1,721,067 research outputs found
Anomalous ion heating from ambipolar-constrained magnetic fluctuation-induced transport
No full textA kinetic theory for the anomalous heating of ions from energy stored in magnetic turbulence is
presented. Imposing self-consistency through the constitutive relations between particle
distributions and fields, a turbulent Kirchhoff’s Law is derived that expresses a direct connection
between rates of ion heating and electron thermal transport. This connection arises from the
kinematics of electron motion along turbulent fields, which results in granular structures in the
electron distribution. The drag exerted on these structures through emission into collective modes
mediates an effective ambipolar constraint on transport. Resonant damping of the collective modes
by ions produces the heating. In collisionless plasmas the rate of ion damping controls the rate of
emission, and hence the ambipolar-constrained electron heat flux. The heating rate is calculated for
both a resonant and nonresonant magnetic fluctuation spectrum and compared with observations.
The theoretical heating rate is sufficient to account for the observed twofold rise in ion temperature
during sawtooth events in experimental discharges
Nonlinear inward particle flux component in trapped electron mode turbulence
No full textTrapped electron turbulence is shown to have a significant inward particle flux component associated with nonlinear deviations of the density-potential cross correlation from the quasilinear value. The cross correlation is altered because the density advection nonlinearity mixes a linearly stable eigenmode with the eigenmode of the instability. The full nonlinear flux is evaluated by solving spectrum balance equations in a complete basis spanning the fluctuation space. An ordered
expansion for small collisionality, perpendicular wave number, and temperature/density-gradient instability threshold parameter enables an analytic solution for a weakly driven regime. The solution
quantifies the role of zonal modes on transport via their saturation of the turbulence under intensely anisotropic transfer. The inward transport is neither diffusive nor convective, but is driven by temperature gradient and enhanced by flat density gradients. It is slightly smaller than the outwardly
directed flux associated with the growing eigenmode, making the flux a small fraction of the quasilinear value
Nonlinear Inward Particle Flux in Trapped Electron Turbulence.
No full textWeakly collisional trapped electron mode (TEM) turbulence has a robust inward particle flux component associated with a linearly stable eigenmode that is excited nonlinearly by spectral energy transfer from the unstable TEM mode. The nonlinear mixture of the two eigenmodes achieved in saturation cannot be described by the quasilinear approximation, hence the inward flux component, which combines with the outward quasilinear flux to produce the net flux, is fundamentally nonlinear. The net flux, which remains outward but is significantly reduced by the inward component, depends on the gradients of density and temperature. This dependence, which establishes whether the flux is diffusive, convective, or something else, is sensitive to the details of the saturation. Saturation is calculated asymptotically in an ordered expansion in collisionality and the ratio of density to temperature gradient scale length. Spectral transfer is highly anisotropic and saturation must account for the energy transfer to zonal modes with zero poloidal wavenumber. Even though zonal modes do not contribute directly to the particle flux they change the fluctuation level and gradient scaling of both the unstable and stable eigenmodes. The result is a flux that is neither diffusive nor convective, but is driven by temperature gradient and enhanced by density gradients that are flat or nearly so. Near the instability threshold the inward component is particularly strong
Nonlinear damping of zonal modes in anisotropic weakly collisional trapped electron mode turbulence
No full textComprehensive spectral analysis of a fluid model for trapped electron mode (TEM) turbulence reveals that marginally stable zonal modes at infinitesimal amplitude become robustly damped at finite amplitude. Zonal-mode structure, anisotropy, excitation, and wave number spectra are shown to result from interaction of the zero-frequency drift wave with the density advection nonlinearity. Heuristic dimensional balances, closure theory, and simulations manifest the primacy of the interaction, and yield energy transfer rates, fluctuation levels, spectra and finite-amplitude-induced dissipation. Strong sensitivity to the zero-frequency wave induces a marked spectral energy-transfer anisotropy that preferentially drives zonal modes relative to nonzonal modes. Zonal-mode excitation is accompanied by the nonlinear excitation of a spectrum of damped eigenmodes. The mixing of unstable TEM eigenmodes with the damped spectrum subjects zonal modes to finite-amplitude-induced damping. The combination of anisotropic transfer to zonal wave numbers and their nonlinear damping is shown to make this the dominant saturation mechanism for TEM turbulence
Nonlinear dynamics of zonal modes in collisionless trapped electron mode turbulence.
No full textThe dynamics of zonal modes in two-dimensional, collisionless, trapped electron mode turbulence is investigated. The analysis yields scaling predictions for saturated turbulence, and leads to a simple formula for the nonlinear damping of zonal modes
A Comprehensive Spectral Theory of Zonal-Mode Dynamics in Trapped Electron Mode Turbulence.
No full textA
comprehensive,
self-consistent
theory
for
spectral
dynamics
in
trapped
electron
mode
(TEM)
turbulence
offers
critical
new
understanding
and
insights
into
zonal-mode
physics.
This
theory
shows
that
1)
zonal
mode
structure,
anisotropy,
excitation,
and
temporal
behavior
arise
at
and
from
the
interface
of
nonlinear
advection
and
linear
wave
properties;
2)
waves
induce
a
marked
spectral
energy-transfer
anisotropy
that
preferentially
drives
zonal
modes
relative
to
non
zonal
modes;
3)
triplet
correlations
involving
density
(as
opposed
to
those
involving
only
flow)
mediate
the
dominant
energy
transfer
at
long
wavelengths;
4)
energy
transfer
becomes
inverse
in
the
presence
of
wave
anisotropy,
where
otherwise
it
is
forward;
5)
zonal-mode
excitation
is
accompanied
by
excitation
of
a
spectrum
of
damped
eigenmodes,
making
zonal
modes
nonlinearly
damped;
and
6)
the
combination
of
anisotropic
transfer
to
zonal
modes
and
their
nonlinear
damping
make
this
the
dominant
saturation
mechanism
for
TEM
turbulence.
This
accounts
for
the
reduction
of
turbulence
level
by
zonal modes, not zonal-flow E
×
B shearing
Nonlinear stability and instability in collisionless trapped electron mode turbulence
No full textA two-field model for collisionless trapped electron mode turbulence has both finite amplitude-induced stability and instability, depending on wave number. Effects usually identified with nonlinear plasma instability (self-trapping, kinetics, 3D mode structure, magnetic shear) are absent. Nonlinear stability and instability reside in ExB advection of density. It drives modes of a purely damped branch of the dispersion relation to finite amplitude and changes the rate at which free energy is released into the turbulence by shifting the density-potential cross phase. Analysis shows that modes of the purely damped branch cannot be ignored in saturation, and that the linear growth rate is a poor indicator of driving at finite amplitude, invalidating mixing length and quasilinear approximations. Using statistical closure theory, the nonlinear eigenmode and growth rate are determined from the saturation level of modes on all branches, stable and unstable, and the nonlinear cross phase that governs finite-amplitude instability
Tearing mode stability with equilibrium flows in the reversed-field pinch
No full textThe influence of certain equilibrium flows on the stability of tearing modes in the reversed-field pinch is investigated. By solving the linearized magnetohydrodynamic equations in cylindrical geometry, the tearing mode stability factor Δ′ is calculated for a variety of axial flow profiles which have nonzero shear away from the rational surface, including flows localized entirely in the external, ideal region of the tearing mode. It is found that both m = 1 and m = 0 modes are destabilized by an axial flow localized near the edge of the plasma. This is the kind of flow that might be generated by any physical process creating an edge-localized radial electric field. A global flow profile with shear over the middle region of the plasma, simulating the differential rotation of core and edge modes observed in some reversed-field pinch discharges, is found to have a destabilizing effect on the m = 1 mode, while leaving the stability parameter of the m = 0 mode practically unchanged. The possible connection of these results with features of the spontaneous enhanced confinement regime in the Madison Symmetric Torus is discussed
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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