published_or_final_version / Mechanical Engineering / Doctoral / Doctor of Philosophy

published_or_final_version / abstract / toc / Mechanical Engineering / Master / Master of Philosophy

Mitigating space debris with lasers is investigated as a possible mechanism for contactless space debris deflection in Low Earth Orbit (LEO). This deflection mechanism can be carried out by irradiating the space debris surface with a high-power laser beam. The energy absorbed by the surface of the debris, from the laser beam, sublimates the irradiated surface, transforming it to gas from solid. The ablated material is formed into a plume of ejecta, which acts against the orbital debris if the laser is beamed in the right direction; it produces a small push or thrust that deflects the debris by reducing its orbital velocity, altitude and eventually its lifetime in orbit. This approach could also be used to push space debris away from operational satellites paths. Laser ablation depends mainly on understanding the physical properties of both, the laser beam and the space debris. The interaction process for three different commonly used spacecraft materials are illuminated by a laser beam and investigated both experimentally and by using theoretical models. Experimental results and theoretical verifications are employed to evaluate the feasibility of the ablation model and to understand its performance in producing an effective deflection of space debris. This was investigated using Nd3+ Glass laser pulses with three metals: nickel (Ni), aluminium (Al) and copper (Cu). The Nd3+ Glass laser operated at a wavelength of 1.06 μm that provided intensities just below the threshold for plasma formation. This interaction produces surface power intensities ranging between one GW/m2 to one TW/m2, which produces high order temperature gradients that cause non-equilibrium energy transport phenomenon. This phenomenon cannot be explained by classical theories. The results have been used for the enhancement of the ablation model. Additional enhancements included the temperature penetration in the target surface. The surface temperature transients of metals due to laser interaction have also been investigated, and heat transfer is simulated by utilising a kinetic particle model, which captures the dominant energy transport processes. This model of energy transport permits determination of the significant decline in temperature gradients and the non-equilibrium conditions that occur between the Fermi surface conduction electrons and lattice phonons. This results in an accurate temperature distribution calculation within the space debris. The laser pulse specification and the properties of the space debris material were specified for simulation. The kinetic model has been used to simulate the spatial temperature distribution growth in the space debris when illuminated with a 1.06 μm wavelength Nd3+ Glass laser. The evaporation physics are also incorporated into the kinetic model. The average mass flow rate has been evaluated. A critical difference has been discovered between the experimental results and the predicted results using the classical Fourier Theory. The experimental data of the target surface temperatures are compared with Fourier and electron Kinetic theories. The experimental results validate the theoretical results and model improvements. It also illustrated the inaccuracy of Fourier theory regarding its solution of steep energy gradients and its failure to illustrate the non-equilibrium energy transport state, which grows between electrons and lattice phonons. It was noticed that the electron Kinetic theory results provide sufficient agreement with the experimental results below the boiling point and give a much better model than Fourier theory above the boiling temperature. The enhancements have permitted the laser specifications and the performance of the ablation treatment to be characterised. The performance of orbital debris mitigation with pulsed lasers outperformed alternative techniques that can produce a small contactless push on space junk. This method avoids sending complicated spacecraft into orbits to take space debris away from Earth orbits. The laser power that is required to reduce the altitude and the orbital velocity of space debris were predicted and calculated theoretically. The performance has been assessed by its capability to move small debris, centimetre size, by at least a couple of m/s. The results confirmed the possible benefits of using lasers to mitigate space debris in LEO. Employing current technologies together with a high Technology Readiness Level (TRL), an affordable and compact laser system could be successfully constructed and attached to traditional artificial satellites as a space-based laser system. Such a system could demonstrate the method, synergies and techniques of laser ablation. Mission complexity and the extra mass are saved by the direct debris ablation process, which can operate at a relatively small distance compared to a ground-based laser system. The analysis thus confirms the feasibility of utilising space-based laser systems and the applicability of the model's experimental validation.

Many current technologies used in solar cell fabrication have been successfully adapted from the integrated circuits industry. The success of laser processing applications in this industry indicates that such techniques should be considered to reduce manufacturing costs and to improve electrical efficiency of solar cells. This thesis examines the application of advanced laser processing to improve the electrical performance and reduce manufacturing costs of solar cells. It focuses on several different aspects of laser processing; (1) understanding and characterising the effect of laser pulses on silicon, (2) developing new fabrication technologies and (3) integrating laser processes with working solar cell devices. The thesis begins with an overview of existing solar cell structures that incorporate laser processing. A study is then presented that explores the detrimental effects of laser processing, how it can be avoided and how to characterise its influence on solar cell electrical properties. Experimental results combine Yang defect etching, photoconductance decay measurements and a new technique of photoluminescence imaging to isolate the influence of laser ablation and laser melting on silicon wafers. This understanding is used in the development of several laser processes. A laser texturing technique is developed to texture the surface of multicrystalline wafers that cannot be effectively textured with the alkaline etches used on single crystal material. Three advanced laser contacting schemes; laser micro contacts, laser defined aluminium electrodes and laser doping, are assessed as techniques to improve cell efficiency and to reduce fabrication costs. In the final chapter the integration of laser processing with solar cell devices is demonstrated through the fabrication and characterisation of n-type double-sided solar cells with laser doped contacts. Efficiencies of up to 17.4% with an open circuit voltage of 672 mV are reported. This thesis also presents the application of a new characterisation technique, based on photoluminescence, to aid in improving both new and existing fabrication technologies. The work presented in this thesis demonstrates the applicability of advanced laser processing to solar cell fabrication and shows how laser processes can be used in a variety of ways to improve the electrical performance and reduce the fabrication complexity of solar cell devices.

Thesis (Ph. D.)--University of Texas at Austin, 2002. / Vita. Includes bibliographical references. Available also from UMI Company.

Not available / text

published_or_final_version / Mechanical Engineering / Master / Master of Philosophy

published_or_final_version / Mechanical Engineering / Doctoral / Doctor of Philosophy

published_or_final_version / Mechanical Engineering / Doctoral / Doctor of Philosophy

Not available / text