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  • 1
    Electronic Resource
    Electronic Resource
    Toroidal Alfvén eigenmodes in TFTR deuterium–tritium plasmas : The 39th annual meeting of division of plasma physics of APS (1998)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 5 (1998), S. 1703-1711 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Purely alpha-particle-driven toroidal Alfvén eigenmodes (TAEs) with toroidal mode numbers n=1–6 have been observed in deuterium–tritium (D–T) plasmas on the tokamak fusion test reactor [D. J. Grove and D. M. Meade, Nucl. Fusion 25, 1167 (1985)]. The appearance of mode activity following termination of neutral beam injection in plasmas with q(0)〉1 is generally consistent with theoretical predictions of TAE stability [G. Y. Fu et al. Phys. Plasmas 3, 4036 (1996)]. Internal reflectometer measurements of TAE activity is compared with theoretical calculations of the radial mode structure. Core localization of the modes to the region of reduced central magnetic shear is confirmed, however the mode structure can deviate significantly from theoretical estimates. The peak measured TAE amplitude of δn/n∼10−4 at r/a∼0.3−0.4 corresponds to δB/B∼10−5, while δB/B∼10−8 is measured at the plasma edge. Enhanced alpha particle loss associated with TAE activity has not been observed. © 1998 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.872818
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  • 2
    Electronic Resource
    Electronic Resource
    J* optimization of small aspect ratio stellarator/tokamak hybrid devices : The 39th annual meeting of division of plasma physics of APS (1998)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 5 (1998), S. 1752-1758 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A new class of low aspect ratio toroidal hybrid stellarators is found using a more general plasma confinement optimization criterion than quasisymmetrization. The plasma current profile and shape of the outer magnetic flux surface are used as control variables to achieve near constancy of the longitudinal invariant J* on internal flux surfaces (quasiomnigeneity), in addition to a number of other desirable physics target properties. A range of compact (small aspect ratio A), low plasma current devices have been found with significantly improved confinement, both for thermal as well as energetic (collisionless) particle components. With reasonable increases in magnetic field and geometric size, such devices can also be scaled to confine 3.5 MeV alpha particle orbits.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.872844
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  • 3
    Electronic Resource
    Electronic Resource
    Shear Alfvén waves with Landau and collisional effects (1995)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 2 (1995), S. 2033-2043 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Shear Alfvén waves can be driven unstable by hot particles such as alpha particles in an ignited fusion device or hot ions in existing devices. Motivated by rather collisional Wendelstein 7 Advanced Stellarator (W7-AS) [Phys. Rev. Lett. 72, 1220 (1994)] beam-driven global Alfvén instability experiments, the effect of electron and ion collisions on these modes has been examined. Collisions broaden and suppress the peak associated with Landau effects. This broadening makes ion damping more important, while the electron damping is suppressed. Additional resistive effects provide increased damping for the main part of the spectrum, which can have a rather high phase velocity. Of more general interest is the fact that collisional and collisionless resistivity has a numerically stabilizing effect that is known to be important for nonlinear resistive magnetohydrodynamics (MHD). This can preclude the need for introducing and testing the sensitivity to similar ad hoc effects. Numerical and analytic results for both a particle-conserving Krook collision operator and a Lorentz (pitch angle) collision operator are compared and contrasted. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.871291
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  • 4
    Electronic Resource
    Electronic Resource
    Physics of compact stellarators : The 40th annual meeting of the division of plasma physics of the american physical society (1999)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 6 (1999), S. 1858-1864 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Recent progress in the theoretical understanding and design of compact stellarators is described. Hybrid devices, which depart from canonical stellarators by deriving benefits from the bootstrap current which flows at finite beta, comprise a class of low aspect ratio A〈4 stellarators. They possess external kink stability (at moderate beta) in the absence of a conducting wall, possible immunity to disruptions through external control of the transform and magnetic shear, and they achieve volume-averaged ballooning beta limits (4%–6%) similar to those in tokamaks. In addition, bootstrap currents can reduce the effects of magnetic islands (self-healing effect) and lead to simpler stellarator coils by reducing the required external transform. Powerful physics and coil optimization codes have been developed and integrated to design experiments aimed at exploring compact stellarators. The physics basis for designing the national compact stellarator will be discussed.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.873489
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  • 5
    Electronic Resource
    Electronic Resource
    Alternate method for treating Alfvén waves driven unstable by α particles (1995)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 2 (1995), S. 4656-4658 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: For treating Alfvén waves driven unstable by α particles, an alternative to the Oak Ridge National Laboratory gyro-Landau fluid approach is suggested. Its primary advantage is that this method can simultaneously treat fluid nonlinearities and kinetic quasilinear effects. It is argued that more perturbative approaches could overestimate the kinetic quasilinear effects. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.870957
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  • 6
    Electronic Resource
    Electronic Resource
    Nonlinear evolution of the toroidal Alfvén instability using a gyrofluid modelf| (1994)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 1 (1994), S. 1503-1510 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Discrete shear Alfvén modes such as the TAE (toroidal Alfvén eigenmode) are susceptible to destabilization by energetic alpha populations and neutral beams; this can lead to enhanced fast ion losses and degraded heating efficiencies. A gyrofluid model with Landau closure has been developed for understanding both the linear and nonlinear phases of these instabilities. The linear wave–particle resonances necessary to excite Alfvén instabilities are included in a coupled set of fluid equations. This model is used to analyze several nonlinear saturation mechanisms that arise from mode coupling effects. The effects of shear flow velocity generation (through the Reynolds stress) and localized current generation (leading to modifications in the q profile) are specifically examined.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.870700
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  • 7
    Electronic Resource
    Electronic Resource
    Linearized gyrofluid model of the alpha-destabilized toroidal Alfvén eigenmode with continuum damping effects (1992)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 4 (1992), S. 3316-3328 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The toroidicity-induced shear Alfvén eigenmode (TAE) can be destabilized by energetic particle populations through inverse Landau damping. It may also be significantly damped by coupling with adjacent shear Alfvén continua. A gyrofluid model with Landau closure that includes both of these effects is developed and applied to this instability. The model consists of the usual reduced magnetohydrodynamic (MHD) equations for the evolution of the poloidal flux and toroidal component of vorticity, coupled with equations for the density and parallel velocity moments of the energetic species. The latter two equations include Landau damping/growth effects through use of a consistent closure relation, which is equivalent to a two-pole approximation to the plasma dispersion function. These equations are solved numerically using a three-dimensional initial value code (tae/fl) in toroidal geometry. The unstable TAE growth rate and continuum damping rates are compared with recent analytical estimates, and reasonable agreement is obtained.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.860386
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  • 8
    Electronic Resource
    Electronic Resource
    Alpha-Alfvén local dispersion relation and solutions (1992)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 4 (1992), S. 3869-3882 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The local dispersion relation for shear and kinetic Alfvén waves is derived for arbitrary alpha particle distribution functions and analyzed for a set of representative distribution functions. Including both the velocity dependence of magnetic drift and temperature gradients is found to have the most striking effect on the partial growth rate associated with alpha particles for shear Alfvén waves. Parallel electric field effects are included and found to make significant changes in the damping associated with electrons and ions. The formalism parallels that used in reduced magnetohydrodynamics, so that it can be readily extended to calculations incorporating more detailed and realistic equilibrium geometry.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.860344
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  • 9
    Electronic Resource
    Electronic Resource
    Extending the collisional fluid equations into the long mean-free-path regime in toroidal plasmas. I. Plasma viscosity (1990)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 2 (1990), S. 1190-1194 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: An expression for the pressure anisotropy and thus for the viscous stress in the plateau regime is derived for arbitrary toroidal magnetic configurations without assuming incompressibility or the existence of flux surfaces, without neglecting the flow components perpendicular to the magnetic surface, and without restricting the flow velocity to be a constant on the flux surface. It can be employed to study low-frequency instabilities in the long mean-free-path regime. A smoothly connected formula for the pressure anisotropy, valid in both the collisional fluid regime and the plateau regime, is given to facilitate the numerical computation. An alternative interpretation of the neoclassical transport theory is also obtained. It is found that if the effects of the temperature gradient are neglected, neoclassical transport fluxes can be interpreted as driven by the velocity stress.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.859255
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  • 10
    Electronic Resource
    Electronic Resource
    Effects of trapped alpha particles on ballooning modes in tokamaks (1985)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Fluids 28 (1985), S. 2494-2501 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The effects of a trapped, precessing alpha particle population on ballooning modes are examined for a large aspect ratio, shifted circular flux surface tokamak equilibrium. The alphas are modeled in the deeply trapped limit and with a Maxwellian distribution in energy. The resulting kinetic ballooning equation is solved numerically, and the dependence of the eigenvalues and stability boundaries on shear, background pressure gradient, and ratio of hot-to-background tempera- tures (and densities) is investigated. In the low-frequency regime (ω (very-much-less-than) ωdH =alpha precessional drift frequency), the alpha component has a stabilizing influence, while in the intermediate frequency range (ω(approximately-less-than)ωdH) the alphas destabilize ballooning modes through interaction with the trapped particle precessional drift resonance. Parameter ranges which should be typical of alpha production in near term tokamak devices such as the Tokamak Fusion Test Reactor (TFTR) [Phys. Rev. Lett. 52, 1492 (1984)] are considered.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.865257
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