The transient analysis of large-scale systems is often difficult even when the systems belong to the simplest M/M/n type of queues. To address analytical difficulties, previous studies have been conducted under various asymptotic regimes by suitably accelerating parameters, thereby establishing some useful mathematical frameworks and giving insights into important characteristics and intuitions. However, some studies show significant limitations when used to approximate real service systems: (i) they are more relevant to steady-state analysis; (ii) they emphasize proofs of convergence results rather than numerical methods to obtain system performance; and (iii) they provide only one set of limit processes regardless of actual system size. Attempting to overcome the drawbacks of previous studies, this dissertation studies the transient analysis of large-scale service systems with time-dependent parameters. The research goal is to develop a methodology that provides accurate approximations based on a technique called uniform acceleration, utilizing the theory of strong approximations. We first investigate and discuss the possible inaccuracy of limit processes obtained from employing the technique. As a solution, we propose adjusted fluid and diffusion limits that are specifically designed to approximate large, finite-sized systems. We find that the adjusted limits significantly improve the quality of approximations and hold asymptotic exactness as well. Several numerical results provide evidence of the effectiveness of the adjusted limits. We study both a call center which is a canonical example of large-scale service systems and an emerging peer-based Internet multimedia service network known as P2P. Based on our findings, we introduce a possible extension to systems which show non-Markovian behavior that is unaddressed by the uniform acceleration technique. We incorporate the denseness of phase-type distributions into the derivation of limit processes. The proposed method offers great potential to accurately approximate performance measures of non-Markovian systems with less computational burden.

The second report from the Project-Group ESRF-Beamline of the Forschungszentrum Rossendorf covers the period from July 1999 until December 2000. The ROssendorf BeamLine (ROBL) at the European Synchrotron Radiation Facility (ESRF) in Grenoble, France performed quite well during this time. In the beamtime used by the FZR and collaborating institutes 44 scheduled experiments were performed, while in the ESRF scheduled beamtime 12 experiments. Additionally, a distinct amount of beamtime was devoted to in-house research of the FZR and methodical experiments. Since February 2000 ROBL is part of the European Commission's programme "Access to Research Infrastructure" which supports user groups from member and associated states of the EU during experiments at ROBL. In the year 2000 ROBL hosted 6 groups for experiments. The report is organised in three main parts. The first part contains extended contributions on results obtained at ROBL. The second part gives an overview about the scheduled experiments, publications, guests having visited ROBL with support of the EC, and some other information. Finally, the third part collects the experimental reports of the user groups received.

Thesis (Ph. D.)--School of Electrical and Computer Engineering, Georgia Institute of Technology, 2004. Directed by Douglas M. Blough. / Includes bibliographical references (leaves 62-63).

An advancement of the two stage growth recipe for the fabrication of CIGS solar cells was developed. The developed advancement was inconsistent in producing samples of similar stoichiometry. This was a huge barrier for up scaling the process as the behavior of devices would be different due to variation in stoichiometry. Samples with reproducible stoichiometry were obtained once the heating rate of elements, selenium in particular was better understood. This is mainly attributed to the exponential increase of selenium flux after its evaporation temperature. Monitoring the selenium flux was vital in getting constant selenium fluxes. Few changes to the growth recipe were induced to optimize the amount of selenium being used. Depositions were done using constant selenium to metal flux ratio of 5. Elemental tradeoffs were observed as a result of the growth recipe change. These tradeoffs are in favor of the two stage growth recipe. The solar cells were fabricated on a soda lime glass substrate with a molybdenum back contact. Improper sample cleaning and storage were found to affect the deposition outcome of the molybdenum back contact. This also had a cascading effect on the absorber layer. Residual precipitates during deposition of CdS were avoided by increasing the spinner speed which increased the reaction rate. This is attributed to the growth of CdS either by cluster-by-cluster growth or by ion-by-ion growth. SEM, EDS were some important tools used to characterize the devices. EDS in particular, was used extensively at different stages throughout the growth process to ensure that we were heading in the right direction. Current-voltage (I-V) measurements were done to study the solar cell performance under light and dark.

This dissertation presents two efficient circuit simulation techniques for very large scale integrated (VLSI) circuits. Model order reduction (MOR) plays a significant role in VLSI circuit simulation as nowadays the system model may contain millions of equations or variables. MOR is needed to reduce the order of the original system to allow the simulation to be performed with an acceptable amount of time, reasonable storage and reliable accuracy. Multi-point moment matching is one of the state-of-the-art methods for MOR. However, the moment order and expansion points are usually selected in a heuristic way, which cannot guarantee the global accuracy of the reduced-order model (ROM). Therefore, it is important to utilize an adaptive algorithm in exercising multi-point moment matching. In this regard, we propose a novel automatic adaptive multi-point moment matching algorithm for MOR of linear descriptor systems. The algorithm implements both adaptive frequency expansion point selection and automatic moment order control guided by a transfer function-based error metric. Without a priori information of the system response, the proposed algorithm leads to a much higher global accuracy compared with standard multipoint moment matching without adaptation. The moments are computed via a generalized Sylvester equation which is subsequently solved by a newly proposed generalized alternating direction implicit (GADI) method. Another technique for circuit simulation proposed in this thesis is the matrix exponential (MEXP) method. MEXP method has been demonstrated to be a competitive candidate for transient simulation of VLSI circuits. Nevertheless, the performance of MEXP based on ordinary Krylov subspace is unsatisfactory for stiff circuits, because the underlying Arnoldi process tends to oversample the high magnitude part of the system spectrum while under-sampling the low magnitude part that is important to the final accuracy. In this thesis, we explore the use of extended Krylov subspace to generate more accurate and efficient approximation for MEXP.We also develop a formulation, called generalized extended Krylov subspace, that allows unequal positive and negative dimensions in the subspace for better performance, and propose an adaptive scheme based on the generalized extended Krylov subspace to select the ratio between the positive and negative dimensions. / published_or_final_version / Electrical and Electronic Engineering / Master / Master of Philosophy

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