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  • 1
    Electronic Resource
    Electronic Resource
    Magnetization curling in perpendicular iron particle arrays (alumite media) (1988)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 64 (1988), S. 2016-2021 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Alumite has been investigated by measuring the hysteresis loops for different field directions to determine the influence of the particle interactions in a perpendicular oriented particle medium on the angular dependence of the coercivity and the hysteresis loss. Alumite is an ideal model material for this purpose because the particle size and particle distance are well defined and can be varied independently. It is shown that the coercivity of the particle array can be identified with the nucleation field of one single particle for perpendicular fields. It is clear from the radius dependence of the coercivity that the reversal mode is the curling mode. The angular dependence of the coercivity does not agree with the well-known theoretical curve for one particle, which is caused by the particle interactions. The same can be said about the angular dependence of the hysteresis loss. Calculations of hysteresis loops of particle arrays with interacting particles show the influence of these interactions on the angular dependence of the coercivity.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.341758
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  • 2
    Electronic Resource
    Electronic Resource
    The pseudospectrum of the resistive magnetohydrodynamics operator: Resolving the resistive Alfvén paradox (1994)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 1 (1994), S. 3151-3160 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The "Alfvén paradox'' is that as resistivity decreases, the discrete eigenmodes do not converge to the generalized eigenmodes of the ideal Alfvén continuum. To resolve the paradox, the ε-pseudospectrum of the resistive magnetohydrodynamic (RMHD) operator is considered. It is proven that for any ε, the ε-pseudospectrum contains the Alfvén continuum for sufficiently small resistivity. Formal ε-pseudoeigenmodes are constructed using the formal Wentzel–Kramers–Brillouin–Jeffreys solutions, and it is shown that the entire stable half-annulus of complex frequencies with ρ||ω||2=||k⋅B(x)||2 is resonant to order ε, i.e., belongs to the ε-pseudospectrum. The resistive eigenmodes are exponentially ill-conditioned as a basis and the condition number is proportional to exp(R1/2M), where RM is the magnetic Reynolds number.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.870468
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  • 3
    Electronic Resource
    Electronic Resource
    Modeling the excitation of global Alfvén modes by an external antenna in the Joint European Torus (JET) (1995)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 2 (1995), S. 1605-1613 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The active excitation of global Alfvén modes using the saddle coils in the Joint European Torus (JET) [Plasma Physics and Controlled Nuclear Fusion Research 1984, Proceedings of the 10th International Conference, London (International Atomic Energy Agency, Vienna, 1985), Vol. 1, p. 11] as the external antenna, will provide information on the damping of global modes without the need to drive the modes unstable. For the modeling of the Alfvén mode excitation, the toroidal resistive magnetohydrodynamics (MHD) code CASTOR (Complex Alfvén Spectrum in TORoidal geometry) [18th EPS Conference On Controlled Fusion and Plasma Physics, Berlin, 1991, edited by P. Bachmann and D. C. Robinson (The European Physical Society, Petit-Lancy, 1991), Vol. 15, Part IV, p. 89] has been extended to calculate the response to an external antenna. The excitation of a high-performance, high beta JET discharge is studied numerically. In particular, the influence of a finite pressure is investigated. Weakly damped low-n global modes do exist in the gaps in the continuous spectrum at high beta. A pressure-driven global mode is found due to the interaction of Alfvén and slow modes. Its frequency scales solely with the plasma temperature, not like a pure Alfvén mode with a density and magnetic field.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.871310
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  • 4
    Electronic Resource
    Electronic Resource
    More on core-localized toroidal Alfvén eigenmodes (1995)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 2 (1995), S. 3401-3406 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A novel type of ideal toroidal Alfvén eigenmode, localized in the low-shear core region of a tokamak plasma, is shown to exist, whose frequency is near the upper continuum of the toroidal Alfvén gap. This mode converts to a kinetic-type toroidal Alfvén eigenmode above a critical threshold that depends on aspect ratio, pressure gradient, and shear. Opposite to the usual ideal toroidal Alfvén eigenmode, this new mode is peaked in amplitude on the small-major-radius side of the plasma. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.871174
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  • 5
    Electronic Resource
    Electronic Resource
    Nonlinear interaction of fast particles with Alfvén waves in toroidal plasmas (1997)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 4 (1997), S. 2597-2611 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A numerical algorithm to study the nonlinear, resonant interaction of fast particles with Alfvén waves in tokamak geometry has been developed. When the instability is sufficiently weak, it is known that the wave-particle trapping nonlinearity will lead to mode saturation before wave–wave nonlinearities are appreciable. The spectrum of linear modes can thus be calculated using a magnetohydrodynamic normal-mode code, then nonlinearly evolved in time in an efficient way according to a two-timescale Lagrangian dynamical wave model. The fast particle kinetic equation, including the effect of orbit nonlinearity arising from the mode perturbation, is simultaneously solved for the deviation, δf=f−f0, from an initial analytic distribution f0. High statistical resolution allows linear growth rates, frequency shifts, resonance broadening effects, and nonlinear saturation to be calculated quickly and precisely. The results have been applied to an International Thermonuclear Experimental Reactor [ITER EDA Doc. Series No. 7 (International Atomic Energy Agency, Vienna, 1996), p. V-32] instability scenario. Results show that weakly damped core-localized modes alone cause negligible alpha transport in these reactor-like plasmas—even with growth rates one order of magnitude higher than expected values. However, the possibility of significant transport in reactor-type plasmas due to weakly unstable global modes remains an open question. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.872348
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  • 6
    Electronic Resource
    Electronic Resource
    Free boundary resistive modes in tokamaks (1993)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 5 (1993), S. 1545-1558 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: There exist a number of observations of magnetohydrodynamic (MHD) activity that can be related to resistive MHD modes localized near the plasma boundary. To study the stability of these modes, a free boundary description of the plasma is essential. The resistive plasma–vacuum boundary conditions have been implemented in the fully toroidal resistive spectral code castor (Complex Alfvén Spectrum in Toroidal Geometry) [Proceedings of the 18th Conference on Controlled Fusion and Plasma Physics, Berlin, edited by P. Bachmann and D. C. Robinson (European Physical Society, Petit-Lancy, Switzerland, 1991), p. 89]. The influence of a free boundary, as compared to a fixed boundary on the stability of low-m tearing modes, is studied. It is found that the stabilizing (toroidal) effect of a finite pressure due the plasma compression is lost in the free boundary case for modes localized near the boundary. Since the stabilization due to the favorable average curvature in combination with a pressure gradient near the boundary is small, the influence of the pressure on the stability is much less important for free boundary modes than for fixed boundary modes.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.860894
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  • 7
    Electronic Resource
    Electronic Resource
    High density, high performance high-confinement-mode plasmas in the Joint European Torus (JET) : Review,Tutorial and Invited Papers from the 43rd Annual Meeting of the APS Division of Plasma Physics (2002)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 9 (2002), S. 2103-2112 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Recent experiments at the Joint European Torus [Rebut et al., Fusion Eng. Des. 22, 7 (1993)] aim to improve confinement quality in high-confinement-mode (H-mode) plasmas at high densities. Energy confinement time as predicted by the International Thermonuclear Experimental Reactor ITER-H98(y,2) scaling at densities near or in excess of 85% of the Greenwald density limit scaling has been obtained by (i) strong plasma shaping (triangularity 0.35〈δ〈0.5), or (ii) impurity seeding, or (iii) high-field side pellet injection. Slow peaking of central density without confinement degradation is observed. Loss of sawteeth and core impurity accumulation is prevented by central ion cyclotron resonance heating. In high triangularity and impurity seeded plasmas, reduction of average power loss associated with type I edge localized modes (ELMs) is found which is attributed to the occurrence of additional losses in between ELMs. Broad band magnetic fluctuations are seen which are reminiscent of regimes with small ELMs in other tokamaks. Plasma configurations have been varied to find best combinations of edge pedestal parameters and small ELM losses. © 2002 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1456067
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  • 8
    Electronic Resource
    Electronic Resource
    Modeling of diamagnetic stabilization of ideal magnetohydrodynamic instabilities associated with the transport barrier (2001)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 8 (2001), S. 4292-4305 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A new code, MISHKA-D (Drift MHD), has been developed as an extension of the ideal magnetohydrodynamics (MHD) code MISHKA-1 in order to investigate the finite gyroradius stabilizing effect of ion diamagnetic drift frequency, ω*i, on linear ideal MHD eigenmodes in tokamaks in general toroidal geometry. The MISHKA-D code gives a self-consistent computation of both stable and unstable eigenmodes with eigenvalues |γ|≅ω*i in plasmas with strong radial variation in the ion diamagnetic frequency. Test results of the MISHKA-D code show good agreement with the analytically obtained ω*i spectrum and stability limits of the internal kink mode, n/m=1/1, used as a benchmark case. Finite-n ballooning and low-n kink (peeling) modes in the edge transport barrier just inside the separatrix are studied for high confinement mode (H-mode) plasmas with the ω*i effect included. The ion diamagnetic stabilization of the ballooning modes is found to be most effective for narrow edge pedestals. For low enough plasma density the ω*i stabilization can lead to a second zone of ballooning stability, in which all the ballooning modes are stable for any value of the pressure gradient. For internal transport barriers typical of the Joint European Torus [JET, P. H. Rebut et al., Proceedings of the 10th International Conference, Plasma Physics and Controlled Nuclear Fusion, London (International Atomic Energy Agency, Vienna, 1985), Vol. I, p. 11] optimized shear discharges, the stabilizing influence of ion diamagnetic frequency on the n=1 global pressure driven disruptive mode is studied. A strong radial variation of ω*i is found to significantly decrease the stabilizing ω*i effect on the n=1 mode, in comparison with the case of constant ω*i estimated at the foot of the internal transport barrier. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1398573
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