7,889 research outputs found
Monitoring stimulated emission at the single photon level in one-dimensional atoms
We theoretically investigate signatures of stimulated emission at the single
photon level for a two-level atom interacting with a one-dimensional light
field. We consider the transient regime where the atom is initially excited,
and the steady state regime where the atom is continuously driven with an
external pump. The influence of pure dephasing is studied, clearly showing that
these effects can be evidenced with state of the art solid state devices. We
finally propose a scheme to demonstrate the stimulation of one optical
transition by monitoring another one, in three-level one-dimensional atoms.Comment: 4 pages, 4 figures. Improved introduction; Comments adde
Plasma Processing of Large Curved Surfaces for SRF Cavity Modification
Plasma based surface modification of niobium is a promising alternative to
wet etching of superconducting radio frequency (SRF) cavities. The development
of the technology based on Cl2/Ar plasma etching has to address several crucial
parameters which influence the etching rate and surface roughness, and
eventually, determine cavity performance. This includes dependence of the
process on the frequency of the RF generator, gas pressure, power level, the
driven (inner) electrode configuration, and the chlorine concentration in the
gas mixture during plasma processing. To demonstrate surface layer removal in
the asymmetric non-planar geometry, we are using a simple cylindrical cavity
with 8 ports symmetrically distributed over the cylinder. The ports are used
for diagnosing the plasma parameters and as holders for the samples to be
etched. The etching rate is highly correlated with the shape of the inner
electrode, radio-frequency (RF) circuit elements, chlorine concentration in the
Cl2/Ar gas mixtures, residence time of reactive species and temperature of the
cavity. Using cylindrical electrodes with variable radius, large-surface
ring-shaped samples and d.c. bias implementation in the external circuit we
have demonstrated substantial average etching rates and outlined the
possibility to optimize plasma properties with respect to maximum surface
processing effect
Correlation between magnetic interactions and domain structure in A1 FePt ferromagnetic thin films
We have investigated the relationship between the domain structure and the
magnetic interactions in a series of FePt ferromagnetic thin films of varying
thickness. As-made films grow in the magnetically soft and chemically
disordered A1 phase that may have two distinct domain structures. Above a
critical thickness nm the presence of an out of plane
anisotropy induces the formation of stripes, while for planar
domains occur.
Magnetic interactions have been characterized using the well known DCD-IRM
remanence protocols, plots, and magnetic viscosity measurements. We
have observed a strong correlation between the domain configuration and the
sign of the magnetic interactions. Planar domains are associated with positive
exchange-like interactions, while stripe domains have a strong negative
dipolar-like contribution. In this last case we have found a close correlation
between the interaction parameter and the surface dipolar energy of the stripe
domain structure. Using time dependent magnetic viscosity measurements, we have
also estimated an average activation volume for magnetic reversal, nm which is approximately
independent of the film thickness or the stripe period.Comment: 25 pages, 11 figure
Effect of Self-Bias on Cylindrical Capacitive Discharge for Processing of Inner Walls of Tubular Structures-Case of SRF Cavities
Cylindrical capacitive discharge is a convenient medium for generating reactive ions to process inner walls superconductive radio-frequency (SRF) cavities. These cavities, used in particle accelerators, presents a three-dimensional structure made of bulk Niobium, with axial cylindrical symmetry. Manufactured cavity walls are covered with Niobium oxides and scattered particulates, which must be removed for desired SRF performance. Cylindrical capacitive discharge in a mixture of Ar and Cl2 is a sole and natural non-wet acid choice to purify the inner surfaces of SRF cavities by reactive ion etching. Coaxial cylindrical discharge is generated between a powered inner electrode and the grounded outer electrode, which is the cavity wall to be etched. Plasma sheath voltages were tailored to process the outer wall by providing an additional dc current to the inner electrode with the help of an external compensating dc power supply and corrugated design of the inner electrode. The dc bias potential difference is established between two electrodes to make the set-up favorable for SRF wall processing. To establish guidelines for reversing the asymmetry and establishing the optimal sheath voltage at the cavity wall, the dc self-bias potential and dc current dependence on process parameters, such as gas pressure, rf power and chlorine content in the Ar/Cl2 gas mixture was measured. The process is potentially applicable to all concave metallic surfaces. © 2018 Author(s)
Equilibrium properties of the mixed state in superconducting niobium in a transverse magnetic field: Experiment and theoretical model
Equilibrium magnetic properties of the mixed state in type-II superconductors
were measured with high purity bulk and film niobium samples in parallel and
perpendicular magnetic fields using dc magnetometry and scanning Hall-probe
microscopy. Equilibrium magnetization data for the perpendicular geometry were
obtained for the first time. It was found that none of the existing theories is
consistent with these new data. To address this problem, a theoretical model is
developed and experimentally validated. The new model describes the mixed state
in an averaged limit, i.e. %without detailing the samples' magnetic structure
and therefore ignoring interactions between vortices. It is quantitatively
consistent with the data obtained in a perpendicular field and provides new
insights on properties of vortices. % and the entire mixed state. At low values
of the Ginzburg-Landau parameter, the model converts to that of Peierls and
London for the intermediate state in type-I superconductors. It is shown that
description of the vortex matter in superconductors in terms of a 2D gas is
more appropriate than the frequently used crystal- and glass-like scenarios.Comment: 8 pages, 9 figure
Modelling diffusion of innovations in a social network
A new simple model of diffusion of innovations in a social network with
upgrading costs is introduced. Agents are characterized by a single real
variable, their technological level. According to local information agents
decide whether to upgrade their level or not balancing their possible benefit
with the upgrading cost. A critical point where technological avalanches
display a power-law behavior is also found. This critical point is
characterized by a macroscopic observable that turns out to optimize
technological growth in the stationary state. Analytical results supporting our
findings are found for the globally coupled case.Comment: 4 pages, 5 figures. Final version accepted in PR
ALDEHYDE EMISSIONS FROM A STATIONARY DIESEL ENGINE OPERATING WITH CASTOR OIL BIODIESEL – DIESEL OIL BLENDS
The presence of aldehyde in the exhaust gas of a stationary, direct injection, compression ignition engine operating with castor oil biodiesel/diesel oil blends (B5, B10, B20 and B35) is analyzed. The diesel engine was operated with constant speed of 1800 rev/min and load of 37.5 kW. The gas sample was collected directly from the exhaust. Aldehydes were identified and quantified using gas chromatography (GC) with flame ionization detector analyzer (FID). Acetaldehyde presented higher exhaust concentration than formaldehyde for all fuel blends tested. In general, the exhaust aldehyde levels were very low and did not present significant differences between the fuel blends tested
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