68 research outputs found
Quantum Quenches in Extended Systems
We study in general the time-evolution of correlation functions in a extended
quantum system after the quench of a parameter in the hamiltonian. We show that
correlation functions in d dimensions can be extracted using methods of
boundary critical phenomena in d+1 dimensions. For d=1 this allows to use the
powerful tools of conformal field theory in the case of critical evolution.
Several results are obtained in generic dimension in the gaussian (mean-field)
approximation. These predictions are checked against the real-time evolution of
some solvable models that allows also to understand which features are valid
beyond the critical evolution.
All our findings may be explained in terms of a picture generally valid,
whereby quasiparticles, entangled over regions of the order of the correlation
length in the initial state, then propagate with a finite speed through the
system. Furthermore we show that the long-time results can be interpreted in
terms of a generalized Gibbs ensemble. We discuss some open questions and
possible future developments.Comment: 24 Pages, 4 figure
Entanglement and correlation functions following a local quench: a conformal field theory approach
We show that the dynamics resulting from preparing a one-dimensional quantum
system in the ground state of two decoupled parts, then joined together and
left to evolve unitarily with a translational invariant Hamiltonian (a local
quench), can be described by means of quantum field theory. In the case when
the corresponding theory is conformal, we study the evolution of the
entanglement entropy for different bi-partitions of the line. We also consider
the behavior of one- and two-point correlation functions. All our findings may
be explained in terms of a picture, that we believe to be valid more generally,
whereby quasiparticles emitted from the joining point at the initial time
propagate semiclassically through the system.Comment: 19 pages, 4 figures, v2 typos corrected and refs adde
Zonal flows and long-distance correlations during the formation of the edge shear layer in the TJ-II stellarator
A theoretical interpretation is given for the observed long-distance
correlations in potential fluctuations in TJ-II. The value of the correlation
increases above the critical point of the transition for the emergence of the
plasma edge shear flow layer. Mean (i.e. surface averaged, zero-frequency)
sheared flows cannot account for the experimental results. A model consisting
of four envelope equations for the fluctuation level, the mean flow shear, the
zonal flow amplitude shear, and the averaged pressure gradient is proposed. It
is shown that the presence of zonal flows is essential to reproduce the main
features of the experimental observations.Comment: 19 pages, 7 figure
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Density and Temperature Profile Modifications with Electron Cyclotron Power Injection in Quiescent Double Barrier Discharges on DIII-D
Quiescent double barrier (QDB) conditions often form when an internal transport barrier is created with high-power neutral-beam injection into a quiescent H-mode (QH) plasma. These QH-modes offer an attractive, high-performance operating scenario for burning plasma experiments due to their quasi-stationarity and lack of edge localized modes (ELMs). Our initial experiments and modeling using ECH/ECCD in QDB shots were designed to control the current profile and, indeed, we have observed a strong dependence on the q-profile when EC-power is used inside the core transport barrier region. While strong electron heating is observed with EC power injection, we also observe a drop in the other core parameters; ion temperature and rotation, electron density and impurity concentration. These dynamically changing conditions provide a rapid evolution of T{sub e} T{sub i} profiles accessible with 0.3 < (T{sub e} T{sub i}){sub axis} < 0.8 observed in QDB discharges. We are exploring the correlation and effects of observed density profile changes with respect to these time-dependent variations in the temperature ratio. Thermal and particle diffusivity calculations over this temperature ratio range indicate a consistency between the rise in temperature ratio and an increase in transport corresponding to the observed change in density
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The Physics of Edge Resonant Magnetic Perturbations in Hot Tokamak Plasmas
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ELM Suppression in Low Edge Collisionality H-Mode Discharges Using n=3 Magnetic Perturbations
Using resonant magnetic perturbations with toroidal mode number n = 3, we have produced H-mode discharges without edge localized modes (ELMs) which run with constant density and radiated power for periods up to about 2550 ms (17 energy confinement times). These ELM suppression results are achieved at pedestal collisionalities close to those desired for next step burning plasma experiments such as ITER and provide a means of eliminating the rapid erosion of divertor components in such machines which could be caused by giant ELMs. The ELM suppression is due to an enhancement in the edge particle transport which reduces the edge pressure gradient and pedestal current density below the threshold for peeling-ballooning modes. These n = 3 magnetic perturbations provide a means of active control of edge plasma transport
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