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Plasma physics is rich in phenomena, occurrences and applications: many instabilities exist due to the relatively long-range Coloumb forces that mediate constituent particle dynamics, most of the visible universe is in the plasma state, and plasmas have been used from lighting to computer chip manufacturing to (attempted) fusion power generation. Within plasma fusion research, Thomson scattering is a commonly used diagnostic. It allows the temperature and density of the plasma electrons to be measured without distorting the plasma. However, the scattering cross-section is small. Thomson scattering signals can be difficult to detect against the background emission of the plasma. In this thesis, the Thomson scattering diagnostic data from the COMPASS-D experiment is analysed. Several aspects of the diagnostic are presented along with detailed explanation of the inference procedure for determining the plasma's electron temperature. This temperature analysis was achieved by utilising a Bayesian inference model that allowed prior information about likely values to be systematically included. This prior information was found to remove the degeneracy present due to the low signal-to-noise ratio of the data. A genetic algorithm (GA) library, called ELGAR, was developed and used to solve the minimisation problem resulting from the Bayesian inference. The GA proved to be a reliable method of solving such problems. ELGAR was also used to investigate certain characteristics of the GA such as optimal choice of key parameters. These were found to be in disagreement with theoretical results but the difference was explained by the different mode of operation of ELGAR. The Thomson scattering analysis was extended to include two-temperature considerations. The set of observations consistent with an n-temperature distribution function was found to be bounded by a curve. Some data from the COMPASS-D experiment lay outside this boundary, but was bounded by a similar curve. This suggested that some systematic error had occurred. Some explanations of possible causes of this bias were suggested. Constraints were found for interpretations of any observations. These indicated that, for some observations, a set of temperatures is unavailable (as either to hotter or colder component) for a distribution function which is consistent with that observation. For certain observations, the least-squares temperature estimate is contained within the set of impossible temperatures. This indicates that the presence of a hotter species of electrons can bias the observations towards higher temperatures. The thesis concludes with a summary and a discussion of possible future work.

Selective laser sintering with femtosecond (fs) lasers and metal nanoparticles (NPs) can achieve high precision and dense sub-micron features with lower porosity, low oxidation and reduced residual stress, due to the high peak power and highly localized heat affected zone. Successful sintering of metal NPs with fs laser is challenging due to the ablation caused by hot electron effects, such as blast and Coulomb explosion. Electron firstly absorbs the laser energy and becomes a hot electron gas with a temperature of several thousand Kelvins. These hot electrons can ablate nanoparticles within a couple of picoseconds, way before the energy is passed to the lattice, and hence hinder the sintering process. In this study, we proposed a dual-step sintering strategy that deposits the laser energy into metal NPs via multiple steps to lower the electron temperature significantly and while still maintaining a high enough lattice temperature for sintering in order to avoid the ablation. To demonstrate this concept, we utilized a pair of fs laser pulses with a time delay at picosecond scale to sinter Cu NPs. The delay time is chosen to be comparable with the electron-phonon coupling time so that the highest electron temperature would not be too high to cause ablation, while the final lattice temperature is still enough for sintering. When the delay time is slightly longer than the electron-phonon coupling time of Cu NPs, it is shown that the ablation area is drastically reduced and the power window for successful sintering can be extended by about 2 times. At the same time, the heat affected zone can be reduced by 66% (area). Our results showed that comparing with sintering with single femtosecond laser pulse train, with this dual-step sintering strategy: (i) the ablation could be suppressed significantly and the sintering window could be extended by twice; (ii) the heat affected zone in the glass substrate can be reduced by more than half; (iii) there is clear improvement on surface smoothness. This new strategy can be adopted for all the SLS processes with fs laser and unlock the power of fs-SLS for future applications.

Laser-induced micro-plasma dynamics are investigated in laboratory air, ultra-high-pure hydrogen gas, and hydrogen-nitrogen gas mixtures. The dissertation focuses on atomic spectroscopy of hydrogen in the visible region. Line-of-sight measurements are analyzed to obtain spatial distributions of electron densities and excitation temperatures. The studies include evaluation of self-absorption phenomena. The plasma dynamics occur initially well above re-entry speeds and diminish to hypersonic and then supersonic expansions. Expansion velocities are measured that are above three hundred times the speed of sound in standard atmosphere. Optical breakdown is induced by using pulsed laser radiation. Emission spectra are collected by employing a spectrometer equipped with an intensified charge-coupled device. Atomic emission profiles for hydrogen alpha, hydrogen beta and hydrogen gamma lines are utilized to determine plasma characteristics such as electron density and excitation temperature for specific time delays. The duplicating mirror approach is applied for the evaluation of self-absorption. The extent of self-absorption is investigated for various time delays from plasma generation. The electron density is also determined from singly ionized nitrogen lines and compared with values obtained from the hydrogen lines to further evaluate self-absorption. Experimental records are modeled using computational physics including inversions of integral equations to infer radial distributions from line-of-sight measurements. The presented work contributes to the fundamental understanding of laser-induced breakdown spectroscopy widely utilized for analytical diagnostics of gases, liquids or solids.