Ciprian Dumitrache

Laser plasma formation assisted by ultraviolet pre-ionization

We present experimental and modeling studies of air pre-ionization using ultraviolet (UV) laser pulses and its effect on laser breakdown of an overlapped near-infrared (NIR) pulse. Experimental studies are conducted with a 266 nm beam (fourth harmonic of Nd:YAG) for UV pre-ionization and an overlapped 1064 nm NIR beam (fundamental of Nd:YAG), both having pulse duration of ∼10 ns. Results show that the UV beam produces a pre-ionized volume which assists in breakdown of the NIR beam, leading to reduction in NIR breakdown threshold by factor of >2. Numerical modeling is performed to examine the ionization and breakdown of both beams. The modeled breakdown threshold of the NIR, including assist by pre-ionization, is in reasonable agreement with the experimental results.

High Power Spark Delivery System Using Hollow Core Kagome Lattice Fibers

This study examines the use of the recently developed hollow core kagome lattice fibers for delivery of high power laser pulses. Compared to other photonic crystal fibers (PCFs), the hollow core kagome fibers have larger core diameter (~50 µm), which allows for higher energy coupling in the fiber while also maintaining high beam quality at the output (M2 = 1.25). We have conducted a study of the maximum deliverable energy versus laser pulse duration using a Nd:YAG laser at 1064 nm. Pulse energies as high as 30 mJ were transmitted for 30 ns pulse durations. This represents, to our knowledge; the highest laser pulse energy delivered using PCFs. Two fiber damage mechanisms were identified as damage at the fiber input and damage within the bulk of the fiber. Finally, we have demonstrated fiber delivered laser ignition on a single-cylinder gasoline direct injection engine.

Laser Plasma Formation in Air Using Dual Pulse Pre-Ionization

This contribution details an experimental and theoretical study of tailoring laser plasma temperature using two overlapped laser pulses of nanosecond duration. The method utilizes an ultraviolet pulsed laser beam to achieve pre-ionization, followed by a second, nearinfrared, superimposed pulse to provide energy addition in a controllable fashion through electron avalanche ionization (EAI). Experiments were conducted in atmospheric pressure air. It was observed during experiments, and through numerical simulations, that the preionization beam influences the breakdown threshold of the overlapped second beam. A subsequent study used laser Schlieren images as a diagnostic tool to aid the understanding of the temporal and spatial evolution of the breakdown kernel. Schlieren images of the overlapped ultraviolet and near-infrared beams were obtained at various time-delays for both breakdown and below breakdown conditions.

Threshold characteristics of ultraviolet and near infrared nanosecond laser induced plasmas

The present contribution compares the energy absorption, optical emission, temperature, and fluid dynamics of ultraviolet (UV) λ = 266 nm and near infrared (NIR) λ = 1064 nm nanosecond laser induced plasmas in ambient air. For UV pulses at the conditions studied, energy absorption by the plasmas increases relatively gradually with laser pulse energy starting at delivered energy of E ∼ 8 mJ. Corresponding measurements of plasma luminosity show that the absorption of UV radiation does not necessarily result in visible plasma emission. For the NIR induced plasmas, the energy absorption profile is far more abrupt and begins at ∼55 mJ. In contrast with UV, the absorption of NIR radiation is always accompanied by intense optical emission. The temperatures of both types of plasma have been measured with Rayleigh scattering thermometry (at times after the Thomson signal sufficiently diminishes). The UV plasmas can attain a wider range of temperatures, including lower temperatures, depending on the pulse energy (e...

Development of a Photonic Crystal Fiber Delivery System for Laser Ignition in Engines

We report the use of hollow core kagome lattice Photonic Crystal Fibers (PCFs) for fiberoptic delivery of laser induced sparks and engine ignition. Beam quality characterizations of the kagome fiber using 1064 nm light showed near single-mode output with M=1.2 at low power and M=1.4 at high power. High power characterizations showed maximum pulse energy (before onset of damage) of 12 mJ for pulse durations of 12 ns, and 30 mJ for pulse durations of 30 ns. These energies should be sufficient for laser ignition of even lean fuel-air mixtures. Finally, we have developed an initial prototype of a practical laser delivery system using 3 m length of coiled kagome fiber. We have demonstrated operation of a gasoline diesel injection (GDI) single-cylinder engine with 100% ignition reliability and coefficient of variation of the indicated mean effective pressure of 1.84%.

Laser-Induced Heating Using a Non-Resonant Dual-Pulse Approach with Application to Laser Ignition

The present paper addresses laser ignition using a dual pulse pre-ionization technique. The approach uses an initial ultraviolet (UV) laser pulse at 266 nm to provide pre-ionization (but not full breakdown) along with an overlapped near-infrared (NIR) pulse at 1064 nm to add energy to the pre-ionized gas. Experiments to characterize the UV pre-ionization and pulse overlap are conducted in atmospheric pressure air. Rayleigh scattering is used to measure the air temperature in the focal region of the pulses. At intensities slightly lower than the breakdown threshold, the UV pulse increases the air temperature from ~295 K to ~600 K. Overlapping of the NIR pulse provides additional temperature increase of 10s of Kelvin. The UV pulse is also observed to create weak optical emission from the focal volume even at intensities below the breakdown threshold.

Laser Generated Plasma Using a Dual Pulse Approach with Application to Laser Ignition

The present paper discusses an alternative approach to laser induced ignition using a dual pulse pre-ionization technique. The method uses a 266nm UV laser pulse to achieve initial gas pre-ionization followed by an overlapped 1064 nm NIR pulse for energy addition into the pre-ionized gas. The experimental and numerical studies presented here investigate the requirements for achieving ignition conditions using the overlapped pulses configuration without optical breakdown. Results have shown that the UV pulse is effective in achieving gas pre-ionization thus allowing good energy absorption through the subsequent NIR pulse. Even at a low intensity of the 266 nm pulse of ~1 GW/cm, the breakdown requirement for the 1064 nm beam drops ~25%, while 266 nm pulses of ~5 GW/cm lead to a drop of ~50%. Rayleigh scattering has been employed for measuring the gas temperature increase due to the two laser pulses with preliminary results showing increase of ~200 K.

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

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.

Laser Ignition of Methane-Air Mixtures: An Investigation of the Lean Limit and Minimum Ignition Energy

This study examines the laser ignition of methane-air mixtures in a Rapid Compression Machine (RCM). The lean limit and the Minimum Ignition Energy (MIE) were investigated in the engine-like conditions provided by the RCM.

Laser Ignition of Methane-Air Mixtures with a Rapid Compression Machine

We present here a study of laser ignition in a rapid compression machine (RCM). Laser induced ignition of natural gas-air mixtures at various test conditions was investigated. An Nd:YAG laser (1064 nm, 12 ns pulse duration) was employed as the laser source. Methaneair mixtures ranging from stoichiometric conditions to equivalence ratio of 0.4 were ignited using the laser as part of a lean limit study where the fuel-energy was kept constant for all cases. A study of minimum laser spark energy (MSE) and minimum ignition energy (MIE) was also conducted. We show that both MSE and MIE exhibit a stochastic behavior and, as a consequence, results can only be interpreted statistically. For our conditions, we found MSE at 90 % probability of MSE90=2.3 mJ. The MIE study was conducted at an equivalence ratio of φ=0.4 yielding an MIE (90% probability) of MIE90=7.2 mJ.