma formation) from a nanosecond near-infrared (NIR) laser pulse in air. The simulations focus on the post-discharge kernel dynamics with the goal of developing a better understanding on how vorticity is generated during the kernel cooling phase. Initial conditions (ICs) of kernel shape, temperature and pressure (corresponding to the end of the laser pulse) are found from experimental Rayleigh scattering data. It is shown that this method for determining ICs is preferred versus use of the Taylor-Sedov blast wave theory as it provides a more accurate description of the starting field. Past experimental observations have revealed that the gas dynamics of nanosecond laser sparks typically lead to the formation of an asymmetric torus with a frontal lobe propagating towards the laser source. We show that the development of the asymmetric torus is governed by strong vorticity generated through baroclinic torque arising from the blast wave that forms at the kernel boundary. Initially, the blast takes the shape of the teardrop kernel but then evolves into a spherical front during the first ~10 µs because the blast wave strength varies along its circumference. This spatial variation leads to a misalignment between the pressure and density gradients and generation of vorticity by baroclinic torque. Ultimately, the observed flow-field is dictated by how the energy was initially deposited around the beam waist during breakdown. As such, one can tailor the aerodynamics induced during the cooling and recombination phase by controlling the energy deposition profile.

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Section: A Determination Of Initial Conditionsmentioning

confidence: 99%

Gas dynamics and vorticity generation in laser-induced breakdown of air

Dumitrache¹,

Yalin²

2020

Opt. Express

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“…This decoupling of plasma formation steps allows access to a larger space of plasma parameters, effectively providing a means of tailoring the plasma temperature and density. For example, while typical single-pulse laser sparks in air are fully ionized with temperatures in excess of 10,000K 36 , 37 , the dual-pulse technique allows generation of plasmas with varying ionization fraction (between ~0.0001 and 0.01) within a wide range of temperatures (between ~400 and 10,000 K) 37 . The use of pre-ionization for laser ignition of methane-air mixtures was also demonstrated by Michael et al .…”

Section: Introductionmentioning

confidence: 99%

Control of Early Flame Kernel Growth by Multi-Wavelength Laser Pulses for Enhanced Ignition

Dumitrache

VanOsdol

Limbach

et al. 2017

Sci Rep

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The present contribution examines the impact of plasma dynamics and plasma-driven fluid dynamics on the flame growth of laser ignited mixtures and shows that a new dual-pulse scheme can be used to control the kernel formation process in ways that extend the lean ignition limit. We perform a comparative study between (conventional) single-pulse laser ignition (λ = 1064 nm) and a novel dual-pulse method based on combining an ultraviolet (UV) pre-ionization pulse (λ = 266 nm) with an overlapped near-infrared (NIR) energy addition pulse (λ = 1064 nm). We employ OH* chemiluminescence to visualize the evolution of the early flame kernel. For single-pulse laser ignition at lean conditions, the flame kernel separates through third lobe detachment, corresponding to high strain rates that extinguish the flame. In this work, we investigate the capabilities of the dual-pulse to control the plasma-driven fluid dynamics by adjusting the axial offset of the two focal points. In particular, we find there exists a beam waist offset whereby the resulting vorticity suppresses formation of the third lobe, consequently reducing flame stretch. With this approach, we demonstrate that the dual-pulse method enables reduced flame speeds (at early times), an extended lean limit, increased combustion efficiency, and decreased laser energy requirements.

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“…However, once formed, the NIR plasma is found to be almost an order of magnitude denser as compared to the (lower www.nature.com/scientificreports/ pulse energy) REMPI case, for example n e = (8.3 ± 1.3) × 10 17 cm -3 at 110 ns after the pulse. These results are largely explained by the different mechanism involved in non-resonant breakdown at 1064 nm where most of the free-electrons are generated by electron avalanche ionization (as opposed to MPI or REMPI) which also results in a stronger threshold behavior 19 . For the dual-pulse REMPI case, the resulting plasma is similarly dense as compared to the single-pulse NIR pulse but using approximately 2.5 times less total pulse energy, i.e.…”

Section: Electron Density Of Dual-pulse Rempi Plasmas 1-d Rayleigh Amentioning

confidence: 99%

Resonant dual-pulse laser ignition technique based on oxygen REMPI pre-ionization

Dumitrache

Butte

Yalin

2020

Sci Rep

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This contribution investigates a novel laser ignition method based on a dual-pulse resonant pre-ionization scheme. The first laser pulse efficiently creates initial gas ionization (seed electrons) through a 2 + 1 resonantly-enhanced multiphoton ionization (REMPI) scheme targeting molecular oxygen (λ ~ 287.6 nm). This pulse is followed by a second non-resonant near-infrared pulse (λ = 1064 nm) for energy addition into the gas via inverse bremsstrahlung absorption. The sequence of two pulses creates a laser induced plasma that exhibits high peak electron number density and temperature (ne ~ 8 × 1017 cm-3 at t = 100 ns and T ~ 8000 K at t = 10 μs, respectively). These plasma parameters are similar to those attained for typical single-pulse near-infrared laser plasmas but with the advantage of substantially lower pulse energy (by factor of ~ 2.5) in the dual-pulse REMPI case. A combustion study focusing on ignition of propane/air mixtures shows that the dual-pulse REMPI method leads to an extension of the lean flammability limit, and an increase in combustion efficiency near the lean limit, as compared to laser ignition with a single NIR pulse. The measurement results and observed gas dynamics are discussed in the context of their impact on combustion applications.

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Threshold characteristics of ultraviolet and near infrared nanosecond laser induced plasmas

Cited by 33 publications

References 34 publications

Gas dynamics and vorticity generation in laser-induced breakdown of air

Gas dynamics and vorticity generation in laser-induced breakdown of air

Control of Early Flame Kernel Growth by Multi-Wavelength Laser Pulses for Enhanced Ignition

Resonant dual-pulse laser ignition technique based on oxygen REMPI pre-ionization

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