37,192 research outputs found
Modeling the effect of anisotropic pressure on tokamak plasmas normal modes and continuum using fluid approaches
Extending the ideal MHD stability code MISHKA, a new code, MISHKA-A, is
developed to study the impact of pressure anisotropy on plasma stability. Based
on full anisotropic equilibrium and geometry, the code can provide normal mode
analysis with three fluid closure models: the single adiabatic model (SA), the
double adiabatic model (CGL) and the incompressible model. A study on the
plasma continuous spectrum shows that in low beta, large aspect ratio plasma,
the main impact of anisotropy lies in the modification of the BAE gap and the
sound frequency, if the q profile is conserved. The SA model preserves the BAE
gap structure as ideal MHD, while in CGL the lowest frequency branch does not
touch zero frequency at the resonant flux surface where , inducing a
gap at very low frequency. Also, the BAE gap frequency with bi-Maxwellian
distribution in both model becomes higher if with a q
profile dependency. As a benchmark of the code, we study the m/n=1/1 internal
kink mode. Numerical calculation of the marginal stability boundary with
bi-Maxwellian distribution shows a good agreement with the generalized
incompressible Bussac criterion [A. B. Mikhailovskii, Sov. J. Plasma Phys 9,
190 (1983)]: the mode is stabilized(destabilized) if
Analysing the impact of anisotropy pressure on tokamak equilibria
Neutral beam injection or ion cyclotron resonance heating induces pressure
anisotropy. The axisymmetric plasma equilibrium code HELENA has been upgraded
to include anisotropy and toroidal flow. With both analytical and numerical
methods, we have studied the determinant factors in anisotropic equilibria and
their impact on flux surfaces, magnetic axis shift, the displacement of
pressures and density contours from flux surface. With , can vary 20% on flux surface, in a MAST like
equilibrium. We have also re-evaluated the widely applied approximation to
anisotropy in which , the average of parallel
and perpendicular pressure, is taken as the approximate isotropic pressure. We
find the reconstructions of the same MAST discharge with , using isotropic and anisotropic model respectively, to have a 3%
difference in toroidal field but a 66% difference in poloidal current
Anisotropic diffusion of galactic cosmic ray protons and their steady-state azimuthal distribution
Galactic transport models for cosmic rays involve the diffusive motion of
these particles in the interstellar medium. Due to the large-scale structured
galactic magnetic field this diffusion is anisotropic with respect to the local
field direction. We included this transport effect along with continuous loss
processes in a quantitative model of galactic propagation for cosmic ray
protons which is based on stochastic differential equations. We calculated
energy spectra at different positions along the Sun's galactic orbit and
compared them to the isotropic diffusion case. The results show that a larger
amplitude of variation as well as different spectral shapes are obtained in the
introduced anisotropic diffusion scenario and emphasize the need for accurate
galactic magnetic field models.Comment: 7 pages, 5 figures, accepted for publication in A&
Pseudorapidity shape of elliptic flow as signature for fast equilibration in relativistic heavy-ion collisions at energies up to sqrt(s) = 200 GeV
The implications of parton recombination processes on the dynamics of
ultrarelativistic heavy-ion reactions are investigated. To do so, the
quark-gluon string transport model has been extended for partonic recombination
and fusion processes. Parton recombination leads to short equilibration times
and improves significantly on the theoretical description of measured directed
and elliptic flow, i.e., v_1 and v_2, distributions in Au+Au collisions at
sqrt(s) = 200 GeV, in particular what concerns their pseudorapidity dependence.
The shape of v_2(eta) is found to be closely related to fast thermalization.Comment: 7 pages (revtex4) with 4 figures, v3: substantially extended
description and discussion of the model and its results, accepted for
publication in Phys. Rev.
Directed force chain networks and stress response in static granular materials
A theory of stress fields in two-dimensional granular materials based on
directed force chain networks is presented. A general equation for the
densities of force chains in different directions is proposed and a complete
solution is obtained for a special case in which chains lie along a discrete
set of directions. The analysis and results demonstrate the necessity of
including nonlinear terms in the equation. A line of nontrivial fixed point
solutions is shown to govern the properties of large systems. In the vicinity
of a generic fixed point, the response to a localized load shows a crossover
from a single, centered peak at intermediate depths to two propagating peaks at
large depths that broaden diffusively.Comment: 18 pages, 12 figures. Minor corrections to one figur
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