DSpace Community:
http://hdl.handle.net/10174/31
Wed, 21 Oct 2020 19:49:04 GMT2020-10-21T19:49:04ZExploring cosmic origins with CORE: Survey requirements and mission design
http://hdl.handle.net/10174/27651
Title: Exploring cosmic origins with CORE: Survey requirements and mission design
Authors: Delabrouille, J; de Aviillez, Miguel
Abstract: Future observations of cosmic microwave background (CMB) polarisation have the potential to answer some of the most fundamental questions of modern physics and cos- mology, including: what physical process gave birth to the Universe we see today? What are the dark matter and dark energy that seem to constitute 95% of the energy density of the Universe? Do we need extensions to the standard model of particle physics and fundamental interactions? Is the ⇤CDM cosmological scenario correct, or are we missing an essential piece of the puzzle? In this paper, we list the requirements for a future CMB polarisation survey addressing these scientific objectives, and discuss the design drivers of the CORE space mission proposed to ESA in answer to the “M5” call for a medium-sized mission. The rationale and options, and the methodologies used to assess the mission’s performance, are of interest to other future CMB mission design studies. CORE has 19 frequency channels, distributed over a broad frequency range, spanning the 60–600 GHz interval, to control astro- physical foreground emission. The angular resolution ranges from 20 to 180, and the aggregate CMB sensitivity is about 2 μK·arcmin. The observations are made with a single integrated focal-plane instrument, consisting of an array of 2100 cryogenically-cooled, linearly-polarised detectors at the focus of a 1.2-m aperture cross-Dragone telescope. The mission is designed to minimise all sources of systematic e↵ects, which must be controlled so that no more than 10 4 of the intensity leaks into polarisation maps, and no more than about 1% of E-type polarisa- tion leaks into B-type modes. CORE observes the sky from a large Lissajous orbit around the Sun-Earth L2 point on an orbit that o↵ers stable observing conditions and avoids contamina- tion from sidelobe pick-up of stray radiation originating from the Sun, Earth, and Moon. The entire sky is observed repeatedly during four years of continuous scanning, with a combination of three rotations of the spacecraft over di↵erent timescales. With about 50% of the sky cov- ered every few days, this scan strategy provides the mitigation of systematic e↵ects and the internal redundancy that are needed to convincingly extract the primordial B-mode signal on large angular scales, and check with adequate sensitivity the consistency of the observations in several independent data subsets. CORE is designed as a “near-ultimate” CMB polarisation mission which, for optimal complementarity with ground-based observations, will perform the observations that are known to be essential to CMB polarisation science and cannot be ob- tained by any other means than a dedicated space mission. It will provide well-characterised, highly-redundant multi-frequency observations of polarisation at all the scales where fore- ground emission and cosmic variance dominate the final uncertainty for obtaining precision CMB science, as well as 20 angular resolution maps of high-frequency foreground emission in the 300–600GHz frequency range, essential for complementarity with future ground-based observations with large telescopes that can observe the CMB with the same beamsize.Wed, 04 Apr 2018 23:00:00 GMThttp://hdl.handle.net/10174/276512018-04-04T23:00:00ZRelativistic electron impact ionization cross sections of carbon ions and application to an optically thin plasma
http://hdl.handle.net/10174/27650
Title: Relativistic electron impact ionization cross sections of carbon ions and application to an optically thin plasma
Authors: de Avillez, Miguel; Guerra, Mauro; Santos, Jose; Breitschwerdt, Dieter
Abstract: Context. Ionization through electron impact is a fundamental process associated with the evolution of the ionic structure and emis- sivity of astrophysical plasmas. Over several decades substantial efforts have been made to measure and calculate the ionization cross sections of ionization through electron impact of different ions shell by shell, in particular, of carbon ions. Spectral emission codes use electron-impact ionization cross sections and/or rates taken from different experimental and theoretical sources. The theoretical cross sections are determined numerically and include a diversity of quantum mechanical methods. The electron-impact ionization database therefore is not uniform in the methods, which makes it hard to determine the reason for the deviations with regard to experimental data. In many cases only total ionization rates for Maxwell–Boltzmann plasmas are available, which makes calculating inner-shell ionization in collisional-radiative models using thermal and nonthermal electron distribution functions difficult. A solution of this problem is the capability of generating the cross sections with an analytical method using the minimum number of atomic parameters. In this way, uniformity in the database is guaranteed, and thus deviations from experiments are easily identified and traced to the root of the method.
Aims. The modified relativistic binary encounter Bethe (MRBEB) method is such a simple analytical scheme based on one atomic parameter that allows determining electron-impact ionization cross sections. This work aims the determination of K- and L-shell cross sections of the carbon atom and ions using the MRBEB method and show their quality by: (i) comparing them with those obtained with the general ionization processes in the presence of electrons and radiation (GIPPER) code and the flexible atomic code (FAC), and (ii) determining their effects on the ionic structure and cooling of an optically thin plasma.
Methods. The MRBEB method was used to calculate the inner-shells cross sections, while the plasma calculations were carried out with the collisional+photo ionization plasma emission software (CPIPES). The mathematical methods used in this work comprise a modified version of the double-exponential over a semi-finite interval method for numerical integrations, Gauss-elimination method with scaled partial pivoting for the solution of systems of linear equations, and an iterative least-squares method to determine the fits of ionization cross sections.
Results. The three sets of cross sections show deviations among each other in different energy regions. The largest deviations occur near and in the peak maximum. Ion fractions and plasma emissivities of an optically thin plasma that evolves under collisional ioniza- tion equilibrium, derived using each set of cross sections, show deviations that decrease with increase in temperature and ionization degree. In spite of these differences, the calculations using the three sets of cross sections agree overall.
Conclusions. A simple model like the MRBEB is capable of providing cross sections similar to those calculated with more sophisti- cated quantum mechanical methods in the GIPPER and FAC codes.Sat, 07 Sep 2019 23:00:00 GMThttp://hdl.handle.net/10174/276502019-09-07T23:00:00ZRelatórios DMat (2017-2019)
http://hdl.handle.net/10174/27586
Title: Relatórios DMat (2017-2019)
Authors: Correia, Joaquim M C
Abstract: Relatório de atividade e comprovativos, 2017-2019Fri, 10 Jan 2020 00:00:00 GMThttp://hdl.handle.net/10174/275862020-01-10T00:00:00ZUn algorithme pour la simulation du transport convectif et de la propagation acoustique à tout nombre de Mach
http://hdl.handle.net/10174/27493
Title: Un algorithme pour la simulation du transport convectif et de la propagation acoustique à tout nombre de Mach
Authors: Moguen, Yann; Bruel, Pascal; Correia, Paulo; Dick, Erik
Abstract: A pressure-correction algorithm is presented for compressible fluid flow regimes. It is well-suited to si-
mulate flows at all levels of Mach number with smooth and discontinuous flow field changes, by providing
a precise representation of convective transport and acoustic propagation. The co-located finite volume
space discretization is used with the AUSM flux splitting. It is demonstrated that two ingredients are
essential for obtaining good quality solutions : the presence of an inertia term in the transporting velo-
city expression ; a velocity difference diffusive term in the face pressure expression, with a correct Mach
number scaling to recover the hydrodynamic and acoustic low Mach number limits. To meet these two
requirements, a new flux scheme, named MIAU, for Momentum Interpolation with Advection Upstream
splitting is proposed.Wed, 31 Jul 2019 23:00:00 GMThttp://hdl.handle.net/10174/274932019-07-31T23:00:00Z