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Scheme for Low Energy Beam Transport with a non-neutralized section
https://cds.cern.ch/record/2011960
A typical Low Energy Beam Transport (LEBT) design relies on dynamics with nearly complete beam space charge neutralization over the entire length of the LEBT. This paper argues that, for a beam with modest perveance and uniform current density distribution when generated at the source, a downstream portion of the LEBT can be un-neutralized without significant emittance growth.Shemyakin, AProst, LFri, 24 Apr 2015 05:49:43 GMT23 Apr 2015arXiv:1504.06302https://cds.cern.ch/record/2011960Intensity correlations between reflected and transmitted speckle patterns
https://cds.cern.ch/record/2011959
We study theoretically the spatial correlations between the intensities measured at the input and output planes of a disordered scattering medium. We show that short-range and long-range correlations coexist, with relative weights that depend on the optical thickness of the medium. At large optical thicknesses, the long-range spatial correlation persists, and takes negative values. These results may have direct implications for the control of wave transmission through complex media by wavefront shaping, thus finding applications in sensing, imaging and information transfer.Fayard, NCazé, APierrat, RCarminati, RFri, 24 Apr 2015 05:49:42 GMT23 Apr 2015arXiv:1504.06267https://cds.cern.ch/record/2011959Configuration Interaction with Antisymmetrized Geminal Powers
https://cds.cern.ch/record/2011958
To avoid the combinatorial computational cost of configuration interaction (CI), we have previously introduced the symmetric tensor decomposition CI (STD-CI) method, where we take advantage of the antisymmetric nature of the electronic wave function and express the CI coefficients compactly as a series of Kronecker product states (STD series) [W. Uemura and O. Sugino, Phys. Rev. Lett. 109, 253001 (2012)]. Here we extend the variational degrees of freedom by using different molecular orbitals for different terms in the STD series. This scheme is equivalent to the linear combination of the Hartree-Fock-Bogoliubov state or the antisymmetrized geminal powers (AGP). The total energy converges very rapidly within 0.72 $\mu$Hartree taking only 10 terms for the water molecule, and the convergence is likewise fast for Hubbard tetramers. The computational cost scales as the fifth power of the number of electrons and the square of the number of terms in the STD series, indicating the promise of this AGP-based scheme for highly accurate and efficient computation of quantum systems.Uemura, WataruKasamatsu, ShusukeSugino, OsamuFri, 24 Apr 2015 05:49:42 GMT23 Apr 2015arXiv:1504.06250https://cds.cern.ch/record/2011958Mathematical Modeling and Stability of Predator-Prey Systems
https://cds.cern.ch/record/2011957
This work investigated the stability of some Lotka-Volterra type models. We used the Liapunov method, which consists in analyzing the stability of systems of ordinary differential equations (ODE's), around the equilibrium, when submitted to perturbations in the initial conditions.Sobrinho, Altair Santos de Oliveirade Oliveira, Camila FogaçaKita, Carolina MassaeNatti, Érica Regina TakanoNatti, Paulo LaerteFri, 24 Apr 2015 05:49:42 GMT23 Apr 2015arXiv:1504.06244https://cds.cern.ch/record/2011957Improving detection of influential nodes in complex networks
https://cds.cern.ch/record/2011956
Recently an increasing amount of research is devoted to the question of how the most influential nodes (seeds) can be found effectively in a complex network. There are a number of measures proposed for this purpose, for instance, high-degree centrality measure reflects the importance of the network topology and has a reasonable runtime performance to find a set of nodes with highest degree, but they do not have a satisfactory dissemination potentiality in the network due to having many common neighbors ($\mbox{CN}^{(1)}$) and common neighbors of neighbors ($\mbox{CN}^{(2)}$). This flaw holds in other measures as well. In this paper, we compare high-degree centrality measure with other well-known measures using ten datasets in order to find a proportion for the common seeds in the seed sets obtained by them. We, thereof, propose an improved high-degree centrality measure (named \textit{DegreeDistance}) and improve it to enhance accuracy in two phases, FIDD and SIDD, by put a threshold on the number of common neighbors of already-selected seed nodes and a non-seed node which is under investigation to be selected as a seed as well as considering the influence score of seed nodes directly or through their common neighbors over the non-seed node. To evaluate the accuracy and runtime performance of DegreeDistance, FIDD, and SIDD, they are applied to eight large-scale networks and it finally turns out that SIDD dramatically outperforms other well-known measures and evinces comparatively more accurate performance in identifying the most influential nodes.Sheikhahmadi, AmirNematbakhsh, Mohammad AShokrollahi, ArmanFri, 24 Apr 2015 05:49:42 GMT23 Apr 2015arXiv:1504.06236https://cds.cern.ch/record/2011956The Side-Effects of the Space Charge Field Introduced by Hollow Electron Beam in the Electron Cooler of CSRm
https://cds.cern.ch/record/2011955
Electron cooler is used to improve the quality of the beam in synchrotron, however it also introduces nonlinear electromagnetic field, which cause tuneshift, tunespread and may drive resonances leading to beam loss. In this paper the tuneshift and the tunespread caused by nonlinear electromagnetic field of the hollow electron beam was investigated, and the resonance driving terms of the nonlinear electromagnetic field was analysed. The differences were presented comparing with the solid electron beam. The calculations were performed for $^{238}U^{32+}$ ions of energy 1.272MeV stored in CSRm, using the parameters given in table1. The conclusion is that in this situation nonlinear field caused by the hollow electron beam do not lead to serious resonances.Tang, Mei-TangYang, Xiao-DongMao, Li-JunLi, JieMa, Xiao-MingYan, Tai-LaiZheng, Wen-HengZhao, HeWu, BoWang, GengRuan, ShuangSha, Xiao-PingFri, 24 Apr 2015 05:49:42 GMT23 Apr 2015arXiv:1504.06233https://cds.cern.ch/record/2011955Centrifugal instability of Stokes layers in crossflow: the case of a forced cylinder wake
https://cds.cern.ch/record/2011954
The wake flow around a circular cylinder at $Re\approx100$ performing rotatory oscillations has been thoroughly discussed in the literature, mostly focusing on the modifications to the natural B\'enard-von K\'arm\'an vortex street that result from the forced shedding modes locked to the rotatory oscillation frequency. The usual experimental and theoretical frameworks at these Reynolds numbers are quasi-two-dimensional, since the secondary instabilities bringing a three-dimensional structure to the cylinder wake flow occur only at higher Reynolds numbers. In the present paper we show that a three-dimensional structure can appear below the usual three-dimensionalization threshold, when forcing with frequencies lower than the natural vortex shedding frequency, at high amplitudes, as a result of a previously unreported mechanism: a pulsed centrifugal instability of the oscillating Stokes layer at the wall of the cylinder. The present numerical investigation lets us in this way propose a physical explanation for the turbulence-like features reported in the recent experimental study of D'Adamo et al. (2011).D'Adamo, JuanGodoy-Diana, RamiroWesfreid, José EduardoFri, 24 Apr 2015 05:49:42 GMT23 Apr 2015arXiv:1504.06229https://cds.cern.ch/record/2011954A quantum cascade laser-based mid-IR frequency metrology system with ultra-narrow linewidth and $1\times 10^{-13}$-level absolute frequency stability
https://cds.cern.ch/record/2011953
We demonstrate a powerful tool for high-resolution mid-IR spectroscopy and frequency metrology with quantum cascade lasers (QCLs). We have implemented frequency stabilization of a QCL to an ultra-low expansion (ULE) reference cavity, via upconversion to the near-IR spectral range, at a level of $1\times10^{-13}$. The absolute frequency of the QCL is measured relative to a hydrogen maser, with instability $<1\times10^{-13}$ and inaccuracy $5\times10^{-13}$, using a frequency comb phase-stabilized to an independent ultrastable laser. The QCL linewidth is determined to be 60 Hz, dominated by fiber noise. Active suppression of fiber noise could result in sub-10 Hz linewidth.Hansen, Michael GMagoulakis, EvangelosChen, Qun-FengErnsting, IngoSchiller, StephanFri, 24 Apr 2015 05:49:42 GMT23 Apr 2015arXiv:1504.06216https://cds.cern.ch/record/2011953Octonacci Photonic Quasicrystals
https://cds.cern.ch/record/2011952
We study theoretically the transmission spectra in one-dimensional photonic quasicrystals, made up of SiO$_2$($A$) and TiO$_2$($B$) materials, organized following the Octonacci sequence, where the $n$th-stage of the multilayer $S_{n}$ is given by the rule $S_{n}=S_{n-1}S_{n-2}S_{n-1}$, for $n\geq 3$ and with $S_{1}=A$ and $S_{2}=B$. The expression for transmittance was obtained by employing a theoretical calculation based in the transfer-matrix method. To normally incident waves, we observe that, for a same generation, the transmission spectra for TE and TM waves are equal, at least qualitatively, and they present a scaling property where a self-similar behavior is obtained, as an evidence that these spectra are fractals. The spectra show regions where the omnidirectional band gaps emerges for specific generations of Octonacci photonic structure, except to TM waves. For TE waves, we note that all of them have the almost same width, for different generations. We also report the localization of modes as a consequence of the quasiperiodicity of the heterostructureBrand, E RCosta, C HVasconcelos, M SAnselmo, D H A LMello, V DFri, 24 Apr 2015 05:49:42 GMT23 Apr 2015arXiv:1504.06214https://cds.cern.ch/record/2011952Order Reduction of the Radiative Heat Transfer Model for the Simulation of Plasma Arcs
https://cds.cern.ch/record/2011951
An approach to derive low-complexity models describing thermal radiation for the sake of simulating the behavior of electric arcs in switchgear systems is presented. The idea is to approximate the (high dimensional) full-order equations, modeling the propagation of the radiated intensity in space, with a model of much lower dimension, whose parameters are identified by means of nonlinear system identification techniques. The low-order model preserves the main structural aspects of the full-order one, and its parameters can be straightforwardly used in arc simulation tools based on computational fluid dynamics. In particular, the model parameters can be used together with the common approaches to resolve radiation in magnetohydrodynamic simulations, including the discrete-ordinate method, the P-N methods and photohydrodynamics. The proposed order reduction approach is able to systematically compute the partitioning of the electromagnetic spectrum in frequency bands, and the related absorption coefficients, that yield the best matching with respect to the finely resolved absorption spectrum of the considered gaseous medium. It is shown how the problem's structure can be exploited to improve the computational efficiency when solving the resulting nonlinear optimization problem. In addition to the order reduction approach and the related computational aspects, an analysis by means of Laplace transform is presented, providing a justification to the use of very low orders in the reduction procedure as compared with the full-order model. Finally, comparisons between the full-order model and the reduced-order ones are presented.Fagiano, LorenzoGati, RudolfFri, 24 Apr 2015 05:49:42 GMT23 Apr 2015arXiv:1504.06204https://cds.cern.ch/record/2011951Broadband Impedance Calculations and Single Bunch Instabilities Estimations of the HLS-II storage ring
https://cds.cern.ch/record/2011950
The upgrade project of Hefei Light Source storage ring is under way. In this paper, the broadband impedances of resistive wall and coated ceramic vacuum chamber were calculated using the analytic formula, the wake fields and impedances of other designed vacuum chambers have been simulated by the CST code, and then the broadband impedances model was obtained. Using the theoretical formula, longitudinal and transverse single bunch instabilities were discussed. With the carefully-designed vacuum chamber, we find that the thresholds of the beam instabilities are higher than the beam current goal.Zhang, QingkunWang, LinLi, WeiminGao, WeiweiFri, 24 Apr 2015 05:49:42 GMT23 Apr 2015arXiv:1504.06202https://cds.cern.ch/record/2011950Fast production of Bose-Einstein condensates of metastable Helium
https://cds.cern.ch/record/2011949
We report on the Bose-Einstein condensation of metastable Helium-4 atoms using a hybrid approach, consisting of a magnetic quadrupole and a crossed optical dipole trap. In our setup we cross the phase transition with 2x10^6 atoms, and we obtain pure condensates of 5x10^5 atoms in the optical trap. This novel approach to cooling Helium-4 provides enhanced cycle stability, large optical access to the atoms and results in production of a condensate every 6 seconds - a factor 3 faster than the state-of-the-art. This speed-up will dramatically reduce the data acquisition time needed for the measurement of many particle correlations, made possible by the ability of metastable Helium to be detected individually.Bouton, QChang, RHoendervanger, A LNogrette, FAspect, AWestbrook, C IClément, DFri, 24 Apr 2015 05:49:42 GMT23 Apr 2015arXiv:1504.06197https://cds.cern.ch/record/2011949Delaying natural transition of a boundary layer using smooth steps
https://cds.cern.ch/record/2011948
The boundary layer flow over a smooth forward-facing stepped plate is studied with particular emphasis on the delay of the transition to turbulence. The interaction between the Tollmien-Schlichting (T-S) waves and the base flow over a single/two forward facing smooth steps is conducted by linear analysis indicating the amplitude of the T-S waves are attenuated in the boundary layer over a single smooth plate. Furthermore, we show that two smooth forward facing steps give rise to a further reduction of the amplitude of the T-S waves. A direct numerical simulation (DNS) is performed for the two smooth forward steps correlating favourably with the linear analysis and showing that for the investigated parameters, the K-type transition is inhibited whereas the turbulence onset of the H-type transition is postponed albeit not suppressed. Transition is indeed delayed and drag reduced for both these transition scenarios suggesting smooth forward facing steps could be leveraged as a passive flow control strategy to delay natural transition.Xu, HuiLombard, Jean-Eloi WSherwin, Spencer JFri, 24 Apr 2015 05:49:40 GMT23 Apr 2015arXiv:1504.06195https://cds.cern.ch/record/2011948Size dependent phase diagrams of Nickel-Carbon nanoparticles
https://cds.cern.ch/record/2011947
The carbon rich phase diagrams of nickel-carbon nanoparticles, relevant to catalysis and catalytic chemical vapor deposition synthesis of carbon nanotubes, are calculated for system sizes up to about 3 nanometers (807 Ni atoms). A tight binding model for interatomic interactions drives the Grand Canonical Monte Carlo simulations used to locate solid, core/shell and liquid stability domains, as a function of size, temperature and carbon chemical potential or concentration. Melting is favored by carbon incorporation from the nanoparticle surface, resulting in a strong relative lowering of the eutectic temperature and a phase diagram topology different from the bulk one. This should be taken into account in our understanding of the nanotube growth mechanisms.Magnin, YannZappelli, AlexandreAmara, HakimDucastelle, FrançoisBichara, ChristopheFri, 24 Apr 2015 05:49:40 GMT23 Apr 2015arXiv:1504.06193https://cds.cern.ch/record/2011947Discrete solitons in self-defocusing systems with $\mathcal{PT}$-symmetric defects
https://cds.cern.ch/record/2011946
We construct families of discrete solitons (DSs) in an array of self-defocusing waveguides with an embedded $\mathcal{PT}$ (parity-time)-symmetric dimer, which is represented by a pair of waveguides carrying mutually balanced gain and loss. Four types of states attached to the embedded defect are found, namely, staggered and unstaggered bright localized modes and gray or anti-gray DSs. Their existence and stability regions expand with the increase of the strength of the coupling between the dimer-forming sites. The existence of the gray and staggered bright DSs is qualitatively explained by dint of the continuum limit. All the gray and anti-gray DSs are stable (some of them are unstable if the dimer carries the nonlinear $\mathcal{PT}$ symmetry, represented by balanced nonlinear gain and loss; in that case, the instability does not lead to a blowup, but rather creates oscillatory dynamical states). The boundary between the gray and anti-gray DSs is predicted in an approximate analytical form.Chen, ZhiqiangHuang, JiashengChai, JingleiZhang, XiangyuLi, YongyaoMalomed, Boris AFri, 24 Apr 2015 05:49:40 GMT23 Apr 2015arXiv:1504.06191https://cds.cern.ch/record/2011946Two New Variations on the Twin Pseudoparadox
https://cds.cern.ch/record/2011945
Two new scenarios are proposed which generalize the standard story leading to the pseudoparadox of the Einsteinian relativistic twins, thereby enabling some deeper understanding. First, the fable by Aesop 'The Hare and the Tortoise' is considered in the light of Einsteinian chronogeometry. It is then shown that the Hare, while arriving later than the Tortoise, may still be the winner of the race (or at least may consider itself to be). Second, the situation is considered where the twin initially left at home decides to catch up his brother during his travel. Can they meet so that they may celebrate a common anniversary and recover the same age?Leblond, Jean Marc LevyFri, 24 Apr 2015 05:49:40 GMT20 Mar 2015arXiv:1504.06186https://cds.cern.ch/record/2011945Information-Probabilistic description of the Universe
https://cds.cern.ch/record/2011944
We describe the universe as a single entangled ensemble of quantum particles. Total entropy of this world ensemble, which can be expressed as a sum of information, thermodynamic and entanglement components, is assumed to be always zero. This condition suggests information quantization, which we associate with Planck's action. Then the entropy neutrality condition for the universe leads to the zero-action principle. We show that main concepts of classical space-time and gravity naturally emerge in this picture. Generalized least action principle, which embraces maximal entropy principles of information theory, is introduced.Gogberashvili, MerabFri, 24 Apr 2015 05:49:40 GMT22 Mar 2015arXiv:1504.06183https://cds.cern.ch/record/2011944Polygonal micro-whirlpools induced in ferrofluids
https://cds.cern.ch/record/2011941
We report on the observation of the polygonal whirlpools in the thin layer of ferrofluid under illumination with a laser beam carrying optical vortex and in the presence of a vertical magnetic field. This kind of structures have attracted attention after discovering a hexagonal storm in Saturns atmosphere. Our polygonal whirlpools were created in a closed system (no free surfaces) in micro scale (whirlpool diameter less than 20 micrometers) by the use of holographic optical tweezers. The polygonal shape was changed by varying the magnetic field strength or value of the optical vortex topological charge.Bacia, MarcinLamperska, WeronikaMasajada, JanDrobczynski, SlawomirMarc, MaciejFri, 24 Apr 2015 05:49:40 GMT23 Apr 2015arXiv:1504.06172https://cds.cern.ch/record/2011941Laplace-Fourier analysis and instabilities of a gainy slab
https://cds.cern.ch/record/2011940
The idealization of monochromatic plane waves leads to considerable simplifications in the analysis of electromagnetic systems. However, for active systems this idealization may be dangerous due to the presence of growing waves. Here we consider a gainy slab, and use a realistic incident beam, which is both causal and has finite width. This clarifies some apparent paradoxes arising from earlier analyses of this setup. In general it turns out to be necessary to involve complex frequencies $\omega$ and/or complex transversal wavenumbers $k_x$. Simultaneously real $\omega$ and $k_x$ cannot describe amplified waves in a slab which is infinite in the transversal direction. We also show that the only possibility to have an absolute instability for a finite width beam, is if a normally incident plane wave would experience an instability.Hågenvik, Hans OlafSkaar, JohannesFri, 24 Apr 2015 05:49:40 GMT23 Apr 2015arXiv:1504.06171https://cds.cern.ch/record/2011940Low-rank approach to the computation of path integrals
https://cds.cern.ch/record/2011939
We present a method for solving the reaction-diffusion equation with general potential in free space. It is based on approximation of the Feynman-Kac formula by a sequence of convolutions on sequentially diminishing grids. For computation of the convolutions we propose a fast algorithm based on the low-rank approximation of Hankel matrices. The requirements of the method hold $\mathcal{O}(n r M \log M + n r^2 M)$ flops in complexity and $\mathcal{O}(M r)$ elements in memory, where $n$ is the dimension of the integral, $r \ll n$, and $M$ is the size of mesh in one dimension. The presented technique can be generalized to the higher-order diffusion processes.Litsarev, M SOseledets, I VFri, 24 Apr 2015 05:49:40 GMT22 Apr 2015arXiv:1504.06149https://cds.cern.ch/record/2011939Low cost and high performance light trapping structure for thin-film solar cells
https://cds.cern.ch/record/2011938
Nano-scaled dielectric and metallic structures are popular light tapping structures in thin-film solar cells. However, a large parasitic absorption in those structures is unavoidable. Most schemes based on such structures also involve the textured active layers that may bring undesirable degradation of the material quality. Here we propose a novel and cheap light trapping structure based on the prism structured SiO2 for thin-film solar cells, and a flat active layer is introduced purposefully. Such a light trapping structure is imposed by the geometrical shape optimization to gain the best optical benefit. By examining our scheme, it is disclosed that the conversion efficiency of the flat a-Si:H thin-film solar cell can be promoted to exceed the currently certified highest value. As the cost of SiO2-based light trapping structure is much cheaper and easier to fabricate than other materials, this proposal would have essential impact and wide applications in thin-film solar cells.Wang, DongLinCui, HuijuanSu, GangFri, 24 Apr 2015 05:49:40 GMT23 Apr 2015arXiv:1504.06142https://cds.cern.ch/record/2011938Short range DFT combined with long-range local RPA within a range-separated hybrid DFT framework
https://cds.cern.ch/record/2011937
Selecting excitations in localized orbitals to calculate long-range correlation contributions to range-separated density-functional theory can reduce the overall computational effort significantly. Beyond simple selection schemes of excited determinants, the dispersion-only approximation, which avoids counterpoise-corrected monomer calculations, is shown to be particularly interesting in this context, which we apply to the random-phase approximation. The approach has been tested on dimers of formamide, water, methane and benzene.Chermak, EReinhardt, PeterMussard, BastienAngyan, JanosFri, 24 Apr 2015 05:49:39 GMT23 Apr 2015arXiv:1504.06139https://cds.cern.ch/record/2011937Fowler-Nordheim Plot Analysis: a Progress Report
https://cds.cern.ch/record/2011936
The commonest method of characterising a cold field electron emitter is to measure its current-voltage characteristics, and the commonest method of analysing these characteristics is by means of a Fowler-Nordheim (FN) plot. This tutorial/review-type paper outlines a more systematic method of setting out the Fowler-Nordheim-type theory of cold field electron emission, and brings together and summarises the current state of work by the authors on developing the theory and methodology of FN plot analysis. This has turned out to be far more complicated than originally expected. Emphasis is placed in this paper on: (a) the interpretation of FN-plot slopes, which is currently both easier and of more experimental interest than the analysis of FN-plot intercepts; and (b) preliminary explorations into developing methodology for interpreting current-voltage characteristics when there is series resistance in the conduction path from the high-voltage generator to the emitter's emitting regions. This work reinforces our view that FN-plot analysis is best carried out on the raw measured current-voltage data, without pre-conversion into another data format, particularly if series resistance is present in the measuring circuit. Relevant formulae are given for extracting field-enhancement-factor values from such an analysis.Forbes, Richard GDeane, Jonathan H BFischer, AndreasMousa, Marwan SFri, 24 Apr 2015 05:49:39 GMT23 Apr 2015arXiv:1504.06134https://cds.cern.ch/record/2011936Experimental exploration of the optomechanical attractor diagram and its dynamics
https://cds.cern.ch/record/2011935
We demonstrate experimental exploration of the attractor diagram of an optomechanical system where the optical forces compensate for the mechanical losses. In this case stable self-induced oscillations occur but only for specific mirror amplitudes and laser detunings. We demonstrate that we can amplify the mechanical mode to an amplitude 500 times larger than the thermal amplitude at 300K. The lack of unstable or chaotic motion allows us to manipulate our system into a non-trivial steady state and explore the dynamics of self-induced oscillations in great detail.Buters, Frank MEerkens, Hedwig JHeeck, KierWeaver, Matthew JPepper, Briande Man, SvenBouwmeester, DirkFri, 24 Apr 2015 05:49:39 GMT23 Apr 2015arXiv:1504.06119https://cds.cern.ch/record/2011935Evaluating the Force Concept Inventory for different student groups at the Norwegian University of Science and Technology
https://cds.cern.ch/record/2011934
The Force Concept Inventory (FCI) was developed by Hestenes, Wells and Swackhamer, in order to assess student understanding of the concept of force. FCI has been used for over 20 years and in different countries. When applying the inventory in a new context it is important to evaluate the reliability and discrimination power of this assessment tool. In this study the reliability and discrimination power are evaluated in the context of Engineering education at a Norwegian university, using statistical tests, focusing on both item analysis and on the entire test. The results indicate that FCI is a reliable and discriminating tool in most cases. As there are exceptions, statistical tests should always be done when FCI is administered in a new context.Persson, J RFri, 24 Apr 2015 05:49:39 GMT23 Apr 2015arXiv:1504.06099https://cds.cern.ch/record/2011934