220 research outputs found
Phase imaging in scanning transmission electron microscopy using bright-field balanced divergency method
We introduce a phase imaging mechanism for scanning transmission electron
microscopy that exploits the complementary intensity changes of transmitted
disks at different scattering angles. For scanning transmission electron
microscopy, this method provides a straightforward, dose-efficient, and
noise-robust phase imaging, from atomic resolution to intermediate length
scales, as a function of scattering angles and probe defocus. At atomic
resolution, we demonstrate that the phase imaging using the method can detect
both light and heavy atomic columns. Furthermore, we experimentally apply the
method to the imaging of nanoscale magnetic phases in FeGe samples. Compared
with conventional methods, phase retrieval using the new method has higher
effective spatial resolution and robustness to non-phase background contrast.
Our method complements traditional phase imaging modalities in electron
microscopy and has the potential to be extended to other scanning transmission
techniques and to characterize many emerging material systems.Comment: Updated link to codes; Fixed wrong labeling in Figures 3; Updated
Figure
Energy transfer analysis for capacitor voltage balancing of modular multilevel converters
Voltage balancing between sub-module (SM) capacitors is essential for reliable operation of the modular multilevel converter (MMC). To facilitate design and understanding of the balancing controllers, this study presents an energy transfer analysis for MMC, which explains how the energy can be independently transmitted from/to one phase, between the upper and lower arms, and among the SMs, of an MMC. Using this analysis, the variables which can be utilized to achieve capacitor voltage balancing are identified. Validity of this study has been verified by experimental results based on a three-phase MMC prototype
A hybrid modular multilevel converter for medium-voltage variable-speed motor drives
Modular multilevel converters (MMC) have revolutionized the voltage-sourced converter-based high-voltage direct current transmission, but not yet got widespread application in medium-voltage variable-speed motor drives, because of the large capacitor voltage ripples at low motor speeds. In this paper, a novel hybrid MMC topology is introduced, which significantly reduces the voltage ripple of capacitors, particularly at low motor speeds. Moreover, this topology does not introduce any motor common-mode voltage; as a result, there are no insulation and bearing current problems. Additionally, the current stress can remain at rated value throughout the whole speed range; thus, no device needs to be oversized and converter efficiency can be ensured. Operating principle of this hybrid topology is explained, and control schemes are also developed. Validity and performance of the proposed topology are verified by simulation and experimental results
Novel Bacterial Diversity and Fragmented eDNA Identified in Hyperbiofilm-Forming Pseudomonas aeruginosa Rugose Small Colony Variant
Pseudomonas aeruginosa biofilms represent a major threat to health care. Rugose small colony variants (RSCV) of P. aeruginosa, isolated from chronic infections, display hyperbiofilm phenotype. RSCV biofilms are highly resistant to antibiotics and host defenses. This work shows that RSCV biofilm aggregates consist of two distinct bacterial subpopulations that are uniquely organized displaying contrasting physiological characteristics. Compared with that of PAO1, the extracellular polymeric substance of RSCV PAO1ĪwspF biofilms presented unique ultrastructural characteristics. Unlike PAO1, PAO1ĪwspF released fragmented extracellular DNA (eDNA) from live cells. Fragmented eDNA, thus released, was responsible for resistance of PAO1ĪwspF biofilm to disruption by DNaseI. When added to PAO1, such fragmented eDNA enhanced biofilm formation. Disruption of PAO1ĪwspF biofilm was achieved by aurine tricarboxylic acid, an inhibitor of DNA-protein interaction. This work provides critical novel insights into the contrasting structural and functional characteristics of a hyperbiofilm-forming clinical bacterial variant relative to its own wild-type strain
Development of Drive Control Strategy for Front-and-Rear-Motor-Drive Electric Vehicle (FRMDEV)
In order to achieve both high-efficiency drive and low-jerk mode switch in FRMDEVs, a drive control strategy is proposed, consisting of top-layer torque distribution aimed at optimal efficiency and low-layer coordination control improving mode-switch jerk. First, with the use of the off-line particle swarm optimization algorithm (PSOA), the optimal switching boundary between single-motor-drive mode (SMDM) and dual-motor drive mode (DMDM) was modelled and a real-time torque distribution model based on the radial basis function (RBF) was created to achieve the optimal torque distribution. Then, referring to the dynamic characteristics of mode switch tested on a dual-motor test bench, a torque coordination strategy by controlling the variation rate of the torque distribution coefficient during the mode-switch process was developed. Finally, based on a hardware-in-loop (HIL) test platform and an FRMDEV, the proposed drive control strategy was verified. The test results show that both drive economy and comfort were improved significantly by the use of the developed drive control strategy
Faint high-energy gamma-ray photon emission of GRB 081006A from Fermi observations
Since the launch of the Fermi gamma - ray Space Telescope on June 11, 2008,
the LAT instrument has solidly detected more than 20 GRBs with high energy
photon emission above 100 MeV. Using the matched filter technique, 3 more GRBs
have also shown evidence of correlation with high energy photon emission as
demonstrated by Akerlof et al. In this paper, we present another GRB
unambiguously detected by the matched filter technique, GRB 081006A. This event
is associated with more than 13 high energy photons above 100 MeV. The
likelihood analysis code provided by the Science Support Center (FSSC)
generated an independent verification of this detection by comparison of the
Test Statistics (TS) value with similar calculations for random LAT data
fields. We have performed detailed temporal and spectral analysis of photons
from 8 keV up to 0.8 GeV from the GBM and the LAT. The properties of GRB
081006A can be compared to the other two long duration GRBs detected at similar
significance, GRB 080825C and GRB 090217A. We find that GRB 081006A is more
similar to GRB 080825C with comparable appearances of late high energy photon
emission. As demonstrated previously, there appears to be a surprising dearth
of faint LAT GRBs, with only one additional GRB identified in a sample of 74
GRBs. In this unique period when both and are operational,
there is some urgency to explore this aspect of GRBs as fully as possible.Comment: ApJ, 745, 7
Resonant scattering of energetic electrons by unusual low-frequency hiss
Abstract We quantify the resonant scattering effects of the unusual low-frequency dawnside plasmaspheric hiss observed on 30 September 2012 by the Van Allen Probes. In contrast to normal (ā¼100-2000 Hz) hiss emissions, this unusual hiss event contained most of its wave power at ā¼20-200 Hz. Compared to the scattering by normal hiss, the unusual hiss scattering speeds up the loss of ā¼50-200 keV electrons and produces more pronounced pancake distributions of ā¼50-100 keV electrons. It is demonstrated that such unusual low-frequency hiss, even with a duration of a couple of hours, plays a particularly important role in the decay and loss process of energetic electrons, resulting in shorter electron lifetimes for ā¼50-400 keV electrons than normal hiss, and should be carefully incorporated into global modeling of radiation belt electron dynamics during periods of intense injections
GRB 110709A, 111117A and 120107A: Faint high-energy gamma-ray photon emission from Fermi/LAT observations and demographic implications
Launched on June 11, 2008, the LAT instrument onboard the Gamma-ray
Space Telescope has provided a rare opportunity to study high energy photon
emission from gamma-ray bursts. Although the majority of such events (27) have
been iden tified by the Fermi LAT Collaboration, four were uncovered by using
more sensiti ve statistical techniques (Akerlof et al 2010, Akerlof et al 2011,
Zheng et al 2 012). In this paper, we continue our earlier work by finding
three more GRBs ass ociated with high energy photon emission, GRB 110709A,
111117A and 120107A. To s ystematize our matched filter approach, a pipeline
has been developed to identif y these objects in near real time. GRB 120107A is
the first product of this anal ysis procedure. Despite the reduced threshold
for identification, the number of GRB events has not increased significantly.
This relative dearth of events with low photon number prompted a study of the
apparent photon number distribution. W e find an extremely good fit to a simple
power-law with an exponent of -1.8 0.3 for the differential
distribution. As might be expected, there is a substa ntial correlation between
the number of lower energy photons detected by the GBM and the number observed
by the LAT. Thus, high energy photon emission is associ ated with some but not
all of the brighter GBM events. Deeper studies of the pro perties of the small
population of high energy emitting bursts may eventually yi eld a better
understanding of these entire phenomena.Comment: accepted to Ap
A Comparison of Different Protocols for the Extraction of Microbial DNA Inhabiting Synthetic Mars Simulant Soil
Compared with typical Earth soil, Martian soil and Mars simulant soils have distinct properties, including pH > 8.0 and high contents of silicates, iron-rich minerals, sulfates, and metal oxides. This unique soil matrix poses a major challenge for extracting microbial DNA. In particular, mineral adsorption and the generation of destructive hydroxyl radicals through cationic redox cycling may interfere with DNA extraction. This study evaluated different protocols for extracting microbial DNA from Mars Global Simulant (MGS-1), a Mars simulant soil. Two commercial kits were tested: the FastDNA SPIN Kit for soil (āMP kitā) and the DNeasy PowerSoil Pro Kit (āPowerSoil kitā). MGS-1 was incubated with living soil for five weeks, and DNA was extracted from aliquots using the kits. After extraction, the DNA was quantified with a NanoDrop spectrophotometer and used as the template for 16S rRNA gene amplicon sequencing and qPCR. The MP kit was the most efficient, yielding approximately four times more DNA than the PowerSoil kit. DNA extracted using the MP kit with 0.5 g soil resulted in 28,642ā37,805 16S rRNA gene sequence reads and 30,380ā42,070 16S rRNA gene copies, whereas the 16S rRNA gene could not be amplified from DNA extracted using the PowerSoil kit. We suggest that the FastDNA SPIN Kit is the best option for studying microbial communities in Mars simulant soils
Regulation of denitrification at the cellular level: a clue to the understanding of N2O emissions from soils
Denitrifying prokaryotes use NOx as terminal electron acceptors in response to oxygen depletion. The process emits a mixture of NO, N2O and N2, depending on the relative activity of the enzymes catalysing the stepwise reduction of NO3ā to N2O and finally to N2. Cultured denitrifying prokaryotes show characteristic transient accumulation of NO2ā, NO and N2O during transition from oxic to anoxic respiration, when tested under standardized conditions, but this character appears unrelated to phylogeny. Thus, although the denitrifying community of soils may differ in their propensity to emit N2O, it may be difficult to predict such characteristics by analysis of the community composition. A common feature of strains tested in our laboratory is that the relative amounts of N2O produced (N2O/(N2+N2O) product ratio) is correlated with acidity, apparently owing to interference with the assembly of the enzyme N2O reductase. The same phenomenon was demonstrated for soils and microbial communities extracted from soils. Liming could be a way to reduce N2O emissions, but needs verification by field experiments. More sophisticated ways to reduce emissions may emerge in the future as we learn more about the regulation of denitrification at the cellular level
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