Femtosecond Intense Laser-induced Ablation Ignition And Its Sensing Detection

According to the development goal of China’s double carbon policy,clean and efficient use of energy is the key to energy conservation and emission reduction.Leanburn combustion technology has the potential for high combustion efficiency and low pollutant emissions.Under lean-burn conditions,the traditional spark ignition technology needs to increase the ignition voltage to ensure a stable breakdown,which would shorten lifespan of spark plug,reduce the reliability of ignition system and increase the probability of misfire.Laser ignition has attracted extensive attention because of its advantages such as reliable ignition ability under lean-burn conditions,non-invasive electrodes,and flexible control of ignition timing and position.At present,the nanosecond laser-induced plasma ignition has been mostly adopted in laser ignition,but due to the instability of the breakdown process,the potential misfire is inescapable,and the minimum ignition laser pulse energy(MPE)is generally up to tens of millijoules,which restrict its engineering application.Recently,femtosecond laser ignition has made breakthroughs.Due to the femtosecond laser nonlinear filamenting propagation,the high-intensity laser filament establishes a high-density free radical pool,and the longitudinally extended in filament core also gives rise to simultaneous ignition along the filament line,which help achieve an ultra-low MPE at sub-millijoule with 100% ignition success rate in the lean methane/air mixture.However,further reducing the MPE of femtosecond laser ignition and extending the ignition distance is of great significance to promote its engineering application under various actual working conditions.In addition,as a non-invasive,fast-response and high-sensitivity diagnostic technology,optical sensing detection technology has been widely used in environmental detection,biomedical and industrial manufacturing,especially in the field of combustion diagnosis,which has been used for measuring various combustion parameters such as species concentration,temperature distribution,equivalence ratio.In this thesis,we propose a scheme of femtosecond intense laser-induced ablation ignition,and investigate the characteristics and mechanism of femtosecond intense laser-induced ablation ignition by using optical sensing detection technology.The main research contents are listed as follows:Firstly,a femtosecond laser with a central wavelength of 800 nm and a pulse width of 45 fs was employed to generate filament ablated a tantalum target,which was used to perform the ignition experiment in a methane/air mixture flow.The ignition success rate is characterized by measuring the flame fluorescence image after each fs laser pulse ablation.It is proved that the femtosecond intense laser-induced ablation ignition is feasible,and the technology can achieve 100% ignition success rate with ultra-low 1m J laser pulse energy,which is about 50% lower than femtosecond laser ignition.Secondly,during the process of femtosecond intense laser-induced ablation ignition,the plasma components were detected by fs filament-induced nonlinear spectroscopy technology,and the dynamic evolution of flame kernel and optical emission spectroscopy spectra were measured by time-resolved spectroscopy and imaging.It was found that the filament contained a large number of active radicals such including OH and O,and the high-energy plasmas generated by the laser breakdown target coupled the laser energy into the free radicals through collision,which promote the continuous combustion chemical reaction,and finally forming the flame kernel.Therefore,the mechanism of femtosecond intense laser-induced ablation ignition is revealed.Thirdly,a lens with focal length of 1 m is used to generate an about 70-mm-long single filament,and we found that the effective ablation position for robust ignition was about 40 mm in the whole length of filament,that is,this ablation ignition technology is not sensitive to the interaction position between the filament and target surface.Meanwhile,we have also exhibited its remote ignition ability.Fourth,the damage of filament to the ablation target was recorded by scanning electron microscope and ultra-depth three-dimensional microscope,revealing that the damage of the target ablated by fs filament was tiny(only about 80 ng/pulse),which was less than one hundredth of that ablated by the nanosecond laser.The target can maintain the robust ignition ability for thousands of ablation events,and thus the target is not required to keep moving and replace frequently in laser ignition system,and its lifetime is greatly extended.