262 research outputs found
On the nature of reconnection at a solar coronal null point above a separatrix dome
Three-dimensional magnetic null points are ubiquitous in the solar corona,
and in any generic mixed-polarity magnetic field. We consider magnetic
reconnection at an isolated coronal null point, whose fan field lines form a
dome structure. We demonstrate using analytical and computational models
several features of spine-fan reconnection at such a null, including the fact
that substantial magnetic flux transfer from one region of field line
connectivity to another can occur. The flux transfer occurs across the current
sheet that forms around the null point during spine-fan reconnection, and there
is no separator present. Also, flipping of magnetic field lines takes place in
a manner similar to that observed in quasi-separatrix layer or slip-running
reconnection.Comment: Accepted for publication in the Astrophysical Journa
Shrinkage of Three Tropical Hardwoods Below and Above the Fiber Saturation Point
Two experimental techniques were used to perform moisture sorption tests at 25°C on samples of three tropical hardwood species: tornillo (Cedrelinga cateniformis Ducke), pumaquiro (Aspidosperma macrocarpon Mart.), and huayruro (Ormosia coccinea Jackson) woods. The first technique used saturated salt solutions at a relative humidity from 0% to 90%, and the second one used the pressure membrane method at above 96% relative humidity. These sorption tests were combined with shrinkage measurements. The fiber saturation point (FSP), estimated by interpolation to zero volumetric shrinkage, was 28%, 22.5%, and 21.5% for tornillo, pumaquiro, and huayruro, respectively. Results confirmed that at equilibrium moisture content, radial, tangential, and volumetric shrinkage occur above the actual FSP. This behavior can be explained by the effect of hysteresis at saturation on wood properties. This hysteresis indicates that loss of bound water takes place in the presence of liquid or capillary water, which contradicts the traditional concept of FSP. The initial equilibrium moisture content at which bound water starts to leave cell walls varied largely among the species: 52%, 36%, and 77% for tornillo, pumaquiro, and huayruro, respectively. The liquid water remaining in wood could be principally located in the least permeable flow paths of these wood species
Inhomogeneous mechanical losses in micro-oscillators with high reflectivity coating
We characterize the mechanical quality factor of micro-oscillators covered by
a highly reflective coating. We test an approach to the reduction of mechanical
losses, that consists in limiting the size of the coated area to reduce the
strain and the consequent energy loss in this highly dissipative component.
Moreover, a mechanical isolation stage is incorporated in the device. The
results are discussed on the basis of an analysis of homogeneous and
non-homogeneous losses in the device and validated by a set of Finite-Element
models. The contributions of thermoelastic dissipation and coating losses are
separated and the measured quality factors are found in agreement with the
calculated values, while the absence of unmodeled losses confirms that the
isolation element integrated in the device efficiently uncouples the dynamics
of the mirror from the support system. Also the resonant frequencies evaluated
by Finite-Element models are in good agreement with the experimental data, and
allow the estimation of the Young modulus of the coating. The models that we
have developed and validated are important for the design of oscillating
micro-mirrors with high quality factor and, consequently, low thermal noise.
Such devices are useful in general for high sensitivity sensors, and in
particular for experiments of quantum opto-mechanics
Dynamical Relaxation of Coronal Magnetic Fields. III. 3D Spiral Nulls
Context: The majority of studies on stressed 3D magnetic null points consider
magnetic reconnection driven by an external perturbation, but the formation of
a genuine current sheet equilibrium remains poorly understood. This problem has
been considered more extensively in two-dimensions, but lacks a generalization
into 3D fields.
Aims: 3D magnetic nulls are more complex than 2D nulls and the field can take
a greater range of magnetic geometries local to the null. Here, we focus on one
type and consider the dynamical non-resistive relaxation of 3D spiral nulls
with initial spine-aligned current. We aim to provide a valid
magnetohydrostatic equilibrium, and describe the electric current accumulations
in various cases, involving a finite plasma pressure.
Methods: A full MHD code is used, with the resistivity set to zero so that
reconnection is not allowed, to run a series of experiments in which a
perturbed spiral 3D null point is allowed to relax towards an equilibrium, via
real, viscous damping forces. Changes to the initial plasma pressure and other
magnetic parameters are investigated systematically.
Results: For the axi-symmetric case, the evolution of the field and the
plasma is such that it concentrates the current density in two cone-shaped
regions along the spine, thus concentrating the twist of the magnetic field
around the spine, leaving a radial configuration in the fan plane. The plasma
pressure redistributes in order to maintain the current density accumulations.
However, it is found that changes in the initial plasma pressure do not modify
the final state significantly. In the cases where the initial magnetic field is
not axi-symmetric, a infinite-time singularity of current perpendicular to the
fan is found at the location of the null
Detection of weak stochastic force in a parametrically stabilized micro opto-mechanical system
Measuring a weak force is an important task for micro-mechanical systems,
both when using devices as sensitive detectors and, particularly, in
experiments of quantum mechanics. The optimal strategy for resolving a weak
stochastic signal force on a huge background (typically given by thermal noise)
is a crucial and debated topic, and the stability of the mechanical resonance
is a further, related critical issue. We introduce and analyze the parametric
control of the optical spring, that allows to stabilize the resonance and
provides a phase reference for the oscillator motion, yet conserving a free
evolution in one quadrature of the phase space. We also study quantitatively
the characteristics of our micro opto-mechanical system as detector of
stochastic force for short measurement times (for quick, high resolution
monitoring) as well as for the longer term observations that optimize the
sensitivity. We compare a simple, naive strategy based on the evaluation of the
variance of the displacement (that is a widely used technique) with an optimal
Wiener-Kolmogorov data analysis. We show that, thanks to the parametric
stabilization of the effective susceptibility, we can more efficiently
implement Wiener filtering, and we investigate how this strategy improves the
performance of our system. We finally demonstrate the possibility to resolve
stochastic force variations well below 1% of the thermal noise
An ultra-low dissipation micro-oscillator for quantum opto-mechanics
Generating non-classical states of light by opto-mechanical coupling depends
critically on the mechanical and optical properties of micro-oscillators and on
the minimization of thermal noise. We present an oscillating micro-mirror with
a mechanical quality factor Q = 2.6x10^6 at cryogenic temperature and a Finesse
of 65000, obtained thanks to an innovative approach to the design and the
control of mechanical dissipation. Already at 4 K with an input laser power of
2 mW, the radiation-pressure quantum fluctuations become the main noise source,
overcoming thermal noise. This feature makes our devices particularly suitable
for the production of pondero-motive squeezing.Comment: 21 pages including Supplementary Informatio
Dynamical two-mode squeezing of thermal fluctuations in a cavity opto-mechanical system
We report the experimental observation of two-mode squeezing in the
oscillation quadratures of a thermal micro-oscillator. This effect is obtained
by parametric modulation of the optical spring in a cavity opto-mechanical
system. In addition to stationary variance measurements, we describe the
dynamic behavior in the regime of pulsed parametric excitation, showing
enhanced squeezing effect surpassing the stationary 3dB limit. While the
present experiment is in the classical regime, our technique can be exploited
to produce entangled, macroscopic quantum opto-mechanical modes
Frequency noise cancellation in optomechanical systems for ponderomotive squeezing
Ponderomotive squeezing of the output light of an optical cavity has been
recently observed in the MHz range in two different cavity optomechanical
devices. Quadrature squeezing becomes particularly useful at lower spectral
frequencies, for example in gravitational wave interferometers, despite being
more sensitive to excess phase and frequency noise. Here we show a
phase/frequency noise cancellation mechanism due to destructive interference
which can facilitate the production of ponderomotive squeezing in the kHz range
and we demonstrate it experimentally in an optomechanical system formed by a
Fabry-P\'{e}rot cavity with a micro-mechanical mirror.Comment: 11 pages, 9 figures. Physical explanation expanded. Modified figure
Control of Recoil Losses in Nanomechanical SiN Membrane Resonators
In the context of a recoil damping analysis, we have designed and produced a
membrane resonator equipped with a specific on-chip structure working as a
"loss shield" for a circular membrane. In this device the vibrations of the
membrane, with a quality factor of , reach the limit set by the intrinsic
dissipation in silicon nitride, for all the modes and regardless of the modal
shape, also at low frequency. Guided by our theoretical model of the loss
shield, we describe the design rationale of the device, which can be used as
effective replacement of commercial membrane resonators in advanced
optomechanical setups, also at cryogenic temperatures
Calibrated quantum thermometry in cavity optomechanics
Cavity optomechanics has achieved the major breakthrough of the preparation
and observation of macroscopic mechanical oscillators in peculiarly quantum
states. The development of reliable indicators of the oscillator properties in
these conditions is important also for applications to quantum technologies. We
compare two procedures to infer the oscillator occupation number, minimizing
the necessity of system calibrations. The former starts from homodyne spectra,
the latter is based on the measurement of the motional sidebands asymmetry in
heterodyne spectra. Moreover, we describe and discuss a method to control the
cavity detuning, that is a crucial parameter for the accuracy of the latter,
intrinsically superior procedure
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