Automated sample exchange and tracking system for neutron research at cryogenic temperatures

An automated system for sample exchange and tracking in a cryogenic environment and under remote computer control was developed. Up to 24 sample "cans" per cycle can be inserted and retrieved in a programed sequence. A video camera acquires a unique identification marked on the sample can to provide a record of the sequence. All operations are coordinated via a LABVIEW program that can be operated locally or over a network. The samples are contained in vanadium cans of 6-10 mm in diameter and equipped with a hermetically sealed lid that interfaces with the sample handler. The system uses a closed-cycle refrigerator (CCR) for cooling. The sample was delivered to a precooling location that was at a temperature of approximately 25 K, after several minutes, it was moved onto a "landing pad" at approximately 10 K that locates the sample in the probe beam. After the sample was released onto the landing pad, the sample handler was retracted. Reading the sample identification and the exchange operation takes approximately 2 min. The time to cool the sample from ambient temperature to approximately 10 K was approximately 7 min including precooling time. The cooling time increases to approximately 12 min if precooling is not used. Small differences in cooling rate were observed between sample materials and for different sample can sizes. Filling the sample well and the sample can with low pressure helium is essential to provide heat transfer and to achieve useful cooling rates. A resistive heating coil can be used to offset the refrigeration so that temperatures up to approximately 350 K can be accessed and controlled using a proportional-integral-derivative control loop. The time for the landing pad to cool to approximately 10 K after it has been heated to approximately 240 K was approximately 20 min.     

In situ synchrotron X-ray micro-tomography study of pitting corrosion in stainless steel

Pitting corrosion of stainless steel has been investigated with high-resolution in situ X-ray microtomography. The growth of pits at the tip of stainless steel pins has been observed with 3D microtomography under different conditions of applied current and cell potential. The results demonstrate how pits evolve in stainless steel, forming a characteristic “lacy” cover of perforated metal. In addition, it is shown how the shape of pits becomes modified by MnS inclusions.     

Analysis of variable-frequency currents superimposed on DC currentsin asynchronous HVDC links in stressing turbine-generator-exciter shafts

The paper makes an indepth analysis of the excitation of shaft torsional vibrations in steam turbine-generator-exciter shafts by variable-frequency ripple currents superimposed on DC currents in asynchronous links using the finite element and reduced models of the machine shafts. Frequencies at which sympathetic shaft torsional vibrations would be excited by modulation product harmonics in 50 Hz/50 Hz and 50 Hz/60 Hz asynchronous links as a function of deviation in system frequency are illustrated. It is shown that amplitude of shaft torque due to steady resonant torque excitation is a function of initial rate of increase of vibrations at adjacent cells, the time constant for decay of the vibration, and stiffness between adjacent shaft cells     

Monolithic, multiple-wavelength vertical-cavity surface-emitting laser arrays by surface-controlled MOCVD growth rate enhancement and reduction

Monolithic, multiple-wavelength vertical-cavity surface-emitting laser (VCSEL) arrays have been obtained by the surface-controlled enhancement and reduction of the MOCVD epitaxial growth rate, achieving a periodic, graded wavelength span greater than 30 nm. Room-temperature (RT), electrically pumped continuous-wave (CW) lasing is demonstrated, with uniform threshold currents of 5.5/spl plusmn/0.5 mA with typical output powers of 0.5 mW. We show here for the first time both the enhancement and the reduction of the growth rate of the entire VCSEL structure and demonstrate the controlled variation of the VCSEL lasing wavelength over a widened spectral range by exploiting both effects.     

Renewable energy alternatives for developed countries

This paper examines renewable energy alternatives in developed countries: environmental mechanisms; future energy alternatives; green electricity marketing and its potential; pricing; and limitations on the eve of open access. It discusses objectives in electricity restructuring; the role of resources planning in forming long range energy environmental policies; feasibility of electricity trading mechanisms; green marketing in the United States and Australia; green energy offers in Canada; and the results of green pricing programs in Europe and the United States. The role of existing and planned mechanisms to achieve environmental benefits in restructured electricity markets are reviewed. Technological and institutional challenges of achieving real, long-term reductions in carbon dioxide and other emissions from the electric sector are discussed. Factors associated with infrastructure turnover together with technology development and deployment are addressed, where attention is given to policies which promote highly integrated and coordinated reductions in emissions. The paper then focuses on the green pool and trends in power marketing where status of competitive markets, green pricing programs for franchise customers, green power products for contestable customers, credibility of green power marketing, and public policy for renewable energy technologies in competitive markets are discussed. It then reviews green energy in Ontario on the eve of open access, and shows there is a market for green energy if customers have a choice     

On the design of space-time and space-frequency codes for MIMO frequency-selective fading channels

The authors introduced an algebraic design framework for space-time coding in flat-fading channels . We extend this framework to design algebraic codes for multiple-input multiple-output (MIMO) frequency-selective fading channels. The proposed codes strive to optimally exploit both the spatial and frequency diversity available in the channel. We consider two design approaches: The first uses space-time coding and maximum likelihood decoding to exploit the multi-path nature of the channel at the expense of increased receiver complexity. Within this time domain framework, we also propose a serially concatenated coding construction which is shown to offer a performance gain with a reasonable complexity iterative receiver in some scenarios. The second approach utilizes the orthogonal frequency division multiplexing technique to transform the MIMO multipath channel into a MIMO flat block fading channel. The algebraic framework is then used to construct space-frequency codes (SFC) that optimally exploit the diversity available in the resulting flat block fading channel. Finally, the two approaches are compared in terms of decoder complexity, maximum achievable diversity advantage, and simulated frame error rate performance in certain representative scenarios.     

Brightness-Scaling Potential of Actively Phase-Locked Solid-State Laser Arrays

Recent progress in developing phased arrays of high-brightness solid-state lasers is summarized. We address the prospects for continued brightness-scaling via a model that extrapolates measured results to large numbers of array elements and provides a quantitative illustration of the features of coherent beam combination. This demonstrates that with present-day technology, both slab and fiber lasers have the capability to scale to unprecedented brightness levels.     

Scaling of longitudinally diode-pumped self-frequency-doublingNd:YCOB lasers

We report the scaling of a new self-frequency-doubling laser, based on the crystal Nd³⁺:YCa₄O(BO₃) ₃, to higher powers. The power scaling is achieved by diode pumping using a novel technology of combining the output of up to four high-brightness laser diodes. Spectroscopic, thermo-mechanical, and laser properties are investigated for use in designing high-power self-frequency-doubling lasers. Using a method of angular-multiplexing individual laser diodes for pumping, we demonstrate output powers of more than 1.9 W of fundamental (1060 nm) radiation and 245 mW at the second harmonic. Experimental investigation rendered a thermal stress resistance figure-of-merit for this material to be between 210-280 W/m     

IEEE Power Engineering Review

A better understanding of the SI principles can make the conversion process easier and help promote proper use of SI as it increasingly becomes a part of daily life. Although published guidelines explain the conversion methods and proper use of the SI system, the actual change to SI units of measure has been slow and painful. Many IEEE standards have missed the directive deadline because of difficulty in actually applying the conversion guidelines' principles. One impediment to conversion seems to be a lack of full understanding of the SI system. Another is unfamiliarity with the methods of converting from one system to another while maintaining consistent accuracy. The IEEE conversion directive does not require that manufacturers change their factory equipment or assembly processes, only that referenced standards use SI dimensions to specify the equipment purchased. This paper gives guidelines for metric conversion.