Author: Publisher:
ISBN:
Category :
Languages : en
Pages : 24
Book Description
Numerical Simulations of Ultrashort Laser Pulse Ablation and Plasma Expansion in Ambient Air
Author: Numerical Model of Laser Ablation and Plasma Expansion for Ultrashort Laser Pulses
Author: Kinetic and Hydrodynamic Simulations of Laser Ablation and Plasma Plume Expansion Induced by Bursts of Short Laser Pulses
Author: Omid A. RanjbarPublisher:
ISBN:
Category : Electronic dissertations
Languages : en
Pages :
Book Description
Ablation of materials by nanosecond laser pulses involves expansion of a laser-induced vapor plume into a background gas. The absorption of the incident laser radiation by the plume can substantially decrease the amount of laser energy absorbed directly by the target, and, correspondingly, the amount of the ablated material. This plasma shielding effect limits the overall efficiency of industrial laser systems designed for material removal applications. The goal of the present work is to numerically study the expansion process of plumes induced by irradiation of a metal target by bursts or groups of nanosecond laser pulses and to reveal the implications of the interaction between plumes induced by individual pulses for the efficiency of material removal. The plume expansion induced by irradiation of a copper target in argon background gas is studied based on one- and two-dimensional hybrid computational models that include a hydrodynamic or kinetic model of plasma plumes. The hydrodynamic model is based on finite-difference solution of gas dynamics equations. The kinetic model is implemented in the form of the direct simulation Monte Carlo (DSMC) method. In this work, the generalization of the DSMC method for plasma flows is developed. The effects of laser fluence, spot size, inter-pulse separation, and background gas pressure are thoroughly studied. The numerical simulations of plume expansion induced by a burst of pulses indicate the formation of complicated flow structures with cascades of the primary and secondary shock waves and strong interaction between plumes induced by individual pulses. The simulations reveal the plume accumulation effect when the plumes induced by preceding pulses in a burst change conditions of propagation of plumes generated by subsequent pulses. The degree of plasma shielding increases with increasing number of laser pulses due to the plume accumulation effect. It results in reduction of the effectiveness of material removal by the subsequent pulses. The degrees of the plasma shielding and plume accumulation effects strongly depend on the inter-pulse separation and laser spot size. The trade-off between the plume accumulation and thermal accumulation effects maximizes the ablation depth per pulse at a certain value of the time delay between pulses.
Ultrafast dynamics of melting and ablation at large laser intensities
Author: Ilja MingareevPublisher: Cuvillier Verlag
ISBN: 3736928785
Category : Science
Languages : en
Pages : 158
Book Description
Diese Arbeit leistet einen Beitrag zum Verständnis der ultraschnellen Abtrags- und Schmelzphänomene von Festkörpern bei Anregung mit Laserstrahlung großer Intensität. Fundamentale Aspekte des laserinduzierten Abtrags von Reinmetallen (Au, Al, Cu, Fe, W) mit Ultrakurzpuls-Laserstrahlung wie z.B. Laser-Materie-Wechselwirkung, Plasmabildung, Verdampfung und Schmelzdynamik wurden untersucht. Darüber hinaus wurde Schmelzen und Schweißen von technischem Borosilikatglas mittels hochrepetierender Ultrakurzpuls-Laserstrahlung untersucht. Für Untersuchungen der transienten laserinduzierten Vorgänge auf unterschiedlichen Zeitskalen wurden neuartige experimentelle Verfahren entwickelt und eingesetzt. Pumpprobe Photographie wurde für zeitaufgelöste Messungen auf einem erweiterten zeitlichen Detektionsbereich bis ca. 2 Mikrosekunden mit Sub-Pikosekunden Auflösung realisiert. Für Detektion von transienten Brechungsindexmodifikationen und Morphologieänderungen wurde ein neuartiges, zeitaufgelöstes Verfahren zur quantitativen Phasenmikroskopie (TQPm) entwickelt. Die geometrischen und zeitlichen Profile der eingesetzten Laserstrahlung großer Intensität wurden beim Abtragen von Metallen untersucht. Aufheizung des Materials bedingt durch spontane verstärkte Emission mit Pulsdauer im Nanosekundenbereich führt zu einem materialabhängigen Temperaturanstieg von mehreren hundert Kelvin und wurde numerisch untersucht. Zeitaufgelöste Schattenphotographie und quantitative Messungen des Abtragsvolumens von Metallen wurden in unterschiedlichen Umgebungen durchgeführt. Im untersuchten zeitlichen Detektionsbereich kann die beobachtete Abtragsdynamik in mindestens vier charakteristischen Zeitregimes klassifiziert werden: Ausbreitung von dichtem Materialdampf und Plasma, Verdampfung aufgrund der Nukleationseffekte, Abtrag in Form von flüssigen Schmelzstrahlen und Erstarrung. Basierend auf experimentellen Ergebnissen wurde ein qualitatives Modell für laserinduzierten Abtrag von Metallen bei großen Strahlungsintensitäten aufgestellt, welches bedeutende Unterschiede zum Abtragen bei schwellennahen Intensitäten aufweist. Insbesondere sind physikalische Vorgänge die im Zusammenhang mit Materieüberhitzung stehen wie z.B. Phasenexplosion und “boiling crisis”, als entscheidende Abtragsphänomene suggeriert worden. Laserinduziertes Schmelzen von technischem Borosilikatglas mit hochrepetierender Ultrakurzpuls-Laserstrahlung wurde mittels TQPm zeitaufgelöst untersucht. Experimentelle Ergebnisse weisen transiente Brechungsindexmodifikationen auf welche auf Ionisationsprozesse und Verdichtung der Materie zurückzuführen sind. Als eine wichtige Anwendung dieser Prozesse wurde das Mikroschweißen von dünnen Glas- und Silizium-Platten demonstriert. Beim Schmelzen von Material an Substrat-Grenzflächen können konsistente Schweißnähte im Mikrometerbereich erzeugt werden.
Numerical Simulation of Ultra-short Laser Pulse Energy Deposition and Transport for Material Processing
Author: Photon Processing in Microelectronics and Photonics
Author: Simulation of Laser-tissue Thermal Interaction and Plasma-mediated Ablation
Author: Jian JiaoPublisher:
ISBN:
Category : Laser ablation
Languages : en
Pages : 156
Book Description
In this dissertation, two types of laser-tissue interactions - ultra-short pulsed (USP) laser-tissue thermal interaction and plasma-mediated ablation - are numerically investigated. The thermal interaction is focused on laser hyperthermia skin cancer treatment. A combined ultrafast radiative transfer and bioheat transfer model is developed to predict the temperature distribution in a 2-D axisymmetric skin tissue cylinder subjected to a train of short pulsed irradiations. An appropriate mathematical model to describe the focused laser beam transport is considered. The laser propagation in the tissue is simulated via solving the transient equation of radiative transfer with the Transient Discrete-Ordinates Method. The bioheat transfer equation is calculated by the Alternative Directional Implicit Method. A numerical scheme is introduced to deal with the multi-time-scale heat transfer problems in a three-layer model skin tissue exposed to multiple short laser pulses. Parametric studies including influences of lens diameter, laser focused depth, wavelength and laser power on killing different types of skin tumors are carried out. For the study of USP laser induced plasma-mediated ablation in transparent media, an ionization rate equation is adopted, which consists of the multiphoton and avalanche ionizations, and diffusion and recombination effects. The present 2-D axisymmetric numerical model is validated via extensive comparisons with experimental measurements and other 1-D numerical models available in the literature. The dynamic process of plasma formation for ablation in pure water can be better visualized by the current modeling. To determine plasma-mediated ablation threshold, we propose that a certain number of free electrons are required to trigger the avalanche ionization. Based on this assumption, the ablation thresholds for pulse widths down to the femtoseconds range at wavelength 800 nm are calculated for transparent corneal epithelium. It is found that the critical seed free electron-density decreases as the pulse width increases, obeying a tp-5.65 rule. A complete numerical model with all ionization and loss mechanisms is also employed to elucidate and validate the conditions when avalanche ionization occurs and dominates in the plasma-mediated ablation process. Moreover, the crater sizes ablated in a PDMS by a 900 fs pulsed laser at wavelength 1552 nm are modeled using the present model, and the results match with the existing experimental measurements. The study of plasma-mediated ablation is further extended to turbid media. For ablation in absorbing and highly-scattering biological tissues, combined modeling of ultrafast radiative transfer and plasma formation is needed. We studied plasma-mediated ablation in a model human skin tissue, in which the rate equation is modified with chromophore ionization in addition to the multiphoton and avalanche ionizations. A parametric study on the influences of band gap, critical free-electron density and recombination rate on the prediction of ablation threshold in the model skin tissue is carried out. Moreover, investigation on crater formation is conducted, and the modeled crater sizes are compared with available experimental measurements on corundum and sapphire materials. For ablation in corundum and sapphire, the free electrons are generated via photoionization (multiphoton and tunneling ionizations) and avalanche ionization. The onset of mass ablation in the material is assumed when the local absorbed laser energy is sufficient to cause evaporation. Good agreements between the modeling and experiments are observed.
Modeling of Plasma Dynamics During Pulsed Electron Beam Ablation of Graphite
Author: Muddassir AliPublisher:
ISBN:
Category :
Languages : en
Pages :
Book Description
Recent advances in the field of plasma nanofabrication suggest that plasma-based technologies may replace many of the conventional chemical and thermal routes in the synthesis of nanomaterials (with at least one dimension below 100 nm) and thin films. In contrast to the conventional processing routes, where only neutral species are involved, a plasma is made up of energetic species including ions, electrons, and excited molecules in addition to neutrals. Due to the highly energetic nature of interactions among these species and with other surfaces (substrates), a plasma allows for the formation of materials at higher rates even though their concentrations might be low as compared with those of neutral species in non-plasma based methods. While the mechanisms of the various interactions in a plasma are undoubtedly complex and require a fundamental understanding, they offer new opportunities for material nanofabrication. Pulsed electron beam ablation (PEBA) has recently emerged as a novel and promising technique for high quality thin films growth. Pulsed electron beam film deposition consists of many physical processes including target material heating, target ablation, plasma plume expansion, and film growth on a substrate. Electron beam ablation is a complex process, which comprises heating, phase change, and removal of a fine fraction from the target surface. Ablation strongly affects the space distribution, composition, mass transfer processes, which in turn has a critical bearing on the structure, stoichiometry and properties of thin films. Plasma plume expansion into an ambient gas is a fundamental issue in PEBA as the quality of thin films deposited onto the substrate depends on the composition, energy and density of particles ejected from the target. A one-dimensional heat conduction model is presented to investigate the heating and ablation of a graphite target upon interaction with a polyenergetic electron beam. The effect of electron beam efficiency, power density, accelerating voltage, and Knudsen layer just above the target surface during ablation are taken into account in the model. Phase transition induced during ablation is considered through the temperature dependent thermodynamic properties of graphite. The temperature distribution, surface receding velocity, melting depth, ablation depth, and ablated mass per unit area are numerically simulated. Upon ablation, plasma expansion, induced by interaction of a nanosecond electron beam pulse (~100 ns) with a graphite target in an argon atmosphere at reduced pressure, was investigated by solving gas-dynamics equations. The spatiotemporal profiles of the temperature, pressure, velocity, and density of the plasma plume are numerically simulated for a beam efficiency of 0.6 and accelerating voltage of 15 kV. Each model is validated by comparing some of the obtained simulation results with experimental data available in the literature.
Pulsed Laser Ablation
Author: Ion N. MihailescuPublisher: CRC Press
ISBN: 1351733532
Category : Science
Languages : en
Pages : 564
Book Description
Pulsed laser–based techniques for depositing and processing materials are an important area of modern experimental and theoretical scientific research and development, with promising, challenging opportunities in the fields of nanofabrication and nanostructuring. Understanding the interplay between deposition/processing conditions, laser parameters, as well as material properties and dimensionality is demanding for improved fundamental knowledge and novel applications. This book introduces and discusses the basic principles of pulsed laser–matter interaction, with a focus on its peculiarities and perspectives compared to other conventional techniques and state-of-the-art applications. The book starts with an overview of the growth topics, followed by a discussion of laser–matter interaction depending on laser pulse duration, background conditions, materials, and combination of materials and structures. The information outlines the foundation to introduce examples of laser nanostructuring/processing of materials, pointing out the importance of pulsed laser–based technologies in modern (nano)science. With respect to similar texts and monographs, the book offers a comprehensive review including bottom-up and top-down laser-induced processes for nanoparticles and nanomicrostructure generation. Theoretical models are discussed by correlation with advanced experimental protocols in order to account for the fundamentals and underline physical mechanisms of laser–matter interaction. Reputed, internationally recognized experts in the field have contributed to this book. In particular, this book is suitable for a reader (graduate students as well as postgraduates and more generally researchers) new to the subject of pulsed laser ablation in order to gain physical insight into and advanced knowledge of mechanisms and processes involved in any deposition/processing experiment based on pulsed laser–matter interaction. Since knowledge in the field is given step by step comprehensively, this book serves as a valid introduction to the field as well as a foundation for further specific readings.
Numerical Simulation of Ultrashort Laser Pulses Using the Smoothed Pseudo Wigner Distribution
Author: Ramon Maximilian SpringerPublisher:
ISBN: 9783843946162
Category :
Languages : en
Pages :
Book Description