5,066 research outputs found
Influence of the rotational sense of two colliding laser beams on the radiation of an ultrarelativistic electron
With analytical treatment, the classical dynamics of an ultrarelativistic
electron in two counter-propagating circularly polarized strong laser beams
with either co-rotating or counter-rotating direction are considered. Assuming
that the particle energy is the dominant scale in the setup, an approximate
solution is derived and the influence of the rotational sense on the dynamics
is analyzed. Qualitative differences in both electron energy and momentum are
found for the laser beams being co-rotating or counter-rotating and are
confirmed by the exact numerical solution of the classical equation of motion.
Despite of these differences in the electron trajectory, the radiation spectra
of the electron do not deviate qualitatively from each other for configurations
with varying rotational directions of the laser beams. Here, the radiation of
an ultrarelativistic electron interacting with counterpropagating laser beams
is given in the framework of the Baier-Katkov semi-classical approximation.
Several parameter regimes are considered and the spectra resulting from the two
scenarios all have the same shape and only differ quantitatively by a few
percent.Comment: 13 pages, 8 figure
FLASH: Randomized Algorithms Accelerated over CPU-GPU for Ultra-High Dimensional Similarity Search
We present FLASH (\textbf{F}ast \textbf{L}SH \textbf{A}lgorithm for
\textbf{S}imilarity search accelerated with \textbf{H}PC), a similarity search
system for ultra-high dimensional datasets on a single machine, that does not
require similarity computations and is tailored for high-performance computing
platforms. By leveraging a LSH style randomized indexing procedure and
combining it with several principled techniques, such as reservoir sampling,
recent advances in one-pass minwise hashing, and count based estimations, we
reduce the computational and parallelization costs of similarity search, while
retaining sound theoretical guarantees.
We evaluate FLASH on several real, high-dimensional datasets from different
domains, including text, malicious URL, click-through prediction, social
networks, etc. Our experiments shed new light on the difficulties associated
with datasets having several million dimensions. Current state-of-the-art
implementations either fail on the presented scale or are orders of magnitude
slower than FLASH. FLASH is capable of computing an approximate k-NN graph,
from scratch, over the full webspam dataset (1.3 billion nonzeros) in less than
10 seconds. Computing a full k-NN graph in less than 10 seconds on the webspam
dataset, using brute-force (), will require at least 20 teraflops. We
provide CPU and GPU implementations of FLASH for replicability of our results
Ultrarelativistic electrons in counterpropagating laser beams
The dynamics and radiation of ultrarelativistic electrons in strong
counterpropagating laser beams are investigated. Assuming that the particle
energy is the dominant scale in the problem, an approximate solution of
classical equations of motion is derived and the characteristic features of the
motion are examined. A specific regime is found with comparable strong field
quantum parameters of the beams, when the electron trajectory exhibits
ultrashort spike-like features, which bears great significance to the
corresponding radiation properties. An analytical expression for the spectral
distribution of spontaneous radiation is derived in the framework of the
Baier-Katkov semiclassical approximation based on the classical trajectory. All
the analytical results are further validated by exact numerical calculations.
We consider a non-resonant regime of interaction, when the laser frequencies in
the electron rest frame are far from each other, avoiding stimulated emission.
Special attention is devoted to settings when the description of radiation via
the local constant field approximation fails and to corresponding spectral
features. Periodic and non-periodic regimes are considered, when lab
frequencies of the laser waves are always commensurate. The sensitivity of
spectra with respect to the electron beam spread, focusing and finite duration
of the laser beams is explored.Comment: 23 papes, 10 figure
High-brilliance ultra-narrow-band x-rays via electron radiation in colliding laser pulses
A setup of a unique x-ray source is put forward employing a relativistic
electron beam interacting with two counter-propagating laser pulses in the
nonlinear few-photon regime. In contrast to Compton scattering (CS) sources,
the envisaged x-ray source exhibits an extremely narrow relative bandwidth of
to , comparable to the x-ray free-electron laser (XFEL). The
brilliance of the x-rays can be orders of magnitude higher than a
state-of-the-art CS source, while the angle spreading of the radiation is much
smaller. By tuning the laser intensities and the electron energy, one can
realize either a single peak or a comb-like x-ray source around keV energy. The
laser intensity and the electron energy in the suggested setup are rather
moderate, rendering this scheme compact and table-top size, as opposed to XFEL
and synchrotron infrastructures
Enhancement of electron-positron pair creation due to transient excitation of field-induced bound states
We study the creation of electron-positron pairs induced by two spatially separated electric fields that vary periodically in time. The results are based on large-scale computer simulations of the time-dependent Dirac equation in reduced spatial dimensions. When the separation of the fields is very large, the pair creation is caused by multiphoton transitions and mainly determined by the frequency of the fields. However, for small spatial separations a coherence effect can be observed that can enhance or reduce the particle yield compared to the case of two infinitely separated fields. If the travel time for a created electron or positron between both field locations becomes comparable to the period of the oscillating fields, we observe peaks in the energy spectrum which can be explained in terms of field-induced transient bound states
A new 111 type iron pnictide superconductor LiFeP
A new iron pnictide LiFeP superconductor was found. The compound crystallizes
into a Cu2Sb structure containing an FeP layer showing superconductivity with
maximum Tc of 6K. This is the first 111 type iron pnictide superconductor
containing no arsenic. The new superconductor is featured with itinerant
behavior at normal state that could helpful to understand the novel
superconducting mechanism of iron pnictide compounds.Comment: 3 figures + 1 tabl
Diversity of eukaryotic plankton of aquaculture ponds with Carassius auratus gibelio, using denaturing gradient gel electrophoresis
PCR-denaturing gradient gel electrophoresis (DGGE) and canonical correspondence analysis (CCA) were used to explore the relationship between eukaryotic plankton community succession and environmental factors in two aquaculture pond models with gibel carp Carassius auratus gibelio. The main culture species of pond 1 were gibel carp and grass carp, and the combined density was 46224 fingerling/ha (gibel carp/grass carp/silver carp/bighead carp, 17:4:6:1). The main culture species of pond 2 was gibel carp, and the combined density was 37551 fingerling/ha (gibel carp/silver carp/bighead carp, 52:1:1). Water samples were collected monthly. The results showed that the annual average concentrations of TP and PO_4-P in pond 1 were significantly higher than pond 2 (p>0.05). The concentration of chlorophyll a (chl a) has no significantly difference between pond 1 and pond 2. DGGE profiles of 18S rRNA gene fragments from the two ponds revealed that the diversity of eukaryotic plankton assemblages was highly variable. 91 bands and 71 bands were detected in pond 1 and pond 2, respectively. The average Shannon–Wiener index of pond 1 was significantly higher than pond 2. Canonical correspondence analysis (CCA) revealed that temperature played a key role in the structure of the eukaryotic plankton community in both ponds, but the nutrient concentration did not affect it. Our results suggest that DGGE method is a cost-effective way to gain insight into seasonal dynamics of eukaryotic plankton communities in culture ponds, and the increase in the number of filter-feeding silver carp and bighead carp could increase the diversity of the eukaryotic plankton community
Suppression of pair creation due to a steady magnetic field
We investigate the electron-positron pair creation process in a supercritical static electric field in the presence of a static magnetic field that is perpendicular. If both fields vary spatially in one direction the dynamics can be reduced to a set of one-dimensional systems. Using a generalized computational quantum field theoretical procedure, we calculate the time dependence of the spatial density for the created electrons. In the presence of the magnetic field, a significant amount of suppression of pair creation is observed in the simulations and confirmed by an analytical analysis for the limits of short-range fields and long interaction times. This suppression might be interpreted in terms of Pauli blocking by the electron during its return to the creation region as it performs a cyclotronlike motion in the magnetic field
Enhancement of pair creation due to locality in bound-continuum interactions
Electron-positron pair production from vacuum is studied in combined
background fields, a binding electric potential well and a laser field. The
production process is triggered by the interactions between the bound states in
the potential well and the continuum states in the Dirac sea. By tuning the
binding potential well, the pair production can be strongly affected by the
locality of the bound states. The narrower bound states in position space are
more efficient for pair production. This is in contrast to what is commonly
expected that the wider extended bound states have larger region to interact
with external fields and would thus create more particles. This surprise can be
explained as the more localized bound states have a much wider extension in the
momentum space, which can enhance the bound-continuum interactions in the
creation process. This enhancement manifests itself in both perturbative and
non-perturbative production regimes
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