State-of-the-art insulator and electrode materials for use in highcurrent high energy switching

An investigation into the failure of ceramic insulators that are used in a surface discharge switch (SDS) was conducted. The materials analyzed are Al₂O₃-25% SiC, MTF (modified alumina titanate), and CZA 500 (zirconia-alumina composite) ceramics. These insulators were subjected to high-current (~300 kA) surface discharges in atmospheric air and nitrogen. Energy-dispersive X-ray surface analysis was performed on the insulator surfaces in order to determine the contaminants that are present and the possible failure modes associated with the plasma arc environments mentioned above. Electrode erosion rates have been measured as a function of total charge transfer (up to 50 C/shot) for several in-situ materials including Cu-Nb, Cu-Nb+LaB₆, and Cu-Ta. Results from comparisons with standard Cu and CuW materials indicate that the in-situ materials represent an efficient method of retaining the copper in the bulk until it vaporizes and thus yield significantly lower erosion rates at high Coulomb transfer rates     

Controlling Spin Qubits in Quantum Dots

We review progress on the spintronics proposal for quantum computing where the quantum bits (qubits) are implemented with electron spins. We calculate the exchange interaction of coupled quantum dots and present experiments, where the exchange coupling is measured via transport. Then, experiments on single spins on dots are described, where long spin relaxation times, on the order of a millisecond, are observed. We consider spin-orbit interaction as sources of spin decoherence and find theoretically that also long decoherence times are expected. Further, we describe the concept of spin filtering using quantum dots and show data of successful experiments. We also show an implementation of a read out scheme for spin qubits and define how qubits can be measured with high precision. Then, we propose new experiments, where the spin decoherence time and the Rabi oscillations of single electrons can be measured via charge transport through quantum dots. Finally, all these achievements have promising applications both in conventional and quantum information processing. PACS: 03.67.Lx, 03.67.Mn, 73.23.Hk, 85.35.Be     

Diagnosability of repairable faults

The diagnosis problem for discrete event systems consists in deciding whether some fault event occurred or not in the system, given partial observations on the run of that system. Diagnosability checks whether a correct diagnosis can be issued in bounded time after a fault, for all faulty runs of that system. This problem appeared two decades ago and numerous facets of it have been explored, mostly for permanent faults. It is known for example that diagnosability of a system can be checked in polynomial time, while the construction of a diagnoser is exponential. The present paper examines the case of transient faults, that can appear and be repaired. Diagnosability in this setting means that the occurrence of a fault should always be detected in bounded time, but also before the fault is repaired, in order to prepare for the detection of the next fault or to take corrective measures while they are needed. Checking this notion of diagnosability is proved to be PSPACE-complete. It is also shown that faults can be reliably counted provided the system is diagnosable for faults and for repairs.     

The intelligent help system COMFOHELP

The paper is concerned with the question of whether and under what conditions active help systems with plan recognition components that have been developed in the environment of artificial intelligence research are able to prove their value in the real context of commercial application programs. The question is investigated using the development of the COMFOHELP intelligent help system as an example. COMFOHELP supports the COMFOTEX graphical text processing program and has been developed by the Linguistic Information Science Group at the University of Regensburg since 1988. The system recognizes erroneous and suboptimal plans pursued by the user by analyzing the dialog history and comparing them with the correct plan for achieving the user's goal.Section 2 discusses the research situation and elaborates on those problems which up to now prevented research concepts for plan recognition and intelligent help systems from being practically applied. Testing error situations empirically is a first prerequisite since potential erroneous plans can only be established in real-world tests. The second prerequisite is a special system architecture which counteracts the problem of ambiguities in plan recognition. Section 3 introduces a first still restricted prototype version of COMFOHELP whose efficiency was verified in a statistical hypothesis test. The users performing their text processing tasks with the support of COMFOHELP came off significantly better than members of a reference group working without the intelligent help. Section 4 shows that the proposed COMFOHELP system architecture is reconfirmed by the results of extensive empirical investigations (with more than 100 users) of erroneous plans when using a more complex version of COMFOTEX. The architecture still proves to be worthwhile even when functionality is increased by a factor of three to four.     

Classification of slice-based VBR video traffic and estimation of link loss by exceedance

Classification of a video stream is an essential preliminary step to estimate the bit loss when the video stream is transmitted over a communication network. In this paper, we classify the video frames by the average frame size and estimate the bit loss for each class when the bitrate exceeds the capacity of the bottleneck link. The video stream under study is encoded using the explicit slice-based H.264/AVC encoding scheme. This scheme reduces the burstiness of regular H.264/AVC encoded video by removing the traditional GOP structure. Instead, a repetitive combination of intracoded and predicted slices is employed, thereby introducing a specific dependence structure in the video data. We consider a bufferless model of the communication system and evaluate the channel capacity required to give a maximum allowed loss rate for each class.Due to the high variability, non-stationarity and non-homogeneity of the underlying video data, the obtained classes are checked regarding the dependence and distribution structure of the data. The high quantiles of the losses are estimated for each class.     

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Book reviews

     

Historical perspective and current trends in emission microscopy, mirror electron microscopy and low-energy electron microscopy. An introduction to the proceedings of the Second International Symposium and Workshop on Emission microscopy and Related Techniques

Emission microscopes and related instruments comprise a specialized class of electron microscopes that have in common an acceleration field in combination with the first stage of imaging (i.e., an immersion objective lens, also called a cathode lens or emission lens). These imaging techniques include photoelectron emission microscopy (PEEM or PEM), electron emission induced by heat, ions, or neutral particles, mirror electron microscopy (MEM), and low-energy electron microscopy (LEEM), among others. In these instruments the specimen is placed on a flat cathode or is the cathode itself. The low-energy electrons that are emitted, reflected, or backscattered from the specimen are first accelerated and then imaged by means of an electron lens system resembling that of a transmission electron microscope. The image is formed in a parallel mode in all of the above instruments, in contrast to the image in scanning electron microscopes, where the information is collected sequentially by scanning the specimen. A brief history and introduction to emission microscopy, MEM, and LEEM is presented as a background for the Proceedings of the Second International Symposium and Workshop on this subject, held in Seattle, Washington, August 16-17, 1990. Current trends in this field gleaned from the presentations at that meeting are discussed.     

Discrete-time analysis of linear and nonlinear systems using analogcircuit simulators

A method to teach discrete-time systems analysis concepts and skills to engineering students is presented and discussed. The method presented in this investigation uses commercially available analog-digital circuit simulators and a delay element and sampler constructed from analog components. The delay element and sampler used with standard, continuous-time amplifiers and summers, allow the user to simulate discrete-time systems in a manner identical to that used for continuous-time systems. Construction of the discrete-time delay element and sampler using the PSpice circuit simulator is presented as is their use in a variety of well-known linear discrete-time systems including problems from economics, population dynamics and digital signal processing. The well-known nonlinear quadratic model of chaos is also simulated in this investigation. The results of the linear and nonlinear discrete-time system analysis are compared to the analytical results and show that the delay element and sampler are stable and accurate components useful in modeling a wide variety of linear and nonlinear discrete-time systems