The plasma vertical position system on the COMPASS–D tokamak is studied in this thesis. An analogue P+D controller is used to regulate the plasma vertical position which is open loop unstable. Measurements from inside the vessel are used for the derivative component of the control signal and external measurements for the proportional component. Two main sources of disturbances are observed on COMPASS–D. One source is 600Hz noise from thyristor power supplies which cause large oscillations at the control amplifier output. Another source is impulse–like disturbances due to ELMs (Edge Localized Modes) and this can occasionally lead to loss of control when the control amplifier saturates. Models of the plasma open loop dynamics were obtained using the process of system identification. Experimental data is used to fit the coefficients of a mathematical model. The frequency response of the model is strongly dependent on the shape of the plasma. The effect of shielding by the vessel wall on external measurements when compared with internal measurements is also observed. The models were used to predict values of gain margins and phase crossover frequencies which were found to be in good agreement with measured values. The harsh reactor conditions on the proposed ITER tokamak preclude the use of internal measurements. On COMPASS–D the stability margins of the loop decrease when using only external flux loops. High order controllers were designed to stabilize the system using only external measurements and to reduce the effect of 600Hz noise on the control amplifier voltage. The controllers were tested on COMPASS–D and demonstrated the improved performance of high order controllers over the simple P+D controller. ELMs cause impulse–like disturbances on the plasma position. The optimal controller minimizing the peak of the impulse response can be calculated analytically for COMPASS–D. A multiobjective controller which combines a small peak impulse response with robust stability and noise attenuation can be obtained using a numerical search.

<p>Wavelength division multiplexing (WDM) is the key technology of the current generation fiber-optics network. To build agile and intelligent next generation optical networks, optical wavelength conversion and signal regeneration are crucial new functions under intense research and development. These new functions call for innovative, low cost and high performance optoelectronic devices. One of such enabling devices is quantum-well electroabsorption modulators (EAM) that are appealing in terms of structural simplicity and low noise and are potentially advantageous on high-speed operation and low power consumption. The goal of this thesis is to systematically study EAM for optical signal functions in optical networks from various perspectives, including fundamental device physics, comprehensive models, innovative design, and experimental prototyping.</p> <p>After the first chapter of introduction, Chapter 2 and 3 are devoted to device models. In Chapter 2, a self-consistent and physics-based model has been developed for two key nonlinear optical mechanisms in quantum-well EAM: exciton saturation and electric field screening. Presented in Chapter 3 is a simplified but efficient model for EAM with a feature of handling strong electric field.</p> <p>Next, the fundamental physics relevant to nonlinear EAM are studied in Chapter 4 and 5. Exciton state mixing effects on intersubband transitions in quantum well have been investigated in Chapter 4 and a drastic different picture from that of the previous studies has been revealed. Studies have also been done in Chapter 5 on valence band mixing effects in exciton capture and escape in quantum well structures. And it is found that much faster capture and escape processes can be resulted from the band mixing effects.</p> <p>Then, the two key design issues of nonlinear EAM have been addressed. In Chapter 6, different saturation dynamics of electrons and holes in quantum wells have been thoroughly analyzed and utilized to achieve the best compromise between high-speed and low power consumption of EAM in optical wavelength conversion and signal regeneration. In Chapter 7, the polarization issue of transverse electric (TE) mode and transverse magnetic (TM) mode is addressed from two different perspectives: design for the most effective optical saturation by using TM mode absorption and design for TE and TM polarization insensitive operation.</p> <p>Finally, Chapter 8 presents the results of experimental proto typing on the design concept to enhance exciton absorption saturation using light-hole excitation through TM optical mode.</p> / Doctor of Philosophy (PhD)

For precise measurement of new TeV-scale physics and precision studies of the Higgs Boson, a new lepton collider is required. To enable meaningful analysis, a centre of mass energy of 500GeV and luminosity of 10³⁴cm^-2s^-1 is needed. The planned 31km long International Linear Collider is capable of meeting these targets, requiring a final emittance of 10 micro-radians horizontally and 35nmrad vertically. To achieve these demanding emittance values, the accelerator components in the main linacs must be aligned against an accurately mapped network of reference markers along the entire tunnel. An automated system could map this tunnel network quickly, accurately, safely and repeatedly; the Linear Collider Alignment and Survey (LiCAS) Rapid Tunnel Reference Surveyor (RTRS) is a working prototype of such a system. The LiCAS RTRS is a train of measurement units that accurately locate regularly spaced retro-reflector markers using Frequency Scanning Interferometry (FSI). The unit locations with respect to each other are precisely reconstructed using a Laser Straightness Monitor (LSM) and tilt sensor system, along with a system of internal FSI lines. The design, commissioning, practical usage, calibration, and reconstruction performance of the LSM is addressed in this work. The commissioned RTRS is described and the properties of the LSM components are investigated in detail. A method of finding the position of laser beam spots on the LSM cameras is developed, along with a process of combining individual spot positions into a more robust measurement compatible with the data from other sub-systems. Laser beam propagation along the LSM is modelled and a robust method of reconstructing CCD beam spot position measurements into positions and orientations of the LSM units is described. A method of calibrating LSM units using an external witness system is presented, along with a way of using the overdetermined nature of the LSM to improve calibration constant errors by including data taken from unwitnessed runs. The reconstruction uncertainty, inclusive of both statistical and systematic effects, of the LSM system is found to be of 5.8 microns × 5.3 microns in lateral translations and 27.6 microradians × 34.1 microradians in rotations perpendicular to the beam, with an uncertainty of 51.1 microradians in rotations around the beam coming from a tilt-sensor arrangement.