Effect of colliding plasmas dynamics, evolution, and stagnation on carbon molecular formation
The major theme of this dissertation is to investigate the dynamics of expanding laser ablation plumes generated in vacuum as well as in the presence of an ambient gas with a special emphasis on the fascinating field of colliding laser plasmas. In order to understand the physical nature of the mechanisms taking place during laser produced plasmas (LPP) evolution like recombination, collisional excitation, and plasma-laser interaction, time-space resolved studies offered the most logical approach. The thesis is divided into eight chapters and a brief description of each chapter is given below. Chapter 1 provides a brief introduction of LPP, its properties, and applications. The chapter also discusses the fundamental theories describing laser-materials interaction and provides a literature survey on colliding plasma. In Chapter 2, the description of experimental methods used for the present work is given. Details of the experimental set up used for the visible emission spectroscopy and optical time of flight, studies are also discussed. Chapter 3 gives a numerical model for estimating basic laser-mater interaction and plasma parameters such as surface temperature, ablation rate, laser absorption by the generated plasma and its temperature and density. Chapter 4 contains a study on the ambient gas effects on nanosecond laser-produced graphite plasma molecular dynamics formation. The results showed weak C 2 emission zone limited to very close distance to the target surface in vacuum conditions. In contrast, C2 formation in the plasma plume was profoundly enhanced in the presence of ambient gas pressure where C 2 intensity oscillations were observed in both axial and radial directions with increasing ambient gas pressure. By studying these oscillations it was concluded that recombination is the major mechanism for C2 formation. In chapter 5, spatio-temporal mappings of ionic, neutral, and molecular species were generated under varied ambient gas pressures conditions for carbon plasmas generated by 40fs pulses of 800 nm radiation from a femtosecond Ti:Sapphire laser. The results show that the molecular species spatial extension and lifetime are directly correlated to the evolution of excited ions in the presence of an ambient gas. Compared to ns graphite LPPs where carbon dimers are represented by complex intensity contours, the contours of fs laser plumes showed more uniform C2 emission. Chapter 6 of this thesis show that graphite colliding plasmas can be used for generating stagnation layer as a potential source for cluster production and as an alternative to introducing ambient gas for decelerating or controlling the plume dynamics. In this study, it was found that C2 dimer emission intensity improved significantly using the colliding scheme compared to conventional single laser ablation plumes. In chapter 7, a special colliding plasma experimental scheme was designed to study fusion reactor plasma facing component material ablation. A special experimental setup was designed where the laser is split into two perpendicular line-like beams focused onto a hemi-circular targets composed of varied elements. Single plume and stagnation layer dynamics of candidate fusion wall materials, viz., carbon and tungsten, and other materials, viz., aluminum, and molybdenum were investigated. The results highlight different characteristics of tungsten and carbon colliding plasmas under similar conditions. While tungsten plumes did not show stagnation clouds at the colliding zone, an intense stagnation layer formed from carbon colliding plumes. This stagnation layer, which may be a source of nanoparticles and aerosols generation, could limit the reactor performance by slowing down the repetition rate. (Abstract shortened by UMI.)
Harilal, Purdue University.
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