11,267 research outputs found
Initial carrier-envelope phase of few-cycle pulses determined by THz emission from air plasma
The evolution of THz waveform generated in air plasma provides a sensitive
probe to the variation of the carrier envelope phase (CEP) of propagating
intense few-cycle pulses. Our experimental observation and calculation reveal
that the number and positions of the inversion of THz waveform are dependent on
the initial CEP, which is near 0.5{\pi} constantly under varied input pulse
energies when two inversions of THz waveform in air plasma become one. This
provides a method of measuring the initial CEP in an accuracy that is only
limited by the stability of the driving few-cycle pulses.Comment: 13 pages, 4 figure
Simulation for field emission images of micrometer-long SWCNTs
The electron distribution of open-ended single-walled carbon nanotubes with
chirality indexes (7,0) and (5,5) in the field emission conditions was
calculated via a multi-scaled algorithm. The field emission images were
produced numerically. It was found that the emission patterns change with the
applied macroscopic field. Especially, the symmetry of the emission pattern of
the (7,0) carbon nanotube is breaking in the lower field but the breaking is
less obvious in the higher field. The enlargement factor increases with the
applied macroscopic field.Comment: 8 pages, 4 figure
Waveform-Controlled Terahertz Radiation from the Air Filament Produced by Few-Cycle Laser Pulses
Waveform-controlled Terahertz (THz) radiation is of great importance due to
its potential application in THz sensing and coherent control of quantum
systems. We demonstrated a novel scheme to generate waveform-controlled THz
radiation from air plasma produced when carrier-envelope-phase (CEP) stabilized
few-cycle laser pulses undergo filamentation in ambient air. We launched
CEP-stabilized 10 fs-long (~ 1.7 optical cycles) laser pulses at 1.8 {\mu}m
into air and found that the generated THz waveform can be controlled by varying
the filament length and the CEP of driving laser pulses. Calculations using the
photocurrent model and including the propagation effects well reproduce the
experimental results, and the origins of various phase shifts in the filament
are elucidated.Comment: 5pages, 5 figure
Segatron: Segment-Aware Transformer for Language Modeling and Understanding
Transformers are powerful for sequence modeling. Nearly all state-of-the-art
language models and pre-trained language models are based on the Transformer
architecture. However, it distinguishes sequential tokens only with the token
position index. We hypothesize that better contextual representations can be
generated from the Transformer with richer positional information. To verify
this, we propose a segment-aware Transformer (Segatron), by replacing the
original token position encoding with a combined position encoding of
paragraph, sentence, and token. We first introduce the segment-aware mechanism
to Transformer-XL, which is a popular Transformer-based language model with
memory extension and relative position encoding. We find that our method can
further improve the Transformer-XL base model and large model, achieving 17.1
perplexity on the WikiText-103 dataset. We further investigate the pre-training
masked language modeling task with Segatron. Experimental results show that
BERT pre-trained with Segatron (SegaBERT) can outperform BERT with vanilla
Transformer on various NLP tasks, and outperforms RoBERTa on zero-shot sentence
representation learning.Comment: Accepted by AAAI 202
Transition of lithium growth mechanisms in liquid electrolytes
Next-generation high-energy batteries will require a rechargeable lithium metal anode, but lithium dendrites tend to form during recharging, causing short-circuit risk and capacity loss, by mechanisms that still remain elusive. Here, we visualize lithium growth in a glass capillary cell and demonstrate a change of mechanism from root-growing mossy lithium to tip-growing dendritic lithium at the onset of electrolyte diffusion limitation. In sandwich cells, we further demonstrate that mossy lithium can be blocked by nanoporous ceramic separators, while dendritic lithium can easily penetrate nanopores and short the cell. Our results imply a fundamental design constraint for metal batteries (“Sand's capacity”), which can be increased by using concentrated electrolytes with stiff, permeable, nanoporous separators for improved safety.MIT Energy Initiative (Robert Bosch GmbH)National Science Foundation (U.S.) (Grant DMR-1410636)Stanford University. Global Climate and Energy ProjectUnited States. Dept. of Energy. Office of Basic Energy Sciences (Stanford University. SUNCAT Center for Interface Science and Catalysis
Numerical Simulation Study on Propeller Slipstream Interference of High Altitude Long Endurance Unmanned Air Vehicle
AbstractIn this paper,thecontrol equation of Multiple Reference Frame(MRF) as the propeller calculation model was present and analyzed, the propeller slipstream interference on HALE UAV was studied with three-dimensional numerical simulation. It is shown that the flow field of the MRF model is good consistent with true propeller flow, and MRF can accurately simulate aerodynamic interference on the aircraft. The stream traces on the V-tail surface were deflected and shrank, pressure distribution,Cmx and Cmzon V-tail surface was changed apparently too.Butslipstream had little effect on wing. The influence of propeller slipstream on the aerodynamic performance of the UAV at the status of taking off is biggest, become weaker at status of climbing and smallest at the status of cruising. The influence of propeller slipstream is enhanced with increment of propeller thrust and basically familiar in the same thrust between the two blade attack angle. The pressure drag on aft of UAV fuselage increased rapidly by the interference of propeller slipstream, leading aerodynamic performance of UAV become badly
Repairing Process Models with Non-Free-Choice Constructs Based on Token Replay
A method of repairing process models with non-free-choice constructs is proposed based on logical Petri nets, aiming at the problem of low precision in the existing repair methods. An extended successor matrix of transitions is determined according to the distance between any two transitions. There are two types of choice-construct transitions. One is a non-free-choice construct transition, and the other is a general choice construct transition. The type of choice-construct transitions can be determined based on the extended successor matrix and the relationship between the front and back sets of transitions. The location of the deviations is calculated by an improved replaying method. Finally, a model can be repaired according to remaining-token places and missing-token places. Based on the experiments on real event logs, the method proposed in this paper has a better performance in fitness, precision, and simplicity compared with its peers
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