A high-performance cross-current type cycloconverter-fed inductionmotor drive system

Making use of the latest techniques, the authors have successfully developed a cross-current-type cycloconverter-fed, squirrel-cage induction motor drive system, which far exceeds DC drive systems over a wide range from zero speed to high speed. It is best suited for tandem cold mill main and various other large-scale rolling mills     

Finite element iterative techniques for determining the interface boundary between Laplace and Poisson domains—characteristic analysis of a field effect transistor

Two kinds of techniques for solving a shape determination problem are proposed. The determination of the interface boundary between two domains governed by Poisson and Laplace equations under the compatible and constraint condition is considered. Influence coefficient and inverse variational approaches are examined by using the iterative finite element procedure. A two-dimensional model of a junction-type field effect transistor is a test example. The determination of its interface boundary and the prediction of the potential distribution and static characteristic are demonstrated.     

Degradation and recovery of AlGaAs/GaAs p-HEMT irradiated by high-energy particle

Results of an extensive study on the irradiation damage and its recovery behavior resulting from thermal annealing in AlGaAs/GaAs pseudomorphic high electron mobility transistors (HEMTs) subjected to a 220-MeV carbon, 1-MeV electrons and 1-MeV fast neutrons are presented. The drain current and effective mobility decrease after irradiation, while the threshold voltage increases in positive direction. The decrease of the drain current and mobility is thought to be due to the scattering of channel electrons with the induced lattice defects and also to the decrease of the electron density in the two dimensional electron gas region. Isochronal thermal annealing shows that the device performance degraded by the irradiation recovers. The decreased drain current for output characteristics recovers by 75% of pre-rad value after 300°C thermal annealing for AlGaAs HEMTs irradiated by carbon particles with a fluence of 1×10¹² cm⁻². The influence of the materials and radiation source on the degradation is also discussed with respect to the nonionizing energy loss. Those are mainly attributed to the difference of particle mass and the probability of nuclear collision for the formation of lattice defect in Si-doped AlGaAs donor layer. A comparison is also made with results obtained on irradiated InGaP/InGaAs p-HEMTs in order to investigate the effect of the constituent atom. The damage coefficient of AlGaAs HEMTs is also about one order greater than that of InGaP HEMTs for the same radiation source. The materials and radiation source dependence of performance degradation is mainly thought to be attributed to the difference of mass and the possibility of nuclear collision for the formation of lattice defects in Si-doped donor layer.     

Impact of high energy particle irradiation on the electrical performance of Si1−x Ge x epitaxial diodes

The impact of 20-MeV alpha ray irradiation on the electrical characteristics of strained-layer epitaxial Si_1–x Ge _x diodes is investigated as a function of fluence and Ge content (x = 0.08, 0.12 and 0.16). The reverse current at a fixed bias increases with fluence, although the rate of increase decreases with increasing fluence and/or Ge content. The reduction of the capacitance with fluence points to a strong deactivation of the boron (B) dopant atoms, which decreases with increasing Ge content. The close to square root dependence between the B-deactivation and the reverse current increase, suggests that in the irradiated diodes the latter is dominated by deep levels associated with interstitial B complexes. This is confirmed by deep level transient spectroscopy, revealing that the trap introduction rate at a given fluence reduces with increasing Ge content, similar to that for the reverse diode current.     

Electrical Impedance Computed Tomography Based on a Finite Element Model

A simulation study of electrical impedance computed tomography is presented. This is an inverse problem. A field is discretized by the finite element method and an iterative approach derived from the sensitivity theorem is examined for leads taken on the field surface. It is shown that the conductivity distribution in the field can be estimated from the impedance data obtained for the body surface leads. Simulation suggests the availability and the limitation for impedance plethysmography application. The finite element model must be chosen properly to provide the unique solution.     

Seismic PSA method for multiple nuclear power plants in a site

The maximum number of nuclear power plants in a site is eight and about 50% of power plants are built in sites with three or more plants in the world. Such nuclear sites have potential risks of simultaneous multiple plant damages especially at external events. Seismic probabilistic safety assessment method (Level-1 PSA) for multi-unit sites with up to 9 units has been developed. The models include Fault-tree linked Monte Carlo computation, taking into consideration multivariate correlations of components and systems from partial to complete, inside and across units. The models were programmed as a computer program CORAL reef. Sample analysis and sensitivity studies were performed to verify the models and algorithms and to understand some of risk insights and risk metrics, such as site core damage frequency (CDF per site-year) for multiple reactor plants. This study will contribute to realistic state of art seismic PSA, taking consideration of multiple reactor power plants, and to enhancement of seismic safety.     

Boundary element iterative techniques for determining the interface boundary between two Laplace domains—a basic study of impedance plethysmography as an inverse problem

The interface boundary between two domains with different conductivity is to be determined from the impedance measured at the domain surface. This is an inverse problem. Two approaches, influence coefficient approach and boundary integral approach, using a boundary element model are presented. To examine the validity of the approaches, numerical experiments are carried out for a two-dimensional field, and the suitability and limitation of the methods for application to impedance plethysmography are discussed.