276 research outputs found
Magnetic island merger as a mechanism for inverse magnetic energy transfer
Magnetic energy transfer from small to large scales due to successive
magnetic island coalescence is investigated. A solvable analytical model is
introduced and shown to correctly capture the evolution of the main quantities
of interest, as borne out by numerical simulations. Magnetic reconnection is
identified as the key mechanism enabling the inverse transfer, and setting its
properties: magnetic energy decays as , where is time
normalized to the (appropriately defined) reconnection timescale; and the
correlation length of the field grows as . The magnetic energy
spectrum is self-similar, and evolves as ,
where the -dependence is imparted by the formation of thin current sheets.Comment: 6 pages, 5 figures, submitted for publicatio
Multi-scale dynamics of magnetic flux tubes and inverse magnetic energy transfer
We report on an analytical and numerical study of the dynamics of a
three-dimensional array of identical magnetic flux tubes in the
reduced-magnetohydrodynamic description of the plasma. We propose that the
long-time evolution of this system is dictated by flux-tube mergers and that
such mergers are dynamically constrained by the conservation of the pertinent
(ideal) invariants, {\it viz.} the magnetic potential and axial fluxes of each
tube. We also propose that in the direction perpendicular to the merging plane,
flux tubes evolve in critically-balanced fashion. These notions allow us to
construct an analytical model for how quantities such as the magnetic energy
and the energy-containing scale evolve as functions of time. Of particular
importance is the conclusion that, like its two-dimensional counterpart, this
system exhibits an inverse transfer of magnetic energy that terminates only at
the system scale. We perform direct numerical simulations that confirm these
predictions and reveal other interesting aspects of the evolution of the
system. We find, for example, that the early time evolution is characterized by
a sharp decay of the initial magnetic energy, which we attribute to the
ubiquitous formation of current sheets. We also show that a quantitatively
similar inverse transfer of magnetic energy is observed when the initial
condition is a random, small-scale magnetic seed field.Comment: 33 pages, 19 figures, accepted for publication in Journal of Plasma
Physic
Patching laser-reduced graphene oxide with carbon nanodots
Three-dimensional graphenes are versatile materials for a range of electronic applications and considered among the most promising candidates for electrodes in future electric double layer capacitors (EDLCs) as they are expected to outperform commercially used activated carbon. Parameters such as electrical conductivity and active surface area are critical to the final device performance. By adding carbon nanodots to graphene oxide in the starting material for our standard laser-assisted reduction process, the structural integrity (i.e. lower defect density) of the final 3D-graphene is improved. As a result, the active surface area in the hybrid starting materials was increased by 130% and the electrical conductivity enhanced by nearly an order of magnitude compared to pure laser-reduced graphene oxide. These improved material parameters lead to enhanced device performance of the EDLC electrodes. The frequency response, i.e. the minimum phase angle and the relaxation time, were significantly improved from â82.2° and 128 ms to â84.3° and 7.6 ms, respectively. For the same devices the specific gravimetric device capacitance was increased from 110 to a maximum value of 214 F gâ1 at a scan rate of 10 mV sâ1
Role of coupling delay in oscillatory activity in autonomous networks of excitable neurons with dissipation
We study numerically the effects of time delay in networks of delay-coupled
excitable FitzHugh Nagumo systems with dissipation. The generation of periodic
self-sustained oscillations and its threshold are analyzed depending on the
dissipation of a single neuron, the delay time, and random initial conditions.
The peculiarities of spatiotemporal dynamics of time-delayed bidirectional
ring-structured FitzHugh-Nagumo neuronal systems are investigated in cases of
local and nonlocal coupling topology between the nodes, and a first-order
nonequilibrium phase transition to synchrony is established. It is shown that
the emergence of oscillatory activity in delay-coupled FitzHugh-Nagumo neurons
is observed for smaller values of the coupling strength as the dissipation
parameter decreases. This can provide the possibility of controlling the
spatiotemporal behavior of the considered neuronal networks. The observed
effects are quantified by plotting distributions of the maximal Lyapunov
exponent and the global order parameter in terms of delay and coupling
strength.Comment: 14 pages, 17 figure
Preparation of Long-Lived, Non-Autoionizing Circular Rydberg States of Strontium
Alkaline earth Rydberg atoms are very promising tools for quantum
technologies. Their highly excited outer electron provides them with the
remarkable properties of Rydberg atoms and, notably, with a huge coupling to
external fields or to other Rydberg atoms while the ionic core retains an
optically active electron. However, low angular-momentum Rydberg states suffer
almost immediate autoionization when the core is excited. Here, we demonstrate
that strontium circular Rydberg atoms with a core excited in a metastable
level are impervious to autoionization over more than a few millisecond time
scale. This makes it possible to trap and laser-cool Rydberg atoms. Moreover,
we observe singlet to triplet transitions due to the core optical
manipulations, opening the way to a quantum microwave to optical interface
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