10,465 research outputs found
Particle Acceleration and Plasma Dynamics during Magnetic Reconnection in the Magnetically-dominated Regime
Magnetic reconnection is thought to be the driver for many explosive
phenomena in the universe. The energy release and particle acceleration during
reconnection have been proposed as a mechanism for producing high-energy
emissions and cosmic rays. We carry out two- and three-dimensional kinetic
simulations to investigate relativistic magnetic reconnection and the
associated particle acceleration. The simulations focus on electron-positron
plasmas starting with a magnetically dominated, force-free current sheet
(). For this limit, we demonstrate
that relativistic reconnection is highly efficient at accelerating particles
through a first-order Fermi process accomplished by the curvature drift of
particles along the electric field induced by the relativistic flows. This
mechanism gives rise to the formation of hard power-law spectra and approaches for sufficiently large and
system size. Eventually most of the available magnetic free energy is converted
into nonthermal particle kinetic energy. An analytic model is presented to
explain the key results and predict a general condition for the formation of
power-law distributions. The development of reconnection in these regimes leads
to relativistic inflow and outflow speeds and enhanced reconnection rates
relative to non-relativistic regimes. In the three-dimensional simulation, the
interplay between secondary kink and tearing instabilities leads to strong
magnetic turbulence, but does not significantly change the energy conversion,
reconnection rate, or particle acceleration. This study suggests that
relativistic reconnection sites are strong sources of nonthermal particles,
which may have important implications to a variety of high-energy astrophysical
problems.Comment: 18 pages, 13 figures, slightly modified after submitted to Ap
A loop unrolling method based on machine learning
In order to improve the accuracy of loop unrolling factor in the compiler, we propose a loop unrolling method based on improved random decision forest. First, we improve the traditional random decision forest through adding weight value. Second, BSC algorithm based on SMOTE algorithm is proposed to solve the problem of unbalanced data sets. Nearly 1000 loops are selected from several benchmarks, and features extracted from these loops constitute the training set of the loop unrolling factor prediction model. The model has a prediction accuracy of 81 % for the unrolling factor, and the existing Open64 compiler gives 36 % only
Momentum-resolved radio-frequency spectroscopy of a spin-orbit coupled atomic Fermi gas near a Feshbach resonance in harmonic traps
We theoretically investigate the momentum-resolved radio-frequency
spectroscopy of a harmonically trapped atomic Fermi gas near a Feshbach
resonance in the presence of equal Rashba and Dresselhaus spin-orbit coupling.
The system is qualitatively modeled as an ideal gas mixture of atoms and
molecules, in which the properties of molecules, such as the wavefunction,
binding energy and effective mass, are determined from the two-particle
solution of two-interacting atoms. We calculate separately the radio-frequency
response from atoms and molecules at finite temperatures by using the standard
Fermi golden rule, and take into account the effect of harmonic traps within
local density approximation. The total radio-frequency spectroscopy is
discussed, as functions of temperature and spin-orbit coupling strength. Our
results give a qualitative picture of radio-frequency spectroscopy of a
resonantly interacting spin-orbit coupled Fermi gas and can be directly tested
in atomic Fermi gases of K40 atoms at Shanxi University and of Li6 atoms at
MIT.Comment: 11 pages, 9 Figure
Two-channel model description of confinement-induced Feshbach molecules
Using a two-channel model, we investigate theoretically the binding energy of
confinement-induced Feshbach molecules in two- and one-dimensional ultracold
atomic systems, near a Feshbach resonance. We show that the two-channel
prediction will evidently deviate from the simple single-channel theory as the
width of Feshbach resonances decreases. For one-dimensional system, we perform
a full two-channel calculation, with the inclusion of bare interatomic
interactions in the open channel. Away from the resonance, we find a sizable
correction to the binding energy, if we neglect incorrectly the bare
interatomic interactions as in the previous work [Dickerscheid and Stoof, Phys.
Rev. A 72, 053625 (2005)]. We compare our theoretical results with existing
experimental data and present predictions for narrow Feshbach resonances that
could be tested in future experiments.Comment: 8 pages, 5 figure
Radio-frequency spectroscopy of weakly bound molecules in spin-orbit coupled atomic Fermi gases
We investigate theoretically radio-frequency spectroscopy of weakly bound
molecules in an ultracold spin-orbit-coupled atomic Fermi gas. We consider two
cases with either equal Rashba and Dresselhaus coupling or pure Rashba
coupling. The former system has been realized very recently at Shanxi
University [Wang et al., arXiv:1204.1887] and MIT [Cheuk et al.,
arXiv:1205.3483]. We predict realistic radio-frequency signals for revealing
the unique properties of anisotropic molecules formed by spin-orbit coupling.Comment: 11 pages, 7 figure
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