6,606 research outputs found

### Minimum-energy broadcast in random-grid ad-hoc networks: approximation and distributed algorithms

The Min Energy broadcast problem consists in assigning transmission ranges to
the nodes of an ad-hoc network in order to guarantee a directed spanning tree
from a given source node and, at the same time, to minimize the energy
consumption (i.e. the energy cost) yielded by the range assignment. Min energy
broadcast is known to be NP-hard.
We consider random-grid networks where nodes are chosen independently at
random from the $n$ points of a $\sqrt n \times \sqrt n$ square grid in the
plane. The probability of the existence of a node at a given point of the grid
does depend on that point, that is, the probability distribution can be
non-uniform.
By using information-theoretic arguments, we prove a lower bound
$(1-\epsilon) \frac n{\pi}$ on the energy cost of any feasible solution for
this problem. Then, we provide an efficient solution of energy cost not larger
than $1.1204 \frac n{\pi}$.
Finally, we present a fully-distributed protocol that constructs a broadcast
range assignment of energy cost not larger than $8n$,thus still yielding
constant approximation. The energy load is well balanced and, at the same time,
the work complexity (i.e. the energy due to all message transmissions of the
protocol) is asymptotically optimal. The completion time of the protocol is
only an $O(\log n)$ factor slower than the optimum. The approximation quality
of our distributed solution is also experimentally evaluated.
All bounds hold with probability at least $1-1/n^{\Theta(1)}$.Comment: 13 pages, 3 figures, 1 tabl

### Quadrupole scan emittance measurements for the ELI-NP compton gamma source

The high brightness electron LINAC of the Compton
Gamma Source at the ELI Nuclear Physics facility in Roma-
nia is accelerating a train of 32 bunches with a nominal total
charge of
250 pC
and nominal spacing of
16 ns
. To achieve
the design gamma flux, all the bunches along the train must
have the designed Twiss parameters. Beam sizes are mea-
sured with optical transition radiation monitors, allowing a
quadrupole scan for Twiss parameters measurements. Since
focusing the whole bunch train on the screen may lead to
permanent screen damage, we investigate non-conventional
scans such as scans around a maximum of the beam size
or scans with a controlled minimum spot size. This paper
discusses the implementation issues of such a technique in
the actual machine layou

### Efficient plasma wakefield acceleration simulations via kinetic-hydro code

Start-to-end simulations are needed for sensitivity stud- ies and online analysis of experimental data of the Plasma Wakefield Acceleration experiment COMB at SPARC_LAB facility, Frascati (Italy). Ad hoc tools are needed for the plasma section modeling. Particle in cell codes are the most widely used tools for this purpose, but they suffer from the considerable amount of computational resources they re- quire. We seek for a simple, portable, quick-to-run approach. For this purpose we introduce a time-explicit cylindrical hybrid fluid-kinetic code: Architect. The beam particles are treated with PIC-like kinetic approach, while the plasma wake is treated as a fluid. Since the number of computational particles used by the hybrid model is significantly reduced with respect of full PIC codes with the same number of di- mensions, the time required for a simulation is reduced as well

### Fabrication of flexible silicon nanowires by self-assembled metal assisted chemical etching for surface enhanced Raman spectroscopy

A homogenous array of flexible gold coated silicon nanowires was
fabricated by the combination of nano spheres lithography and metal
assisted chemical etching to obtain highly effective Surface Enhanced
Raman Spectroscopy (SERS) substrates. 3D nanostructures with different
aspect ratios and well-defined geometries were produced by adjusting the
fabrication parameters in order to select the best configuration for
SERS analysis. The optimum flexible nanowires with an aspect ratio of 1
: 10 can self-close driven by the microcapillary force under exposure to
liquid and trap the molecules at their metallic coated ``fingertips'',
thus generating hot spots with ultrahigh field enhancement. The
performance of these SERS substrates was evaluated using melamine as the
analyte probe with various concentrations from the millimolar to the
picomolar range. Flexible gold coated SiNWs demonstrated high uniformity
of the Raman signal over large area with a variability of only 10% and
high sensitivity with a limit of detection of 3.20 x 10(-7) mg L-1
(picomolar) which promotes its application in several fields such food
safety, diagnostic and pharmaceutical. Such an approach represents a
low-cost alternative to the traditional nanofabrication processes to
obtain well ordered silicon nanostructures, offering multiple degrees of
freedom in the design of different geometries such as inter-wire
distance, density of the wires on the surface as well as their length,
thus showing a great potential for the fabrication of SERS substrates

### Endovascular management of giant visceral artery aneurysms

Endovascular management of small visceral artery aneurysms is an established treatment with satisfactory outcomes. However, when size exceeds 5 cm visceral aneurysms are considered as âgiantâ (giant visceral artery aneurysms or GVAAs) and management is significantly more complex. Between August 2007 and June 2019 eleven cases of GVAAs that were endovascularly treated were retrospectively reviewed and included in this single center study. Mean size was 80 mm (Â± 26.3 mm) x 46 mm (+ -11.8 mm). Nine of the lesions were true aneurysms, and two were pseudoaneurysms. In 8 patients, the lesion was causing compression symptoms in the surrounding organs, one patient developed a contained rupture while 2 patients were completely asymptomatic. However, all patients were hemodynamically stable at the time of treatment. Technical success was defined as immediate complete exclusion of the aneurysmal sac, and clinical success as complete relief from clinical symptoms. Follow-up was performed with CT angiography, ultrasound and clinical examination. Mean follow-up was 45 months (range 6â84). Technical and clinical success were both 91%. Complications were one lack of control of contained rupture that was subsequently operated, one case of self-limiting non-target spleen embolization and one case of splenic abscess. Three patients died, one due to the contained rupture 15 days after procedure, the other two for other causes and occurred during the long-term follow-up. This series suggests that endovascular treatment of giant visceral artery aneurysms and pseudoaneuryms is a valid minimally invasive solution with very satisfactory immediate and long-term outcomes unless the aneurysm is already ruptured. A variety of endovascular tools may be required for successful treatment

### Coherence properties and diagnostics of betatron radiation emitted by an externally-injected electron beam propagating in a plasma channel

A 3-dimensional time-domain simulation of X-ray produced by a laser wakefield accelerated electron beam was performed in order to know its properties like intensity, spectrum, divergence and coherence. Particular attention was paid to the coherence around the acceleration axis. The broad spectrum of betatron radiation (1â10 keV) leads to a short coherence length. Nevertheless we observe that under particular detection condition the spatial coherence has a characteristic enlargement. We give a simplified interpretation of this effect in terms of phase shift of the electric field on a virtual detector. Moreover we describe a near field scattering technique to characterize the betatron radiation. This diagnostics will be used to map the transverse spatio-temporal coherence of X-ray radiation in the laser wakefield accelerator under development at Frascati National Laboratories (LNF)

### A strong-coupling analysis of two-dimensional O(N) sigma models with $N\geq 3$ on square, triangular and honeycomb lattices

Recently-generated long strong-coupling series for the two-point Green's
functions of asymptotically free ${\rm O}(N)$ lattice $\sigma$ models are
analyzed, focusing on the evaluation of dimensionless renormalization-group
invariant ratios of physical quantities and applying resummation techniques to
series in the inverse temperature $\beta$ and in the energy $E$. Square,
triangular, and honeycomb lattices are considered, as a test of universality
and in order to estimate systematic errors. Large-$N$ solutions are carefully
studied in order to establish benchmarks for series coefficients and
resummations. Scaling and universality are verified. All invariant ratios
related to the large-distance properties of the two-point functions vary
monotonically with $N$, departing from their large-$N$ values only by a few per
mille even down to $N=3$.Comment: 53 pages (incl. 5 figures), tar/gzip/uuencode, REVTEX + psfi

### Exact results in planar N=1 superconformal Yang-Mills theory

In the \beta-deformed N=4 supersymmetric SU(N) Yang-Mills theory we study the
class of operators O_J = Tr(\Phi_i^J \Phi_k), i\neq k and compute their exact
anomalous dimensions for N,J\to\infty. This leads to a prediction for the
masses of the corresponding states in the dual string theory sector. We test
the exact formula perturbatively up to two loops. The consistency of the
perturbative calculation with the exact result indicates that in the planar
limit the one--loop condition g^2=h\bar{h} for superconformal invariance is
indeed sufficient to insure the {\em exact} superconformal invariance of the
theory. We present a direct proof of this point in perturbation theory. The O_J
sector of this theory shares many similarities with the BMN sector of the N=4
theory in the large R--charge limit.Comment: LaTex, 14 pages, 3 figures; v2: minor corrections and one reference
adde

### Crossover scaling from classical to nonclassical critical behavior

We study the crossover between classical and nonclassical critical behaviors.
The critical crossover limit is driven by the Ginzburg number G. The
corresponding scaling functions are universal with respect to any possible
microscopic mechanism which can vary G, such as changing the range or the
strength of the interactions. The critical crossover describes the unique flow
from the unstable Gaussian to the stable nonclassical fixed point. The scaling
functions are related to the continuum renormalization-group functions. We show
these features explicitly in the large-N limit of the O(N) phi^4 model. We also
show that the effective susceptibility exponent is nonmonotonic in the
low-temperature phase of the three-dimensional Ising model.Comment: 5 pages, final version to appear in Phys. Rev.

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