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  • 1
    Electronic Resource
    Electronic Resource
    Stability of a compact three-period stellarator with quasiaxial symmetry features (2000)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 7 (2000), S. 2546-2553 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The ideal magnetohydrodynamic stability properties of a three-field period stellarator with quasiaxial symmetry features proposed by Garabedian and Ku [Phys. Plasmas 6, 645 (1999)] are investigated. The stability limit imposed by local ballooning and Mercier modes allows β∼5.3% to be achieved by tailoring the pressure profile. For this system, the global external kink modes are shown to be stable. To demonstrate this, a sequence of configurations has been developed that varies from the reference system proposed to the equivalent axisymmetric tokamak. A scan of kink stability reveals that the configuration with a plasma boundary shape that corresponds to 80% of the basic three-period stellarator and 20% of the tokamak becomes marginal to global external modes. The normal magnetic field line curvature and the local magnetic shear play crucial roles in the stability properties. The local magnetic shear develops a helical stripe with increasing three-dimensional plasma deformation that inhibits the formation of global structures. The normal magnetic field line curvature, however, becomes locally amplified by the three-dimensional plasma deformation in regions where the local magnetic shear vanishes. As a result, the susceptibility to localized ballooning mode destabilization is enhanced. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.874095
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  • 2
    Electronic Resource
    Electronic Resource
    Recent advances in the design of quasiaxisymmetric stellarator plasma configurations : The 42nd annual meeting of the division of plasma physics of the American Physical Society and the 10th international congress on plasma physics (2001)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 8 (2001), S. 2083-2094 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Strategies for the improvement of quasiaxisymmetric stellarator configurations are explored. Calculations of equilibrium flux surfaces for candidate configurations are also presented. One optimization strategy is found to generate configurations with improved neoclassical confinement, simpler coils with lower current density, and improved flux surface quality relative to previous designs. The flux surface calculations find significant differences in the extent of islands and stochastic regions between candidate configurations. (These calculations do not incorporate the predicted beneficial effects of perturbed bootstrap currents.) A method is demonstrated for removing low order islands from candidate configurations by relatively small modifications of the configuration. One configuration is identified as having particularly desirable properties for a proposed experiment. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1351826
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  • 3
    Electronic Resource
    Electronic Resource
    External kinks in plasmas with helical boundary deformation and net toroidal current (1997)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 4 (1997), S. 3482-3492 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The global ideal magnetohydrodynamic (MHD) stability of plasmas with a prescribed helical boundary deformation and nonvanishing toroidal current is investigated with respect to the (m,n) external modes with n=1,2,3, and m=n+1. L= 2, 3 and mixtures (of both) configurations were studied by systematically varying parameters such as the aspect ratio, the number of equilibrium field periods, the toroidal current density, and the pressure profiles. The study focuses mainly on the q〈2 area. It is shown that a fixed helical boundary deformation can stabilize the (m,n) external modes with n=1,2,3, and m=n+1 at values of β(similar, equals)1%–2%. These modes are unstable in the circular tokamak at the same value of β. If δ is a measure of the plasma boundary deformation, then windows of stability [δminδmax] may exist depending strongly on equilibrium parameters. The results are described in terms of stability zones in the (qaxis,qedge) plane. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.872245
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  • 4
    Electronic Resource
    Electronic Resource
    Erratum: "Spectrum of ballooning instabilities in a stellarator'' [Phys. Plasmas 3, 275 (1996)] (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 3520-3520 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.872084
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  • 5
    Electronic Resource
    Electronic Resource
    Local ideal magnetohydrodynamic stability of a quasihelically symmetric stellarator (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 3713-3719 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The ideal magnetohydrodynamic stability with respect to localized ballooning modes and the Mercier criterion of a quasihelically symmetric stellarator configuration is investigated. Configurations with an enhanced magnetic mirror and a deeper magnetic well that can be achieved by energizing a set of auxiliary coils have also been analyzed. In all cases, the Mercier criterion imposes a slightly stricter β limit than the ballooning modes. Using relatively broad pressure profiles, the Mercier criterion yields β=0.4% for the standard configuration, β=0.3% for the high mirror configuration and β=1.3% for the deep well configuration. The comparable ballooning limits are β=0.7%, β=0.6%, and β=1.66% for the standard, high mirror, and deep well configurations, respectively. With a more peaked pressure profile, ballooning instabilities near the center of the plasma and Mercier modes near the edge of the plasma limit the performance of the standard configuration at β(approximately-equal-to)0.8%. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.871504
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  • 6
    Electronic Resource
    Electronic Resource
    Spectrum of ballooning instabilities in a stellarator (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 275-280 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The recent revival of interest in the application of the "ballooning formalism'' to low-frequency plasma instabilities has prompted a comparison of the Wentzel–Kramers–Brillouin (WKB) ballooning approximation with an (in principle) exact normal mode calculation for a three-dimensional plasma equilibrium. Semiclassical quantization, using the ideal magnetohydrodynamic (MHD) ballooning eigenvalue to provide a local dispersion relation, is applied to a ten-field period stellarator test case. Excellent qualitative agreement, and good quantitative agreement is found with predictions from the TERPSICHORE code [D. V. Anderson et al., Int. J. Supercomput. Appl. 4, 34 (1990)] for toroidal mode numbers from 1 to 14 and radial mode numbers from 0 to 2. The continuum bands predicted from three-dimensional WKB theory are too narrow to resolve. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.871853
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  • 7
    Electronic Resource
    Electronic Resource
    Effect of quadrupole fields on local three-dimensional (3-D) ideal magnetohydrodynamic (MHD) stability in torsatrons (1994)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 1 (1994), S. 793-795 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The local three-dimensional (3-D) ideal magnetohydrodynamic (MHD) stability properties of a 12-field period torsatron device of moderate aspect ratio are investigated as a function of an externally applied quadrupole magnetic field. It is demonstrated that with a parabolic pressure profile the volume-averaged β limit due to the ballooning instabilities decreases from β(approximately-equal-to)2% to β(approximately-equal-to)1.5% as the current Imid in the quadrupole field coils normalized to that in the main helical coils increases from 0 to 0.19.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.870773
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  • 8
    Electronic Resource
    Electronic Resource
    Normal curvature, local magnetic shear and parallel current density in tokamaks and torsatrons (1997)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 4 (1997), S. 153-161 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Reversed magnetic shear is associated with large negative surface averaged parallel current densities that increase in magnitude towards the plasma edge and guarantee stability at large β in tokamaks. The surface averaged parallel current density is smaller and decreases in magnitude radially in conventional shear tokamaks. This results in much lower magnetohydrodynamic (MHD) stable β. The parallel current density in torsatrons is dominated by the Pfirsch–Schlüter currents, the global shear contributes negligibly to the local shear and the β limits are comparable to that of a conventional tokamak. A simplistic evaluation of the local sign of the driving terms in the energy principle does not provide a useful guide to even qualitatively describe the MHD stability of a device. Ballooning instabilities concentrate in regions where the curvature is most destabilizing. Their localization does not appear to depend on the sign of the interaction between the parallel current density and the local magnetic shear contribution to the driving term. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.872126
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  • 9
    Electronic Resource
    Electronic Resource
    Global linear gyrokinetic simulations in quasi-symmetric configurations (2001)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 8 (2001), S. 3321-3333 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: First global linear study of electrostatic drift waves in two realistic quasisymmetric configurations, namely the Quasi-Axially symmetric Stellarator with three fields periods (QAS3) [P. Garabian and L. P. Ku, Phys. Plasma 6, 645 (1999)] and the Helically Symmetric eXperiment (HSX) [F. S. B. Anderson et al., Trans. Fusion Technol. 27, 273 (1995)], are presented. Effects of the shape of the plasma on the growth rate and frequency of the ion temperature gradient (ITG) driven mode are investigated by varying the quasi-symmetric configurations to an equivalent symmetric system. The calculations have been performed using a three-dimensional (3D) global gyrokinetic code in the magnetic configurations provided by the magnetohydrodynamic (MHD) equilibrium code VMEC [S. P. Hirshman and D. K. Lee, Comput. Phys. Commun. 39, 161 (1986)]. The plasma is modeled by gyrokinetic ions and adiabatic electrons. In QAS3, results are very close to those obtained for a tokamak. The drift waves are only slightly affected by the shape of the plasma or the local magnetic shear. On the other hand, results for the HSX configuration show a clear 3D effect, namely a strong toroidal variation of the drift wave mode structure. This variation is a clear structure of the 3D plasma shape. However, first results show that the growth rate of the ITG driven mode is largely unaffected by this effect. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1374585
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  • 10
    Electronic Resource
    Electronic Resource
    Helical axis stellarator equilibrium and resistive ballooning (1986)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Fluids 29 (1986), S. 1948-1958 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: An approximate analytic solution of the magnetohydrodynamic (MHD) equilibrium equation for large-aspect-ratio, helically symmetric systems is obtained. The resistive ballooning stability of these equilibria is studied numerically and analytically. We find that the ballooning modes scale as the resistivity to the third power. They are driven by the helical nature of the magnetic axis and by the elliptic distortion of the flux surfaces away from a circle.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.865622
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