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  • 1
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    Role of density gradient driven trapped electron mode turbulence in the H-mode inner core with electron heating (2016)
    American Institute of Physics (AIP)
    Details
    Publication Date: 2016-05-11
    Description: A series of DIII-D [J. L. Luxon, Nucl. Fusion 42 , 614 (2002)] low torque quiescent H-mode experiments show that density gradient driven trapped electron mode (DGTEM) turbulence dominates the inner core of H-mode plasmas during strong electron cyclotron heating (ECH). Adding 3.4 MW ECH doubles T e / T i from 0.5 to 1.0, which halves the linear DGTEM critical density gradient, locally reducing density peaking, while transport in all channels displays extreme stiffness in the density gradient. This suggests that fusion α -heating may degrade inner core confinement in H-mode plasmas with moderate density peaking and low collisionality, with equal electron and ion temperatures, key conditions expected in burning plasmas. Gyrokinetic simulations using GYRO [J. Candy and R. E. Waltz, J. Comput. Phys. 186 , 545 (2003)] (and GENE [Jenko et al. , Phys. Plasmas 7 , 1904 (2000)]) closely match not only particle, energy, and momentum fluxes but also density fluctuation spectra from Doppler backscattering (DBS), with and without ECH. Inner core DBS density fluctuations display discrete frequencies with adjacent toroidal mode numbers, which we identify as DGTEMs. GS2 [Dorland et al ., Phys. Rev. Lett. 85 , 5579 (2000)] predictions show the DGTEM can be suppressed, to avoid degradation with electron heating, by broadening the current density profile to attain q 0 〉 q min 〉 1 .
    Print ISSN: 1070-664X
    Electronic ISSN: 1089-7674
    Topics: Physics
    Published by American Institute of Physics (AIP)
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  • 2
    Electronic Resource
    Electronic Resource
    Orientation of chiral heptahelicene C30H18 on copper surfaces: An x-ray photoelectron diffraction study (2001)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 115 (2001), S. 1020-1027 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The orientation and the intramolecular relaxation due to adsorption of the chiral phenanthrene-derivative heptahelicene, C30H18, on Cu(111) and Cu(332) surfaces have been investigated by means of angle-scanned full-hemispherical x-ray photoelectron diffraction. Although the C 1s diffraction patterns of the adsorbed submonolayer coverage helicene films exhibit scattering anisotropies of less than two percent, a detailed analysis involving simple molecular mechanics calculations of the atomic coordinates, photoelectron diffraction single-scattering cluster calculations and an R-factor analysis permits the determination of the helicene molecular orientation. On Cu(111), the molecules are found to bind to the substrate surface via their terminal phenanthrene group oriented parallel to the surface plane, while on Cu(332) the three terminal C-6 rings are oriented parallel to the (111) terrace plane. Six azimuthal molecular orientations are found to coexist on Cu(111), on Cu(332), however, the step–molecule interaction leads to a unique azimuthal alignment of the heptahelicene molecules. The heptahelicene on Cu(332) system thus represents a chiral surface with single-phase orientational order. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1377886
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  • 3
    Electronic Resource
    Electronic Resource
    The interaction of hydrogen with a cobalt(101¯0) surface (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 5388-5401 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The adsorption of hydrogen on a cobalt(101¯0) surface was investigated in ultrahigh vacuum (UHV) between 85 and 500 K using Video-LEED, temperature-programmed thermal desorption (TPD), work function (ΔΦ) measurements, and high-resolution electron energy loss spectroscopy (HREELS). Between 90 and 200 K, hydrogen adsorbs dissociatively with high sticking coefficient (s0≥0.8) via precursor kinetics and forms, with increasing exposure, a c(2×4), a p2mg (2×1) and a (1×2) LEED structure (hydrogen coverages aitch-thetaH=0.5, 1.0, and 1.5, respectively). While the first two structures represent true ordered hydrogen phases there is strong evidence that the (1×2) phase is reconstructed, likely in a paired-row configuration. The formation of the (1×2) phase is slightly thermally activated; its decomposition produces a sharp thermal desorption maximum (α state) appearing on the low-energy side of a β-TPD signal which reflects the hydrogen desorbing from the unreconstructed surface. The activation energies for desorption from the α and β states are 62 and 80 kJ/mol, respectively. Chemisorption in the β state [(2×1) phase up to aitch-thetaH=1.0] is associated with a ΔΦ of +207 meV, while the fully developed (1×2) reconstructed phase (α state) causes a ΔΦ of approximately −122 meV resulting in an overall work function change of +85 meV at saturation. From HREELS, we determine the H adsorption site in all superstructures to be threefold with a local CS symmetry. Our results are discussed and compared with previous findings for similar metal–hydrogen interaction systems.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.467392
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  • 4
    Electronic Resource
    Electronic Resource
    Hexagonal nanotubes of ZnS by chemical conversion of monocrystalline ZnO columns (2001)
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 78 (2001), S. 3687-3689 
    Details
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Monocrystalline ZnO columns grown in electrodeposition were converted to ZnS using ion exchange reactions in H2S or S vapor. At ∼400 °C the reaction with H2S only affects a thin layer of 10–30 nm thickness at the surface of the ZnO crystallites, and ZnS-coated ZnO columns are produced. Exploiting the large difference in etch resistance between ZnS and ZnO, the ZnO core of the columns can be removed, and a tubular structure of ZnS can be prepared. Typical dimensions of the ZnS tubes are a length of 1–3 μm, a diameter of 100–300 nm, and a wall thickness of 10–30 nm. The ZnS tubes have the same distribution, alignment, and surface morphology as the original ZnO columns. The reaction in S vapor is suitable to produce solid ZnS columns. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1376427
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