65 research outputs found
High-energy acceleration phenomena in extreme radiation-plasma interactions
We simulate, using a particle-in-cell code, the chain of acceleration
processes at work during the Compton-based interaction of a dilute electron-ion
plasma with an extreme-intensity, incoherent gamma-ray flux with a photon
density several orders of magnitude above the particle density. The plasma
electrons are initially accelerated in the radiative flux direction through
Compton scattering. In turn, the charge-separation field from the induced
current drives forward the plasma ions to near-relativistic speed and
accelerates backwards the non-scattered electrons to energies easily exceeding
those of the driving photons. The dynamics of those energized electrons is
determined by the interplay of electrostatic acceleration, bulk plasma motion,
inverse Compton scattering and deflections off the mobile magnetic fluctuations
generated by a Weibel-type instability. The latter Fermi-like effect notably
gives rise to a forward-directed suprathermal electron tail. We provide simple
analytical descriptions for most of those phenomena and examine numerically
their sensitivity to the parameters of the problem
Automated Quality Assessment of Space-Continuous Models for Pedestrian Dynamics
In this work we propose a methodology for assessment of pedestrian models
continuous in space. With respect to the Kolmogorov-Smirnov distance between
two data clouds, representing for instance simulated and the corresponding
empirical data, we calculate an evaluation factor between zero and one. Based
on the value of the herein developed factor, we make a statement about the
goodness of the model under evaluation. Moreover this process can be repeated
in an automatic way in order to maximize the above mentioned factor and hence
determine the optimal set of model parameters.Comment: 8 pages, 3 figures, accepted at the Proceedings of Traffic and
Granular Flow '1
Geometry of lipid vesicle adhesion
The adhesion of a lipid membrane vesicle to a fixed substrate is examined
from a geometrical point of view. This vesicle is described by the Helfrich
hamiltonian quadratic in mean curvature; it interacts by contact with the
substrate, with an interaction energy proportional to the area of contact. We
identify the constraints on the geometry at the boundary of the shared surface.
The result is interpreted in terms of the balance of the force normal to this
boundary. No assumptions are made either on the symmetry of the vesicle or on
that of the substrate. The strong bonding limit as well as the effect of
curvature asymmetry on the boundary are discussed.Comment: 7 pages, some major changes in sections III and IV, version published
in Physical Review
Beam based calibration of X-ray pinhole camera in SSRF
The Shanghai Synchrotron Radiation Facility (SSRF) contains a 3.5-GeV storage
ring serving as a national X-ray synchrotron radiation user facility
characterized by a low emittance and a low coupling. The stability and quality
of the electron beams are monitored continuously by an array of diagnostics. In
particular, an X-ray pinhole camera is employed in the diagnostics beamline of
the ring to characterize the position, size, and emittance of the beam. The
performance of the measurement of the transverse electron beam size is given by
the width of the point spread function (PSF) of the X-ray pinhole camera.
Typically the point spread function of the X-ray pinhole camera is calculated
via analytical or numerical method. In this paper we will introduce a new beam
based calibration method to derive the width of the PSF online
High repetition-rate electro-optic sampling: Recent studies using photonic time-stretch
Single-shot electro-optic sampling (EOS) is a powerful characterization tool for monitoring the shape of electron bunches, and coherent synchrotron radiation pulses. For reaching high acquisition rates, an efficient possibility consists to associate classic EOS systems with the so-called photonic time-stretch technique [1]. We present recent results obtained at SOLEIL and ANKA using this strategy. In particular, we show how a high sensitivity variant of photonic time stretch [2] EOS enabled to monitor the CSR pulses emitted by short electron bunches at SOLEIL [3]. We could thus confirm in a very direct way the theories predicting an interplay between two physical processes. Below a critical bunch charge, we observe a train of identical THz pulses stemming from the shortness of the electron bunches. Above this threshold, CSR emission is dominated by drifting structures appearing through spontaneous self-organization. We also consider the association of time-stretch and EOS for recording electron bunch near fields at high repetition rate. We present preliminary results obtained at ANKA, aiming at recording the electron bunch shape evolution during the microbunching instability
A NEW SCHEME FOR ELECTRO-OPTIC SAMPLING AT RECORD REPETITION RATES: PRINCIPLE AND APPLICATION TO THE FIRST (TURN-BY-TURN) RECORDINGS OF THz CSR BURSTS AT SOLEIL
Abstract The microbunching instability is an ubiquitous problem in storage rings at high current density. However, the involved fast time-scales hampered the possibility to make direct real-time recordings of theses structures. When the structures occur at a cm scale, recent works at UVSOR [1], revealed that direct recording of the coherent synchrotron radiation (CSR) electric field with ultra-high speed electronics (17 ps) provides extremely precious informations on the microbunching dynamics. However, when CSR occurs at THz frequencies (and is thus out of reach of electronics), the problem remained largely open. Here we present a new opto-electronic strategy that enabled to record series of successive electric field pulses shapes with picosecond resolution (including carrier and envelope), every 12 ns, over a total duration of several milliseconds. We also present the first experimental results obtained with this method at Synchrotron SOLEIL, above the microbunching instability threshold. The method can be applied to the detection of ps electric fields in other situations where high repetition rate is also an issue
Invariance and homogenization of an adaptive time gap car-following model
In this paper we consider a microscopic model of traffic flow called the adaptive time gap carfollowing model. This is a system of ODEs which describes the interactions between cars moving on a single line. The time gap is the time that a car needs to reach the position of the car in front of it (if the car in front of it would not move and if the moving car would not change its velocity). In this model, both the velocity of the car and the time gap satisfy an ODE. We study this model and show that under certain assumptions, there is an invariant set for which the dynamics is defined for all times and for which we have a comparison principle. As a consequence, we show rigorously that after rescaling, this microscopic model converges to a macroscopic model that can be identified as the classical LWR model for traffic
Stationary state properties of a traffic flow model mixing stochastic transport and car-following
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