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  • American Institute of Physics (AIP)  (3)
  • American Association for the Advancement of Science (AAAS)
  • 2005-2009
  • 1990-1994  (4)
  • 1980-1984  (1)
  • 1965-1969
  • 1991  (4)
  • 1980  (1)
  • 1
    Digitale Medien
    Digitale Medien
    High-purity ion beam production at high current densities with a liquid-helium-cooled series-field-coil extraction ion diode (1991)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 70 (1991), S. 2926-2938 
    Details
    ISSN: 1089-7550
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: Experiments are described in which a high-purity, high-power (0.15 TW, 1 MeV) proton beam is generated from an ion source consisting of H2 gas frozen onto a liquid-helium-cooled copper anode at 4.2 K in a series-field-coil extraction diode on the 0.7 TW HydraMITE-II accelerator. Peak anode proton current densities of 2 kA/cm2 were measured. This current density is a factor of 100 higher than those obtained in previous liquid-helium-cooled cryogenic diode experiments on small accelerators and is in the range required for high-power ion beam applications. Thomson parabola, Faraday cup, and carbon activation measurements indicate an ion beam proton fraction close to 100% for the cryogenic source, compared to 50–70% for the standard hydrocarbon anode tested. The cryogenic proton source is believed to consist of no more than a few monolayers of molecular hydrogen. The hydrogen-coated cryogenic anode shows a faster initial anode turn-on than other materials. However, source-limited emission from the thin hydrogen layer results in a somewhat longer current risetime, reduced ion diode efficiency, lower proton current enhancement over the Child–Langmuir limit, and a proton spectrum of lower average energy than for the hydrocarbon anode. Techniques to overcome these limitations are discussed. Cryogenic ion sources consisting of frozen N2, CH4, and Ne have also been studied. In each case, high intensity beams consisting predominantly of components of the refrigerated gas were produced.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.349317
    Standort Signatur Erwartet Verfügbarkeit
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    Artikel (Nationallizenzen)
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  • 2
    Digitale Medien
    Digitale Medien
    Twistor representation of Yang–Mills fields on Minkowski space (1991)
    College Park, Md. : American Institute of Physics (AIP)
    Journal of Mathematical Physics 32 (1991), S. 259-265 
    Details
    ISSN: 1089-7658
    Quelle: AIP Digital Archive
    Thema: Mathematik , Physik
    Notizen: A correspondence between (not necessarily self-dual nor anti-self-dual) Yang–Mills fields on Minkowski space, and pairs of cohomology classes γ, φ, on null twistor space PN(large-closed-square) is established; γ∈H1CR (PN(large-closed-square),@sg) defines a deformed Cauchy–Riemman (C–R)(γ) structure on a principal bundle over PN, and φ∈H1CR(γ) (PN(large-closed-square),O(−4)⊗@sg). The correspondence depends on the choice of a spacelike hyperplane in Minkowski space. Here, γ and φ provide initial values in the spin bundle over this hyperplane for a system of evolution equations, along the null geodesic spray congruence in the spin bundle over Minkowski space.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.529129
    Standort Signatur Erwartet Verfügbarkeit
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  • 3
    Digitale Medien
    Digitale Medien
    An experimental study of the molecular mixing process in an axisymmetric laminar vortex ring (1991)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 3 (1991), S. 1385-1392 
    Details
    ISSN: 1089-7666
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: Results are presented from an experimental study of the molecular mixing of a dynamically passive conserved scalar quantity in an axisymmetric laminar vortex ring. The experiments are based on highly resolved laser-induced fluorescence imaging measurements of the scalar field ζ(x,t) in the diametral plane of the ring, from which the evolution of the molecular mixing rate field ∇ζ⋅∇ζ(x,t) can be directly examined. In particular, the structure and dynamics of the mixing process are addressed during the three characteristic stages in the ring evolution, namely, (i) the ring generation stage, (ii) the ring pinch-off stage, and (iii) the asymptotic stage of the ring. Results show a layering of the mixing process in which the diffusional cancellation term ∇(∇ζ):∇(∇ζ) plays a major role in setting the overall mixing rate achieved. The scalar field measurements are also used to extract detailed information about the underlying velocity field in the ring.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.858069
    Standort Signatur Erwartet Verfügbarkeit
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  • 4
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    Unbekannt
    Evidence for biased gene conversion in concerted evolution of ribosomal DNA (1991)
    American Association for the Advancement of Science (AAAS)
    Details
    Publikationsdatum: 1991-01-18
    Beschreibung: Concerted evolution is the production and maintenance of homogeneity within repeated families of DNA. Two mechanisms--unequal crossing over and biased gene conversion--have been the principal explanations of concerted evolution. Concerted evolution of ribosomal DNA (rDNA) arrays is thought to be largely the result of unequal crossing over. However, concerted evolution of rDNA in parthenogenetic lizards of hybrid origin is strongly biased toward one of two parental sequences, which is consistent with biased gene conversion as the operative mechanism. The apparent gene conversions are independent of initial genome dosage and result in homogenization of rDNA arrays across all nucleolar organizer regions.〈br /〉〈span class="detail_caption"〉Notes: 〈/span〉Hillis, D M -- Moritz, C -- Porter, C A -- Baker, R J -- New York, N.Y. -- Science. 1991 Jan 18;251(4991):308-10.〈br /〉〈span class="detail_caption"〉Author address: 〈/span〉Department of Zoology, University of Texas, Austin 78712.〈br /〉〈span class="detail_caption"〉Record origin:〈/span〉 〈a href="http://www.ncbi.nlm.nih.gov/pubmed/1987647" target="_blank"〉PubMed〈/a〉
    Schlagwort(e): Animals ; *Biological Evolution ; Blotting, Southern ; DNA, Ribosomal/*genetics ; Gene Conversion ; Karyotyping ; Lizards ; Nucleic Acid Hybridization ; Parthenogenesis ; Restriction Mapping
    Print ISSN: 0036-8075
    Digitale ISSN: 1095-9203
    Thema: Biologie , Chemie und Pharmazie , Informatik , Medizin , Allgemeine Naturwissenschaft , Physik
    Publiziert von American Association for the Advancement of Science (AAAS)
    Standort Signatur Erwartet Verfügbarkeit
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  • 5
    facet.materialart.
    Unbekannt
    Between disciplines (1980)
    American Association for the Advancement of Science (AAAS)
    Details
    Publikationsdatum: 1980-08-29
    Beschreibung: 〈br /〉〈span class="detail_caption"〉Notes: 〈/span〉Porter, A L -- Rossini, F A -- Chubin, D E -- Connolly, T -- New York, N.Y. -- Science. 1980 Aug 29;209(4460):966.〈br /〉〈span class="detail_caption"〉Record origin:〈/span〉 〈a href="http://www.ncbi.nlm.nih.gov/pubmed/17747211" target="_blank"〉PubMed〈/a〉
    Print ISSN: 0036-8075
    Digitale ISSN: 1095-9203
    Thema: Biologie , Chemie und Pharmazie , Informatik , Medizin , Allgemeine Naturwissenschaft , Physik
    Publiziert von American Association for the Advancement of Science (AAAS)
    Standort Signatur Erwartet Verfügbarkeit
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