130 research outputs found
Multi-machine analysis of termination scenarios with comparison to simulations of controlled shutdown of ITER discharges
To improve our understanding of the dynamics and control of ITER terminations, a study has
been carried out on data from existing tokamaks. The aim of this joint analysis is to compare
the assumptions for ITER terminations with the present experience basis. The study examined
the parameter ranges in which present day devices operated during their terminations, as
well as the dynamics of these parameters. The analysis of a database, built using a selected
set of experimental termination cases, showed that, the H-mode density decays slower than
the plasma current ramp-down. The consequential increase in fGW limits the duration of the
H-mode phase or result in disruptions. The lower temperatures after the drop out of H-mode
will allow the plasma internal inductance to increase. But vertical stability control remains
manageable in ITER at high internal inductance when accompanied by a strong elongation
reduction. This will result in ITER terminations remaining longer at low q (q95 ~ 3) than
most present-day devices during the current ramp-down. A fast power ramp-down leads
to a larger change in ÎČp at the HâL transition, but the experimental data showed that these are manageable for the ITER radial position control. The analysis of JET data shows that
radiation and impurity levels significantly alter the HâL transition dynamics. Self-consistent
calculations of the impurity content and resulting radiation should be taken into account when
modelling ITER termination scenarios. The results from this analysis can be used to better
prescribe the inputs for the detailed modelling and preparation of ITER termination scenariosDoE Awards DE-FC02-99ER54512DoE Awards DE-AC02- 76CH03073DoE Awards DE-FC02-04ER54698EURATOM 63305
Patchiness and Demographic Noise in Three Ecological Examples
Understanding the causes and effects of spatial aggregation is one of the
most fundamental problems in ecology. Aggregation is an emergent phenomenon
arising from the interactions between the individuals of the population, able
to sense only -at most- local densities of their cohorts. Thus, taking into
account the individual-level interactions and fluctuations is essential to
reach a correct description of the population. Classic deterministic equations
are suitable to describe some aspects of the population, but leave out features
related to the stochasticity inherent to the discreteness of the individuals.
Stochastic equations for the population do account for these
fluctuation-generated effects by means of demographic noise terms but, owing to
their complexity, they can be difficult (or, at times, impossible) to deal
with. Even when they can be written in a simple form, they are still difficult
to numerically integrate due to the presence of the "square-root" intrinsic
noise. In this paper, we discuss a simple way to add the effect of demographic
stochasticity to three classic, deterministic ecological examples where
aggregation plays an important role. We study the resulting equations using a
recently-introduced integration scheme especially devised to integrate
numerically stochastic equations with demographic noise. Aimed at scrutinizing
the ability of these stochastic examples to show aggregation, we find that the
three systems not only show patchy configurations, but also undergo a phase
transition belonging to the directed percolation universality class.Comment: 20 pages, 5 figures. To appear in J. Stat. Phy
Recent results on multiplicative noise
Recent developments in the analysis of Langevin equations with multiplicative
noise (MN) are reported. In particular, we:
(i) present numerical simulations in three dimensions showing that the MN
equation exhibits, like the Kardar-Parisi-Zhang (KPZ) equation both a weak
coupling fixed point and a strong coupling phase, supporting the proposed
relation between MN and KPZ;
(ii) present dimensional, and mean field analysis of the MN equation to
compute critical exponents;
(iii) show that the phenomenon of the noise induced ordering transition
associated with the MN equation appears only in the Stratonovich representation
and not in the Ito one, and
(iv) report the presence of a new first-order like phase transition at zero
spatial coupling, supporting the fact that this is the minimum model for noise
induced ordering transitions.Comment: Some improvements respect to the first versio
Entornos personales de aprendizaje: un estudio comparativo entre profesores costarricenses en formaciĂłn y en ejercicio
This research makes a comparative analysis of the frequency of use of various ICT tools that 381 students in Education degrees and 218 Costa Rican teachers incorporate in their Personal Learning Environments (PLE) in order to search for information, generate content, and communicate. An ex post facto study was conducted using the survey method. The results revealed poor integration of technological resources into the PLEs of both groups, although it has been found that it is the students who use the greatest number of tools. It proposes the need to implement measures to enrich the PLEs.Esta investigaciĂłn analiza comparativamente la frecuencia de uso de diversas herramientas TIC que 381 estudiantes de las carreras de EducaciĂłn y 218 profesores costarricenses incorporan en sus Entornos Personales de Aprendizaje (PLE) para buscar informaciĂłn, generar contenido y comunicarse. Se realizĂł un estudio ex post facto utilizando el mĂ©todo de encuesta. Los resultados desvelaron una escasa integraciĂłn de los recursos tecnolĂłgicos en los PLE de ambos colectivos, aunque se ha constatado que son los estudiantes quienes usan un mayor nĂșmero de herramientas. Se propone la necesidad de arbitrar medidas que contribuyan a enriquecer los PLE
Geometric Approach to Pontryagin's Maximum Principle
Since the second half of the 20th century, Pontryagin's Maximum Principle has
been widely discussed and used as a method to solve optimal control problems in
medicine, robotics, finance, engineering, astronomy. Here, we focus on the
proof and on the understanding of this Principle, using as much geometric ideas
and geometric tools as possible. This approach provides a better and clearer
understanding of the Principle and, in particular, of the role of the abnormal
extremals. These extremals are interesting because they do not depend on the
cost function, but only on the control system. Moreover, they were discarded as
solutions until the nineties, when examples of strict abnormal optimal curves
were found. In order to give a detailed exposition of the proof, the paper is
mostly self\textendash{}contained, which forces us to consider different areas
in mathematics such as algebra, analysis, geometry.Comment: Final version. Minors changes have been made. 56 page
Exploiting oxidative phosphorylation to promote the stem and immunoevasive properties of pancreatic cancer stem cells
Pancreatic ductal adenocarcinoma (PDAC), the fourth leading cause of cancer death, has a 5-year survival rate of approximately 7â9%. The ineffectiveness of anti-PDAC therapies is believed to be due to the existence of a subpopulation of tumor cells known as cancer stem cells (CSCs), which are functionally plastic, and have exclusive tumorigenic, chemoresistant and metastatic capacities. Herein, we describe a 2D in vitro system for long-term enrichment of pancreatic CSCs that is amenable to biological and CSC-specific studies. By changing the carbon source from glucose to galactose in vitro, we force PDAC cells to utilize OXPHOS, resulting in enrichment of CSCs defined by increased CSC biomarker and pluripotency gene expression, greater tumorigenic potential, induced but reversible quiescence, increased OXPHOS activity, enhanced invasiveness, and upregulated immune evasion properties. This CSC enrichment method can facilitate the discovery of new CSC-specific hallmarks for future development into targets for PDAC-based therapies
Modelagem matemĂĄtica e parĂąmetros qualitativos da secagem de folhas de capim-limĂŁo [Cymbopogon citratus (DC.) Stapf]
Quasi-static numerical study of the breathing mechanism of an elliptical crack in an unbalanced rotating shaft
Prediction of nitrogen excretion from data on dairy cows fed a wide range of diets compiled in an intercontinental database: a meta-analysis
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