245 research outputs found
Brownian Carnot engine
The Carnot cycle imposes a fundamental upper limit to the efficiency of a
macroscopic motor operating between two thermal baths. However, this bound
needs to be reinterpreted at microscopic scales, where molecular bio-motors and
some artificial micro-engines operate. As described by stochastic
thermodynamics, energy transfers in microscopic systems are random and thermal
fluctuations induce transient decreases of entropy, allowing for possible
violations of the Carnot limit. Despite its potential relevance for the
development of a thermodynamics of small systems, an experimental study of
microscopic Carnot engines is still lacking. Here we report on an experimental
realization of a Carnot engine with a single optically trapped Brownian
particle as working substance. We present an exhaustive study of the energetics
of the engine and analyze the fluctuations of the finite-time efficiency,
showing that the Carnot bound can be surpassed for a small number of
non-equilibrium cycles. As its macroscopic counterpart, the energetics of our
Carnot device exhibits basic properties that one would expect to observe in any
microscopic energy transducer operating with baths at different temperatures.
Our results characterize the sources of irreversibility in the engine and the
statistical properties of the efficiency -an insight that could inspire novel
strategies in the design of efficient nano-motors.Comment: 7 pages, 7 figure
Numerical Studies of Cosmic Ray Injection and Acceleration
A numerical scheme that incorporates a thermal leakage injection model into a
combined gas dynamics and cosmic ray (CR, hereafter) diffusion-convection code
has been developed. The particle injection is followed numerically by filtering
the diffusive flux of suprathermal particles across the shock to the upstream
region according to a velocity-dependent transparency function that controls
the fraction of leaking particles. We have studied CR injection and
acceleration efficiencies during the evolution of CR modified planar shocks for
a wide range of initial shock Mach numbers, , assuming a Bohm-like
diffusion coefficient. The injection process is very efficient when the
subshock is strong, leading to fast and significant modification of the shock
structure. As the CR pressure increases, the subshock weakens and the injection
rate decreases accordingly, so that the subshock does not disappear. Although
some fraction of the particles injected early in the evolution continue to be
accelerated to ever higher energies, the postshock CR pressure reaches an
approximate time-asymptotic value due to a balance between fresh
injection/acceleration and advection/diffusion of the CR particles away from
the shock. We conclude that the injection rates in strong parallel shocks are
sufficient to lead to rapid nonlinear modifications to the shock structures and
that self-consistent injection and time-dependent simulations are crucial to
understanding the non-linear evolution of CR modified shocks.Comment: 28 pages, To appear in ApJ November 1, 2002 issu
Optically levitated nanoparticle as a model system for stochastic bistable dynamics
Nano-mechanical resonators have gained an increasing importance in nanotechnology owing to their contributions to both fundamental and applied science. Yet, their small dimensions and mass raises some challenges as their dynamics gets dominated by nonlinearities that degrade their performance, for instance in sensing applications. Here, we report on the precise control of the nonlinear and stochastic bistable dynamics of a levitated nanoparticle in high vacuum. We demonstrate how it can lead to efficient signal amplification schemes, including stochastic resonance. This work contributes to showing the use of levitated nanoparticles as a model system for stochastic bistable dynamics, with applications to a wide variety of fields.inancial support from the ERC- QnanoMECA (Grant No. 64790), the Spanish Ministry of Economy and Competitiveness, under grant FIS2016-80293-R and through the ‘Severo Ochoa’ Programme for Centres of Excellence in R&D (SEV-2015-0522), Fundació Privada CELLEX and from the CERCA Programme/Generalitat de Catalunya. J.G. has been supported by H2020-MSCA-IF-2014 under REA grant Agreement No. 655369. L.R. acknowledges support from an ETH Marie Curie Cofund Fellowship
Efficiency of Nonlinear Particle Acceleration at Cosmic Structure Shocks
We have calculated the evolution of cosmic ray (CR) modified astrophysical
shocks for a wide range of shock Mach numbers and shock speeds through
numerical simulations of diffusive shock acceleration (DSA) in 1D quasi-
parallel plane shocks. The simulations include thermal leakage injection of
seed CRs, as well as pre-existing, upstream CR populations. Bohm-like diffusion
is assumed. We model shocks similar to those expected around cosmic structure
pancakes as well as other accretion shocks driven by flows with upstream gas
temperatures in the range K and shock Mach numbers spanning
. We show that CR modified shocks evolve to time-asymptotic states
by the time injected particles are accelerated to moderately relativistic
energies (p/mc \gsim 1), and that two shocks with the same Mach number, but
with different shock speeds, evolve qualitatively similarly when the results
are presented in terms of a characteristic diffusion length and diffusion time.
For these models the time asymptotic value for the CR acceleration efficiency
is controlled mainly by shock Mach number. The modeled high Mach number shocks
all evolve towards efficiencies %, regardless of the upstream CR
pressure. On the other hand, the upstream CR pressure increases the overall CR
energy in moderate strength shocks (). (abridged)Comment: 23 pages, 12 ps figures, accepted for Astrophysical Journal (Feb. 10,
2005
Modelling the spectral evolution of classical double radio sources
The spectral evolution of powerful double radio galaxies (FR II's) is thought
to be determined by the acceleration of electrons at the termination shock of
the jet, their transport through the bright head region into the lobes and the
production of the radio emission by synchrotron radiation in the lobes. Models
presented to date incorporate some of these processes in prescribing the
electron distribution which enters the lobes. We have extended these models to
include a description of electron acceleration at the relativistic termination
shock and a selection of transport models for the head region. These are
coupled to the evolution of the electron spectrum in the lobes under the
influence of losses due to adiabatic expansion, by inverse Compton scattering
on the cosmic background radiation and by synchrotron radiation. The
evolutionary tracks predicted by this model are compared to observation using
the power/source-size (P-D) diagram. We find that the simplest scenario, in
which accelerated particles suffer adiabatic losses in the head region which
become more severe as the source expands produces P-D-tracks which conflict
with observation, because the power is predicted to decline too steeply with
increasing size. Agreement with observation can be found by assuming that
adiabatic losses are compensated during transport between the termination shock
and the lobe by a re-acceleration process distributed throughout the head
region.Comment: 14 pages, 6 figures, accepted for publication in Astronomy and
Astrophysic
Connecting remote and in situ observations of shock-accelerated electrons associated with a coronal mass ejection
One of the most prominent sources for energetic particles in our solar system
are huge eruptions of magnetised plasma from the Sun called coronal mass
ejections (CMEs), which usually drive shocks that accelerate charged particles
up to relativistic energies. In particular, energetic electron beams can
generate radio bursts through the plasma emission mechanism, for example, type
II and accompanying herringbone bursts. Here, we investigate the acceleration
location, escape, and propagation directions of various electron beams in the
solar corona and compare them to the arrival of electrons at spacecraft. To
track energetic electron beams, we use a synthesis of remote and direct
observations combined with coronal modelling. Remote observations include
ground-based radio observations from the Nancay Radioheliograph (NRH) combined
with space-based extreme-ultraviolet and white-light observations from the
Solar Dynamics Observatory (SDO), the Solar Terrestrial Relations Observatory
(STEREO) and Solar Orbiter (SolO). We also use direct observations of energetic
electrons from the STEREO and Wind spacecraft. These observations are then
combined with a three-dimensional (3D) representation of the electron
acceleration locations that combined with results from magneto-hydrodynamic
models of the solar corona is used to investigate the origin and link of
electrons observed remotely at the Sun to in situ electrons. We observed a type
II radio burst followed by herringbone bursts that show single-frequency
movement through time in NRH images. The movement of the type II burst and
herringbone radio sources seems to be influenced by the regions in the corona
where the CME is more capable of driving a shock. We also found similar
inferred injection times of near-relativistic electrons at spacecraft to the
emission time of the type II and herringbone bursts.Comment: 16 pages, 15 figure
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