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  • IOPscience (Institute of Physics)  (51)
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  • 1
    Language: English
    In: Journal of Physics B: Atomic, Molecular and Optical Physics, 2014, Vol.47(20), p.204015 (7pp)
    Description: Interaction of a laser pulse with a centrally symmetric medium, such as an isotropic gas of atoms, leads to the generation of harmonic emission which contains exclusively odd harmonics of the incident field. This result is the consequence of both the central symmetry of the medium and the temporal symmetry of the oscillating electric field, , where ω l is the laser frequency. In the case of oriented heteronuclear molecules, the spatial symmetry no longer holds and both odd and even harmonics become allowed. Here we show, by solving the time-dependent Schrödinger equation for H , D , and T , that even-order harmonic generation is also possible for sufficiently long infrared (IR) laser pulses in homonuclear molecules. The appearance of even harmonics is a signature of the coupled electron-nuclear dynamics and reflects field-induced electron localization initiated by the strong laser field, which breaks the spatial symmetry in the system. The analysis of even harmonics generated by pulses of different durations might therefore provide information on correlated electron-nuclear dynamics and charge migration in more complex un-oriented molecular ensembles.
    Keywords: Texts ; Harmonics ; Dynamical Systems ; Symmetry ; Lasers ; Schroedinger Equation ; Position (Location) ; Dynamics ; Atomic and Molecular Physics (General) (So) ; Atomic and Molecular Physics (Ah) ; High Harmonic Generation ; Ultrafast Molecular Dynamics ; Electron Localization;
    ISSN: 0953-4075
    E-ISSN: 1361-6455
    Source: IOPscience (IOP Publishing)
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  • 2
    Language: English
    In: Nuclear Fusion, 2014, Vol.54(6), p.064005 (17pp)
    Description: First experiments on edge-localized mode (ELM) mitigation with the help of ITER-like coils on ASDEX Upgrade are analysed using linear and quasilinear kinetic models to describe the interaction of resonant magnetic field perturbations (RMP) with the plasma. The gyrokinetic derivation of RMP-driven transport coefficients is given in detail. The role of fluid resonances is studied, in particular the role of the resonance associated with the equilibrium electric field reversal point E r  = 0. Like the electron fluid resonance associated with the zero of the total perpendicular electron fluid velocity, the E r  = 0 resonance may lead to enhanced transport due to the reduction of RMP shielding in the pedestal region where the RMP field can even be amplified by this resonance. The conditions on the RMP coil spectrum resulting from the analysis are discussed.
    Keywords: Computational Fluid Dynamics ; Mathematical Models ; Magnetic Resonance ; Fluids ; Fluid Flow ; Erbium ; Elm ; Coils ; Nuclear and High-Energy Physics (General) (So) ; Nuclear and High-Energy Physics (Ah) ; Rmps ; Elm Mitigation ; Tokamak;
    ISSN: 0029-5515
    E-ISSN: 1741-4326
    Source: IOPscience (IOP Publishing)
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  • 3
    Language: English
    In: Journal of Physics B: Atomic, Molecular and Optical Physics, 2017, Vol.50(14), p.144001 (8pp)
    Description: The ultrafast decay of highly excited electronic states is resolved with a molecular clock technique, using the vibrational motion associated to the ionic bound states as a time-reference. We demonstrate the validity of the method in the context of autoionization of the hydrogen molecule, where nearly exact full dimensional ab-initio calculations are available. The vibrationally resolved photoionization spectrum provides a time–energy mapping of the autoionization process into the bound states that is used to fully reconstruct the decay in time. A resolution of a fraction of the vibrational period is achieved. Since no assumptions are made on the underlying coupled electron–nuclear dynamics, the reconstruction procedure can be applied to describe the general problem of the decay of highly excited states in other molecular targets.
    Keywords: Physics;
    ISSN: 0953-4075
    E-ISSN: 1361-6455
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  • 4
    Language: English
    In: IOP Conference Series: Materials Science and Engineering, 2010, Vol.14(1), p.012014 (10pp)
    Description: Micro-beam X-ray diffraction has been used to investigate the texture of banded spherulites of melt-crystallized poly(trimethylene terephthalate), PTT, formed in films of approximately 30 to 50 m thickness. The WAXS micro-diffraction patterns show that at the local scale, the PTT texture is close to that of a single crystal. In agreement with previous studies using selected-area electron diffraction, it is shown that the crystal growth direction is parallel to the a -axis of the unit cell. When plotted as a function of the distance to the spherulite center, the intensity of different diffraction peaks reveals the same periodicity. This means that the lamellar twist is strictly uniform. The latter observation is more compatible with the model explaining the twist as a result of unbalanced surface stresses than that of isochiral giant screw dislocations. The main features of the experimental diffractograms can be understood using the numerical approach, which is developed in the approximation of a purely geometric broadening of X-ray reflections. In particular, the simulation can predict the sequence of appearance of different diffraction peaks and their shape on the 2D micro-diffraction patterns.
    Keywords: X-Rays ; Mathematical Models ; Surface Layer ; Spherulites ; Unit Cell ; Diffraction ; Texture ; Mathematical Analysis ; Theoretical Mechanics and Dynamics (Mt) ; Engineering Components and Structures (MD) ; Engineering Components and Structures (Ep) ; Engineering Components and Structures (Ed) ; Engineering Components and Structures (EC) ; Theoretical Mechanics and Dynamics (CE);
    ISSN: 1757-8981
    E-ISSN: 1757-899X
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  • 5
    Language: English
    In: Journal of Physics: Conference Series, 2015, Vol.635(9), p.092089 (1pp)
    Description: In contrast to expectations, we observe that the photoionization time delay from the 1 s 2 spherically symmetric ground state of He depend on the electron emission direction with respect to an external reference. We attribute the observed anisotropy to the interplay between different final quantum states, which become accessible once two photons are involved in the photoionization process. This is a universal effect, which needs to be taken into account for any study dealing with photoionization dynamics.
    Keywords: Physics - Atomic Physics;
    ISSN: 1742-6588
    E-ISSN: 1742-6596
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  • 6
    Language: English
    In: Nuclear Fusion, 2006, Vol.46(4), pp.S159-S169
    Description: The interaction of rotating magnetic fields (RMFs) with a plasma is modelled in the linear approximation. A kinetic Hamiltonian model for the rf plasma conductivity is used. A radially inhomogeneous periodic cylindrical plasma with a rotational transform of the magnetic field is studied with parameters relevant to the dynamic ergodic divertor (DED) of TEXTOR. For the case of a finite electron diamagnetic velocity it is shown that the torque resulting from the RMF tends to bring the electron fluid approximately to the rest frame of this field. This result is in qualitative agreement with long mean-free path drift MHD theory. In contrast to that theory where a resonant behaviour is found at electron and ion diamagnetic frequencies, in the present kinetic analysis, the RMF frequency where the torque passes through zero is smaller than the electron diamagnetic frequency if there is an electron temperature gradient present. The relation of these results with recent experimental measurements of the DED-induced plasma rotation in TEXTOR is discussed.
    Keywords: Physics;
    ISSN: 0029-5515
    E-ISSN: 1741-4326
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  • 7
    Language: English
    In: Nuclear Fusion, 2008, Vol.48(2), p.024005 (14pp)
    Description: Effects of linear plasma response currents on non-axisymmetric magnetic field perturbations from the I-coil used for edge localized mode mitigation in DIII-D tokamak are analysed with the help of a kinetic plasma response model developed for cylindrical geometry. It is shown that these currents eliminate the ergodization of the magnetic field in the core plasma and reduce the size of the ergodic layer at the edge. A simple balance model is proposed which qualitatively reproduces the evolution of the plasma parameters in the pedestal region with the onset of the perturbation. It is suggested that the experimentally observed density pump-out effect in the long mean free path regime is the result of a combined action of ion orbit losses and magnetic field ergodization at the edge.
    Keywords: Physics;
    ISSN: 0029-5515
    E-ISSN: 1741-4326
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  • 8
    Language: English
    In: Plasma Physics and Controlled Fusion, 2009, Vol.51(5), p.055021 (17pp)
    Description: Frequency locking of edge localized modes (ELMs) to the vertical plasma movements induced by magnetic perturbations first demonstrated in TCV was successfully repeated in ASDEX Upgrade. However, the ELMs were triggered in ASDEX Upgrade when the plasma was moving down towards the X-point with a consequent decrease in the plasma current density in the edge region, in contrast to the previous observation on TCV in which ELMs were triggered when the edge current was increased by an upward plasma movement. This opposite behaviour observed in the magnetic triggering of ELMs has been investigated by using a free-boundary tokamak simulator, DINA-CH. The passive stabilization loops (PSLs) located inside the vacuum vessel of ASDEX Upgrade produce similar external linking flux changes to those generated by the G-coil sets in TCV for opposite vertical plasma movements. Therefore, both plasmas experience similar local flux surface expansions near the upper G-coil set and PSL when the ELMs are triggered. In ASDEX Upgrade, however, the localized expansion of the plasma flux surfaces near the upper PSL is observed with the global shrinkage of the plasma column accompanied by the downward plasma movement.
    Keywords: Physics;
    ISSN: 0741-3335
    E-ISSN: 1361-6587
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  • 9
    Language: English
    In: The Astrophysical Journal Letters, 2018, Vol.853(1), p.L8 (14pp)
    Description: We present a photometric detection of the first brightness dips of the unique variable star KIC 8462852 since the end of the Kepler space mission in 2013 May. Our regular photometric surveillance started in 2015 October, and a sequence of dipping began in 2017 May continuing on through the end of 2017, when the star was no longer visible from Earth. We distinguish four main 1%–2.5% dips, named “ Elsie ,” “ Celeste ,” “ Skara Brae ,” and “ Angkor ,” which persist on timescales from several days to weeks. Our main results so far are as follows: (i) there are no apparent changes of the stellar spectrum or polarization during the dips and (ii) the multiband photometry of the dips shows differential reddening favoring non-gray extinction. Therefore, our data are inconsistent with dip models that invoke optically thick material, but rather they are in-line with predictions for an occulter consisting primarily of ordinary dust, where much of the material must be optically thin with a size scale ≪1 μ m, and may also be consistent with models invoking variations intrinsic to the stellar photosphere. Notably, our data do not place constraints on the color of the longer-term “secular” dimming, which may be caused by independent processes, or probe different regimes of a single process.
    Keywords: Astrophysics - Solar And Stellar Astrophysics ; Astrophysics - Earth And Planetary Astrophysics;
    ISSN: 2041-8205
    E-ISSN: 2041-8213
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  • 10
    Language: English
    In: The Astronomical Journal, 2004, Vol.128(6), pp.3034-3052
    Description: We present optical and/or infrared photometry of the Type Ia supernovae SN 1991T, SN 1991bg, SN 1999ek, SN 2001bt, SN 2001cn, SN 2001cz, and SN 2002bo. All but one of these supernovae have decline rate parameters, m 15 ( B ), close to the median value of 1.1 for the whole class of Type Ia supernovae. The addition of these supernovae to the relationship between the near-infrared absolute magnitudes and m 15 ( B ) strengthens the previous relationships we have found in that the maximum light absolute magnitudes are essentially independent of the decline rate parameter. (SN 1991bg, the prototype of the subclass of fast-declining Type Ia supernovae, is a special case.) The dispersion in the Hubble diagram in JHK is only 0.15 mag. The near-infrared properties of Type Ia supernovae continue to be excellent measures of the luminosity distances to the supernova host galaxies because of the need for only small corrections from the epoch of observation to maximum light, low dispersion in absolute magnitudes at maximum light, and the minimal reddening effects in the near-infrared.
    Keywords: Astrophysics;
    ISSN: 0004-6256
    E-ISSN: 1538-3881
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