962 research outputs found
Glycemic index and glycemic load: Application in observational studies and association with hepatocellular carcinoma risk. Meaningful or error prone?
No full textmissin
Study of trapping effect on ion-acoustic solitary waves based on a fully kinetic simulation approach
No full textA fully kinetic simulation approach, treating each plasma component based on the Vlasov equation, is adopted to study the disintegration of an initial density perturbation into a number of ion-acoustic solitary waves (IASWs) in the presence of the trapping effect of electrons. The nonlinear fluid theory developed by Schamel [Plasma Phys. 13, 491 (1971); J. Plasma Phys. 7, 1 (1972); Plasma Phys. 14, 905 (1972); J. Plasma Phys. 9, 377 (1973); Phys. Scr. 20, 306 (1979)] has identified three separate regimes of ion-acoustic solitary waves based on the trapping parameter. Here, the disintegration process and the resulting self-consistent IASWs are studied in a wide range of trapping parameters covering all the three regimes continuously. The dependency of features such as the time of disintegration, the number, speed, and size of IASWs on the trapping parameter are focused upon. It is shown that an increase in this parameter slows down the propagation of IASWs while decreases their sizes in the phase space. These features of IASWs tend to saturate for large values of trapping parameters. The disintegration time shows a more complicated behavior than what was predicted by the theoretical approach. Also for the case of trapping parameters bigger than one, propagation of IASWs is observed in contrast with the theoretical predictions. The kinetic simulation results unveil a smooth and well-defined dependency of solitary waves' features on the trapping parameter, showing the possibility of bridging all the three regimes. Finally, it is shown that for β around zero, the electron phase space structure of the accompanying vortex stays symmetric. The effect of the electron-to-ion temperature ratio on the disintegration and the propagation of IASWs are considered as a benchmarking test of the simulation code (in the nonlinear regime
Scattering of electron holes in the context of ion-acoustic regime
No full textMutual collisions between ion-acoustic solitary waves are studied based on a fully kinetic simulation approach. Two cases, small and large relative
velocities, are studied, and the effect of trapped electron population on the collision process is focused upon. It is shown that, for the case of small
relative velocity, the repelling force between the trapped populations of electrons results in scattering of electron holes. However, this phenomenon
cannot be witnessed if the relative velocity is considerably high since the impact of trapped population remains very wea
Fully kinetic simulation study of ion-acoustic solitons in the presence of trapped electrons
No full textThe nonlinear fluid theory developed by Schamel suggests a modified KdV equation to describe the temporal evolution of ion acoustic (IA) solitons in the presence of trapped electrons. The validity of this theory is studied here by verifying solitons' main characteristic, i.e., stability against successive mutual collisions. We have employed a kinetic model as a more comprehensive theory than the fluid one, and utilized a fully kinetic simulation approach (both ions and electrons are treated based on the Vlasov equation). In the simulation approach, these solitons are excited self-consistently by employing the nonlinear process of IA solitons formation from an initial density perturbation (IDP). The effect of the size of IDPs on the chain formation is proved by the simulation code as a benchmark test. It is shown that the IA solitons, in the presence of trapped electrons, can retain their features (both in spatial and velocity direction) after successive mutual collisions. The collisions here include encounters of IA solitons with the same trapping parameter, while differing in size. Kinetic simulation results reveal a complicated behavior during a collision between IA solitons in contrast to the fluid theory predictions and simulations. In the range of parameters considered here, two oppositely propagating solitons rotate around their collective center in the phase space during a collision, independent of their trapping parameters. Furthermore, they exchange some portions of their trapped population
Simulation study of overtaking of ion-acoustic solitons in the fully kinetic regime
No full textThe overtaking collisions of ion-acoustic solitons in the presence of trapping effects of electrons are studied based on a fully kinetic simulation approach. The method is able to provide all the kinetic details of the process alongside the fluid-level quantities self consistently. Solitons are produced naturally by utilizing the chain formation phenomenon, and then are arranged in a new simulation box to test the different scenarios of overtaking collisions. Three achievements are reported here. First, simulations prove the long-time life span of the ion-acoustic solitons in the presence of trapping effect of electrons (kinetic effects), which serves as the benchmark of the simulation code. Second, their stability against overtaking mutual collisions is established by creating collisions between solitons with different number and shapes of trapped electrons, i.e., different trapping parameter. Finally, details of solitons during collisions for both ions and electrons are provided on both fluid and kinetic levels. These results show that on the kinetic level, trapped electron population accompanying each of the solitons are exchanged between the solitons during the collision. Furthermore, the behavior of electron holes accompanying solitons contradicts the theory about the electron holes interaction developed based on kinetic theory. They also show behaviors much different from other electron holes witnessed in processes such as nonlinear Landau damping (Bernstein-Greene-Kruskal -BGK- modes) or beam-plasma interaction (like two-beam instability
Dietary intake of total polyphenol and polyphenol classes and the risk of colorectal cancer in the European Prospective Investigation into Cancer and Nutrition (EPIC) cohort
No full textThe coordination of EPIC is financially supported by the European Commission (DG-SANCO) and the International Agency for Research on Cancer. European Research Council (ERC-2009-AdG 232997); Health Research Fund (FIS): PI13/00061 to Granada; PI13/01162 to EPICMurcia, Regional Governments of Andalucı´a, Asturias, Basque Country, Murcia and Navarra, AGAUR - Generalitat de Catalunya (exp. 2014 SGR 726), The Health Research Funds RD12/0036/0018, cofunded by European Regional Development Fund (ERDF) ‘‘A way to build Europe (Spain);RZ-R was supported by the ‘‘Miguel Servet’’ program (CP15/00100) from the Institute of Health Carlos III and European Social Fund (ESF). (...)Zamora-Ros, R., Cayssials, V., Jenab, M., Rothwell, J.A., Fedirko, V., Aleksandrova, K., Tjønneland, A., Kyrø, C., Overvad, K., Boutron-Ruault, M.-C., Carbonnel, F., Mahamat-Saleh, Y., Kaaks, R., Kühn, T., Boeing, H., Trichopoulou, A., Valanou, E., Vasilopoulou, E., Masala, G., Pala, V., Panico, S., Tumino, R., Ricceri, F., Weiderpass, E., Lukic, M., Sandanger, T.M., Lasheras, C., Agudo, A., Sánchez, M.-J., Amiano, P., Navarro, C., Ardanaz, E., Sonestedt, E., Ohlsson, B., Nilsson, L.M., Rutegård, M., Bueno-de-Mesquita, B., Peeters, P.H., Khaw, K.-T., Wareham, N.J., Bradbury, K., Freisling, H., Romieu, I., Cross, A.J., Vineis, P., Scalbert, A
A study of the stability properties of Sagdeev solutions in the ion-acoustic regime using kinetic simulations
No full textThe Sagdeev pseudo-potential approach has been employed extensively in theoretical studies to determine large-amplitude (fully) nonlinear solutions in a variety of multi-species plasmas. Although these solutions are repeatedly considered as solitary waves (and even solitons), their temporal stability has never been proven. In this paper, a numerical study of the Vlasov-Poisson system is made to follow their temporal evolution in the presence of numerical noise and thereby test their long-time propagation stability. Considering the ion-acoustic regime, both constituents of the plasma, i.e., electrons and ions are treated following their distribution functions in these sets of fully-kinetic simulations. The findings reveal that the stability of the Sagdeev solution depends on a combination of two parameters, i.e., velocity and trapping parameter. It is shown that there exists a critical value of trapping parameter for both fast and slow solutions which separates stable from unstable solutions. In the case of stable solutions, it is shown that these nonlinear structures can propagate for long periods, which confirms their status as solitary waves. Stable solutions are reported for both Maxwellian and Kappa distribution functions. For unstable solutions, it is demonstrated that the instability causes the Sagdeev solution to decay by emitting ion-acoustic wave-packets on its propagation trail. The instability is shown to take place in a large range of velocities and even for Sagdeev solutions with a velocity much higher than the ion-sound speed. Besides, in order to validate our simulation code, two precautionary measures are taken. First, the well-known effect of the ion dynamics on a stationary electron hole solution is presented as a benchmarking test of the approach. Second, In order to verify the numerical accuracy of the simulations, the conservation of energy and entropy is presented.</p
Random-effects meta-regression models for studying nonlinear dose-response relationship, with an application to alcohol and esophageal squamous cell carcinoma
No full textA fundamental challenge in meta-analyses of published epidemiological dose-response data is the estimate of the function describing how the risk of disease varies across different levels of a given exposure. Issues in trend estimate include within studies variability, between studies heterogeneity, and nonlinear trend components. We present a method, based on a two-step process, that addresses simultaneously these issues. First, two-term fractional polynomial models are fitted within each study included in the meta-analysis, taking into account the correlation between the reported estimates for different exposure levels. Second, the pooled dose-response relationship is estimated considering the between studies heterogeneity, using a bivariate random-effects model. This method is illustrated by a meta-analysis aimed to estimate the shape of the dose-response curve between alcohol consumption and esophageal squamous cell carcinoma (SCC). Overall, 14 case-control studies and one cohort study, including 3000 cases of esophageal SCC, were included. The meta-analysis provided evidence that ethanol intake was related to esophageal SCC risk in a nonlinear fashion. High levels of alcohol consumption resulted in a substantial risk of esophageal SCC as compared to nondrinkers. However, a statistically significant excess risk for moderate and intermediate doses of alcohol was also observed, with no evidence of a threshold effect
A random-effects meta-regression model for studying nonlinear dose-response relationship
No full textINTRODUCTION. A fundamental challenge in meta-analysis of published epidemiological dose-response data is the estimation of the function describing how the risk of disease varies across different levels of a given exposure. The usual approach consists of estimating the linear change in the natural logarithm of the relative risk estimate per unit of exposure within each study, and then combining these estimates across studies [1]. Three major statistical issues to deal with when using this approach have been reported in literature: (i) the correlation among reported dose-specific logRRs estimates due to the common reference group within the same study (ii) the heterogeneity between studies and (iii) the nonlinear trend components of the dose response relationship. AIMS. The aim of our work is to develop a method that addresses simultaneously the three statistical issues cited above by implementing a random-effects meta-regression model in a nonlinear dose-response relationship framework. To illustrate the proposed methodology, the results of a meta-analysis to study the effect of alcohol on the risk of esophageal cancer are showed. CONCLUSION. In this work, we have discussed a flexible curve-regressing method to perform random effects meta-analysis of epidemiological dose- response data. The proposed model has been found to perform satisfactorily in estimating the dose-response relationship between alcohol consumption and esophageal cancer risk, taking into account both the within-study and between-studies variances
Alcohol drinking and pancreatic cancer risk : a meta-analysis of the dose-risk relation
No full textIn order to provide a more precise quantification of the association between alcohol consumption and pancreatic cancer risk, we performed a meta-analysis of relevant dose-risk results. We conducted a PubMed search of all case-control (N=21) and cohort (N=11) studies published up to March 2009. We computed summary relative risk (RR) estimates using either fixed- or, in the presence of heterogeneity, random-effects models. The pooled RR was 0.92 (95% confidence interval, 95% CI, 0.86-0.97) for <3 drinks/day and 1.22 (95% CI, 1.12-1.34) for ≥3 drinks/day. The increased risk for heavy drinking was similar in women and men, but apparently stronger in cohort studies (RR=1.29), in studies with high quality index (RR=1.30), and did not appear to be explained by residual confounding by either history of pancreatitis or tobacco smoking. This meta-analysis provides strong evidence for the absence of a role of moderate drinking in pancreatic carcinogenesis, coupled to an increased risk for heavy alcohol drinking. Given the moderate increase in risk and the low prevalence of heavy drinkers in most populations, alcohol appears to be responsible only for a small fraction of all pancreatic cancers
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