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  • 1.
    Andersson, Ole
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Heydari, Hoshang
    Stockholm University, Faculty of Science, Department of Physics.
    Geometry of quantum evolution for mixed quantum states2014In: Physica Scripta, ISSN 0031-8949, E-ISSN 1402-4896, Vol. T160, p. 014004-Article in journal (Refereed)
    Abstract [en]

    The geometric formulation of quantum mechanics is a very interesting field of research which has many applications in the emerging field of quantum computation and quantum information, such as schemes for optimal quantum computers. In this work we discuss a geometric formulation of mixed quantum states represented by density operators. Our formulation is based on principal fiber bundles and purifications of quantum states. In our construction, the Riemannian metric and symplectic form on the total space are induced from the real and imaginary parts of the Hilbert-Schmidt Hermitian inner product, and we define a mechanical connection in terms of a locked inertia tensor and moment map. We also discuss some applications of our geometric framework.

  • 2.
    Andersson, Ole
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Heydari, Hoshang
    Stockholm University, Faculty of Science, Department of Physics.
    Spectral weighted geometric phase for mixed quantum states2014In: Physica Scripta, ISSN 0031-8949, E-ISSN 1402-4896, Vol. T163, article id 014027Article in journal (Refereed)
    Abstract [en]

    Geometric phase has found a broad spectrum of applications in both classical and quantum physics. In this work we discuss a geometric phase for mixed quantum states based on traces of spectral weighted holonomies. Our approach applies to general unitary evolutions of both nondegenerate and degenerate mixed states, and it generalizes the standard definition of geometric phase for mixed states, which is based on quantum interferometry. We provide an explicit formula for the geometric phase that can be easily implemented for computations in quantum physics, and we discuss higher order analogs of the geometric phase that might be defined at points where the ordinary geometric phase is undefined.

  • 3.
    Brandenburg, Axel
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Chatterjee, P.
    Del Sordo, Fabio
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Hubbard, A.
    Käpylä, P. J.
    Rheinhardt, M.
    Turbulent transport in hydromagnetic flows2010In: Physica Scripta, ISSN 0031-8949, E-ISSN 1402-4896, Vol. T142, article id 14028Article in journal (Refereed)
    Abstract [en]

    The predictive power of mean-field theory is emphasized by comparing theory with simulations under controlled conditions. The recently developed test-field method is used to extract turbulent transport coefficients both in the kinematic and the nonlinear or quasi-kinematic cases. A striking example of the quasi-kinematic method is provided by magnetic buoyancy-driven flows that produce an alpha effect and turbulent diffusion.

  • 4. Dassanayake, B. S.
    et al.
    Das, Susanta
    Stockholm University, Faculty of Science, Department of Physics, The Manne Siegbahn Laboratory.
    Ayyad, A.
    Tanis, J. A.
    Electron transmission through a single glass macrocapillary: dependence on energy and time2011In: Physica Scripta, ISSN 0031-8949, E-ISSN 1402-4896, Vol. T144, p. 014041-Article in journal (Refereed)
    Abstract [en]

    The transmission of electrons through an insulating single cylindrically shaped glass capillary of macroscopic dimensions has been investigated for electron energies from 300 to 1000 eV using a high-resolution electrostatic parallel-plate analyzer (spectrometer). The transmitted intensity decreased with increasing sample tilt angle relative to the beam direction, and had two regions: direct, where there is no interaction of the beam with the inner capillary wall, and indirect, where it does interact. From the full-width-at-half-maximum of the angular distributions, the indirect region was found to reveal a further two distinct areas of characteristics versus tilt angle with respect to elasticity/inelasticity of transmitted electrons. Electron transmission for the case of no tilt of the sample was found to be time dependent, due to charge-up of the capillary inner surface. The new results are compared with previous experimental data obtained using a 10x lower resolution spectrometer.

  • 5.
    Gatchell, Michael
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Zettergren, Henning
    Stockholm University, Faculty of Science, Department of Physics.
    Seitz, Fabian
    Stockholm University, Faculty of Science, Department of Physics.
    Chen, Tao
    Stockholm University, Faculty of Science, Department of Physics.
    Alexander, John D.
    Stockholm University, Faculty of Science, Department of Physics.
    Stocket, Mark H.
    Stockholm University, Faculty of Science, Department of Physics.
    Schmidt, Henning T.
    Stockholm University, Faculty of Science, Department of Physics.
    Lawicki, A.
    Rangama, J.
    Rousseau, P.
    Capron, M.
    Maclot, S.
    Maisonny, R.
    Domaracka, A.
    Adoui, L.
    Mery, A.
    Chesnel, J-Y
    Manil, B.
    Huber, B. A.
    Cederquist, Henrik
    Stockholm University, Faculty of Science, Department of Physics.
    Ions colliding with polycyclic aromatic hydrocarbon clusters2013In: Physica Scripta, ISSN 0031-8949, E-ISSN 1402-4896, Vol. T156, p. 014062-Article in journal (Refereed)
    Abstract [en]

    We have measured the ionization and fragmentation of polycyclic aromatic hydrocarbon (PAH) molecules and their clusters. We find that PAH clusters containing up to roughly 100 individual molecules fragment strongly following collisions with keV ions in low or high charge states (q). For both types of collisions, singly charged PAH molecules are found to be the dominant products but for very different reasons. A high-q ion projectile charge leads to strong multiple ionization of the PAH clusters and subsequent Coulomb explosions. A low-q ion projectile charge often leads to single ionization but stronger internal heating and long evaporation sequences with a singly charged PAH monomer as the end product. We have developed a Monte Carlo method for collision-induced heating of PAH clusters and present an evaporation model where the clusters cool slowly as most of the internal energies are stored in intramolecular vibrations and not in molecule-molecule vibrations.

  • 6. Gorda, O.
    et al.
    Braeuning-Demian, A.
    Danared, Håkan
    Stockholm University, Faculty of Science, Department of Physics.
    Dolinskii, A.
    Engström, Mats
    Stockholm University, Faculty of Science, Department of Physics.
    Herfurth, F.
    Källberg, Anders
    Stockholm University, Faculty of Science, Department of Physics.
    Kleffner, C-M
    Lestinsky, M.
    Simonsson, Ansgar
    Stockholm University, Faculty of Science, Department of Physics.
    Sjöholm, Johannes
    Stockholm University, Faculty of Science, Department of Physics.
    Vorobjev, G.
    Ion-optical design of CRYRING@ESR2015In: Physica Scripta, ISSN 0031-8949, E-ISSN 1402-4896, Vol. T166, article id 014043Article in journal (Refereed)
    Abstract [en]

    In 2012 the CRYRING storage ring was delivered from Stockholm to Darmstadt as a part of the Swedish in-kind contribution to the FAIR project. The ring lattice has been slightly changed for optimal injection and to provide additional space for experiment equipment. For the injection from the experimental storage ring (ESR), a new transfer line has been designed. The local injector line has been significantly modified compared to the previous one in Stockholm taking into account the geometry of the existing GSI building. In this paper we present the ion-optical properties of CRYRING@ESR after the described modifications. Single-turn injection from the ESR and multi-turn injection from the local injector are discussed. Ion-optical calculations of fast and slow extraction from CRYRING are also presented. The closed orbit correction scheme is considered taking into account the future arrangement of the beam position monitors and correction magnets. Based on the results of the calculations the requirements for the magnet alignment are finally discussed.

  • 7.
    Gurell, Jonas
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Wahlgren, Glenn
    Lunds universitet, Department of Physics Catholic University of American, NASA Goddard Space Flight Center.
    Nave, Gillian
    National Institute of Standards and Technology, Atomic Physics Division.
    Wyart, Jean-Francois
    Laboratoire Aimé-Cotton, Observatoire de Paris-Meudon.
    Wavelengths, energy levels and HFS in Ho II2009In: Physica Scripta, ISSN 0031-8949, E-ISSN 1402-4896, Vol. 79, no 3, p. 035306-Article in journal (Refereed)
    Abstract [en]

    We recorded spectra of Ho II emitted by hollow-cathode lamps, using the ultraviolet Fourier transform spectrometer (FTS) at Lund Observatory and the 2-m FTS at the National Institute of Standards and Technology. The combined wavenumber coverage spans the interval from 8000 cm-1 to 49600 cm-1(12500 Å to 2016 Å). We measured 303 lines in these spectra and used them to determine energies and hyperfine structure constants of 100 levels. Ofthese, 41 energy levels have not been reported previously and hyperfine structure constants have not been reported previously for 83 levels. Comparisonsbetween the hyperfine structure constants and energy levels presented in this article and those previously determined from other studies are included anddiscrepancies are discussed. A theoretical study of the odd-parity levels leads to hyperfine structure parameters and to predictions for unknown excited levelsof the ground-state configuration.

  • 8.
    Heydari, Hoshang
    Stockholm University, Faculty of Science, Department of Physics.
    A class of quantum gate entanglers2010In: Physica Scripta, ISSN 0031-8949, E-ISSN 1402-4896, Vol. T140, p. 14048-Article in journal (Refereed)
    Abstract [en]

    We construct quantum gate entanglers for different classes of multipartite states based on the definition of W and GHZ concurrence classes. First, we review the basic construction of concurrence classes based on the orthogonal complement of a positive operator valued measure (POVM) on quantum phase. Then, we construct quantum gate entanglers for different classes of multi-qubit states. In particular, we show that these operators can entangle multipartite states if they satisfy some conditions for W and GHZ classes of states. Finally, we explicitly give the W class and GHZ classes of quantum gate entanglers for four-qubit states.

  • 9.
    Heydari, Hoshang
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Andersson, Ole
    Stockholm University, Faculty of Science, Department of Physics.
    Geometric uncertainty relation for quantum ensembles2015In: Physica Scripta, ISSN 0031-8949, E-ISSN 1402-4896, Vol. 90, no 2, article id 025102Article in journal (Refereed)
    Abstract [en]

    Geometrical structures of quantum mechanics provide us with new insightful results about the nature of quantum theory. In this work we consider mixed quantum states represented by finite rank density operators. We review our geometrical framework that provide the space of density operators with Riemannian and symplectic structures, and we derive a geometric uncertainty relation for observables acting on mixed quantum states. We also give an example that visualizes the geometric uncertainty relation for spin-1/2 particles.

  • 10.
    Hobein, Matthias
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Liu, Yanfang
    Stockholm University, Faculty of Science, Department of Physics.
    Solders, Andreas
    Stockholm University, Faculty of Science, Department of Physics.
    Suhonen, Markus
    Stockholm University, Faculty of Science, Department of Physics.
    Kamalou, Omar
    Stockholm University, Faculty of Science, Department of Physics.
    Schuch, Reinhold
    Stockholm University, Faculty of Science, Department of Physics.
    A compact time-resolving pepperpot emittance meter for low-energy highly charged ions2011In: Physica Scripta, ISSN 0031-8949, E-ISSN 1402-4896, Vol. T144, p. 014062-Article in journal (Refereed)
    Abstract [en]

    An emittance meter for pulsed, low-energy ion beams was developed. Based on the pepperpot method, the device is compact and portable. It has been installed at the S-EBIT Laboratory at AlbaNova, Stockholm University, to measure the emittance of highly charged ions extracted from the electron beam ion trap R-EBIT and the cooling trap of the high-precision Penning trap mass spectrometer SMILETRAP II. Using a fast delay-line anode detector, the emittance and time-of-flight of the extracted ions can be measured simultaneously. In this paper, design and data processing system are described and preliminary results are presented.

  • 11.
    Hobein, Matthias
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Liu, Yuwen
    Stockholm University, Faculty of Science, Department of Physics.
    Solders, Andreas
    Stockholm University, Faculty of Science, Department of Physics.
    Suhonen, Markus
    Stockholm University, Faculty of Science, Department of Physics.
    Kamalou, Omar
    Schuch, Reinhold
    Stockholm University, Faculty of Science, Department of Physics.
    A compact time-resolving pepperpot emittance meter for low energy highly-charged ionsIn: Physica Scripta, ISSN 0031-8949, E-ISSN 1402-4896Article in journal (Refereed)
  • 12. Kahniashvili, Tina
    et al.
    Brandenburg, Axel
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Stockholm University, Faculty of Science, Department of Astronomy. University of Colorado, USA.
    Tevzadze, Alexander G.
    The evolution of primordial magnetic fields since their generation2016In: Physica Scripta, ISSN 0031-8949, E-ISSN 1402-4896, Vol. 91, no 10, article id 104008Article in journal (Refereed)
    Abstract [en]

    We study the evolution of primordial magnetic fields in an expanding cosmic plasma. For this purpose we present a comprehensive theoretical model to consider the evolution of MHD turbulence that can be used over a wide range of physical conditions, including cosmological and astrophysical applications. We model different types of decaying cosmic MHD turbulence in the expanding Universe and characterize the large-scale magnetic fields in such a medium. Direct numerical simulations of freely decaying MHD turbulence are performed for different magnetogenesis scenarios: magnetic fields generated during cosmic inflation as well as electroweak and QCD phase transitions in the early Universe. Magnetic fields and fluid motions are strongly coupled due to the high Reynolds number in the early Universe. Hence, we abandon the simple adiabatic dilution model to estimate magnetic field amplitudes in the expanding Universe and include turbulent mixing effects on the large-scale magnetic field evolution. Numerical simulations have been carried out for non-helical and helical magnetic field configurations. The numerical results show the possibility of inverse transfer of energy in magnetically dominated non-helical MHD turbulence. On the other hand, decay properties of helical turbulence depend on whether the turbulent magnetic field is in a weakly or a fully helical state. Our results show that primordial magnetic fields can be considered as a seed for the observed large-scale magnetic fields in galaxies and clusters. Bounds on the magnetic field strength are obtained and are consistent with the upper and lower limits set by observations of extragalactic magnetic fields.

  • 13.
    Kemel, Koen
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Brandenburg, Axel
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Kleeorin, Nathan
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Ben-Gurion University of the Negev, Israel.
    Rogachevskii, Igor
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Ben-Gurion University of the Negev, Israel.
    Non-uniformity effects in the negative effective magnetic pressure instability2013In: Physica Scripta, ISSN 0031-8949, E-ISSN 1402-4896, Vol. T155, article id 014027Article in journal (Refereed)
    Abstract [en]

    In direct numerical simulations of strongly stratified turbulence we have previously studied the development of large scale magnetic structures starting from a uniform background field. This is caused by an instability resulting from a negative contribution of small-scale turbulence to the effective (mean-field) magnetic pressure, and was qualitatively reproduced in mean-field simulations (MFS) where this pressure reduction was modeled as a function of the mean magnetic field normalized by the equipartition field. We now investigate the effect of mean current density on the turbulent pressure reduction. In our MFS, such currents are associated with sharp gradients of the growing structures. We find that an enhanced mean current density increases the suppression of the turbulent pressure.

  • 14.
    Kemel, Koen
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Brandenburg, Axel
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Kleeorin, Nathan
    Rogachevskii, Igor
    Nonuniformity effects in the negative effective magnetic pressure instabilityIn: Physica Scripta, ISSN 0031-8949, E-ISSN 1402-4896Article in journal (Other academic)
  • 15.
    Kiselman, D.
    Stockholm University, Faculty of Science, Department of Astronomy.
    Solar 3D models versus observations—a few comments2008In: Physica Scripta, ISSN 0031-8949, E-ISSN 1402-4896, Vol. 133, p. 4016-Article in journal (Other academic)
    Abstract [en]

    Some properties of 3D solar photospheric models and the observations used to test them are discussed: granulation contrast and spectral line variation in the granulation pattern and over the solar disc. The recent use of the techniques of solar physics to tackle abundance problems is encouraged.

  • 16.
    Koivisto, Tomi S.
    et al.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Helsinki Institute of Physics, Finland; Helsinki University, Finland.
    Nyrhinen, Hannu J.
    Stability of disformally coupled accretion disks2017In: Physica Scripta, ISSN 0031-8949, E-ISSN 1402-4896, Vol. 92, no 10, article id 105301Article in journal (Refereed)
    Abstract [en]

    The no-hair theorem postulates that the only externally observable properties of a black hole are its mass, its electric charge, and its angular momentum. In scalar-tensor theories of gravity, a matter distribution around a black hole can lead to the so called 'spontaneous scalarisation' instability that triggers the development of scalar hair. In the Brans-Dicke type theories, this effect can be understood as a result of tachyonic effective mass of the scalar field. Here we consider the instability in the generalised class of scalar-theories that feature non-conformal, i.e. 'disformal', couplings to matter. Such theories have gained considerable interest in the recent years and have been studied in a wide variety of systems, both cosmological and astrophysical. In view of the prospects of gravitational wave astronomy, it is relevant to explore the implications of the theories in the strong-gravity regime. In this article, we concentrate on the spontaneous scalarisation of matter configurations around Schwarzschild and Kerr black holes. We find that in the more generic scalar-tensor theories, the instability of the Brans-Dicke theory can be enhanced, suggesting violations of the no-hair theorem. On the other hand, we find that, especially if the coupling is very strong, or if the gradients in the matter distribution are negligible, the disformal coupling tends to stabilise the system.

  • 17.
    Koivisto, Tomi S.
    et al.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Urban, Federico R.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Université Libre de Bruxelles, Belgium.
    Doubly-boosted vector cosmologies from disformal metrics2015In: Physica Scripta, ISSN 0031-8949, E-ISSN 1402-4896, Vol. 90, no 9, article id 095301Article in journal (Refereed)
    Abstract [en]

    A systematic dynamical system approach is applied to study the cosmology of anisotropic Bianchi I universes in which a vector field is assumed to operate on a disformal frame. This study yields a number of new fixed points, among which are anisotropic scaling solutions. Within the simplifying assumption of (nearly) constant-slope potentials these are either not stable attractors, do not describe accelerating expansion or else they feature too large anisotropies to be compatible with observations. Nonetheless, some solutions do have an appeal for cosmological applications in that isotropy is retained due to rapid oscillations of the vector field. Such cosmological scenarios could describe physics beyond standard model such as extra dimensional models that predict disformal couplings between vector and scalar fields.

  • 18.
    Larson, Jonas
    Stockholm University, Faculty of Science, Department of Physics.
    On the rotating wave approximation in the adiabatic limit2013In: Physica Scripta, ISSN 0031-8949, E-ISSN 1402-4896, Vol. T153, p. 014040-Article in journal (Refereed)
    Abstract [en]

    I revisit a longstanding question in quantum optics; when is the rotating wave approximation justified? In terms of the Jaynes-Cummings and Rabi models I demonstrate that the approximation in general breaks down in the adiabatic limit regardless of system parameters. This is explicitly shown by comparing Berry phases of the two models, where it is found that this geometrical phase is strictly zero in the Rabi model contrary to the non-trivial Berry phase of the Jaynes-Cummings model. The source of this surprising result is traced back to different topologies in the two models.

  • 19. Lestinsky, M.
    et al.
    Bräuning-Demian, A.
    Danared, H.
    Engström, Mats
    Stockholm University, Faculty of Science, Department of Physics.
    Enders, W.
    Fedotova, S.
    Franzke, B.
    Heinz, A.
    Herfurth, F.
    Källberg, Anders
    Stockholm University, Faculty of Science, Department of Physics.
    Kester, O.
    Litvinov, Y.
    Steck, M.
    Reistad, D.
    Simonsson, Ansgar
    Stockholm University, Faculty of Science, Department of Physics.
    Skeppstedt, Örjan
    Stockholm University, Faculty of Science, Department of Physics.
    Stöhlker, T.
    Vorobjev, G.
    CRYRING@ESR: present status and future research2015In: Physica Scripta, ISSN 0031-8949, E-ISSN 1402-4896, Vol. T166, article id 014075Article in journal (Refereed)
    Abstract [en]

    The former storage ring CRYRING has been shipped from the Manne Siegbahn Laboratory in Stockholm to Darmstadt as a Swedish in-kind contribution to FAIR. At its new location downstream of ESR all ion species presently accessible in ESR can be transferred to CRYRING, in which ions with rigidities between 1.44 and 0.054 Tm can be stored. The original Swedish layout has been modified by reconfiguring the sequence of straight sections and by slightly increasing the circumference to ESR/2. Ions can be injected from ESR or from an independent 300 keV/u RFQ test injector. The instrumentation of the ring includes an RF drift tube system for acceleration and deceleration (1 T s(-1), with a possibility for an upgrade to 7 T s(-1)), electron cooling, a free experimental section, and both fast and slow extraction of ions. We report on the present progress of this project, give a prospective timeline, and summarize the new research which will be enabled by this project. First beam for commissioning of the storage ring is expected for 2015, final bakeout to restore ultrahigh vacuum conditions in 2016 and ion beams injected through ESR in similar to 2017.

  • 20.
    Orban, Istvan
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Loch, S. D.
    Glans, P.
    Böhm, S.
    Stockholm University, Faculty of Science, Department of Physics.
    Schuch, Reinhold
    Stockholm University, Faculty of Science, Department of Physics.
    Effects of spin-forbidden resonances on the recombination of Be-like Si and Be-like Ne2011In: Physica Scripta, ISSN 0031-8949, E-ISSN 1402-4896, Vol. T144, p. 014035-Article in journal (Refereed)
    Abstract [en]

    Recombination through doubly excited states that can be formed only through spin-flip of the excited electrons can give very strong contributions to the recombination rate of Be-like ions. We demonstrate this, in this paper, with the electron-ion recombination spectra of Be-like Ne(6+) and Be-like Si(10+), recently measured at the CRYRING storage ring. These resonances have significant effects on the plasma rate coefficients. We show that neglect or imprecise calculation of these resonances is responsible for large spreads observed between various theoretical results from the literature.

  • 21.
    Pinheiro, Fernanda
    et al.
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Universidade de São Paulo, Brazil.
    de Toledo Piza, A. F. R.
    Quantum entanglement of bound particles under free center of mass dispersion2012In: Physica Scripta, ISSN 0031-8949, E-ISSN 1402-4896, Vol. 85, no 6Article in journal (Refereed)
    Abstract [en]

    On the basis of the full analytical solution of the overall unitary dynamics, the time evolution of entanglement is studied in a simple bipartite model system evolving unitarily from a pure initial state. The system consists of two particles in one spatial dimension bound by harmonic forces and having its free center of mass initially localized in space in a minimum uncertainty wavepacket. The existence of such initial states in which the bound particles are not entangled is discussed. Galilean invariance of the system ensures that the dynamics of entanglement between the two particles is independent of the wavepacket mean momentum. In fact, as shown, it is driven by the dispersive center of mass free dynamics, and evolves in a time scale that depends on the interparticle interaction in an essential way.

  • 22.
    Schmidt, Henning T.
    Stockholm University, Faculty of Science, Department of Physics.
    Electrostatic storage rings for atomic and molecular physics2015In: Physica Scripta, ISSN 0031-8949, E-ISSN 1402-4896, Vol. T166, article id 014063Article in journal (Refereed)
    Abstract [en]

    A significant number of electrostatic ion-storage rings have been built since the late 1990s or are currently in their construction or commisioning phases. In this short contribution, we attempt to supply an overview of these different facilities, while we also mention a selection of the electrostatic ion-beam traps that has been developed through the same time period and by some of the same research groups.

  • 23. Snellman, J. E.
    et al.
    Rheinhardt, M.
    Käpylä, P. J.
    Mantere, M. J.
    Brandenburg, Axel
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Mean-field closure parameters for passive scalar turbulence2012In: Physica Scripta, ISSN 0031-8949, E-ISSN 1402-4896, Vol. 86, no 1, article id 018406Article in journal (Refereed)
    Abstract [en]

    Direct numerical simulations (DNSs) of isotropically forced homogeneous stationary turbulence with an imposed passive scalar concentration gradient are compared with an analytical closure model which provides evolution equations for the mean passive scalar flux and variance. Triple correlations of fluctuations appearing in these equations are described in terms of relaxation terms proportional to the quadratic correlations. Three methods are used to extract the relaxation timescales tau(i) from DNSs. Firstly, we insert the closure ansatz into our equations, assume stationarity and solve for tau(i). Secondly, we use only the closure ansatz itself and obtain tau(i) from the ratio of quadratic and triple correlations. Thirdly, we remove the imposed passive scalar gradient and fit an exponential law to the decaying solution. We vary the Reynolds (Re) and Peclet numbers (while fixing their ratio at unity) and the degree of scale separation and find for large Re a fair correspondence between the different methods. The ratio of the turbulent relaxation time of the passive scalar flux to the turnover time of the turbulent eddies is of the order of 3, which is in remarkable agreement with earlier work. Finally, we make an effort to extract the relaxation timescales relevant for the viscous and diffusive effects. We find two regimes that are valid for small and large Re, respectively, but the dependence of the parameters on scale separation suggests that they are not universal.

  • 24. Stoehlker, Thomas
    et al.
    Litvinov, Yuri A.
    Bagnoud, Vincent
    Bechstedt, Ulf
    Dimopoulou, Christina
    Dolinskii, Alexei
    Geppert, Christopher
    Hagmann, Siegbert
    Katayama, Takeshi
    Kuehl, Thomas
    Maier, Rudolf
    Noertershaeuser, Wilfried
    Prasuhn, Dieter
    Schuch, Reinhold
    Stockholm University, Faculty of Science, Department of Physics.
    Steck, Markus
    Stockhorst, Hans
    SPARC experiments at the high-energy storage ring2013In: Physica Scripta, ISSN 0031-8949, E-ISSN 1402-4896, Vol. T156Article in journal (Refereed)
    Abstract [en]

    The physics program of the SPARC collaboration at the Facility for Antiproton and Ion Research (FAIR) focuses on the study of collision phenomena in strong and even extreme electromagnetic fields and on the fundamental interactions between electrons and heavy nuclei up to bare uranium. Here we give a short overview on the challenging physics opportunities of the high-energy storage ring at FAIR for future experiments with heavy-ion beams at relativistic energies with particular emphasis on the basic beam properties to be expected.

  • 25. Winters, D. F. A.
    et al.
    Kuehl, Th
    Schneider, D. H.
    Indelicato, P.
    Reuschl, R.
    Schuch, Reinhold
    Stockholm University, Faculty of Science, Department of Physics.
    Lindroth, Eva
    Stockholm University, Faculty of Science, Department of Physics.
    Stoehlker, Th
    Laser spectroscopy of the (1s(2)2s2p) (3)P(0)-(3)P(1) level splitting in Be-like krypton2011In: Physica Scripta, ISSN 0031-8949, E-ISSN 1402-4896, Vol. T144, p. 014013-Article in journal (Refereed)
    Abstract [en]

    Heavy few-electron ions, such as He-, Li- and Be-like ions, are ideal atomic systems to study the effects of correlation, relativity and quantum electrodynamics. Very recently, theoretical and experimental studies of these species achieved a considerable improvement in accuracy. Be-like ions are interesting because their first excited state, i.e. (1s(2)2s2p) (3)P(0), has an almost infinite lifetime (tau(0)) in the absence of nuclear spin (I), as it can only decay by a two-photon E1M1 transition to the (1s(2)2s(2)) (1)S(0) ground state. In addition, the energy difference between the 3P0 and the next higher-lying (3)P(1) state is expected to remain almost completely unaffected by QED effects, and should thus be dominated by the effects of correlation and relativity. Therefore, we want to determine the (1s(2)2s2p) (3)P(0)-(3)P(1) level splitting in Be-like krypton ((84)Kr(32+)), which has I = 0, by means of laser spectroscopy at the experimental storage ring at GSI. In such an experiment, the energy splitting can be obtained with very good accuracy and can be compared with recent calculations.

  • 26. Xiao, J.
    et al.
    Fei, Z.
    Yang, Y.
    Jin, X.
    Lu, D.
    Shen, Y.
    Liljeby, Leif
    Stockholm University, Faculty of Science, Department of Physics, The Manne Siegbahn Laboratory.
    Hutton, R.
    Zou, Y.
    Progress of the permanent magnetic electron beam ion trap in Shanghai2011In: Physica Scripta, ISSN 0031-8949, E-ISSN 1402-4896, Vol. T144, p. 014061-Article in journal (Refereed)
    Abstract [en]

    In this paper, a brief description of the progress of the Shanghai permanent magnetic electron beam ion trap is presented. Some test results regarding the electron beam current versus beam energy are presented. The electron beam width was measured and compared with our simulations, and good agreement was found.

  • 27. Zeng, Hong Li
    et al.
    Hertz, John
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Niels Bohr Institute, Denmark.
    Roudi, Yasser
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Norwegian University of Science & Technology (NTNU), Norway.
    L-1 regularization for reconstruction of a non-equilibrium Ising model2014In: Physica Scripta, ISSN 0031-8949, E-ISSN 1402-4896, Vol. 89, no 10, p. 105002-Article in journal (Refereed)
    Abstract [en]

    The couplings in a sparse asymmetric, asynchronous Ising network are reconstructed using an exact learning algorithm. L-1 regularization is used to remove the spurious weak connections that would otherwise be found by simply maximizing the log likelihood of a finite data set. In order to see how L-1 regularization works in detail, we perform the calculation in several ways including (1) by iterative minimization of a cost function equal to minus the log likelihood of the data plus an L-1 penalty term, and (2) an approximate scheme based on a quadratic expansion of the cost function around its minimum. In these schemes, we track how connections are pruned as the strength of the L-1 penalty is increased from zero to large values. The performance of the methods for various coupling strengths is quantified using receiver operating characteristic curves, showing that increasing the coupling strength improves reconstruction quality.

1 - 27 of 27
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