123,121 research outputs found
Impact of Resonant Magnetic Perturbations on Zonal Modes, Drift-Wave Turbulence and the L-H Transition Threshold
We study the effects of Resonant Magnetic Perturbations (RMPs) on turbulence,
flows and confinement in the framework of resistive drift-wave turbulence. This
work was motivated, in parts, by experiments reported at the IAEA 2010
conference [Y. Xu {\it et al}, Nucl. Fusion \textbf{51}, 062030] which showed a
decrease of long-range correlations during the application of RMPs. We derive
and apply a zero-dimensional predator-prey model coupling the Drift-Wave Zonal
Mode system [M. Leconte and P.H. Diamond, Phys. Plasmas \textbf{19}, 055903] to
the evolution of mean quantities. This model has both density gradient drive
and RMP amplitude as control parameters and predicts a novel type of transport
bifurcation in the presence of RMPs. This model allows a description of the
full L-H transition evolution with RMPs, including the mean sheared flow
evolution. The key results are: i) The L-I and I-H power thresholds \emph{both}
increase with RMP amplitude |\bx|, the relative increase of the L-I threshold
scales as \Delta P_{\rm LI} \propto |\bx|^2 \nu_*^{-2} \gyro^{-2}, where
is edge collisionality and \gyro is the sound gyroradius. ii) RMPs
are predicted to \emph{decrease} the hysteresis between the forward and
back-transition. iii) Taking into account the mean density evolution, the
density profile - sustained by the particle source - has an increased turbulent
diffusion compared with the reference case without RMPs which provides one
possible explanation for the \emph{density pump-out} effect.Comment: 30 pages, IAEA-based articl
Shear stress relaxation and ensemble transformation of shear stress autocorrelation functions revisited
We revisit the relation between the shear stress relaxation modulus ,
computed at finite shear strain , and the shear stress
autocorrelation functions and computed,
respectively, at imposed strain and mean stress . Focusing on
permanent isotropic spring networks it is shown theoretically and
computationally that in general
for with being the static equilibrium shear modulus.
and thus must become different for solids and it is impossible
to obtain alone from as often assumed. We comment
briefly on self-assembled transient networks where must vanish for
a finite scission-recombination frequency . We argue that should reveal an intermediate plateau set by the
shear modulus of the quenched network.Comment: 8 pages, 4 figure
Effects of Ru Substitution on Dimensionality and Electron Correlations in Ba(Fe_{1-x}Ru_x)_2As_2
We report a systematic angle-resolved photoemission spectroscopy study on
Ba(FeRu)As for a wide range of Ru concentrations (0.15
\emph{x} 0.74). We observed a crossover from two-dimension to
three-dimension for some of the hole-like Fermi surfaces with Ru substitution
and a large reduction in the mass renormalization close to optimal doping.
These results suggest that isovalent Ru substitution has remarkable effects on
the low-energy electron excitations, which are important for the evolution of
superconductivity and antiferromagnetism in this system.Comment: 4 pages, 4 figure
Lower Bounds on the Ground State Entropy of the Potts Antiferromagnet on Slabs of the Simple Cubic Lattice
We calculate rigorous lower bounds for the ground state degeneracy per site,
, of the -state Potts antiferromagnet on slabs of the simple cubic
lattice that are infinite in two directions and finite in the third and that
thus interpolate between the square (sq) and simple cubic (sc) lattices. We
give a comparison with large- series expansions for the sq and sc lattices
and also present numerical comparisons.Comment: 7 pages, late
Shear modulus of simulated glass-forming model systems: Effects of boundary condition, temperature and sampling time
The shear modulus G of two glass-forming colloidal model systems in d=3 and
d=2 dimensions is investigated by means of, respectively, molecular dynamics
and Monte Carlo simulations. Comparing ensembles where either the shear strain
gamma or the conjugated (mean) shear stress tau are imposed, we compute G from
the respective stress and strain fluctuations as a function of temperature T
while keeping a constant normal pressure P. The choice of the ensemble is seen
to be highly relevant for the shear stress fluctuations mu_F(T) which at
constant tau decay monotonously with T following the affine shear elasticity
mu_A(T), i.e. a simple two-point correlation function. At variance,
non-monotonous behavior with a maximum at the glass transition temperature T_g
is demonstrated for mu_F(T) at constant gamma. The increase of G below T_g is
reasonably fitted for both models by a continuous cusp singularity, G(T) is
proportional to (1-T/T_g)^(1/2), in qualitative agreement with some recent
replica calculations. It is argued, however, that longer sampling times may
lead to a sharper transition. The additive jump discontinuity predicted by
mode-coupling theory and other replica calculations thus cannot ultimately be
ruled out
- …