26,562 research outputs found
Feedback local optimality principle applied to rocket vertical landing VTVL
Vertical landing is becoming popular in the last fifteen years, a technology known under the acronym VTVL, Vertical Takeoff and Vertical Landing [1,2]. The interest in such landing technology is dictated by possible cost reductions [3,4], that impose spaceshipâs recycling. The rockets are not generally de- signed to perform landing operations, rather their design is aimed at takeoff operations, guaranteeing a very high forward acceleration to gain the velocity needed to escape the gravitational force. In this paper a new control method based on Feedback Local Optimality Principle, named FLOP is applied to the rocket landing problem. The FLOP belongs to a special class of optimal controllers, developed by the mechatronic and vehicle dynamics lab of Sapienza, named Variational Feedback Controllers - VFC, that are part of an ongoing research and are recently applied in different field: nonlinear system [5], marine and terrestrial autonomous vehicles [6,7,8], multi agents interactions and vibration control [9, 10]. The paper is devoted to show the robustness of the nonlinear controlled system, comparing the performances with the LQR, one of the most acknowledged methods in optimal control
Multiphoton Coincidence Spectroscopy
We extend the analysis of photon coincidence spectroscopy beyond bichromatic
excitation and two-photon coincidence detection to include multichromatic
excitation and multiphoton coincidence detection. Trichromatic excitation and
three-photon coincidence spectroscopy are studied in detail, and we identify an
observable signature of a triple resonance in an atom-cavity system.Comment: 6 page, REVTeXs, 6 Postscript figures. The abstract appeared in the
Proceedings of ACOLS9
Electronic heat current rectification in hybrid superconducting devices
In this work, we review and expand recent theoretical proposals for the
realization of electronic thermal diodes based on tunnel-junctions of normal
metal and superconducting thin films. Starting from the basic rectifying
properties of a single hybrid tunnel junction, we will show how the
rectification efficiency can be largely increased by combining multiple
junctions in an asymmetric chain of tunnel-coupled islands. We propose three
different designs, analyzing their performance and their potential advantages.
Besides being relevant from a fundamental physics point of view, this kind of
devices might find important technological application as fundamental building
blocks in solid-state thermal nanocircuits and in general-purpose cryogenic
electronic applications requiring energy management.Comment: 9 pages, 5 color figure
Critical current density and vortex pinning in tetragonal FeSSe ()
We report critical current density () in tetragonal FeS single crystals,
similar to iron based superconductors with much higher superconducting critical
temperatures ('s). The is enhanced 3 times by 6\% Se doping. We
observe scaling of the normalized vortex pinning force as a function of reduced
field at all temperatures. Vortex pinning in FeS and FeSSe
shows contribution of core-normal surface-like pinning. Reduced temperature
dependence of indicates that dominant interaction of vortex cores and
pinning centers is via scattering of charge carriers with reduced mean free
path (), in contrast to KFeSe where spatial
variations in () prevails.Comment: 5 pages, 4 figure
Soot measurement in diluted methane diffusion flames by multi-pass extinction and laser-induced incandescence
Multi-pass cavity line-of-sight extinction (MPC-LOSE) and laser- induced incandescence (LII) techniques are deployed to measure the soot volume fraction in a series of nitrogen-diluted flames, which produce only ppm volume mass fractions of soot. The separate suppression effects on soot formation of direct fuel dilution and indirect effects of temperature and res- idence time are interpreted by using a numerically calculated flow velocity and temperature field using a one-step fast chemistry model. The experi- mentally determined rate of soot formation is shown to obey approximately the same function of the local temperature for all dilution cases. The results show that a simple one-step reaction model using previously measured acti- vation energies can account for the dilution effect with good accuracy. The results show that the direct effect of dilution on concentration is comparable to the effects of changing the temperature estimated local temperature and residence time
Supersymmetric field theory of local light diffusion in semi-infinite media
A supersymmetric field theory of light diffusion in semi-infinite disordered
media is presented. With the help of this technique we justify--at the
perturbative level--the local light diffusion proposed by Tiggelen, Lagendijk,
and Wiersma [Phys. Rev. Lett. \textbf{84}, 4333 (2000)], and show that the
coherent backscattering line shape of medium bar displays a crossover from
two-dimensional weak to quasi-one-dimensional strong localization.Comment: 14 pages, 1 figure. accepted for publication in Phys. Rev.
Phonon quarticity induced by changes in phonon-tracked hybridization during lattice expansion and its stabilization of rutile TiO
Although the rutile structure of TiO is stable at high temperatures, the
conventional quasiharmonic approximation predicts that several acoustic phonons
decrease anomalously to zero frequency with thermal expansion, incorrectly
predicting a structural collapse at temperatures well below 1000\,K. Inelastic
neutron scattering was used to measure the temperature dependence of the phonon
density of states (DOS) of rutile TiO from 300 to 1373\,K. Surprisingly,
these anomalous acoustic phonons were found to increase in frequency with
temperature. First-principles calculations showed that with lattice expansion,
the potentials for the anomalous acoustic phonons transform from quadratic to
quartic, stabilizing the rutile phase at high temperatures. In these modes, the
vibrational displacements of adjacent Ti and O atoms cause variations in
hybridization of electrons of Ti and electrons of O atoms. With
thermal expansion, the energy variation in this "phonon-tracked hybridization"
flattens the bottom of the interatomic potential well between Ti and O atoms,
and induces a quarticity in the phonon potential.Comment: 7 pages, 6 figures, supplemental material (3 figures
Observation of indirect ionization of W7+ in an electron-beam ion-trap plasma
In this work, visible and extreme ultraviolet spectra of W7+ are measured
using the high-temperature superconducting electron-beam ion trap (EBIT) at the
Shanghai EBIT Laboratory under extremely low-energy conditions (lower than the
nominal electron-beam energy of 130 eV). The relevant atomic structure is
calculated using the flexible atomic code package based on the relativistic
configuration interaction method. The GRASP2K code, in the framework of the
multiconfiguration Dirac-Hartree-Fock method, is employed as well for
calculating the wavelength of the M1 transition in the ground configuration of
W7+. A line from the W7+ ions is observed at a little higher electron-beam
energy than the ionization potential for W4+, making this line appear to be
from W5+. A hypothesis for the charge-state evolution of W7+ is proposed based
on our experimental and theoretical results; that is, the occurrence of W7+
ions results from indirect ionization caused by stepwise excitation between
some metastable states of lower-charge-state W ions, at the nominal
electron-beam energy of 59 eV
A compactness theorem for complete Ricci shrinkers
We prove precompactness in an orbifold Cheeger-Gromov sense of complete
gradient Ricci shrinkers with a lower bound on their entropy and a local
integral Riemann bound. We do not need any pointwise curvature assumptions,
volume or diameter bounds. In dimension four, under a technical assumption, we
can replace the local integral Riemann bound by an upper bound for the Euler
characteristic. The proof relies on a Gauss-Bonnet with cutoff argument.Comment: 28 pages, final version, to appear in GAF
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