9,764 research outputs found

    Formula for Fixed Point Resolution Matrix of Permutation Orbifolds

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    We find a formula for the resolution of fixed points in extensions of permutation orbifold conformal field theories by its (half-)integer spin simple currents. We show that the formula gives a unitary and modular invariant S matrix.Comment: 42 page

    UV background fluctuations traced by metal ions at z≈3z\approx3

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    Here we investigate how LyC-opaque systems present in the intergalactic medium at z≈3z\approx3 can distort the spectral shape of a uniform UV background (UVB) through radiative transfer (RT) effects. With this aim in mind, we perform a multi-frequency RT simulation through a cosmic volume of 10h−110h^{-1}~cMpc scale polluted by metals, and self-consistently derive the ions of all the species. The UVB spatial fluctuations are traced by the ratio of He II \, \rm \scriptstyle II\ and H I \, \rm \scriptstyle I\ column density, η\eta, and the ratio of C IV \,{\rm {\scriptstyle IV\ }} and Si IV \,{\rm {\scriptstyle IV\ }} optical depths, ζ\zeta. We find that: (i) η\eta spatially fluctuates through over-dense systems (Δ\Delta) with statistically significant deviations δη>25\delta\eta >25\% in 18\% of the volume ; (ii) same fluctuations in ζ\zeta are also present in 3434\% of the enriched domain (only 8\% of the total volume) and derive from a combination of RT induced effects and in-homogeneous metal enrichment, both effective in systems with Δ>1.5\Delta > 1.5.Comment: Accepted for pub. in MNRAS after very minor re

    Model Order Selection Rules For Covariance Structure Classification

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    The adaptive classification of the interference covariance matrix structure for radar signal processing applications is addressed in this paper. This represents a key issue because many detection architectures are synthesized assuming a specific covariance structure which may not necessarily coincide with the actual one due to the joint action of the system and environment uncertainties. The considered classification problem is cast in terms of a multiple hypotheses test with some nested alternatives and the theory of Model Order Selection (MOS) is exploited to devise suitable decision rules. Several MOS techniques, such as the Akaike, Takeuchi, and Bayesian information criteria are adopted and the corresponding merits and drawbacks are discussed. At the analysis stage, illustrating examples for the probability of correct model selection are presented showing the effectiveness of the proposed rules

    On the thermal dynamic behaviour of the helium-cooled DEMO fusion reactor

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    The EU-DEMO conceptual design is being conducted among research institutions and universities from 26 countries of European Union, Switzerland and Ukraine. Its mission is to realise electricity from nuclear fusion reaction by 2050. As DEMO has been conceived to deliver net electricity to the grid, the choice of the Breeding Blanket (BB) coolant plays a pivotal role in the reactor design having a strong influence on plant operation, safety and maintenance. In particular, due to the pulsed nature of the heat source, the Primary Heat Transfer System (PHTS) becomes a very important actor of the Balance of Plant (BoP) together with the Power Conversion System (PCS). Moreover, aiming to mitigate the potential negative impact of plasma pulsing on BoP equipment, for the DEMO plant is also being investigated a "heat transfer chain" option which envisages an Intermediate Heat Transfer System (IHTS) equipped with an Energy Storage System (ESS) between PHTS and PCS. Within this framework, a preliminary study has been carried out to analyse the thermal dynamic behaviour of the IHTS system for the Helium-Cooled Pebble Bed (HCPB) BB concept during pulse/dwell transition which should be still considered as the normal operating mode of a fusion power plant. Starting from preliminary thermal-hydraulic calculations made in order to size the main BoP components, the global performances of DEMO BoP have been quantitatively assessed focusing the attention on the attitude of the whole IHTS to smooth the sudden power variations which come from the plasma. The paper describes criteria and rationale followed to develop a numerical model which manages to simulate simple transient scenarios of DEMO BoP. Results of numerical simulations are presented and critically discussed in order to point out the main issues that DEMO BoP has to overcome to achieve a viable electricity power output

    Parametric thermal analysis for the optimization of Double Walled Tubes layout in the Water Cooled Lithium Lead inboard blanket of DEMO fusion reactor

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    Within the roadmap that will lead to the nuclear fusion exploitation for electric energy generation, the construction of a DEMOnstration (DEMO) reactor is, probably, the most important milestone to be reached since it will demonstrate the technological feasibility and economic competitiveness of an industrial-scale nuclear fusion reactor. In order to reach this goal, several European universities and research centres have joined their efforts in the EUROfusion action, funded by HORIZON 2020 UE programme. Within the framework of EUROfusion research activities, ENEA and University of Palermo are involved in the design of the Water-Cooled Lithium Lead Breeding Blanket (WCLL BB), that is one of the two BB concepts under consideration to be adopted in the DEMO reactor. It is mainly characterized by a liquid lithium-lead eutectic alloy acting as breeder (lithium) and neutron multiplier (lead), as well as by subcooled pressurized water as coolant. Two separate circuits, both characterized by a pressure of 15.5 MPa and inlet/outlet temperatures of 295 °C/328 °C, are deputed to cool down the First Wall (FW) and the Breeder Zone (BZ). The former consists in a system of radial-toroidal-radial C-shaped squared channels where countercurrent water flow occurs while the latter relies in the use of bundles of poloidal-radial Double Walled Tubes (DWTs) housed within the breeder. A parametric thermal study has been carried out in order to assess the best DWTs' layout assuring that the structural material maximum temperature does not overcome the allowable limit of 550 °C and that the overall coolant thermal rise fulfils the design target value of 33 °C. The study has been performed following a theoretical-numerical approach based on the Finite Element Method (FEM) and adopting the quoted Abaqus FEM code. Main assumptions and models together with results obtained are herewith reported and critically discussed

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    Salah Methnani\u2019s Immigrato: Portrait of a Migrant as a Young Man

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    Although after Unification Italy was predominantly a country of emigration, in recent years it has become a hub for migrants from different parts of the world. Among these migrants a group of writers has contributed to re-configuring Italy's national literary identity. Read by critics primarily as an autobiographical text with remarkable sociological value, Tunisian-born Salah Methnani\u2019s Immigrato is, I argue, first and foremost a classic Bildungsroman. Salah, the 'immigrant' in the title, is the story's protagonist, point of view and leading metaphor. His Bildung follows a double path. On the one hand Italy is a country imagined through TV and books read in school, on the other it is the country of the clandestine migrant forced into illegality. Similarly, official city maps contrast with alternative maps of migration; standard Italian learnt at the University in Tunis is spoken next to the jargon of migrants; writing (a diary) and speaking (broken Italian) are often at odds; and the tough reality of everyday life is followed by the oneiric dimension of dreams, often turning into nightmares. The question accompanying Salah's quest, upon which the entire story is built, remains ultimately unresolved: "Am I leaving my country as a North African migrant or am I just another young man eager to know the world first hand?" Immigrato does not only portray the epic of the new migrant of the Mediterranean, it also offers a profound reflection on the condition of modern man and the gnoseological function of art -- that of writing, in this case
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