83 research outputs found
Ratcheting-up through competition: global environmental governance in the era of rising geopolitical tensions between China and the West
The rise of China has been considered as a parametric change bringing turbulence to world politics in the 21st century. In climate and environmental emergencies, how does a rising China and Western powers’ reaction to it affect systems of environmental governance? While some worry that growing geopolitical tensions will undermine global cooperation on climate change and environmental protection, the authors of this chapter argue that great power competition can be positive-sum for the provision of global public goods when countries promote more sustainable products and services through their competition. Using the case of overseas energy finance, the authors show how rising geopolitical competitions between China and the West have led the Chinese government to strengthen the environmental governance of China’s global engagement. The authors consider the factors operating at multiple levels and propose a theoretical framework to explain how policy changes in China are jointly shaped by Western governments, transnational nonstate actors, and domestic policy entrepreneurs
Ratcheting-up through competition:Global environmental governance in the era of rising geopolitical tensions between China and the West
The rise of China has been considered as a parametric change bringing turbulence to world politics in the 21st century. In climate and environmental emergencies, how does a rising China and Western powers’ reaction to it affect systems of environmental governance? While some worry that growing geopolitical tensions will undermine global cooperation on climate change and environmental protection, the authors of this chapter argue that great power competition can be positive-sum for the provision of global public goods when countries promote more sustainable products and services through their competition. Using the case of overseas energy finance, the authors show how rising geopolitical competitions between China and the West have led the Chinese government to strengthen the environmental governance of China’s global engagement. The authors consider the factors operating at multiple levels and propose a theoretical framework to explain how policy changes in China are jointly shaped by Western governments, transnational nonstate actors, and domestic policy entrepreneurs
Optimal destabilization of K-unstable Fano varieties via stability thresholds
We show that for a K-unstable Fano variety, any divisorial valuation
computing its stability threshold induces a non-trivial special test
configuration preserving the stability threshold. When such a divisorial
valuation exists, we show that the Fano variety degenerates to a uniquely
determined twisted K-polystable Fano variety. We also show that the stability
threshold can be approximated by divisorial valuations induced by special test
configurations. As an application of the above results and the analytic work of
Datar, Sz\'ekelyhidi, and Ross, we deduce that greatest Ricci lower bounds of
Fano manifolds of fixed dimension form a finite set of rational numbers. As a
key step in the proofs, we adapt the process of Li and Xu producing special
test configurations to twisted K-stability in the sense of Dervan.Comment: Final version, to appear in Geom. Topo
Beam Size Measurement by Optical Diffraction Radiation and Laser System for Compton Polarimeter
Beam diagnostics is an essential constituent of any accelerator, so that it is named as "organs of sense" or "eyes of the accelerator." Beam diagnostics is a rich field. A great variety of physical effects or physical principles are made use of in this field. Some devices are based on electro-magnetic influence by moving charges, such as faraday cups, beam transformers, pick-ups; Some are related to Coulomb interaction of charged particles with matter, such as scintillators, viewing screens, ionization chambers; Nuclear or elementary particle physics interactions happen in some other devices, like beam loss monitors, polarimeters, luminosity monitors; Some measure photons emitted by moving charges, such as transition radiation, synchrotron radiation monitors and diffraction radiation-which is the topic of the first part of this thesis; Also, some make use of interaction of particles with photons, such as laser wire and Compton polarimeters-which is the second part of my thesis. Diagnostics let us perceive what properties a beam has and how it behaves in a machine, give us guideline for commissioning, controlling the machine and indispensable parameters vital to physics experiments. In the next two decades, the research highlight will be colliders (TESLA, CLIC, JLC) and fourth-generation light sources (TESLA FEL, LCLS, Spring 8 FEL) based on linear accelerator. These machines require a new generation of accelerator with smaller beam, better stability and greater efficiency. Compared with those existing linear accelerators, the performance of next generation linear accelerator will be doubled in all aspects, such as 10 times smaller horizontal beam size, more than 10 times smaller vertical beam size and a few or more times higher peak power. Furthermore, some special positions in the accelerator have even more stringent requirements, such as the interaction point of colliders and wigglor of free electron lasers. Higher performance of these accelerators increases the difficulty of diagnostics. For most cases, intercepting measurements are no longer acceptable, and nonintercepting method like synchrotron radiation monitor can not be applied to linear accelerators. The development of accelerator technology asks for simutanous diagnostics innovations, to expand the performance of diagnostic tools to meet the requirements of the next generation accelerators. Diffraction radiation and inverse Compton scattering are two of the most promising techniques, their nonintercepting nature avoids perturbance to the beam and damage to the instrumentation. This thesis is divided into two parts, beam size measurement by optical diffraction radiation and Laser system for Compton polarimeter. Diffraction radiation, produced by the interaction between the electric field of charged particles and the target, is related to transition radiation. Even though the theory of diffraction radiation has been discussed since 1960s, there are only a few experimental studies in recent years. The successful beam size measurement by optical diffraction radiation at CEBAF machine is a milestone: First of all, we have successfully demonstrated diffraction radiation as an effective nonintercepting diagnostics; Secondly, the simple linear relationship between the diffraction radiation image size and the actual beam size improves the reliability of ODR measurements; And, we measured the polarized components of diffraction radiation for the first time and I analyzed the contribution from edge radiation to diffraction radiation
Energy recovery transport design for PKU FEL
A SRF linac based free electron laser user facility is under developed at Peking University. Energy recovery Linac technology was chosen for increase of average electron beam current, hence, increase of the free electron laser power. In this paper we present a conceptual design of beam transport line which satisfies requirement of ERL. A chicane consisting of four identical bend magnets is selected for path length adjustment up to ?? 18 degree. R56 of both arcs of the beam line is adjustable for full bunch compression. ?2007 IEEE.EI
AI-Oriented Two-Phase Multi-Factor Authentication in SAGINs: Prospects and Challenges
Space-air-ground integrated networks (SAGINs), which have emerged as an
expansion of terrestrial networks, provide flexible access, ubiquitous
coverage, high-capacity backhaul, and emergency/disaster recovery for mobile
users (MUs). While the massive benefits brought by SAGIN may improve the
quality of service, unauthorized access to SAGIN entities is potentially
dangerous. At present, conventional crypto-based authentication is facing
challenges, such as the inability to provide continuous and transparent
protection for MUs. In this article, we propose an AI-oriented two-phase
multi-factor authentication scheme (ATMAS) by introducing intelligence to
authentication. The satellite and network control center collaborate on
continuous authentication, while unique spatial-temporal features, including
service features and geographic features, are utilized to enhance the system
security. Our further security analysis and performance evaluations show that
ATMAS has proper security characteristics which can meet various security
requirements. Moreover, we shed light on lightweight and efficient
authentication mechanism design through a proper combination of
spatial-temporal factors.Comment: Accepted by IEEE Consumer Electronics Magazin
Accelerated partial separable model using dimension-reduced optimization technique for ultra-fast cardiac MRI
Objective. Imaging dynamic object with high temporal resolution is
challenging in magnetic resonance imaging (MRI). Partial separable (PS) model
was proposed to improve the imaging quality by reducing the degrees of freedom
of the inverse problem. However, PS model still suffers from long acquisition
time and even longer reconstruction time. The main objective of this study is
to accelerate the PS model, shorten the time required for acquisition and
reconstruction, and maintain good image quality simultaneously. Approach. We
proposed to fully exploit the dimension reduction property of the PS model,
which means implementing the optimization algorithm in subspace. We optimized
the data consistency term, and used a Tikhonov regularization term based on the
Frobenius norm of temporal difference. The proposed dimension-reduced
optimization technique was validated in free-running cardiac MRI. We have
performed both retrospective experiments on public dataset and prospective
experiments on in-vivo data. The proposed method was compared with four
competing algorithms based on PS model, and two non-PS model methods. Main
results. The proposed method has robust performance against shortened
acquisition time or suboptimal hyper-parameter settings, and achieves superior
image quality over all other competing algorithms. The proposed method is
20-fold faster than the widely accepted PS+Sparse method, enabling image
reconstruction to be finished in just a few seconds. Significance. Accelerated
PS model has the potential to save much time for clinical dynamic MRI
examination, and is promising for real-time MRI applications.Comment: 23 pages, 11 figures. Accepted as manuscript on Physics in Medicine &
Biolog
Few-mode vertical-cavity surface-emitting laser: Optional emission of transverse modes with different polarizations
Few-mode vertical-cavity surface-emitting lasers that can be controlled to emit certain modes and polarization states simply by changing the biased contacts are proposed and fabricated. By directly etching trenches in the p-doped distributed Bragg reflector, the upper mesa is separated into several submesas above the oxide layer. Individual contacts are then deposited. Each contact is used to control certain transverse modes with different polarization directions emitted from the corresponding submesa. These new devices can be seen as a prototype of compact laser sources in mode division multiplexing communications systems
Macro-scale relationship between body mass and timing of bird migration
Clarifying migration timing and its link with underlying drivers is fundamental to understanding the evolution of bird migration. However, previous studies have focused mainly on environmental drivers such as the latitudes of seasonal distributions and migration distance, while the effect of intrinsic biological traits remains unclear. Here, we compile a global dataset on the annual cycle of migratory birds obtained by tracking 1531 individuals and 177 populations from 186 species, and investigate how body mass, a key intrinsic biological trait, influenced timings of the annual cycle using Bayesian structural equation models. We find that body mass has a strong direct effect on departure date from non-breeding and breeding sites, and indirect effects on arrival date at breeding and non-breeding sites, mainly through its effects on migration distance and a carry-over effect. Our results suggest that environmental factors strongly affect the timing of spring migration, while body mass affects the timing of both spring and autumn migration. Our study provides a new foundation for future research on the causes of species distribution and movement
Graphene/silicon heterojunction for reconfigurable phase-relevant activation function in coherent optical neural networks
Optical neural networks (ONNs) herald a new era in information and
communication technologies and have implemented various intelligent
applications. In an ONN, the activation function (AF) is a crucial component
determining the network performances and on-chip AF devices are still in
development. Here, we first demonstrate on-chip reconfigurable AF devices with
phase activation fulfilled by dual-functional graphene/silicon (Gra/Si)
heterojunctions. With optical modulation and detection in one device, time
delays are shorter, energy consumption is lower, reconfigurability is higher
and the device footprint is smaller than other on-chip AF strategies. The
experimental modulation voltage (power) of our Gra/Si heterojunction achieves
as low as 1 V (0.5 mW), superior to many pure silicon counterparts. In the
photodetection aspect, a high responsivity of over 200 mA/W is realized.
Special nonlinear functions generated are fed into a complex-valued ONN to
challenge handwritten letters and image recognition tasks, showing improved
accuracy and potential of high-efficient, all-component-integration on-chip
ONN. Our results offer new insights for on-chip ONN devices and pave the way to
high-performance integrated optoelectronic computing circuits
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