344 research outputs found
Continuum limit of self-driven particles with orientation interaction
We consider the discrete Couzin-Vicsek algorithm (CVA), which describes the
interactions of individuals among animal societies such as fish schools. In
this article, we propose a kinetic (mean-field) version of the CVA model and
provide its formal macroscopic limit. The final macroscopic model involves a
conservation equation for the density of the individuals and a non conservative
equation for the director of the mean velocity and is proved to be hyperbolic.
The derivation is based on the introduction of a non-conventional concept of a
collisional invariant of a collision operator
Electrostatic extraction of cold molecules from a cryogenic reservoir
We present a method which delivers a continuous, high-density beam of slow
and internally cold polar molecules. In our source, warm molecules are first
cooled by collisions with a cryogenic helium buffer gas. Cold molecules are
then extracted by means of an electrostatic quadrupole guide. For ND the
source produces fluxes up to molecules/s with
peak densities up to molecules/cm. For
HCO the population of rovibrational states is monitored by depletion
spectroscopy, resulting in single-state populations up to .Comment: 4 pages, 4 figures, changes to the text, updated figures and
reference
Velocity-selected molecular pulses produced by an electric guide
Electrostatic velocity filtering is a technique for the production of
continuous guided beams of slow polar molecules from a thermal gas. We extended
this technique to produce pulses of slow molecules with a narrow velocity
distribution around a tunable velocity. The pulses are generated by
sequentially switching the voltages on adjacent segments of an electric
quadrupole guide synchronously with the molecules propagating at the desired
velocity. This technique is demonstrated for deuterated ammonia (ND),
delivering pulses with a velocity in the range of and a
relative velocity spread of at FWHM. At velocities around
, the pulses contain up to molecules each. The data are
well reproduced by Monte-Carlo simulations, which provide useful insight into
the mechanisms of velocity selection.Comment: 8 pages, 6 figure
A parabolic approach to the control of opinion spreading
We analyze the problem of controlling to consensus a nonlinear system
modeling opinion spreading. We derive explicit exponential estimates on the
cost of approximately controlling these systems to consensus, as a function of
the number of agents N and the control time-horizon T. Our strategy makes use
of known results on the controllability of spatially discretized semilinear
parabolic equations. Both systems can be linked through time-rescalin
Measurement of the lifetime of Pb, Pb and Pb beams at 4.2 MeV per nucleon subject to electron cooling
By measuring the lifetime of stored beams, the recombination of the ions with cooling electrons was investigated. Rates found are larger than expected for radiative electron capture and significantly higher for Pb53+ than for Pb54+ and Pb52+. These results are important for the design of the lead ion injection system for the Large Hadron Collider and for recombination theories
Singular Cucker-Smale Dynamics
The existing state of the art for singular models of flocking is overviewed,
starting from microscopic model of Cucker and Smale with singular communication
weight, through its mesoscopic mean-filed limit, up to the corresponding
macroscopic regime. For the microscopic Cucker-Smale (CS) model, the
collision-avoidance phenomenon is discussed, also in the presence of bonding
forces and the decentralized control. For the kinetic mean-field model, the
existence of global-in-time measure-valued solutions, with a special emphasis
on a weak atomic uniqueness of solutions is sketched. Ultimately, for the
macroscopic singular model, the summary of the existence results for the
Euler-type alignment system is provided, including existence of strong
solutions on one-dimensional torus, and the extension of this result to higher
dimensions upon restriction on the smallness of initial data. Additionally, the
pressureless Navier-Stokes-type system corresponding to particular choice of
alignment kernel is presented, and compared - analytically and numerically - to
the porous medium equation
Intense Atomic and Molecular Beams via Neon Buffer Gas Cooling
We realize a continuous guided beam of cold deuterated ammonia with a flux of
3e11 ND3 molecules/s and a continuous free-space beam of cold potassium with a
flux of 1e16 K atoms/s. A novel feature of the buffer gas source used to
produce these beams is cold neon, which, due to intermediate Knudsen number
beam dynamics, produces a forward velocity and low-energy tail that is
comparable to much colder helium-based sources. We expect this source to be
trivially generalizable to a very wide range of atomic and molecular species
with significant vapor pressure below 1000 K. This source has properties that
make it a good starting point for laser cooling of molecules or atoms, cold
collision studies, trapping, or nonlinear optics in buffer-gas-cooled atomic or
molecular gases.Comment: 15 pages, 6 figure
Assignment of resonances in dissociative recombination of HD+ ions: high-resolution measurements compared with accurate computations
The collision-energy resolved rate coefficient for dissociative recombination
of HD+ ions in the vibrational ground state is measured using the photocathode
electron target at the heavy-ion storage ring TSR. Rydberg resonances
associated with ro-vibrational excitation of the HD+ core are scanned as a
function of the electron collision energy with an instrumental broadening below
1 meV in the low-energy limit. The measurement is compared to calculations
using multichannel quantum defect theory, accounting for rotational structure
and interactions and considering the six lowest rotational energy levels as
initial ionic states. Using thermal equilibrium level populations at 300 K to
approximate the experimental conditions, close correspondence between
calculated and measured structures is found up to the first vibrational
excitation threshold of the cations near 0.24 eV. Detailed assignments,
including naturally broadened and overlapping Rydberg resonances, are performed
for all structures up to 0.024 eV. Resonances from purely rotational excitation
of the ion core are found to have similar strengths as those involving
vibrational excitation. A dominant low-energy resonance is assigned to
contributions from excited rotational states only. The results indicate strong
modifications in the energy dependence of the dissociative recombination rate
coefficient through the rotational excitation of the parent ions, and underline
the need for studies with rotationally cold species to obtain results
reflecting low-temperature ionized media.Comment: 15 pages, 10 figures. Paper to appear in Phys. Rev. A (version as
accepted
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