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  • American Institute of Physics (AIP)  (3)
  • 2000-2004  (3)
  • 1
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: The specific size and structure of the edge current profile has important effects on the magnetohydrodynamic stability and ultimate performance of many advanced tokamak (AT) operating modes. This is true for both bootstrap and externally driven currents that may be used to tailor the edge shear. Absent a direct local measurement of j(r), the best alternative is a determination of the poloidal field. Measurements of the precision (0.1°–0.01° in magnetic pitch angle and 1–10 ms) necessary to address issues of stability and control and provide constraints for EFIT are difficult to do in the region of interest (ρ=0.9–1.1). Using Zeeman polarization spectroscopy of the 2S–2P lithium resonance line emission from the DIII-D LIBEAM [D. M. Thomas, Rev. Sci. Instrum. 66, 806 (1995); D. M. Thomas, A. W. Hyatt, and M. P. Thomas, Rev. Sci. Instrum. 61, 340 (1990)] measurements of the various field components may be made to the necessary precision in exactly the region of interest to these studies. Because of the negligible Stark mixing of the relevant atomic levels, this method of determining j(r) is insensitive to the large local electric fields typically found in enhanced confinement (H mode) edges, and thus avoids an ambiguity common to motional Stark effect measurements of B. Key issues for utilizing this technique include good beam quality, an optimum viewing geometry, and a suitable optical prefilter to isolate the polarized emission line. A prospective diagnostic system for the DIII-D AT program will be described. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
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  • 2
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A room temperature free shear strain of 5.7% is reported in a single crystal of Ni–Mn–Ga having a composition close to the Heusler alloy Ni2MnGa. A twin boundary was created in a 2 mm×2 mm×25 mm single crystal using a permanent magnet with surface field strength of about 320 000 A/m. A sharp 6.5° bend occurs in the sample at the twin boundary. The surface magnetization changes abruptly across this boundary. By moving the sample relative to the edge of the magnet, we were able to sweep the boundary back and forth along the crystal length. Surface magnetization was measured using a Hall probe and the results confirm that the easy axis is the tetragonal c axis. Powder x-ray diffraction shows that the fcc to body-centered-tetragonal bct martensitic transition of this material involved a 6% reduction of the bct cell c/a ratio, from (square root of 2) to about 1.33. The maximum achievable strain is thus estimated to be 6.2%. The twin planes in the system are the {112}bct and were observed to lie almost normal to the long axis of the sample tested. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
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  • 3
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We re-examine the ionization-detected ultraviolet absorption spectrum of the 3pπ 2Π←X 2A′ transition in HCO and DCO using a high-power visible laser to enhance the observation of first-photon resonant features. This technique, which we term here, assisted REMPI, significantly improves the signal-to-noise ratio of the spectrum, making many weak vibronic sub-bands visible for the first time. A comprehensive fit to the structure evident in a progression of bending levels from (000) to (040) refines the assignment of Song and Cool [X. M. Song and T. A. Cool, J. Chem. Phys. 96, 8664 (1992)] to yield a set of rotational constants that vary with K in relation to v2, together with a higher-order contribution to the Renner–Teller splitting in HCO, which is mirrored in DCO for all levels but (040). The (040) band falls at a frequency that is commensurate with that of CD stretch, and Fermi resonance between 3pπ 2Π(1000)Π and the higher-energy (040) K=1(Π) component gives rise to an added splitting that increases the energy of this (040) component and causes an apparent increase in the Renner parameter. © 2002 American Institute of Physics.
    Type of Medium: Electronic Resource
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