1,721,085 research outputs found
Investigation of turbulent transport regimes in the tokamak edge by using two-fluid simulations
Full text linkThe results of flux-driven, two-fluid simulations in single-null configurations are used to investigate the processes determining the turbulent transport in the tokamak edge. Three turbulent transport regimes are identified: (i) a developed transport regime with turbulence driven by an interchange instability, which shares a number of features with the standard L-mode of tokamak operation; (ii) a suppressed transport regime, characterized by a higher value of the energy confinement time, low-amplitude relative fluctuations driven by a Kelvin–Helmholtz instability, a strong E × B sheared flow and the formation of a transport barrier, which recalls the H-mode; and (iii) a degraded confinement regime, characterized by a catastrophically large interchange-driven turbulent transport, which recalls the crossing of the Greenwald density limit. We derive an analytical expression of the pressure gradient length in the three regimes. The transition from the developed transport regime to the suppressed transport regime is obtained by increasing the heat source or decreasing the collisionality and vice versa for the transition from the developed transport regime to the degraded confinement regime. An analytical expression of the power threshold to access the suppressed transport regime, linked to the power threshold for H-mode access, as well as the maximum density achievable before entering the degraded confinement regime, related to the Greenwald density, are also derived. The experimental dependencies of the power threshold for H-mode access on density, tokamak major radius and isotope mass are retrieved. The analytical estimate of the density limit contains the correct dependence on the plasma current and on the tokamak minor radius.SP
Some considerations on mixing semantics in abstract argumentation
No full textThis paper discusses the issue of mixing different argumentation semantics in a single Dung's argumentation framework. The general notion of combination schema is defined to model a specific way of mixing argumentation semantics, and several properties that may be desirable for a combination schema are introduced at an abstract level. A specific combination schema is then evaluated in the light of such properties, showing that there are several interesting challenges both from a conceptual and a technical perspective still to be tackled
Turbulent transport regimes in the tokamak boundary and operational limits
Full text linkTwo-fluid, three-dimensional, flux-driven, global, electromagnetic turbulence
simulations carried out by using the GBS code are used to identify the main
parameters controlling turbulent transport in the tokamak boundary and to
delineate an electromagnetic phase space of edge turbulence. Four turbulent
transport regimes are identified: (i) a regime of fully developed turbulence
appearing at intermediate values of collisionality and , with turbulence
driven by resistive ballooning modes, related to the L-mode operation of
tokamaks, (ii) a regime of reduced turbulent transport at low collisionality
and large heat source, with turbulence driven by drift-waves, related to a
high-density H-mode regime, (iii) a regime of extremely large turbulent
transport at high collisionality, which is associated with the crossing of the
density limit, and (iv) a regime above the ideal ballooning limit at high
, with global modes affecting the dynamics of the entire confined
region, which can be associated with the crossing of the limit. The
transition from the reduced to the developed turbulent transport regime is
associated here with the H-mode density limit and an analytical scaling law for
maximum edge density achievable in H-mode is obtained. Analogously, analytical
scaling laws for the crossing of the L-mode density and limits are
provided and compared to the results of GBS simulations
The algebra IA(fuz): a framework for qualitative fuzzy temporal reasoning
Full text linkThe aim of this work is to integrate the ideas of flexibility and uncertainty into Allen's interval-based temporal framework, defining a new formalism, called IAfuz, which extends classical Interval Algebra (IA), in that qualitative fuzzy constraints can be expressed between intervals. We generalize the classical operations between IA-relations to IAfuz-relations, as well as the concepts of minimality and local consistency, referring to the framework of Fuzzy Constraint Satisfaction Problem. We analyze the most interesting reasoning tasks in our framework, which generalize the classical problems of checking consistency, finding a solution, and computing the minimal network in the context of IA. In order to solve these tasks, we devise two constraint propagation algorithms and a Branch & Bound algorithm. Since these tasks are NP-complete, we address the problem of finding tractable sub-algebras of IAfuz, by extending to our fuzzy framework the classical pointizable sub-algebras SAc and SA, as well as the maximal tractable subalgebra H introduced by Nebel. In particular, we prove that the fuzzy extension of the latter, called Hfuz, shares with its classical counterpart a maximality property, in that it is the unique maximal subalgebra of IAfuz which contains the fuzzy extensions of Allen's atomic relations
Tractable Fragments of Fuzzy Qualitative Algebra
Full text linkIn this paper we study the computational complexity of Fuzzy Qualitative
Temporal Algebra (QAfuz), a framework that combines qualitative temporal
constraints between points and intervals, and allows modelling vagueness
and uncertainty. Its tractable fragments can be identified by generalizing the
results obtained for crisp Constraint Satisfaction Problems (CSPs) to fuzzy
CSPs (FCSPs); to do this, we apply a general methodology based on the
notion of -cut. In particular, the results concerning the tractability of Qualitative
Algebra QA, obtained in a recent study by different authors, can be
extended to identify the tractable algebras of the fuzzy Qualitative Algebra
QAfuz in such a way that the obtained set is maximal, namely any maximal
tractable fuzzy algebra belongs to this set
Integrating quantitative and qualitative fuzzy temporal constraints
Full text linkIn this work we address the problem of representing and reasoning with temporal knowledge in a very general and flexible manner. To this aim we propose a model of integration of quantitative and qualitative temporal information affected by vagueness and uncertainty. We extend our fuzzy qualitative temporal framework IAfuz integrating the treatment of fuzzy quantitative constraints modeled as trapezoidal distributions. To do this, we extend the treatment of fuzzy temporal constraints considered in the literature and we generalize in a fuzzy direction the classical hybrid approach of temporal constraints integration proposed by Meiri. To show the full expressiveness of the new system,
we apply it to represent the fuzzy temporal knowledge in a typical scheduling example
An Hybrid Fuzzy Temporal Constraint Approach: a Case Study
No full textIn this paper, we propose an application of an integrated model for temporal reasoning able to handle both quantitative and qualitative constraints affected by vagueness and uncertainty. The integration is ob- tained by merging two existing mo- dels, i.e. Meiri’s system, that hand- les qualitative and quantitative classical temporal constraints, and our IAf uz , that handles fuzzy qualitative constraints. The first approach is generalized with possibility theory, while the second is extended to include quantitative constraints. We present a practical application of our framework in a simplified problem of investigation
Qualitative and Quantitative Fuzzy Temporal Constraints
No full textWe present a integrated framework able to handle both quantitative and qualitative constraints affected by vagueness and uncertainty. We merge two existing models: the Meiri’s system [Meiri, 1996] and IAf uz [Badaloni and Giacomin, 2000]; the first approach is generalized with possibility theory, while the second is extended in order to include qualitative constraints. Two traditional solving techniques such as Path- Consistency and Branch and Bound are taken into account. A simple scheduling example shows the main features of the new system
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