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  • 1
    Electronic Resource
    Electronic Resource
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 3 (1991), S. 1865-1870 
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: It is shown that noncircularity of tokamak flux surfaces leads to frequency gaps in the magnetohydrodynamic Alfvén continuum. Within these gaps discrete modes having macroscopic structure are shown to exist and have many common features with toroidicity induced Alfvén eigenmodes. The present work focuses on ellipticity. Since κ−1〉ε in many tokamaks the ellipticity induced Alfvén eigenmode may indeed be a more robust mode. The most global mode couples the m=1, n=1 and m=3, n=1 "cylindrical'' eigenmodes. The region of strong coupling occurs at the q(r)=2 surface and the width of the coupling region is finite and of order (κ−1)a. Furthermore, for typical limiter q(r) profiles satisfying 1(approximately-less-than)q(approximately-less-than)3, the dominant mode harmonics do not intersect the continuum Alfvén spectrum.
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  • 2
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: This paper reviews the current direct-drive ignition capsule designed for the National Ignition Facility (NIF) [M. D. Campbell and W. J. Hogan, Plasma Phys. Control. Fusion 41, B39 (1999)]. The ignition design consists of a cryogenic deuterium–tritium (DT) shell contained within a very thin CH shell. To maintain shell integrity during the implosion, the target is placed on an isentrope approximately three times that of Fermi-degenerate DT (α=3). One-dimensional studies show that the ignition design is robust. Two-dimensional simulations examine the effects on target performance due to laser imprint, power imbalance, and inner- and outer-target-surface roughness. Results from these studies indicate that the capsule gain can be scaled to the ice/vapor surface deformation at the end of the acceleration stage of the implosion. The physical reason for gain reduction as a function of increasing nonuniformities is examined. Simulations show that direct-drive target gains in excess of 30 can be achieved for an inner-ice-surface roughness of 1 μm rms, an on-target power imbalance of 2% rms, and by using the beam-smoothing technique SSD with 1 THz and two color cycles. © 2001 American Institute of Physics.
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  • 3
    Electronic Resource
    Electronic Resource
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 3 (1991), S. 538-544 
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A stability criterion is derived for energetic particle-Alfvén modes. The criterion is valid for arbitrary aspect ratio, arbitrary β, noncircular, axisymmetric tori. The plasma is modeled by a magnetohydrodynamic (MHD) core plus a fully kinetic Vlasov species of hot particles. Electron kinetic effects are neglected. In spite of the complexity associated with the analysis of the Vlasov species, a simple but exact stability boundary is derived. The criterion is very fluidlike in nature, suggesting that accurate evaluation in realistic geometries can be accomplished with perhaps only minor modifications to any one of the existing ideal MHD stability codes.
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  • 4
    Electronic Resource
    Electronic Resource
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 2 (1990), S. 1565-1573 
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Quasianalytic formulas are calculated for the elongation κ and triangularity δ of the plasma surface of a free-boundary tokamak equilibrium. The final results give κ and δ as functions of five quantities: the inverse aspect ratio ε, the poloidal beta βp, the internal inductance li, and the quadrupole and hexapole moments of the externally applied field. The agreement with numerically computed equilibria is found to be quite good when A≥3, κ≤1.5, and δ≤0.2 and when the plasma is limited by the vacuum vessel wall and not diverted by the presence of a separatrix on the plasma surface.
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  • 5
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A model for the deceleration phase and marginal ignition of imploding capsules is derived by solving a set of ordinary differential equations describing the hot-spot energy balance and the shell dynamics including the return shock propagation. It is found that heat flux leaving the hot spot goes back in the form of internal energy and PdV work of the material ablated off the inner-shell surface. Though the hot-spot temperature is reduced by the heat conduction losses, the hot-spot density increases due to the ablated material in such a way that the hot-spot pressure is approximately independent of heat conduction. For hot-spot temperatures exceeding approximately 7 keV, the ignition conditions are not affected by heat conduction losses that are recycled into the hot spot by ablation. Instead, the only significant internal energy loss is due to the hot-spot expansion tamped by the surrounding shell. The change of adiabat induced by the shock is also calculated for marginally igniting shells, and the relation between the in-flight and stagnation adiabats is in general agreement with the numerical fit of LASNEX simulations by Herrmann et al. [Nucl. Fusion 41, 99 (2001)] and the self-similar solution of Kemp et al. [Phys. Rev. Lett. 15, 3336 (2001)]. The minimum kinetic energy required for ignition is also calculated from the same model and shown to be in good agreement with the numerical fit of LASNEX simulations. It is also found that mass ablation leads to a significant reduction of the deceleration phase Rayleigh–Taylor instability growth rates and to the suppression of short wavelength modes. © 2002 American Institute of Physics.
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  • 6
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 7 (2000), S. 2439-2448 
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: It is shown that transonic poloidal flow leads to ideal magnetohydrodynamic tokamak equilibria with radial discontinuities in the density, pressure, and flow velocity profiles. Transonic profiles are defined as having flow velocities ranging from subsonic to supersonic with respect to the poloidal sound speed (csBp/B). The jump of the equilibrium quantities occurs approximately at the sonic surface and its magnitude is of order ε1/2 (ε is the inverse aspect ratio). Because of the large velocity shear at the sonic surface, transonic profiles may improve energy confinement as suggested by current understanding of tokamak plasma turbulence suppression. © 2000 American Institute of Physics.
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  • 7
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 1402-1414 
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The linear stability analysis of accelerated ablation fronts is carried out self-consistently by retaining the effect of finite thermal conductivity. Its temperature dependence is included through a power law (κ∼Tν) with a power index ν(approximately-greater-than)1. The growth rate is derived for Fr(very-much-greater-than)1 (Fr is the Froude number) by using a boundary layer analysis. The self-consistent Atwood number and the ablative stabilization term depend on the mode wavelength, the density gradient scale length, and the power index ν. The analytic formula for the growth rate is shown to be in excellent agreement with the numerical fit of Takabe, Mima, Montierth, and Morse [Phys. Fluids 28, 3676 (1985)] for ν=2.5 and the numerical results of Kull [Phys. Fluids B 1, 170 (1989)] over a large range of ν's. © 1996 American Institute of Physics.
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  • 8
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 2 (1995), S. 3844-3851 
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The cutoff wave number of the ablative Rayleigh–Taylor instability is calculated self-consistently by including the effects of finite thermal conduction. The derived cutoff wave number is quite different from the one obtained with the incompressible fluid (∇⋅v˜=0) or sharp boundary models, and it is strongly dependent on thermal conductivity (K∼Tν) and the Froude number (Fr). The derivation is carried out for values of ν(approximately-greater-than)1, Fr(approximately-greater-than)1, and it is valid for some regimes of interest to direct and indirect-drive inertial confinement fusion (ICF). The analytic formula for the cutoff wave number is in excellent agreement with the numerical results of Kull [Phys. Fluids B 1, 170 (1989)]. © 1995 American Institute of Physics.
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  • 9
    Electronic Resource
    Electronic Resource
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 4 (1992), S. 1465-1474 
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A stability analysis is carried out for energetic particle-Alfvén gap modes. Three modes have been identified: the toroidicity, ellipticity, and noncircular triangularity induced Alfvén eigenmodes (TAE, EAE, and NAE). In highly elongated plasma cross sections with κ−1∼1, the EAE may be a more robust mode than the TAE and NAE. It is found that electron Landau damping in highly elongated plasmas has a strong stabilizing influence on the n=1 EAE, while ion Landau damping stabilizes the n=1 TAE in high-density regimes. Furthermore, the NAE turns out to be stable for all currently proposed ignition experiments. The stability analysis of a typical burning plasma device, Burning Plasma Experiment (BPX) [Phys. Scr. T16, 89 (1987)] shows that n〉1 gap modes can pose a serious threat to the achievement of ignition conditions.
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  • 10
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 8 (2001), S. 5257-5267 
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A model for the deceleration phase of imploding inertial confinement fusion capsules is derived by solving the conservation equations for the hot spot. It is found that heat flux leaving the hot spot goes back in the form of internal energy and pdV work of the material ablated off the inner shell surface. Though the hot-spot temperature is reduced by the heat conduction losses, the hot-spot density increases due to the ablated material in such a way that the hot-spot pressure is approximately independent of heat conduction. For direct-drive National Ignition Facility-like capsules, the ablation velocity off the shell inner surface is of the order of tens μm/ns, the deceleration of the order of thousands μm/ns2, and the density-gradient scale length of the order a few μm. Using the well-established theory of the ablative Rayleigh–Taylor instability, it is shown that the growth rates of the deceleration phase instability are significantly reduced by the finite ablative flow and the unstable spectrum exhibits a cutoff for mode numbers of about l(approximate)90. © 2001 American Institute of Physics.
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