External Cavity Quantum Cascade Lasers

In this thesis we deploy mid-infrared quantum cascade lasers (QCLs) in external linear cavity (EC) and external ring cavity (ERC) configurations, with the aim to develop the platform as a source for high resolution gas spectroscopy. Initially, the temporal evolution of pulsed ERC-QCL system is compared to that of standard Fabry Perot (FP) QCLs to provide insight into the role played by the additional external feedback provided by ECs and ERCs. Time-resolved spectral measurements show that external feedback promotes single longitudinal mode operation on a shorter timescale than is the case for FP-QCLs. A room temperature wavelength tunable external ring cavity QCL is presented which shows improved performance over its linear counterpart due to its ability to support unidirectional emission regimes. Spectral comparisons between linear and ring cavities show that these regimes lead to improved performance due to the suppression of spatial hole burning (SHB), an instability common to linear laser resonators. An active modulation scheme based on a number of bespoke power amplifiers and bias-tees is developed. These systems are used to produce mode-locked pulse trains in external cavity and external ring cavity QCLs by modulating the QCL gain medium’s drive current at frequencies matching the cavity’s round trip frequency. Spectral and temporal comparisons are made between these two cases. Asymmetric pulse propagation is observed in the case of the ERC-QCL which has been suggested as promising platform for ultrashort mode-locked mid-infrared pulses

Power Converters in Power Electronics

In recent years, power converters have played an important role in power electronics technology for different applications, such as renewable energy systems, electric vehicles, pulsed power generation, and biomedical sciences. Power converters, in the realm of power electronics, are becoming essential for generating electrical power energy in various ways. This Special Issue focuses on the development of novel power converter topologies in power electronics. The topics of interest include, but are not limited to: Z-source converters; multilevel power converter topologies; switched-capacitor-based power converters; power converters for battery management systems; power converters in wireless power transfer techniques; the reliability of power conversion systems; and modulation techniques for advanced power converters

The g-factor of the electron bound in ²⁸Si¹³⁺: The most stringent test of bound-state quantum electrodynamics

This thesis describes the ultra-precise determination of the g-factor of the electron bound to hydrogenlike 28Si13+. The experiment is based on the simultaneous determination of the cyclotron- and Larmor frequency of a single ion, which is stored in a triple Penning-trap setup. The continuous Stern-Gerlach effect is used to couple the spin of the bound electron to the motional frequencies of the ion via a magnetic bottle, which allows the non-destructive determination of the spin state. To this end, a highly sensitive, cryogenic detection system was developed, which allowed the direct, non-destructive detection of the eigenfrequencies with the required precision. The development of a novel, phase sensitive detection technique finally allowed the determination of the g-factor with a relative accuracy of 4 • 10−11, which was previously inconceivable. The comparison of the hereby determined value with the value predicted by quantumelectrodynamics (QED) allows the verification of the validity of this fundamental theory under the extreme conditions of the strong binding potential of a highly charged ion. The exact agreement of theory and experiment is an impressive demonstration of the exactness of QED. The experimental possibilities created in this work will allow in the near future not only further tests of theory, but also the determination of the mass of the electron with a precision that exceeds the current literature value by more than an order of magnitude

Use, Operation and Maintenance of Renewable Energy Systems:Experiences and Future Approaches

The aim of this book is to put the reader in contact with real experiences, current and future trends in the context of the use, exploitation and maintenance of renewable energy systems around the world. Today the constant increase of production plants of renewable energy is guided by important social, economical, environmental and technical considerations. The substitution of traditional methods of energy production is a challenge in the current context. New strategies of exploitation, new uses of energy and new maintenance procedures are emerging naturally as isolated actions for solving the integration of these new aspects in the current systems of energy production. This book puts together different experiences in order to be a valuable instrument of reference to take into account when a system of renewable energy production is in operation

Heat sinks based on liquid metal for power electronics cooling applications

PhD ThesisPower semiconductor devices are key components for efficient power conversion in a wide range of industrial applications. The continuous trend toward increasing the power capability and decreasing the chip area of the semiconductors results in the generation of high heat fluxes, due to the power losses. Also, power electronics are one of the most common components of the power converter to fail, as a result of the thermomechanical stress within the structure of power module caused by large junction temperature swings (∆Tj). Effective and efficient thermal management systems should therefore be employed to dissipate the excess heat to the ambient environment and reduce the thermomechanical stress. Liquid metals received little attention as heat transport agents thus far, in spite of their excellent thermophysical properties. Also, their high electrical conductivity allows for driving them with a magnetohydrodynamics (MHD) pump, which is a reliable and low–power device. Hence, a thermal management system based on Ga68In22Sn10 liquid metal coolant is able to remove high heat fluxes, requires low operating power and provides high reliability; all desirable attributes for modern power electronic applications. This thesis focuses on the design and development of a cooling system based on liquid metal for conventional insulated–gate bipolar transistors (IGBTs), which are the most widely used power electronic switches for medium–to–high power conversion applications. The proposed heat sink is attached to the IGBT power module and liquid metal is impinged directly against the baseplate with the use of an integrated MHD pump, thus eliminating the need for thermal interface material (TIM). Moreover, an adaptive thermal management method based on liquid metal flow control is presented that is able to significantly reduce ∆Tj . Also in this thesis, the design and development of a liquid metal heat sink for press–pack IGBTs (PPIs) is proposed. Traditionally, water is used for cooling PPIs in high–power applications. However, ionised particles are developed in the cooling system that contribute to the corrosion of the piping system. Therefore, the use of a thermal management system based on liquid metal increases the heat dissipation capability without corroding the cooling structure. Analytical work is performed on the design of both heat sinks and the implementation of the temperature control method. The thermal performance of both heat sinks, as well as the adaptive heat sink control, are experimentally validated