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  • 1
    ISSN: 1434-6036
    Keywords: 71.45Gm ; 73.20Mf ; 79.20Kz
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract The response of a spherical metallic particle and a void in a metal on an arbitrary external electrical (gradient) field is given analytically within two models: a macroscopic dielectric description in terms of dielectric functions and a linearized hydrodynamic theory which takes into account boundary effects as well as spatial dispersion. From the response functions, the eigenfrequencies of the collective modes and the electron-energy-loss function are obtained. For small radii we found collective modes extending beyond the classical ℓ = ∞ limit of surface-plasmons up to the volume-plasmon. Concerning electron-energy-losses, our results demonstrate that the “blue-shift” on the collective modes which were observed in potassium clusters or calculated for voids in metals are almost of classical origin and stem from spatial dispersion.
    Type of Medium: Electronic Resource
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  • 2
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: The q-profile measurement presented in this article is based on the observation of pellets injected into the plasma and viewed with a fast-framing camera. The pellets sublimate in the plasma and the ablated material is ionized. The ions move along the magnetic field lines, while an ablation cloud is forming. This ablation cloud contains a small fraction of neutral particles which emit visible light after collisional excitation. It is therefore possible to visualize the magnetic field lines along which the pellet passes. The inclination angle of the magnetic field lines with respect to the torus midplane can be determined from this observation. The results are compared with the inclination angles delivered by an equilibrium code. Further analysis shows that it is not meaningful to determine the q-profile from the data of only the pellet ablation, in the case of an elongated plasma as in ASDEX Upgrade. However, the accuracy of the q-profile determined by an equilibrium code, especially in the plasma center, can be greatly improved by using the pellet measurements as additional input data. © 1997 American Institute of Physics.
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  • 3
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    ISSN: 0570-0833
    Keywords: Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Type of Medium: Electronic Resource
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  • 4
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    ISSN: 0044-8249
    Keywords: Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Type of Medium: Electronic Resource
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  • 5
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Neoclassical tearing modes (NTMs) are studied in the ASDEX Upgrade tokamak [O. Gruber, H.-S. Bosch, S. Günter et al., Nucl. Fusion 39, 1321 (1999)]. The onset βN for NTMs scales with the normalized ion gyroradius ρ*, independent of the normalized collisionality ν* for a wide range of ν*. This scaling is in accordance with both polarization current and χ⊥/χ(parallel) model, if for the latter, the heat flux limit on parallel heat conductivity is taken into account. Analysis of NTM stabilization experiments indicates that the typical seed island size is 1–1.5 cm, again in agreement with both models. Mode coupling to the q=1 surface is a crucial element in determining the impact of the NTM on the discharge performance. Complete stabilization using electron cyclotron current drive (ECCD) has been demonstrated at βN=2.5 and with about 10% of the total heating power. The results are in good qualitative agreement with modeling using the Rutherford equation and in quantitative agreement with a two-dimensional nonlinear cylindrical tearing mode code. A precise positioning of the ECCD microwave beam, so far achieved by feedforward variation of Bt, is required for efficient stabilization. This calls for feedback control of the stabilization scheme. Our plans to implement such a scheme on ASDEX Upgrade are discussed. © 2001 American Institute of Physics.
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  • 6
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 4 (1997), S. 323-328 
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Analysis of transient signals in the diagnostic of fusion plasmas often requires the simultaneous consideration of their time and frequency information. The newly emerging technique of wavelet analysis contains both time and frequency domains. Therefore it can be a valuable tool for the analysis of transients. In this paper the basic method of wavelet analysis is described. As an example, wavelet analysis is applied to the well-known phenomena of mode locking and fishbone instability. The results quantify the current qualitative understanding of these events in terms of instantaneous frequencies and amplitudes and encourage applications of the method to other problems. © 1997 American Institute of Physics.
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  • 7
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The maximum normalized beta achieved in long-pulse tokamak discharges at low collisionality falls significantly below both that observed in short pulse discharges and that predicted by the ideal MHD theory. Recent long-pulse experiments, in particular those simulating the International Thermonuclear Experimental Reactor (ITER) [M. Rosenbluth et al., Plasma Physics and Controlled Nuclear Fusion (International Atomic Energy Agency, Vienna, 1995), Vol. 2, p. 517] scenarios with low collisionality νe*, are often limited by low-m/n nonideal magnetohydrodynamic (MHD) modes. The effect of saturated MHD modes is a reduction of the confinement time by 10%–20%, depending on the island size and location, and can lead to a disruption. Recent theories on neoclassical destabilization of tearing modes, including the effects of a perturbed helical bootstrap current, are successful in explaining the qualitative behavior of the resistive modes and recent results are consistent with the size of the saturated islands. Also, a strong correlation is observed between the onset of these low-m/n modes with sawteeth, edge localized modes (ELM), or fishbone events, consistent with the seed island required by the theory. We will focus on a quantitative comparison between both the conventional resistive and neoclassical theories, and the experimental results of several machines, which have all observed these low-m/n nonideal modes. This enables us to single out the key issues in projecting the long-pulse beta limits of ITER-size tokamaks and also to discuss possible plasma control methods that can increase the soft β limit, decrease the seed perturbations, and/or diminish the effects on confinement. © 1997 American Institute of Physics.
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