88,189 research outputs found
Improved current-regulated delta modulator for reducing switching frequency and low-frequency current error in permanent magnet brushless AC drives
The conventional current-regulated delta modulator (CRDM) results in a high current ripple and a high switching frequency at low rotational speeds, and in low-frequency current harmonics, including a fundamental current error, at high rotational speeds. An improved current controller based on CRDM is proposed which introduces a zero-vector zone and a current error correction technique. It reduces the current ripple and switching frequency at low speeds, without the need to detect the back-emf, as well as the low-frequency error at high speeds. The performance of the modulator is verified by both simulation and measurements on a permanent magnet brushless ac drive
Formation of Compressed Flat Electron Beams with High Transverse-Emittance Ratios
Flat beams -- beams with asymmetric transverse emittances -- have important
applications in novel light-source concepts, advanced-acceleration schemes and
could possibly alleviate the need for damping rings in lepton colliders. Over
the last decade, a flat-beam-generation technique based on the conversion of an
angular-momentum-dominated beam was proposed and experimentally tested. In this
paper we explore the production of compressed flat beams. We especially
investigate and optimize the flat-beam transformation for beams with
substantial fractional energy spread. We use as a simulation example the
photoinjector of the Fermilab's Advanced Superconducting Test Accelerator
(ASTA). The optimizations of the flat beam generation and compression at ASTA
were done via start-to-end numerical simulations for bunch charges of 3.2 nC,
1.0 nC and 20 pC at ~37 MeV. The optimized emittances of flat beams with
different bunch charges were found to be 0.25 {\mu}m (emittance ratio is ~400),
0.13 {\mu}m, 15 nm before compression, and 0.41 {\mu}m, 0.20 {\mu}m, 16 nm
after full compression, respectively with peak currents as high as 5.5 kA for a
3.2-nC flat beam. These parameters are consistent with requirements needed to
excite wakefields in asymmetric dielectric-lined waveguides or produce
significant photon flux using small-gap micro-undulators.Comment: 17
PDMS/PVA composite ferroelectret for improved energy harvesting performance
This paper address the PDMS ferroelectret discharge issue for improved long- term energy harvesting performance. The PDMS/PVA ferroelectret is fabricated using a 3D-printed plastic mould technology and a functional PVA composite layer is introduced. The PDMS/PVA composite ferroelectret achieved 80% piezoelectric coefficient d33 remaining, compared with 40% without the proposed layer over 72 hours. Further, the retained percentage of output voltage is about 73% over 72 hours
Effects of Minijets on Hadronic Spectra and Azimuthal Harmonics in Au-Au Collisions at 200 GeV
The production of hadrons in heavy-ion collisions at RHIC in the low
transverse-momentum () region is investigated in the recombination model
with emphasis on the effects of minijets on the azimuthal anisotropy. Since the
study is mainly on the hadronization of partons at late time, the fluid picture
is not used to trace the evolution of the system. The inclusive distributions
at low are determined as the recombination products of thermal partons.
The dependencies of both pion and proton have a common exponential factor
apart from other dissimilar kinematic and resonance factors, because they are
inherited from the same pool of thermal partons. Instead of the usual
description based on hydrodynamics, the azimuthal anisotropy of the produced
hadrons is explained as the consequence of the effects of minijets, either
indirectly through the recombination of enhanced thermal partons in the
vicinity of the trajectories of the semihard partons, or directly through
thermal-shower recombination. Although our investigation is focussed on the
single-particle distribution at midrapidity, we give reasons why a component in
that distribution can be identified with the ridge, which together with the
second harmonic is due to the semihard partons created near the medium
surface that lead to calculable anisotropy in . It is shown that the
higher azimuthal harmonics, , can also be well reproduced without
reference to flow. The and centrality dependencies of the higher
harmonics are prescribed by the interplay between TT and TS recombination
components. The implication of the success of this drastic departure from the
conventional approach is discussed.Comment: 28 pages and 8 figures, more discussions and references adde
The Vector and Axial-Vector Charmonium-like States
After constructing all the tetraquark interpolating currents with
and in a systematic way, we
investigate the two-point correlation functions to extract the masses of the
charmonium-like states with QCD sum rule. For the
charmonium-like state, GeV, which implies a possible
tetraquark interpretation for the state Y(4660). The masses for both the
and charmonium-like states are
around GeV, which are slightly above the mass of X(3872). For the
charmonium-like state, the extracted mass is GeV. We also discuss the possible decay modes and experimental search of
the charmonium-like states.Comment: 18 pages, 6 figures and 6 table
Continuous quantum phase transition in a Kondo lattice model
We study the magnetic quantum phase transition in an anisotropic Kondo
lattice model. The dynamical competition between the RKKY and Kondo
interactions is treated using an extended dynamic mean field theory (EDMFT)
appropriate for both the antiferromagnetic and paramagnetic phases. A quantum
Monte Carlo approach is used, which is able to reach very low temperatures, of
the order of 1% of the bare Kondo scale. We find that the finite-temperature
magnetic transition, which occurs for sufficiently large RKKY interactions, is
first order. The extrapolated zero-temperature magnetic transition, on the
other hand, is continuous and locally critical.Comment: 4 pages, 4 figures; updated, to appear in PR
Quasi-reversible Magnetoresistance in Exchange Spring Tunnel Junctions
We report a large, quasi-reversible tunnel magnetoresistance in
exchange-biased ferromagnetic semiconductor tunnel junctions wherein a soft
ferromagnetic semiconductor (\gma) is exchange coupled to a hard ferromagnetic
metal (MnAs). Our observations are consistent with the formation of a region of
inhomogeneous magnetization (an "exchange spring") within the biased \gma
layer. The distinctive tunneling anisotropic magnetoresistance of \gma produces
a pronounced sensitivity of the magnetoresistance to the state of the exchange
spring
Coordination motifs and large-scale structural organization in atomic clusters
The structure of nanoclusters is complex to describe due to their
noncrystallinity, even though bonding and packing constraints limit the local
atomic arrangements to only a few types. A computational scheme is presented to
extract coordination motifs from sample atomic configurations. The method is
based on a clustering analysis of multipole moments for atoms in the first
coodination shell. Its power to capture large-scale structural properties is
demonstrated by scanning through the ground state of the Lennard-Jones and
C clusters collected at the Cambridge Cluster Database.Comment: 6 pages, 7 figure
- …