2,900 research outputs found
Accelerator Based Fusion Reactor
A feasibility study of fusion reactors based on accelerators is carried out.
We consider a novel scheme where a beam from the accelerator hits the target
plasma on the resonance of the fusion reaction and establish characteristic
criteria for a workable reactor. We consider the reactions , and in this study. The critical temperature of the plasma is determined
from overcoming the stopping power of the beam with the fusion energy gain. The
needed plasma lifetime is determined from the width of the resonance, the beam
velocity and the plasma density. We estimate the critical beam flux by
balancing the energy of fusion production against the plasma thermo-energy and
the loss due to stopping power for the case of an inert plasma. The product of
critical flux and plasma lifetime is independent of plasma density and has a
weak dependence on temperature. Even though the critical temperatures for these
reactions are lower than those for the thermonuclear reactors, the critical
flux is in the range of for the plasma density
in the case of an inert plasma. Several
approaches to control the growth of the two-stream instability are discussed.
We have also considered several scenarios for practical implementation which
will require further studies. Finally, we consider the case where the injected
beam at the resonance energy maintains the plasma temperature and prolongs its
lifetime to reach a steady state. The equations for power balance and particle
number conservation are given for this case.Comment: To be published in Nuclear Fusion as a letter, 7 pages, 2 figure
Spin-orbit coupling induced fractionalized Skyrmion excitations in rotating and rapidly quenched spin-1 Bose-Einstein condensates
We investigate the fractionalized Skyrmion excitations induced by spin-orbit
coupling in rotating and rapidly quenched spin-1 Bose-Einstein condensates. Our
results show that the fractionalized Skyrmion excitation depends on the
combination of spin-orbit coupling and rotation, and it originates from a
dipole structure of spin which is always embedded in three vortices constructed
by each condensate component respectively. When spin-orbit coupling is larger
than a critical value, the fractionalized Skyrmions encircle the center with
one or several circles to form a radial lattice, which occurs even in the
strong ferromagnetic/antiferromagnetic condensates. We can use both the
spin-orbit coupling and the rotation to adjust the radial lattice. The
realization and the detection of the fractionalized Skyrmions are compatible
with current experimental technology.Comment: 5 pages, 4 figure
Being Negative but Constructively: Lessons Learnt from Creating Better Visual Question Answering Datasets
Visual question answering (Visual QA) has attracted a lot of attention
lately, seen essentially as a form of (visual) Turing test that artificial
intelligence should strive to achieve. In this paper, we study a crucial
component of this task: how can we design good datasets for the task? We focus
on the design of multiple-choice based datasets where the learner has to select
the right answer from a set of candidate ones including the target (\ie the
correct one) and the decoys (\ie the incorrect ones). Through careful analysis
of the results attained by state-of-the-art learning models and human
annotators on existing datasets, we show that the design of the decoy answers
has a significant impact on how and what the learning models learn from the
datasets. In particular, the resulting learner can ignore the visual
information, the question, or both while still doing well on the task. Inspired
by this, we propose automatic procedures to remedy such design deficiencies. We
apply the procedures to re-construct decoy answers for two popular Visual QA
datasets as well as to create a new Visual QA dataset from the Visual Genome
project, resulting in the largest dataset for this task. Extensive empirical
studies show that the design deficiencies have been alleviated in the remedied
datasets and the performance on them is likely a more faithful indicator of the
difference among learning models. The datasets are released and publicly
available via http://www.teds.usc.edu/website_vqa/.Comment: Accepted for Oral Presentation at NAACL-HLT 201
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