A Study of Non-sequential Double Ionization of Atoms in Circularly Polarized Laser Fields Using Classical-trajectory Simulations

The development of ultrashort pulse laser technologies has enabled the studies of ionization of atoms and molecules in intense laser fields (with intensity on the order of 10^14 W/cm^2 or higher) over the last three decades. Many novel physical phenomena have been discovered, including non-sequential double ionization. This process results in a drastic enhancement in the yield of doubly charged ion in measurements of ionization yields of atoms subjected to intense field. Its mechanism can be explained using the semi-classical model of strong field ionization. First, an electron of the atom is ionized by the strong laser field. Then the electron is accelerated by the laser field and recollides with the parent ion and knocks out another electron. This process should be suppressed when the laser field is circularly polarized because recollision is impossible in this case. It is consistent with early experimental results. However, the observation of enhancement in doubly charged ion yield in a later experiment on magnesium atoms seems to contradict with the previous explanation. Further theoretical studies suggested that recollision is in fact possible under certain conditions. However, so far most of the theoretical and experimental studies on this problem have been only focused on the case of magnesium atom at laser wavelength of 800nm. In this thesis, we present a theoretical study of non-sequential double ionization for the case of circularly polarized fields using two-electron classical-trajectory Monte-Carlo simulations at various laser wavelength and ionization potentials. We study the contributions of sequential and non-sequential ionization processes as a function of laser intensity. We also investigate the characteristics of recolliding trajectories.NSFNo embargoAcademic Major: Physic

Promoting Public Participation in Post-Disaster Construction through Wechat Platform

Purpose - How could memory, heritage and post-disaster construction integrated in practice? The purpose of this paper is to introduce our approach in public participation of reconstruction plan, after a raging fire destroyed part of the historic town of Shangri-la, China. Approach – We develop two kind of crowd sourcing platform to collect and also present memory of the vanishing streets which were distroyed completely by fire. One is on Wechat platform. Through secondary development on Wechat Platform, we built a public service account that allowed users to upload photos, hand-painted pictures, and text, all of the files can be saved automaticly in our database. The other platform in on web. The website is designed for users to upload photos based on location where they were taken. All the images collected from the two platforms can be open accessed viewed with location information, which had been sort out by volunteers. The wechat platform is also used to communicate and provide education and information of the historic town to promote awareness of the heritage value. Users can send text to the public account, without privacy risk. Findings – Spreading with help from a local non-government organization, the invations of the wechat public service account received amazing amount of attention, which, according to automatic web statistics, reached up to 40,000. About 150 people followed the Wechat public account. At last we received nearly 1000 photos and hand-painted pictures. About half of our users are from Shangrila local community, Their uploaded files including historical photos of the community, providing us local perspective with long period of concern. The other half users come from travlers from all over the world, mostly from China but also european people. Their photos and paintings also contribute to the memory construction. Implications –The widespread use of smart mobile devices can make individuals more active as participants of public fairs, with the premise of carefully designed infrastructure. In this way, new technologies may contribute to a people centred principle in our conservation and design process. Value – Our approach is so-called Volunteered Geographic Information(VGI)(Goodchild,2007) in collecting memory fragments for post-disaster construction. By convenience of uploading photos and texts from mobile devices, we successfully involved local people and travlers‘participation. The case might bring insight into the field of public participation practice

A modified constitutive model for tensile deformation of 9%Cr steel under prior fatigue loading

Reliable constitutive models are necessary for the precise design and manufacture of complicated components. This study is devoted to developing a modified constitutive model to capture the effects of prior fatigue loading on subsequent tensile deformation of 9%Cr steel. In the proposed model, a strain hardening rule combined with a defined fatigue damage parameter was introduced to represent prior fatigue damage. The defined fatigue damage parameter based on the inelastic strain range of each cycle is capable of describing the evolution of tensile strength, recovery of martensite laths and decline of dislocation density, regardless of the variation in fatigue loading conditions. To validate the predictive capacity of the proposed model, experimental tensile results at different strain amplitudes, lifetime fractions and hold times of prior fatigue loading were compared with the predicted results. Good agreement between experimental and predicted results indicates that the proposed model is robust in describing the tensile behaviour under prior fatigue loading. Moreover, few determined material parameters are required, which makes the proposed model convenient for practical applications

Microstructural damage mechanics-based model for creep fracture of 9%Cr steel under prior fatigue loading

Predicting the remnant creep fracture life precisely is crucial for ensuring safety of high temperature components. This study presents a microstructural damage mechanics-based model for creep fracture of 9%Cr steel under prior fatigue loading. Microstructure observation reveals that the decrease of dislocation density and the growth of martensite lath width occurred during prior fatigue process contribute to the degradation of creep strength. Particularly, coarsening of martensite lath width plays the dominated role. To take into account the effect of the prior fatigue loading, kinematic damage equations that represent the evolution of dislocation density and martensite lath are proposed in the developed model. With the proposed model, creep fracture life and creep failure strain at various lifetime factions, strain amplitudes and hold times of prior fatigue loading can be satisfactorily predicted, which manifests that the proposed model is robust in capturing the effects of various prior fatigue loadings. The proposed model is also shown to be able to accurately predict prolonged creep deformation of other similar steel after different prior fatigue loadings

Evaluation of the effect of various prior creep-fatigue interaction damages on subsequent tensile and creep properties of 9%Cr steel

The degradation of tensile and creep properties is inevitable during high temperature service operation. Hence this work aims to evaluate the effect of prior creep-fatigue interaction damages on remnant tensile and creep properties of 9%Cr steel. Prior creep-fatigue tests interrupted at different lifetime fractions and different tensile hold times are performed at 650 °C. Afterwards, subsequent tensile and creep tests are conducted at the same temperature. Results reveal that high lifetime fraction of prior creep-fatigue loading leads to obvious reduction of remnant tensile strength and creep resistance. However, the increase in tensile hold time hardly alters the remnant properties. Microstructure and fracture surface observations indicate that the deterioration of remnant tensile strength is mainly ascribed to the decline of dislocation density occurred during prior creep-fatigue process, whereas the growth of martensite lath plays the dominated role in the reduction of remnant creep resistance. Moreover, surface crack also accelerates the decline of creep resistance at high lifetime fraction. To quantify the prior creep-fatigue interaction damage, a fatigue damage indicator is proposed. Determined relationships between remnant tensile, creep properties and defined fatigue damage are obtained

A new empirical life prediction model for 9–12%Cr steels under low cycle fatigue and creep fatigue interaction loadings

Low cycle fatigue (LCF) and creep fatigue interaction (CFI) loadings are the main factors resulting in the failure of many critical components in the infrastructure of power plants and aeronautics. Accurate prediction of life spans under specified loading conditions is significant for the design and maintenance of components. In the present study, various LCF and CFI tests are conducted to investigate the effects of temperature, strain amplitude, hold time and hold direction on the fatigue life of P92 steel. To predict fatigue life under different experimental conditions, various conventional life prediction models are evaluated and discussed. Moreover, a new empirical life prediction model is proposed based on the conventional Manson-Coffin-Basquin (MCB) model. The newly proposed model is able to simultaneously consider the effects of temperature, strain amplitude, hold time and hold direction on predicted life. The main advantage is that only the known input experimental parameters are required to perform the prediction. In addition to the validation made through the experimental data of P92 steel conducted in the present paper, the model is also verified through numerous experimental data reported in the literature for various 9-12% Cr steels

Effects of Parental Autonomy Support and Teacher Support on Middle School Students’ Homework Effort: Homework Autonomous Motivation as Mediator

The present study tested whether students’ autonomous motivation mediated the association between adult support (parental autonomy support, teacher support) and students’ homework effort. A sample of 666 Chinese middle school students was recruited to complete the parental autonomy support questionnaire, teacher support questionnaire, homework autonomous motivation questionnaire and homework effort questionnaire. Structural equation modeling showed that both parental autonomy support and teacher support positively predicted mathematics homework effort, and mathematics homework autonomous motivation was a mediator in these associations. The present study reveals the importance of adult support and autonomous motivation, and has theoretical and practical implications

Rethinking Temporal Fusion for Video-based Person Re-identification on Semantic and Time Aspect

Recently, the research interest of person re-identification (ReID) has gradually turned to video-based methods, which acquire a person representation by aggregating frame features of an entire video. However, existing video-based ReID methods do not consider the semantic difference brought by the outputs of different network stages, which potentially compromises the information richness of the person features. Furthermore, traditional methods ignore important relationship among frames, which causes information redundancy in fusion along the time axis. To address these issues, we propose a novel general temporal fusion framework to aggregate frame features on both semantic aspect and time aspect. As for the semantic aspect, a multi-stage fusion network is explored to fuse richer frame features at multiple semantic levels, which can effectively reduce the information loss caused by the traditional single-stage fusion. While, for the time axis, the existing intra-frame attention method is improved by adding a novel inter-frame attention module, which effectively reduces the information redundancy in temporal fusion by taking the relationship among frames into consideration. The experimental results show that our approach can effectively improve the video-based re-identification accuracy, achieving the state-of-the-art performance