4,223 research outputs found
Correlated Exciton Transport in Rydberg-Dressed-Atom Spin Chains
We investigate the transport of excitations through a chain of atoms with
non-local dissipation introduced through coupling to additional short-lived
states. The system is described by an effective spin-1/2 model where the ratio
of the exchange interaction strength to the reservoir coupling strength
determines the type of transport, including coherent exciton motion, incoherent
hopping and a regime in which an emergent length scale leads to a preferred
hopping distance far beyond nearest neighbors. For multiple impurities, the
dissipation gives rise to strong nearest-neighbor correlations and
entanglement. These results highlight the importance of non-trivial
dissipation, correlations and many-body effects in recent experiments on the
dipole-mediated transport of Rydberg excitations.Comment: 5 page
Large-mode-number magnetohydrodynamic instability driven by sheared flows in a tokamak plasma with reversed central shear
The effect of a narrow sub-Alfvenic shear flow layer near the minimum q_min
of the tokamak safety factor profile in a configuration with reversed central
shear is analyzed. Sufficiently strong velocity shear gives rise to a broad
spectrum of fast growing Kelvin-Helmholtz (KH)-like ideal magnetohydrodynamic
(MHD) modes with dominant mode numbers m,n ~ 10. Nonlinear simulations with
finite resistivity show magnetic reconnection near ripples caused by KH-like
vortices, the formation of turbulent structures, and a flattening of the flow
profile. The KH modes are compared to double tearing modes (DTM) which dominate
at lower shearing rates. The possible application of these results in tokamaks
with internal transport barrier is discussed.Comment: 4 pages, 4 figure
Magnetic flux pumping in 3D nonlinear magnetohydrodynamic simulations
A self-regulating magnetic flux pumping mechanism in tokamaks that maintains
the core safety factor at , thus preventing sawteeth, is analyzed
in nonlinear 3D magnetohydrodynamic simulations using the M3D-C code. In
these simulations, the most important mechanism responsible for the flux
pumping is that a saturated quasi-interchange instability generates
an effective negative loop voltage in the plasma center via a dynamo effect. It
is shown that sawtoothing is prevented in the simulations if is
sufficiently high to provide the necessary drive for the
instability that generates the dynamo loop voltage. The necessary amount of
dynamo loop voltage is determined by the tendency of the current density
profile to centrally peak which, in our simulations, is controlled by the
peakedness of the applied heat source profile.Comment: submitted to Physics of Plasmas (23 pages, 15 Figures
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