318 research outputs found
Nonlinear diffusion & thermo-electric coupling in a two-variable model of cardiac action potential
This work reports the results of the theoretical investigation of nonlinear
dynamics and spiral wave breakup in a generalized two-variable model of cardiac
action potential accounting for thermo-electric coupling and diffusion
nonlinearities. As customary in excitable media, the common Q10 and Moore
factors are used to describe thermo-electric feedback in a 10-degrees range.
Motivated by the porous nature of the cardiac tissue, in this study we also
propose a nonlinear Fickian flux formulated by Taylor expanding the voltage
dependent diffusion coefficient up to quadratic terms. A fine tuning of the
diffusive parameters is performed a priori to match the conduction velocity of
the equivalent cable model. The resulting combined effects are then studied by
numerically simulating different stimulation protocols on a one-dimensional
cable. Model features are compared in terms of action potential morphology,
restitution curves, frequency spectra and spatio-temporal phase differences.
Two-dimensional long-run simulations are finally performed to characterize
spiral breakup during sustained fibrillation at different thermal states.
Temperature and nonlinear diffusion effects are found to impact the
repolarization phase of the action potential wave with non-monotone patterns
and to increase the propensity of arrhythmogenesis
Double-carbapenem regimen, alone or in combination with colistin, in the treatment of infections caused by carbapenem-resistant Klebsiella pneumoniae (CR-Kp)
Dynamics of charge-displacement channeling in intense laser-plasma interactions
The dynamics of transient electric fields generated by the interaction of
high intensity laser pulses with underdense plasmas has been studied
experimentally with the proton projection imaging technique. The formation of a
charged channel, the propagation of its front edge and the late electric field
evolution have been characterised with high temporal and spatial resolution.
Particle-in-cell simulations and an electrostatic, ponderomotive model
reproduce the experimental features and trace them back to the ponderomotive
expulsion of electrons and the subsequent ion acceleration.Comment: 5 figures, accepted for publication in New Journal of Physic
Soft laser-plasma X-ray source for differential absorption imaging of tracing elements in thin samples
The differential imaging technique is particularly suitable for the
detection of small concentrations of contrasts agents for biological
and medical applications in samples using X-ray radiography. In this
paper, we present an application of this technique using a laser-plasma
soft X-ray source combined with a bent crystal. Using a Fresnel plate
as a test object, we were able to obtain spatial resolutions of the
order of a few tens of microns. The use of our configuration to perform
differential imaging of a test-sample at the L2 edge of Br
at 1,596 eV is finally demonstrated
Multi-GeV Electron Spectrometer
The advance in laser plasma acceleration techniques pushes the regime of the
resulting accelerated particles to higher energies and intensities. In
particular the upcoming experiments with the FLAME laser at LNF will enter the
GeV regime with almost 1pC of electrons. From the current status of
understanding of the acceleration mechanism, relatively large angular and
energy spreads are expected. There is therefore the need to develop a device
capable to measure the energy of electrons over three orders of magnitude (few
MeV to few GeV) under still unknown angular divergences. Within the PlasmonX
experiment at LNF a spectrometer is being constructed to perform these
measurements. It is made of an electro-magnet and a screen made of
scintillating fibers for the measurement of the trajectories of the particles.
The large range of operation, the huge number of particles and the need to
focus the divergence present unprecedented challenges in the design and
construction of such a device. We will present the design considerations for
this spectrometer and the first results from a prototype.Comment: 7 pages, 6 figures, submitted to NIM
Effects of robotic guidance on the coordination of locomotion
Functional integration of motor activity patterns enables the production of coordinated
movements, such as walking. The activation of muscles by weightened summation of
activation signals has been demonstrated to represent the spatiotemporal components that
determine motor behavior during walking. Exoskeleton robotic devices are now often used in
the rehabilitation practice to assist physical therapy of individuals with neurological
disorders. These devices are used to promote motor recovery by providing guidance force to
the patients. The guidance should in principle lead to a muscle coordination similar to
physiological human walking. However, the influence of robotic devices on locomotor
patterns needs still to be characterized. The aim of this study was to analyze the effect of
force guidance and gait speed on the modular organization of walking in a group of eight
healthy subjects.This project is funded by the European Commission, project "BETTER" (contract number 247935) and Spanish Consolider-Ingenio Programme, project "HYPER" (contract number CSD2009-00067) and Universita Degli Studi di Roma "Foro Italico", research project "Dynamic sensorimotor interaction during locomotion: influences of perturbations and/or body unloading"
Test of candidate light distributors for the muon (g2) laser calibration system
The new muon (g-2) experiment E989 at Fermilab will be equipped with a laser
calibration system for all the 1296 channels of the calorimeters. An
integrating sphere and an alternative system based on an engineered diffuser
have been considered as possible light distributors for the experiment. We
present here a detailed comparison of the two based on temporal response,
spatial uniformity, transmittance and time stability.Comment: accepted to Nucl.Instrum.Meth.
Evidence of resonant surface wave excitation in the relativistic regime through measurements of proton acceleration from grating targets
The interaction of laser pulses with thin grating targets, having a periodic
groove at the irradiated surface, has been experimentally investigated.
Ultrahigh contrast () pulses allowed to demonstrate an enhanced
laser-target coupling for the first time in the relativistic regime of
ultra-high intensity >10^{19} \mbox{W/cm}^{2}. A maximum increase by a factor
of 2.5 of the cut-off energy of protons produced by Target Normal Sheath
Acceleration has been observed with respect to plane targets, around the
incidence angle expected for resonant excitation of surface waves. A
significant enhancement is also observed for small angles of incidence, out of
resonance.Comment: 5 pages, 5 figures, 2nd version implements final correction
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