Time-resolved O- density measurements have been made in the bulk plasma of a unipolar pulsed-DC magnetron using laser photodetachment. The magnetron was operated with a titanium target at a total pressure of 1.3 Pa and a fixed oxygen-to-argon partial pressure ratio of 10%. The duty cycle was maintained at 5% but the peak on-time discharge power was varied from 120 to 720 W.For all discharge powers, both the electron (n(e)) and negative ion (n_) densities increase during the plasma on-time, with the electron density reaching a maximum at the end of this phase. In the off-time, the electron density initially decreases at a rapid rate (characteristic decay time similar to 25 mu s) for the first 50 mu s, followed by a slower rate (similar to 150 mu s) for the remainder of the off-time, however, the negative ion density continues to increase in this phase, reaching a maximum at about similar to 150 mu s after the termination of the discharge power. Both the electron and negative ion densities increase with discharge power. The maximum negative ion density was 2.5 x 10(16) m(-3) for a peak power of 720 W, corresponding to a negative ion-to-electron density ratio n_/n(e) (alpha) of about 3. In the long afterglow, this ratio reaches a maximum value of 12 as the electron density decreases faster than the negative ion density. This shows that in the afterglow the plasma is highly electronegative. (C) 2011 Elsevier B.V. All rights reserved.

The density of negative oxygen ions in the bulk plasma of a reactive dc magnetron has been determined for the first time using a combination of laser photodetachment and Langmuir probing. Experimental results are obtained for various O-2/Ar gas mixtures (0-100%), applied powers (50-600 W) and total discharge pressures (2-25 mTorr). The measurements reveal that the O- ion dominates over O-2(-) with the latter less than 2% of the total observed. Variation of the operating parameters showed clear trends in the negative ion densities with maxima observed at particular powers (200 W) and oxygen partial pressures (10% O-2). The negative ion density was found to increase with the chamber pressure and the main loss reaction for O- was determined to be ion-ion recombination with O+, O-2(+) and Ar+.In this study, the maximum negative ion density obtained was found to be 7.7 x 10(15) m(-3) at 200 W applied power, 25 mTorr total pressure and 50% oxygen partial pressure, giving the ratio of the negative ion to electron density, alpha = 1.4, indicating that the plasma is moderately electronegative. These new results show that significant concentrations of negative ions are present in the bulk magnetron plasma when operated in Ar/O-2 gas mixtures during dc sputtering. The influence of these ions on thin film growth is briefly discussed.

Using Langmuir probe-assisted laser photodetachment, the temporal evolution of the O− density was determined in the bulk plasma of a unipolar pulsed-dc magnetron. The source was operated in reactive mode, at a fixed nominal on-time power of 100 W, sputtering Ti in argon–oxygen atmospheres at 1.3 Pa pressure, but over a variation of duty cycles from 5% to 50% and oxygen partial pressures of 10% and 50% of the total pressure. In the plasma on-time, for all duty cycles the negative ion density (n −) rises marginally reaching values typically less than 2 × 1015 m−3 with negative ion-to-electron density ratios, α < 1. However, immediately after the transition from pulse on-to-off, n − falls by about 20–30% as fast O− species created at the cathode exit the system. This is followed by a rapid rise in n − to values at least 2 or 3 times that in the on-time. The rate of rise of n − and its maximum value both increase with decreasing duty cycle. In the off-time, the electron density falls rapidly (initial decay rates of several tens of μs), and therefore the afterglow plasma becomes highly electronegative, with α reaching 4.6 and 14.4 for 10% and 50% oxygen partial pressure, respectively. The rapid rise in n − in the afterglow (in which the electron temperature falls from about 5 to 0.5 eV) is attributed to the dissociative attachment of highly excited oxygen metastables, which themselves are created in the pulse on-time. At the lowest duty of 5%, the long-term O− decay times are several hundred μs. Langmuir probe characteristics show the clear signature that negative ions dominate over the electrons in the off-time. From the ion and electron saturation current ratios, α has been estimated in some chosen cases and found to agree within a factor between 2 and 10 with those obtained more directly from the photodetachment method.

Various types of optical fibres have been investigated and compared for delivering high power laser beams to an optical plug (comprising of lenses and an optical window) for the application of laser-induced ignition of gasoline and air mixtures in an automotive internal combustion engine. Three main types of optical fibre were examined: multi-mode step index silica, sapphire and photonic crystal. The fibres had various core sizes ranging from 35 to 600 mu m and numerical apertures between 0.046 and 0.64. A Q-switched Nd:YAG laser operating at the fundamental wavelength 1064 nm with a pulse length of 15 ns was used for the testing. Fibre output beam properties, including beam mode quality, output divergence, transmission losses, beam energy thresholds and effects of engine vibration were investigated. These fibre beam properties were compared with known beam parameters for laser ignition to assess the suitability of such fibres for a laser ignition system. Online fibre delivery laser ignition engine tests were performed with the most suitable fibres, which showed that combustion could be achieved with this system despite a relatively large percentage of misfires.

This work involves a study on laser ignition (LI) in an internal combustion (IC) engine and investigates the effects on control of engine combustion performance and stability of varying specific laser parameters (beam energy, beam quality, minimum beam waist size, focal point volume and focal length). A Q-switched Nd : YAG laser operating at the fundamental wavelength 1064 nm was successfully used to ignite homogeneous stoichiometric gasoline and air mixtures in one cylinder of a 1.6 litre IC test engine, where the remaining three cylinders used conventional electrical spark ignition (SI). A direct comparison between LI and conventional SI is presented in terms of changes in coefficient of variability in indicated mean effective pressure (COVIMEP) and the variance in the peak cylinder pressure position (Var(PPP)). The laser was individually operated in three different modes by changing the diameter of the cavity aperture, where the results show that for specific parameters, LI performed better than SI in terms of combustion performance and stability. Minimum ignition energies for misfire free combustion ranging from 4 to 28 mJ were obtained for various optical and laser configurations and were compared with the equivalent minimum optical breakdown energies in air.

The use of laser energy to ignite gas and liquid based fuel-air mixtures has been the subject of a number of studies and laboratory experiments at a fundamental level over the past 30 years. Yet, the practical implementation of this laser application has still to be fully realised in a commercial automotive application. Laser Ignition (LI), as a replacement for Spark Ignition (SI) in the internal combustion (IC) engines of automotive vehicles, offers several potential advantages including extending lean burn capability, reducing the cyclic variants between combustion cycles and reducing the overall ignition package costs, weight and energy requirements. The continued development of increasingly compact and efficient laser sources and new associated laser beam delivery techniques have provided the basis for significant steps forward in research towards practical proof-of-concept demonstration of LI in engines for automotive vehicles. This paper reports on some results of research recently undertaken in the Department of Engineering, University of Liverpool, in which a Q-switched Nd:YAG laser operating at 1064 nm wavelength has been used to successfully ignite and run (for extended periods) one cylinder of a 4-cylinder internal combustion (IC) test engine. The variation of several laser parameters and their effect on the engine performance are reported; namely, pulse energies of 5-20 mJ, pulse lengths of 6-15 ns and focused beam waist diameters at the combustion point of 40-100 mu m. The engine performance was measured in terms of changes in Coefficient of Variant (COV) in both Indicated Mean Effective Pressure (IMEP) and the Peak Cylinder Pressure Position (PPP). Further experiments on the focal position of laser ignition were undertaken.

Recent research in laser-induced ignition (LI) of air-fuel mixtures in internal combustion (IC) engines has shown there to be many potential advantages over conventional electrical spark ignition (SI). Spark plugs offer only limited possibilities for optimising engine efficiency, due to their fixed position within a cylinder and the protrusion of electrodes which disturb the cylinder geometry and can quench the flame kernel. Laser radiation is non-invasive and has greater flexibility in terms of the ignition position, allowing the possibility of multipoint ignition. Other potential benefits of LI include: reduced emissions, more stable combustion and lower idle speeds, when compared to conventional SI. This paper reports on the current research being undertaken at The University of Liverpool, which examines the effects of engine combustion performance and stability when specific laser parameters (beam energy, minimum spot size and focal length/volume) are varied. A Q-switched Nd:YAG laser operating at the fundamental wavelength 1064 nm was used to ignite gasoline and air mixtures in one cylinder of a 1.6 litre IC engine, where the remaining three cylinders used conventional SI. A direct comparison between LI and conventional SI is presented in terms of changes in coefficient of variability (COV) in indicated mean effective pressure (IMEP) and the variance in the peak cylinder pressure position (PPP).