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  • Springer  (54)
  • American Physical Society  (44)
  • American Chemical Society  (30)
  • American Institute of Physics (AIP)  (6)
  • Blackwell Science Ltd  (4)
  • 1995-1999  (138)
  • 1985-1989
  • 1997  (138)
  • 1
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The performance of the Tokamak Fusion Test Reactor [D. Meade and the TFTR Group, in Plasma Physics and Controlled Nuclear Fusion Research, Washington, D.C., 1990 (International Atomic Energy Agency, Vienna, 1991), Vol. I, pp. 9–24], as defined by the maximum fusion power production, has been limited, not by confinement, but by stability to pressure-driven modes. Two classes of current profile modification have been investigated to overcome this limit. A new technique has been developed to increase the internal inductance of low-q (q(approximate)4), high-current (Ip〉2MA) plasmas. As was the case at higher edge q, the disruptive β limit has been found to increase roughly linearly with the internal inductance, li. Plasmas with hollow current profiles, i.e., reversed shear, are also predicted and experimentally observed to have increased stability in the negative shear region to ballooning and kink modes. However, performance of these plasmas is still limited by pressure-driven modes in the normal shear region. © 1997 American Institute of Physics.
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  • 2
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: The fast ion and α-particle diagnostic at JET is based on collective Thomson scattering of high power millimeter-wave radiation. The principal aim of the diagnostic will be the measurement of the spatially resolved velocity distribution of fast α particles when tritium is introduced in JET plasmas, although several other applications are foreseen. The diagnostic uses a 140 GHz, 500 kW, gyrotron as the source of probing radiation and a heterodyne detection system. The diagnostic came into operation during the last JET operational campaign. First measurements were made of the thermal and mildly suprathermal (induced by ion cyclotron resonance heating) ion feature. The results confirmed expectations, indicating that the diagnostic should give the predicted performance for observation of α-particle populations in the DT phase. The signal-to-noise ratio is limited by the noise on the background radiation (electron cyclotron emission), which has a minimum around 140 GHz only when JET is operated at B∼3.4 T. To further improve the S/N and to make the diagnostic suitable for operation at higher field, which is now being considered for some of the DT plasmas, a viewing dump will be installed before the DT phase. © 1997 American Institute of Physics.
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  • 3
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 107 (1997), S. 2738-2751 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The infrared spectrum of the rare gas-spherical top complex Ne–SiH4 has been recorded in a supersonic jet in the region of the SiH4 ν3 triply degenerate stretching vibration at ∼2189 cm−1. The Ne–SiH4 spectrum is complex with no obvious regular band structure; a previous paper has reported the assignment and analysis of bands correlating with the SiH4 R(0) (K=0←0, K=1←0) and P(1) (K=0←0, K=0←1) transitions. This paper concludes the study of Ne–SiH4, with the assignment and analysis of bands correlating with the SiH4 Q(1) (K=0←0, K=1←0, K=0←1, K=1←1) and R(1) (K=0←0, K=1←0, K=1←1, K=2←1) transitions. This was facilitated by a twofold approach: The bands were identified by the use of a theoretical intermolecular potential, with potential parameters determined by reference to the two bands previously fitted; and the relative magnitudes of the Coriolis interactions between the K sublevels within the bands determined by explicit calculation of the Coriolis matrix elements. Having fitted the bands to appropriate energy-level expressions, an effective anisotropic potential for the internal rotation of the SiH4 molecule within the complex was fitted to the total of 15 observed band origins. With a fixed R approximation, the V3 anisotropic term was found to be ∼30 cm−1, approximately one-third that of Ar–SiH4, reflecting the freer rotation of the SiH4 molecule when complexed with the less polarizable Ne atom. However, the overall quality of the fit was an order of magnitude worse than that for Ar–SiH4, indicating increased importance of the radial dependence of the Ne–SiH4 potential compared to that of Ar–SiH4. © 1997 American Institute of Physics.
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  • 4
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 81 (1997), S. 5905-5912 
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The picosecond optical limiting characteristics of the optical limiting dyes hexamethylindotricarbocyanine iodide (HITCI) and chloroaluminum phthalocyanine (CAP) are compared and contrasted at 532 nm. From single pulse transmittance experiments, HITCI is shown to possess a serious limitation in its optical limiting behavior, where the regime of induced absorption becomes one of induced transmission for high irradiances. CAP, on the other hand, although possessing a smaller ratio of the excited-state to ground-state absorption cross-section, continues to exhibit optical limiting over a much broader dynamic range. From cross-polarized time-resolved excitation-probe measurements, HITCI exhibits a sharp negative spike around zero delay which originates from orientational coherent grating effects, while CAP does not exhibit any spike. These nonlinearities are explained theoretically as a complex interplay between a series of excited manifolds for the dyes and the radiation properties of the interacting laser pulse. © 1997 American Institute of Physics.
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  • 5
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 106 (1997), S. 2105-2117 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The "universal bound on spin dynamics'' proposed by Sørensen is examined in detail and shown to be of great assistance in the design of novel multiple-pulse NMR experiments. The efficiency of coherence transfer between all possible states of a spin system, including populations, single-quantum coherences, and multiple-quantum coherences, is investigated. Examples are drawn from coherence transfer processes in quadrupolar coupled spin I=1 and 3/2 nuclei and weakly J coupled systems of two and three spin I=1/2 nuclei. It is found that many of the most commonly used NMR pulse sequences fail to achieve the maximum coherence transfer efficiency when applied to spin I=3/2 or to three spin I=1/2 nuclei. However, it is shown that, with knowledge of the universal bound, novel multiple-pulse NMR experiments that achieve optimal efficiency can be easily derived using computer optimization. The application of the universal bound to two-step coherence transfer experiments presents a number of conceptual difficulties. In particular, examples are presented where the product of the universal bounds on the two individual coherence transfer coefficients is larger than the universal bound on the overall transfer from the initial to the final state. These difficulties are resolved and explained in terms of the presence of a "residue'' that is created together with the intermediate state. The universal bound is used to examine the conditions under which the effect of this residue can be suppressed and the constraints that this places on the design of optimal multi-step coherence transfer NMR experiments. © 1997 American Institute of Physics.
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  • 6
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The first compound for which the previously predicted biaxial nematic phase was claimed is 4-[3,4,5-tris(4-dodecyloxybenzyloxy)benzoyloxy]-4′-(4-dodecyloxybenzoyloxy)-1,1′-biphenyl (I). This assignment was based on the observation of the optical texture and the x-ray diffraction pattern. To confirm this identification of the biaxial nematic phase, we have studied the deuterium nuclear magnetic resonance (NMR) spectra of nonuniformly aligned samples with the deuterons located specifically in the mesogen itself or in the disklike solute, hexamethylbenzene-d18, dissolved in the mesogen. These experiments allow us to determine the biaxiality in the partially averaged quadrupolar tensor. For both systems, the biaxiality parameter is found to be zero within the experimental error which is estimated to be less than ±0.08 for the pure mesogen and ±0.06 for the solute. The orientational order parameters determined from the quadrupolar splitting change discontinuously at the nematic-isotropic transition thus confirming the first order character of the transition and in accord with the uniaxial symmetry of the nematic phase. The order parameters determined for the two groups of equivalent deuterons in the specifically deuteriated oxymethylene links allow us to comment on the conformations of these links. © 1997 American Institute of Physics.
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  • 7
    ISSN: 1365-2958
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Biology , Medicine
    Notes: A transposon (Tn10dCam) insertion mutant of Escherichia coli K-12 was isolated that exhibited hypersensitivity to zinc(II) and cadmium(II) and, to a lesser extent, cobalt(II) and nickel (II). The mutated gene, located between 75.5 and 76.2 min on the chromosome, is named zntA (for Zn(II) transport or tolerance). The metal-sensitive phenotype was complemented by a genomic DNA clone mapping at 3677.90–3684.60 kb on the physical map. Insertion of a kanamycin resistance (KnR) cassette at a Sal I site in a subcloned fragment generated a plasmid that partially complemented the zinc(II)-sensitive phenotype. DNA sequence analysis revealed that the KnR cassette was located within the putative promoter region of an ORF (o732 or yhhO) predicted to encode a protein of 732 amino acids, similar to cation transport P-type ATPases in the Cpx-type family. Inverse PCR and sequence analysis revealed that the Tn10dCam element was located within o732 in the genome of the zinc(II)-sensitive mutant. The zntA mutant had elevated amounts of intracellular and cell surface-bound Zn(II), consistent with the view that zntA+ encodes a zinc(II) efflux protein. Exposure of the zntA mutant to cobalt(II) and cadmium(II) also resulted in elevated levels of intracellular and cell surface-bound metal ions.
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  • 8
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Science Ltd
    Molecular microbiology 25 (1997), S. 0 
    ISSN: 1365-2958
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Biology , Medicine
    Notes: A TnphoA mutant of Proteus mirabilis was isolated, which had lost the ability to swarm, yet was still motile. The transposon had inserted into flgN, a flagella gene encoding a 147-amino-acid protein of undefined function. Proteus flgN is arranged in an operon with the class III anti-σ28 gene, flgM, flanked by the class II genes, flgA, flgBCD and flhBA, and a novel putative virulence-related gene. The flgN mutation caused a substantial reduction in cell surface-associated flagellin, particularly during differentiation to the normally hyperflagellated swarm cell. This was not due to an effect on flagella gene expression or a typical defect in the flagella export apparatus as there was no class III gene downregulation by FlgM feedback, or intracellular flagellin accumulation. Loss of FlgN nevertheless caused a severe reduction in the incorporation of pulse-labelled flagellin into the membrane/flagellum fraction of differentiating cells. Substantial amounts of both non-oligomeric flagellin and flagellin degradation products appeared in the extracellular medium, although the few mature filaments made by the mutant were no more sensitive to proteolysis than those of the wild type. FlgN appeared soluble and active in the cytosol. The data suggest that the function of FlgN is to facilitate the initiation of flagella filament assembly, a role that may be especially critical in attaining the much higher concentration of surface flagellin required for swarming. Proteus FlgN has leucine zipper-like motifs arranged on potential amphipathic helices, a feature conserved in cytosolic chaperones for the exported substrates of flagella-related type III virulence systems. While gel filtration of FlgN from the soluble cell fraction did not establish an interaction with flagellin, it indicated that FlgN may associate with an unknown component and/or form an oligomer.
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  • 9
    ISSN: 1365-2958
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Biology , Medicine
    Notes: The Hin recombinase mediates the site-specific inversion of a segment of the Salmonella chromosome between two flanking 26 bp hix DNA recombination sites. Mutations in two amino acid residues, R43 and R69 of the catalytic domain of the Hin recombinase, were identified that can compensate for loss of binding resulting from elimination of certain major and minor groove contacts within the hix recombination sites. With one exception, the R43 and R69 mutants were also able to bind a hix sequence with an additional 4 bp added to the centre of the site, unlike wild-type Hin. Purified Hin mutants R43H and R69C had both partial cleavage and inversion activities in vitro while mutants R43L, R43C, R69S, and R69P had no detectable cleavage and inversion activities. These data support a model in which the catalytic domain plays a role in DNA-binding specificity, and suggest that the arginine residues at positions 43 and 69 function to position the Hin recombinase on the DNA for a step in the recombination reaction which occurs either at and/or prior to DNA cleavage.
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  • 10
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Science Ltd
    Molecular microbiology 26 (1997), S. 0 
    ISSN: 1365-2958
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Biology , Medicine
    Notes: The RfaH protein controls the transcription of a specialized group of Escherichia coli and Salmonella operons that direct the synthesis, assembly and export of the lipopolysaccharide core, exopolysaccharide, F conjugation pilus and haemolysin toxin. RfaH is a specific regulator of transcript elongation; its loss increases transcription polarity in these operons without affecting initiation from the operon promoters. The operons of the RfaH-dependent regulon contain a short conserved 5′ sequence, the ops element, deletion of which increases operon polarity to an extent similar to that caused by loss of RfaH. The ops element is also present upstream of polysaccharide gene clusters of Shigella flexneri, Yersinia enterocolitica, Vibrio cholerae and Klebsiella pneumoniae and the RP4 fertility operon of Pseudomonas aeruginosa, suggesting that this is a widely spread control system. The mechanistic coupling of RfaH and the ops element has been demonstrated in vitro and in vivo, and we suggest that the ops element recruits RfaH and potentially other factors to the RNA polymerase complex, modifying the complex to increase its processivity and allowing transcription to proceed over long distances.
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