Wave propagation in negative resistance media

When an e1eotromagnetio wave enters a medium having an effective negative resistance, it may be expected to grow spatially. The conditions under which such growth may be observed are examined, and an experimental regime chosen where it should be possible to observe these effects. Suchh observations can throw light not only upon the negative resistance travelling wave amplification process, but also upon the transport process giving rise to the host negative resistance. The experimental work is therefore divided between a study of a new negative differential conductivity process recently discovered in n-Ge and a study of the negative résistancee travelling wave amplification process in GaAs. The diagnostic measurements upon n-Ge provide new data relating to the negative resistance process in that material; data such as the maximum negative slope mobility of 300 cm 2 /V.sec. at 77°K (E / / <100» the threshold electric field for N.D.C. of 2.95KV/cm. (E //<100»), and the ceiling frequency of 5 GHz beyond which N.D.C. disappears, all of which have been subsequently corroborated by other workers. Observations of the emission spectrum from GaAs, when the amplified electromagnetic wave begins to experience round trip gain within the specimen cavity, have allowed some fairly detailed discussion to be undertaken concerning the nature of the emission process, which rely upon careful measurements of the spectral power and line width. Although the maximum pulse power observed of 4mW is low, such observations of emission do provide anew mechanism by which microwave power may be extracted from a suitably biased material having a negative differential conductivity. Finally, attention is directed towards some potentially profitable areas for future work

Quench propagation and protection analysis of the ATLAS Toroids

The ATLAS superconducting magnet system consists of the Barrel Toroid, two End Cap Toroids and the Central Solenoid. However, the Toroids of eight coils each are magnetically separate systems to the Central Solenoid. The Toroids are electrically connected in series and energized by a single power supply. The quench protection system is based on the use of relatively small external dump resistances in combination with quench-heaters activated after a quench event detection to initiate the internal dump of stored energy in all the coils. A rather strong quench-back effect due to eddy-currents in the coil casings at the transport current decay is beneficial for the quench protection efficiency in the event of heater failures. The quench behaviour of the ATLAS Toroids was computer simulated for normal operation of the quench protection system and its complete non-operation (failure) mode. (3 refs)

Quench propagation and detection in the superconducting bus-bars of the ATLAS magnets

The ATLAS superconducting magnet system comprising Barrel (BT) and End-Cap Toroids (ECT) and also Central Solenoid (CS) will store more than 1.5 GJ of magnetic energy. The magnet system will have many superconducting busbars, a few meters long each, running from the current leads to Central Solenoid and Toroids as well as between the coils of each Toroid. Quench development in the busbars, i.e., the normal zone propagation process along the busbar superconductors, is slow and exhibits very low voltages. Therefore, its timely and appropriate detection represents a real challenge. The temperature evolution in the busbars under quench is of primary importance. Conservative calculations of the temperature were performed for all the magnets. Also, a simple and effective method to detect a normal zone in a busbar is presented. A thin superconducting wire, whose normal resistance can be easily detected, is placed in a good thermal contact to busbar. Thus, the wire can operate as straightforward and low-noise quench-detector. (4 refs)

First Cool-down and Test at 4.5 K of the ATLAS Superconducting Barrel Toroid Assembled in the LHC Experimental Cavern

The large ATLAS superconducting magnets system consists of the Barrel, two End-Caps Toroids and the Central Solenoid. The eight separate coils making the Barrel Toroid (BT) have been individually tested with success in a dedicated surface test facility in 2004 and 2005 and afterwards assembled in the underground cavern of the ATLAS experiment. In order to fulfil all the cryogenic scenarios foreseen for these magnets with a cold mass of 370 tons, two separate helium refrigerators and a complex helium distribution system have been used. This paper describes the results of the first cool-down, steady-state operation at 4.5 K and quench recovery of the BT in its final configuration

Contesting language policy for asylum seekers in the Northern periphery: The story of Tailor F

This article is about navigating asylum, employment and language policy in a new country as an asylum seeker. Through the story of one individual, we show that profound inequalities are exacerbated when forced migrants are limited in their choice of language they might study or use. The individual is Tailor F, an Iraqi man seeking asylum, and the country is Finland, officially bilingual, with a majority language (Finnish) and a minority language (Swedish). Finland’s official bilingualism does not extend evenly to language education provided for asylum seekers, who are taught Finnish regardless of the region where they are placed. Upon arrival, Tailor F was housed in a reception centre for asylum seekers located in a Swedish-dominant rural area of the country. Through our linguistic ethnography we examine how he navigates multilingually in his early settlement, his current work and his online life. We relate his story to explicit and implicit official bilingualism in Finland and discuss his lived experiences in relation to the contexts of asylum policy and employment. Tailor F’s story shows how, through his practices, he has contested implicit language policy for asylum seekers in order to gain membership of the local Swedish-dominant community, achieve a sense of belonging, and potentially realise his aspirations for the future

The literacy practices for assessment in the vocational curriculum - the case of Hospitality

This article explores two case studies of the literacy practices of assessment in the vocational curriculum. Previous studies have identified learning in the vocational curriculum as being assessment-driven and that subjects often associated with limited levels of literacy actually require multiple literacy practices from students. Drawing upon studies of the literacy practices of assessment in the teaching of Hospitality, the study explores the academic and occupational forms of reading and writing required of students. These literacy practices are often invisible to both students and staff and yet are central to the success of students in relation to attainment. The article suggests that vocational subjects may actually be more demanding in the range of literacies required for assessment than is often considered to be the case

AmrZ is a major determinant of c-di-GMP levels in Pseudomonas fluorescens F113

The transcriptional regulator AmrZ is a global regulatory protein conserved within the pseudomonads. AmrZ can act both as a positive and a negative regulator of gene expression, controlling many genes implicated in environmental adaption. Regulated traits include motility, iron homeostasis, exopolysaccharides production and the ability to form biofilms. In Pseudomonas fluorescens F113, an amrZ mutant presents a pleiotropic phenotype, showing increased swimming motility, decreased biofilm formation and very limited ability for competitive colonization of rhizosphere, its natural habitat. It also shows different colony morphology and binding of the dye Congo Red. The amrZ mutant presents severely reduced levels of the messenger molecule cyclic-di-GMP (c-di-GMP), which is consistent with the motility and biofilm formation phenotypes. Most of the genes encoding proteins with diguanylate cyclase (DGCs) or phosphodiesterase (PDEs) domains, implicated in c-di-GMP turnover in this bacterium, appear to be regulated by AmrZ. Phenotypic analysis of eight mutants in genes shown to be directly regulated by AmrZ and encoding c-di-GMP related enzymes, showed that seven of them were altered in motility and/or biofilm formation. The results presented here show that in P. fluorescens, AmrZ determines c-di-GMP levels through the regulation of a complex network of genes encoding DGCs and PDEs

MICE: The muon ionization cooling experiment. Step I: First measurement of emittance with particle physics detectors

Copyright @ 2011 APSThe Muon Ionization Cooling Experiment (MICE) is a strategic R&D project intended to demonstrate the only practical solution to providing high brilliance beams necessary for a neutrino factory or muon collider. MICE is under development at the Rutherford Appleton Laboratory (RAL) in the United Kingdom. It comprises a dedicated beamline to generate a range of input muon emittances and momenta, with time-of-flight and Cherenkov detectors to ensure a pure muon beam. The emittance of the incoming beam will be measured in the upstream magnetic spectrometer with a scintillating fiber tracker. A cooling cell will then follow, alternating energy loss in Liquid Hydrogen (LH2) absorbers to RF cavity acceleration. A second spectrometer, identical to the first, and a second muon identification system will measure the outgoing emittance. In the 2010 run at RAL the muon beamline and most detectors were fully commissioned and a first measurement of the emittance of the muon beam with particle physics (time-of-flight) detectors was performed. The analysis of these data was recently completed and is discussed in this paper. Future steps for MICE, where beam emittance and emittance reduction (cooling) are to be measured with greater accuracy, are also presented.This work was supported by NSF grant PHY-0842798