GaN has the potential to revolutionize the high power electronics industry, enabling high voltage applications and better power conversion efficiency due to its intrinsic material properties and newly available high purity bulk substrates. However, unintentional impurity incorporation needs to be reduced. This reduction can be accomplished by reducing the source of contamination and exploration of extreme growth conditions which reduce the incorporation of these contaminants. Newly available bulk substrates with low threading dislocations allow for better study of material properties, as opposed to material whose properties are dominated by structural and chemical defects. In addition, very thick films can be grown without cracking due to exact lattice and thermal expansion coefficient match. Through chemical and electrical measurements, this work aims to find growth conditions which reduces contamination without a severe impact on growth rate, which is an important factor from an industry standpoint. The proposed thicknesses of these devices are on the order of one hundred microns and requires tight control of the intentional dopants. / Doctor of Philosophy / GaN is a compound semiconductor which has the potential to revolutionize the high power electronics industry, enabling new applications and energy savings due to its inherent material properties. However, material quality and purity requires improvement. This improvement can be accomplished by reducing contamination and growing under extreme conditions. Newly available bulk substrates with low defects allow for better study of material properties. In addition, very thick films can be grown without cracking on these substrates due to exact lattice and thermal expansion coefficient match. Through chemical and electrical measurements, this work aims to find optimal growth conditions for high purity GaN without a severe impact on growth rate, which is an important factor from an industry standpoint. The proposed thicknesses of these devices are on the order of one hundred microns and requires tight control of impurities.

Pulse power system is widely used for medical, industrial and military applications. The operational principle of the pulse power system is that the energy from the input source is stored in the capacitor bank or superconducting inductive device through a dc-dc converter. Then, when a discharging signal exists, the stored energy is released to the load through pulse forming network (PFN) generating high peak power pulse up to gigawatts within several tens of or hundreds of microseconds. The pulse power system has been originally developed for the defense application. After the format of the voltage compression and voltage addition stages for the short-pulse high power acceleration has been established, it has been evolved to be common. Then, its application has been extended to food processing, medical equipment sterilization and wastewater treatment since many present environmental problems have been known in the early 70's or even earlier. In addition, the pulse power system is newly spotlighted due to the recent world events. The application examples are to treat anthrax-contaminated mail, and the use of accelerators to produce high power X-rays for security screening. Furthermore, the pulse power system has been applied for the tactical weapon system such as electrothermal-chemical (ETC) gun, coilgun and active armor system. Because the pulse power system applied for the tactical weapon system has the potential to be integrated in the military vehicle, a compact, lightweight pulse power system is strongly required for the future weapon system. In this thesis, a distributed power system (DPS) for the capacitor charger is introduced for the application of the active armor system. Furthermore, a design methodology is presented for the front-end converter to achieve the high efficiency as well as the high power density. Design parameters are identified and their impact on the design result is studied. the optimal operating point is determined based on the loss comparison between different operating points. In order to further improve the power density utilizing the unique operation mode i.e. pulse power operation, transformer design using amorphous-based core is provided and the design result is compared with that using ferrite-based core. A 5 kW prototype converter is built up and the experimentation is performed to verify the design. / Master of Science

This thesis proposes control schemes and converter configurations for high power wound field synchronous motor (WFSM) drives. The model for a salient pole WFSM in any general rotating reference frame is developed which can be used to derive models along known rotor (dq) and stator flux (MT) reference frames. Based on these models, the principle of sensor-less stator flux oriented field-oriented control (FOC) for salient pole WFSM is developed. So far in the literature, control of cylindrical rotor machine only has been addressed and the effects of saliency have generally been neglected. The performance of the proposed sensor-less FOC has been demonstrated by experimentally operating a 15.8 HP salient pole WFSM using a three-level IGBT based voltage source inverter (VSI). The principle of FOC has been later extended to the control of current source load commutated inverter (LCI) fed salient pole WFSM drives, where the drawbacks present in conventional self-control method such as rigorous off-line calculation for generation of look up tables, coupling between flux and torque control etc. are eliminated. This thesis also proposes the combination of a VSI with the LCI power circuit to overcome the different disadvantages that are present in the existing LCI topology. Firstly, a novel starting scheme is proposed, where the LCI fed WFSM is started with the aid of a low power auxiliary VSI converter in a smooth manner with sinusoidal motor currents and voltages. This overcomes the difficulties of the present complex dc link current pulsing technique that has drawbacks such as pulsating torque, long starting time etc. In a second mode of operation, it is shown that the VSI can be connected to the existing LCI fed WFSM drive as a harmonic compensator in On-The-Fly mode; this will make the terminal stator current and voltage sinusoidal apart from cancellation of torque pulsations thus improving the drive performance. The above two schemes have potential as retrofit for existing drives. It is possible to combine both the advantages, mentioned above, by permanently connecting the VSI with the LCI power circuit to feed the WFSM. This proposed hybrid LCI/VSI drive can be regarded as a universal solution for high power synchronous motor drives at all power and speed ranges.

This study focuses on the design, fabrication, and characterization of deterministically engineered, three-dimensional architectures to be used as high-performance electrodes in energy storage applications. These high-surface-area architectures are created by the robotically-assisted sequential electrodeposition of structural and sacrificial layers in an alternating fashion, followed by the removal of the sacrificial layers. The primary goal of this study is the incorporation of these highly laminated architectures into the battery electrodes to improve their power density without compromising their energy density. MEMS technologies, as well as electrochemical techniques, are utilized for the realization of these high-power electrodes with precisely controlled characteristic dimensions. Diffusion-limited models are adopted for the determination of the optimum characteristic dimensions of the electrodes, including the surface area, the thickness of the active material film, and the distance between the adjacent layers of the multilayer structure. The contribution of the resultant structures to the power performance is first demonstrated by a proof-of-concept Zn-air microbattery which is based on a multilayer Ni backbone coated with a conformal Zn film serving as the anode. This primary battery system demonstrates superior performance to its thin-film counterpart in terms of the energy density at high discharge rates. Another demonstration involves secondary battery chemistries, including Ni(OH)2 and Li-ion systems, both of which exhibit significant cycling stability and remarkable power capability by delivering more than 50% of their capacities after ultra-fast charge rates of 60 C. Areal capacities as high as 5.1 mAh cm-2 are reported. This multilayer fabrication approach is also proven successful for realizing high-performance electrochemical capacitors. Ni(OH)2-based electrochemical capacitors feature a relatively high areal capacitance of 1319 mF cm-2 and an outstanding cycling stability with a 94% capacity retention after more than 1000 cycles. The improved power performance of the electrodes is realized by the simultaneous minimization of the internal resistances encountered during the transport of the ionic and electronic species at high charge and discharge rates. The high surface area provided by the highly laminated backbone structures enables an increased number of active sites for the redox reactions. The formation of a thin and conformal active material film on this high surface area structure renders a reduced ionic diffusion and electronic conduction path length, mitigating the power-limiting effect of the active materials with low conductivities. Also, the highly conductive backbone serving as a mechanically stable and electrochemically inert current collector features minimized transport resistance for the electrons. Finally, the highly scalable nature of the multilayer structures enables the realization of high-performance electrodes for a wide range of applications from autonomous microsystems to macroscale portable electronic devices.

Thesis (PhD)--Stellenbosch University, 2015.

In this dissertation, an all-fiber-based single frequency nanosecond pulsed laser system at ~ 1918.4 nm in master-oscillator-power-amplifier (MOPA) configuration is present. The nanosecond pulse seed is achieved by directly modulating a continuous wave (CW) single frequency fiber laser using a fast electro-optical modulator (EOM) driven by an arbitrary waveform generator (AWG). One piece of single mode, large core, polarization-maintaining (PM) highly thulium-doped (Tm-doped) germanate glass fiber (LC-TGF) is used to boost the pulse power and pulse energy of these modulated pulses in the final power amplifier. This laser system can work in both high power and high energy regime: in high power regime, to the best of our knowledge, the highest average power 16 W and peak power 78.1 kW are achieved for single frequency transform-limited ~2.0 ns pulses at 500 kHz and 100 kHz repetition rate, respectively: In high energy regime, nearly 1 mJ and half mJ pulse energy is obtained for ~15 ns pulses at 1 kHz repetition rate and 5 kHz repetition rate, respectively. Theoretical modeling of the large-core highly Tm-doped germanate glass double-cladding fiber amplifier (LC-TG-DC-FA) is also present for 2&mum nanosecond pulse amplification. A good agreement between the theoretical and experimental results is achieved. The model can simulate the evolution of pump power, signal energy, pulse shape and the amplified stimulated emission (ASE) in the amplifier. It can also be utilized to investigate the dependence of the stored energy in the LC-TGF on the pump power, seed energy and repetition rate, which can be used to design and optimize the LC-TG-DC-FA to achieve higher pulse energy and average power. Two channel of high energy nanosecond pulses (at 1918.4 nm and 1938 nm) are utilized to generate THz wave in a quasi-phase-matched (QPM) gallium arsenide (GaAs) based on difference frequency generation. THz wave with ~ 5.4μW average power and ~18 mW peak power has been achieved. Besides, one model is built to simulate a singly resonated THz parametric oscillator. The threshold, the dependence of output THz energy on pump energy has been investigated through this model. One pump enhanced THz parametric oscillator has been proposed. The enhancement factor of the nanosecond pulses in a bow-tie ring cavity has been calculated for different pulse duration, cavity length and the transmission of the coupler. And the laser resonances in the ring cavity have been observed by using a piezo to periodically adjust the cavity length. We also build an all-fiber thulium-doped wavelength tunable mode-locked laser operating near 2&mum. Reliable self-starting mode locking over a large tuning range (>50 nm) using fiber taper based carbon nanotube (FTCNT) saturable absorber (SA) is observed. Spectral tuning is achieved by stretching another fiber taper. To the best of our knowledge, this is the first demonstration of an all-fiber wavelength tunable mode-locked laser near 2&mum.

In plasma assisted thin film growth, control over the energy and direction of the incoming species is desired. If the growth species are ionized this can be achieved by the use of a substrate bias or a magnetic field. Ions may be accelerated by an applied potential, whereas neutral particles may not. Thin films grown by ionized physical vapor deposition (I-PVD) have lately shown promising results regarding film structure and adhesion. High power impulse magnetron sputtering (HIPIMS) is a relatively newly developed technique, which relies on the creation of a dense plasma in front of the sputtering target to produce a large fraction of ions of the sputtered material. In HIPIMS, high power pulses with a length of ~100 μs are applied to a conventional planar magnetron. The highly energetic nature of the discharge, which involves power densities of several kW/cm2, creates a dense plasma in front of the target, which allows for a large fraction of the sputtered material to be ionized. The work presented in this thesis involves plasma analysis using electrostatic probes, optical emission spectroscopy (OES), magnetic probes, energy resolved mass spectrometry, and other fundamental observation techniques. These techniques used together are powerful plasma analysis tools, and used together give a good overview of the plasma properties is achieved. from the erosion zone of the magnetron. The peak plasma density during the active cycle of the discharge exceeds 1019 electrons/m3. The expanding plasma is reflected by the chamber wall back into the center part of the chamber, resulting in a second density peak several hundreds of μs after the pulse is turned off. Optical emission spectroscopy (OES) measurements of the plasma indicate that the degree of ionization of sputtered Ti is very high, over 90 % in the peak of the pulse. Even at relatively low applied target power (~200 W/cm2 peak power) the recorded spectrum is totally dominated by radiation from ions. The recorded HIPIMS spectra were compared to a spectrum taken from a DC magnetron discharge, showing a completely different appearance. Magnetic field measurements performed with a coil type probe show significant deformation in the magnetic field of the magnetrons during the pulse. Spatially resolved measurements show evidence of a dense azimuthally E×B drifting current. Circulating currents mainly flow within 2 away cm from the target surface in an early part of the pulse, to later diffuse axially into the chamber and decrease in intensity. We record peak current densities of the E×B drift to be of the order of 105 A/m2. A mass spectrometry (MS) study of the plasma reveals that the HIPIMS discharge contains a larger fraction of highly energetic ions as compared to the continuous DC discharge. Especially ions of the target material are more energetic. Time resolved studies show broad distributions of ion energies in the early stage of the discharge, which quickly narrows down after pulse switch-off. Ti ions with energies up to 100 eV are detected. The time average plasma contains mainly low energy Ar ions, but during the active phase of the discharge, the plasma is highly metallic. Shortly after pulse switch-on, the peak value of the Ti1+/Ar1+ ratio is over 2. The HIPIMS discharge also contains a significant amount of doubly charged ions.

A study and analysis of high energy laser (HEL) systems aboard tactical aircraft is performed. The FA-18E/F Hornet and F-35 Joint Strike Fighter (JSF), equipped with solid-state HEL systems, are the main subjects of the study. Considerations of power generation and thermal management for a fighter-sized HEL system and aero-optic effects on beam propagation from high and medium altitude platforms are examined. An overview of system capabilities details how the HEL system will be more difficult to incorporate into legacy strike aircraft, but may be feasible for future aircraft such as the JSF. Tactical flight simulations are used to study and develop potential concepts of operation (CONOPS), using realistic scenarios and threat environments. Results show that a tactical HEL will not be a stand-alone weapon in combat, but will have many potentially useful tactical applications. Another study of a high energy free electron laser (FEL) system aboard C-130J-30 Hercules shows that such a system is feasible. Finally, a study of the FEL shows that strong field extraction can be optimized using undulator tapering. / US Marien Corps (USMC) author.

Fiber amplifiers with high power and high pulse energy are strongly in demand for both scientific research and industrial applications. Ytterbium-doped fiber has been an outstanding choice for its broad-gain bandwidth and excellent power conversion efficiencies. In this dissertation, we introduced a compact high power high pulsed energy laser system with chirally coupled core (3C) Yb-doped fibers as the gain media. Traditional standard fibers and photonic crystal fibers are not suitable for compact high power high pulse energy laser systems because of poor higher order modes (HOMs) management and complicated air-hole structure. Newly invented 3C silica fibers solve these problems. A helical side-core around the Yb-doped central core extracts the HOMs from the central core. By adjusting this chirally structure, the core of the 3C fiber can be enlarged and the transverse mode of the fiber can be single mode at certain wavelengths. To simulate the amplification process with high power high pulse energy better, a new modeling method based on a combination of the rate equations and the nonlinear Schrödinger equations is invented. The gain was calculated from rate equations and the pulse evolution was analyzed using nonlinear Schrödinger equation. The simulation provided a good guidance for building compact high power high pulse energy laser systems. To achieve high power and high pulse energy, the system is designed as a two-stage structure. The laser

Author: Bc. Lenka Pitnerová Title: The effectiveness of high intensity laser therapy for plantar fasciitis Objectives: The aim of this work is to assess the effect of high and low intensity laser therapy for plantar fasciitis compared with therapeutic ultrasound. Methods: The study included 50 patients with diagnose of heel spur syndrome. Patients were divided into three groups according to undergoing therapy. Group A (n = 20) received 10 applications of high intensity laser therapy, group B (n = 20) obtained 10 applications of low intensity laser therapy and group C (n=10) received 10 applications of therapeutic ultrasound. Intensity and character of the pain were assessed before and after therapeutic intervention using non- standardized questionnaire and numeric pain rating scale. For the assessment and description of the results were used standard statistical indicators and methods. Results: High intensity laser therapy had according to the obtained results positive effect on the clinical course of the disease and was markedly more effective than therapeutic ultrasound in treatment of plantar fasciitis. Almost all parameters reached the highest percentage improvement in the group treated with high intensity laser. The results are influenced by a smaller sample of patients and by inhomogeneity of...