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  • 1990-1994  (16)
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Material
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Year
  • 1
    Electronic Resource
    Electronic Resource
    A numerical study of the Alfvén continuum damping of toroidal Alfvén eigenmodes (1994)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 1 (1994), S. 1214-1225 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The Alfvén continuum damping of the toroidal Alfvén eigenmodes is explicitly formulated for a large aspect ratio force-free tokamak using analytic continuation. A set of jump conditions across the Alfvén singularities are derived which have to be satisfied by the mode amplitudes. The method is then applied to a simple model equilibrium. The characteristics of the modes and their damping are investigated. The method is expected to be generalizable to models with more complete plasma dynamics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.870718
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    Paper (German National Licenses)
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  • 2
    Electronic Resource
    Electronic Resource
    The magnetic and transport properties of high-critical-current Tl2Ba2Ca2Cu3Ox superconducting thin films (1991)
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 59 (1991), S. 1123-1125 
    Details
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Highly c-axis-oriented Tl2Ba2Ca2Cu3Ox superconducting thin films with the critical current density as high as 1×107 A/cm2 at 87 K were obtained by single-target dc sputtering on LaAlO3(100) substrates followed by encapsulated compensate annealing. The strong linear temperature dependence of zero-field critical current at low temperatures suggests a creep-dominated behavior. Preliminary field dependence studies on the critical current density in low-field regime (〈2 kOe) shows no evidence of hysteresis commonly encountered in films with higher granularity. The scaling behavior of the global pinning force density exhibits strong resemblance to conventional type-II superconductors with the pinning force density of the same order of magnitude as that in strong pinned NbN thin films.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.106364
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    Paper (German National Licenses)
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  • 3
    Electronic Resource
    Electronic Resource
    Effects of ZrO2 on the texturing and properties of melt-processed Y1Ba2Cu3O7−δ (1994)
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 64 (1994), S. 2306-2308 
    Details
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The addition of ZrO2 is found to be very effective as the nucleation sites to promote the oriented growth of Y1Ba2Cu3O7−δ (YBCO) during peritectic solidification. Extended regions of well- textured YBCO grains with superior superconducting properties of the YBCO bulk are readily obtained by a single-step melt process at much lower process temperatures. At 77 K, the platelike Josephson weak-link behaviors observed in pure melt-processed YBCO have been effectively eliminated, indicating that the coherency between the spherulitic superconducting YBCO grains may have been greatly improved by ZrO2 additions.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.111652
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    Paper (German National Licenses)
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  • 4
    Electronic Resource
    Electronic Resource
    Global Alfvén modes: Theory and experimentf| (1993)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 5 (1993), S. 2546-2553 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: It is shown that the theoretical predictions and experimental observations of toroidicity-induced Alfvén eigenmodes (TAE's) are now in good agreement, with particularly detailed agreement in the mode frequencies. Calculations of the driving and damping rates predict the importance of continuum damping for low toroidal mode numbers and this is confirmed experimentally. However, theoretical calculations in finite-β, shaped discharges predict the existence of other global Alfvén modes, in particular the ellipticity-induced Alfvén eigenmode (EAE) and a new mode, the beta-induced Alfvén eigenmode (BAE). The BAE mode is calculated to be in or below the same frequency range as the TAE mode and may contribute to the experimental observations at high β. Experimental evidence and complementary analyses are presented confirming the presence of the EAE mode at higher frequencies.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.860742
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    Paper (German National Licenses)
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  • 5
    Electronic Resource
    Electronic Resource
    High-beta discharges in the DIII-D tokamak (1990)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 2 (1990), S. 1280-1286 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Low-q (q95〈3) double-null divertor discharges with values of the volume-average toroidal beta as high as 9.3% have been operated in the DIII-D tokamak [Fusion Technol. 8, 441 (1985)]. In discharges with q95≈5, values of βT/(I/aB) as high as 5 have been obtained. These discharges are shown to be at or below the stability limit to the value of beta for infinite-n, ideal ballooning modes. The discharges are significantly below the beta limit for ideal,low toroidal mode number kink modes. The kink mode beta limit is shown to be strongly dependent on the radial profiles of plasma pressure and current. The theoretical beta limit in DIII-D is shown to be in the range βT/(I/aB)=4 –5 depending on the value of I/aB, and this is consistent with the experiment. High-beta discharges have been operated with ion temperature up to 17 keV. Steady-state, high-beta, low-q operation is demonstrated by a discharge with I/aB=2.6, q95=2.7, in which βT〉7% is maintained for 1.5 sec.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.859543
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    Paper (German National Licenses)
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  • 6
    Electronic Resource
    Electronic Resource
    A new formulation of the resistive tearing mode stability criterion (1993)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 5 (1993), S. 1593-1604 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A new formulation of the toroidal, finite beta, resistive tearing stability problem is presented. As in standard tearing mode theory, the mode structure throughout most of the configuration is determined by an ideal, inertia-free model. Thus, it is very closely related to that obtained from standard ideal magnetohydrodynamic (MHD) numerical stability programs that depend on an energy principle. The effects of inertia, resistivity, and any other plasma properties are important only in thin layers enclosing resonant surfaces. These surfaces are distinguished by the fact that they are composed of closed field lines. Instability growth rates are obtained from the condition of matching between the inner and outer regions. The data needed from the outer region for matching are conventionally reduced to a quantity Δ', but in toroidally coupled axisymmetric systems the relevant quantity is a matrix. A previous paper [Pletzer and Dewar, J. Plasma Phys. 45, 427 (1991)] presented a relation between an extension of the ideal energy and the information from the outer region that is needed in matching to the inner layers. Here, this is used to construct a relation for the tearing mode growth rates directly in terms of an extension of the ideal energy matrix. This demonstrates a convenient way to extend the numerical programs for ideal stability to include stability against tearing modes.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.860899
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    Paper (German National Licenses)
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  • 7
    Electronic Resource
    Electronic Resource
    An optimization of beta in the DIII-D tokamak (1992)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 4 (1992), S. 3644-3662 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Accurate equilibrium reconstruction and detailed stability analysis of a strongly shaped, double-null, βT=11% discharge shows that the plasma core is in the second stable regime to ideal ballooning modes. The equilibrium reconstruction using all the available data (coil currents, poloidal magnetic loops, motional Stark effect data, the kinetic pressure profile, the magnetic axis location, and the location of the two q=1 surfaces) shows a region of negative magnetic shear near the magnetic axis, an outer positive shear region, and a low shear region connecting the two. The inner negative shear region allows a large positive shear region near the boundary, even at low q (q95=2.6), permitting a large outer region pressure gradient to be first regime stable. The inner region is in the second stable regime, consistent with the observed axial beta [βT(0)=44%]. In the low shear region p' vanishes, consistent with Mercier stability. This is one way to extend the ballooning limit in shaped plasmas while maintaining stability against external kinks. The n=1 analysis shows that the plasma is unstable to an ideal internal mode, consistent with the experimental observations of a saturated internal m/n=1/1 mode. The core plasma pressure, not being limited by ballooning stability, appears to be reaching a local equilibrium limit at the magnetic axis.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.860373
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    Paper (German National Licenses)
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  • 8
    Electronic Resource
    Electronic Resource
    Second-order toroidicity-induced Alfvén eigenmodes and mode splitting in a low-aspect-ratio tokamak (1992)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 4 (1992), S. 3451-3453 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A variational ideal magnetohydrodynamic stability code is used to compute the toroidicity-induced Alfvén eigenmode (TAE) for toroidal mode number n=1 in full toroidal geometry with circular cross section. At finite aspect ratio, toroidicity also couples poloidal mode numbers m with m+2 to produce a higher frequency gap in the shear Alfvén spectrum, which is associated with a new, second-order TAE mode. It is also found that toroidal coupling between the TAE modes and continuum branches induces splitting of the TAE modes into two or more global modes at slightly different frequencies. Both the new modes and splitting of the TAE modes have important consequences for the identification of TAE modes in experiments.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.860352
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  • 9
    Electronic Resource
    Electronic Resource
    Effects of current profile on the ideal ballooning mode (1992)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 4 (1992), S. 232-240 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The effects of current profile on the ideal ballooning mode for circular and shaped poloidal cross-section plasmas in tokamaks are studied analytically and numerically. The results show that for moderately shaped plasmas the critical normalized beta, βNC, against the ballooning mode increases approximately linearly with the plasma internal inductance li. As the plasma becomes more strongly shaped, this dependence on li becomes weaker, and for a divertor plasma βNC shows a very weak dependence on li for the range of moderate li values considered.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.860438
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    Paper (German National Licenses)
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  • 10
    Electronic Resource
    Electronic Resource
    Higher beta at higher elongation in the DIII-D tokamak (1991)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 3 (1991), S. 2220-2229 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A theoretical and experimental evaluation of axisymmetric stability and axisymmetric control has led to a modification of the vertical position control in the DIII-D tokamak, which now allows operation to within a few percent of the ideal magnetohydrodynamic (MHD) n=0 limit. It is found that the onset the departure from rigid shift behavior in D-shaped plasmas limits plasma elongation to 2.5 in DIII-D. The possibility of avoiding the vertical instability in future tokamaks with highly elongated plasmas is discussed. Recent experiments have focused on utilizing this capability for axisymmetric control to construct plasma shapes optimized to increase the achievable beta. Operation near the axisymmetric stability limit allows an increase in the achieved normalized current Ip/aBT, where Ip is the total plasma current, a is the minor radius, and BT is the toroidal field. Based on stability calculations, an equilibrium was developed to achieve marginal stability simultaneously to axisymmetric, kink, and ballooning instabilities. In the experiment, the shape was altered to higher elongation during the high-beta phase as the current profile broadened. A record high beta for DIII-D of 11% was achieved. The high-beta phase of the discharge lasted 40 msec, approximately one confinement time.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.859639
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    Paper (German National Licenses)
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