3,355 research outputs found
The effect of magnetic islands on ITG turbulence driven transport
In this work, we address the question of the influence of magnetic islands on
the perpendicular transport due to steady-state ITG turbulence on the energy
transport time scale. We demonstrate that turbulence can cross the separatrix
and enhance the perpendicular transport across magnetic islands. As the
perpendicular transport in the interior of the island sets the critical island
size needed for growth of neoclassical tearing modes, this increased transport
leads to a critical island size larger than that predicted from considering
collisional conductivities, but smaller than that using anomalous effective
conductivities.
We find that on Bohm time scales, the turbulence is able to re-establish the
temperature gradient across the island for islands widths , the turbulence correlation length. The reduction in the island
flattening is estimated by comparison with simulations retaining only the
perpendicular temperature and no turbulence. At intermediate island widths,
comparable to , turbulence is able to maintain finite
temperature gradients across the island
Perturbative approach to the nonlinear saturation of the tearing mode for any current gradient
Within the traditional frame of reduced MHD, a new rigorous perturbation
expansion provides the equation ruling the nonlinear growth and saturation of
the tearing mode for any current gradient. The small parameter is the magnetic
island width w. For the first time, the final equation displays at once terms
of order w ln(1/w) and w which have the same magnitude for practical purposes;
two new O(w) terms involve the current gradient. The technique is applicable to
the case of an external forcing. The solution for a static forcing is computed
explicitly and it exhibits three physical regimes.Comment: 4 pages, submitted to Physical Review Letter
An alternative approach to field-aligned coordinates for plasma turbulence simulations
Turbulence simulation codes can exploit the flute-like nature of plasma
turbulence to reduce the effective number of degrees of freedom necessary to
represent fluctuations. This can be achieved by employing magnetic coordinates
of which one is aligned along the magnetic field. This work presents an
approach in which the position along the field lines is identified by the
toroidal angle, rather than the most commonly used poloidal angle. It will be
shown that this approach has several advantages. Among these, periodicity in
both angles is retained. This property allows moving to an equivalent
representation in Fourier space with a reduced number of toroidal components.
It will be shown how this duality can be exploited to transform conventional
codes that use a spectral representation on the magnetic surface into codes
with a field-aligned coordinate. It is also shown that the new approach can be
generalised to get rid of magnetic coordinates in the poloidal plane
altogether, for a large class of models. Tests are carried out by comparing the
new approach with the conventional approach employing a uniform grid, for a
basic ion temperature gradient (ITG) turbulence model implemented by the two
corresponding versions of the ETAI3D code. These tests uncover an unexpected
property of the model, that localized large parallel gradients can
intermittently appear in the turbulent regime. This leaves open the question
whether this is a general property of plasma turbulence, which may lead one to
reconsider some of the usual assumptions on micro-turbulence dynamics.Comment: 19 pages (once in pdf format). 1 LaTeX file and 10 eps figures in the
zip folde
Plasma turbulence simulations with X-points using the flux-coordinate independent approach
In this work, the Flux-Coordinate Independent (FCI) approach to plasma
turbulence simulations is formulated for the case of generic, static magnetic
fields, including those possessing stochastic field lines. It is then
demonstrated that FCI is applicable to nonlinear turbulent problems with and
without X-point geometry. In particular, by means of simulations with the
FENICIA code, it is shown that the standard features of ITG modes are recovered
with reduced toroidal resolution. Finally, ITG turbulence under the influence
of a static island is studied on the transport timescale with ITER-like
parameters, showing the wide range of applicability of the method
Implementation of a three-qubit quantum error correction code in a cavity-QED setup
The correction of errors is of fundamental importance for the development of
contemporary computing devices and of robust communication protocols. In this
paper we propose a scheme for the implementation of the three-qubit quantum
repetition code, exploiting the interaction of Rydberg atoms with the quantized
mode of a microwave cavity field. Quantum information is encoded within two
circular Rydberg states of the atoms and encoding and decoding process are
realized within two separate microwave cavities. We show that errors due to
phase noise fluctuations could be efficiently corrected using a
state-of-the-art apparatus.Comment: 9 pages, 5 figures. This is v2. Some misprints corrected, conclusions
section extended, refs added. Accepted for publication on PR
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