Search Results - (Author, Cooperation:M. Persson)

2018-02-01

The American Society for Microbiology (ASM)

0099-2240

1098-5336

Biology

2018-07-12

MDPI Publishing

2073-4441

Energy, Environment Protection, Nuclear Power Engineering

2018-06-29

MDPI Publishing

2073-4441

Energy, Environment Protection, Nuclear Power Engineering

2015-02-28

American Association for the Advancement of Science (AAAS)

0036-8075

1095-9203

Biology

Chemistry and Pharmacology

Computer Science

Medicine

Natural Sciences in General

Physics

2015-01-17

American Association for the Advancement of Science (AAAS)

0036-8075

1095-9203

Biology

Chemistry and Pharmacology

Computer Science

Medicine

Natural Sciences in General

Physics

Atmosphere ; *Biological Evolution ; *Climate Change ; *Earth (Planet) ; Fresh Water ; Humans ; *Ozone Depletion

1089-7674

AIP Digital Archive

Physics

A study of the effect of local magnetic shear on the drift wave stability is presented. The eigenvalue problem for the drift wave equation is solved numerically in fully three-dimensional stellarator plasma using the ballooning mode formalism. It is found that negative local magnetic shear has a stabilizing effect on the drift wave instability and positive local shear is destabilizing. This is in agreement with the effect of negative global magnetic shear in tokamaks and also agrees with the simple estimates. As a consequence the highly unstable modes found on a specific magnetic surface are localized in the region of positive local magnetic shear. © 2001 American Institute of Physics.

Electronic Resource

1089-7674

AIP Digital Archive

Physics

It is shown that the coexistence of toroidally nonlocalized ideal-hydromagnetic ballooning instabilities, with a quasidiscrete spectrum, and toroidally localized ballooning instabilities with a broad continuous spectrum, as predicted by Dewar and Glasser [Phys. Fluids 26, 3038 (1983)] can be realized in a Mercier-unstable equilibrium case modeling the Large Helical Device (LHD) [A. Iiyoshi et al., Fusion Technol. 17, 148 (1990)] with a broad pressure profile. The quasidiscrete, interchange branch corresponds to extended modes that can be understood on the basis of a ripple-averaged ballooning equation, whereas the broad-continuum, ballooning branch corresponds to modes localized along a flux tube. The physical origin of the two branches is discussed. © 1998 American Institute of Physics.

Electronic Resource

1089-7674

AIP Digital Archive

Physics

The stability of finite size magnetic islands is analyzed in configurations with multiple resonant magnetic surfaces. It is demonstrated that there are configurations that are linearly stable which can be unstable to finite size perturbations. Two different examples of single helicity double tearing are given for configurations with two q=2 surfaces. In the first case the destabilization is due to the extension of magnetic separatrices out to regions of destabilizng current gradients. For the second case the modes are linearly stabilized by the suppression of the linear coupling of the rational surfaces by differential plasma rotation, which essentially decouples the perturbations around the different rational surfaces. A finite size magnetic island will interact quasilinearly with initial plasma rotation. The plasma rotation is then equilibrated and the mode destabilized.

Electronic Resource

1089-7674

AIP Digital Archive

Physics

The influence of plasma geometry on the linear stability of electrostatic ion-temperature-gradient driven drift modes (ITG or ηi=Ln/LTi modes) is investigated. An advanced fluid model is used for the ions together with Boltzmann distributed electrons. The derived eigenvalue equation is solved numerically. A comparison is made between an H–1NF [Fusion Technol. 17, 123 (1990)] like stellarator equilibrium, a numerical tokamak equilibrium and the analytical s(circumflex)−α equilibrium. The numerical and the analytical tokamak are found to be in good agreement in the low inverse aspect ratio limit. The growth rates of the tokamak and stellarator are comparable whereas the modulus of the real frequency is substantially larger in the stellarator. The threshold in ηi for the stellarator is found to be somewhat larger. In addition, a stronger stabilization of the ITG mode growth is found for large εn(=Ln/R) in the stellarator case. © 2002 American Institute of Physics.

Electronic Resource

1089-7674

AIP Digital Archive

Physics

The stable and unstable parts of the spectrum of the cold ion drift wave model are discussed in a helically symmetric stellarator model using a ballooning mode formalism. The presence of different types of eigenmodes is discussed. A study of the dependence of the growth of the modes on mode number, and other parameters, is also presented. © 1996 American Institute of Physics.

Electronic Resource

1089-7674

AIP Digital Archive

Physics

The ion temperature gradient driven ballooning mode is investigated using two-fluid, gyrofluid, and gyrokinetic descriptions. The linear eigenmode equation is solved numerically in a model equilibrium with shifted circular magnetic surfaces. The localization of the eigenmodes, which persist in the magnetohydrodynamic (MHD) second stability region, and the mode structure, are displayed. The role of finite-Larmor radius (FLR) and magnetic drift resonance effects on the growth rate are elucidated. Negative magnetic shear is found to have a stabilizing effect on the mode. © 1995 American Institute of Physics.

Electronic Resource

1471-4159

Blackwell Publishing Journal Backfiles 1879-2005

Medicine

Abstract: The antinociceptive actions of intrathecal injections of two α2-adrenergic agonists, UK-14,304 and guanfacine, were investigated in rats after pretreatment of the animals with the noradrenaline neurotoxin N-2-chloroethyl-N-ethyl-2-bromobenzylamine (DSP4) 14 days in advance. The chronic noradrenaline depletion induced by DSP4 caused a marked increase in sensitivity of the antinociceptive action of UK-14,304 in the tail-flick test. By contrast, the antinociceptive effect of guanfacine was not appreciably affected by the DSP4 treatment. The antinociceptive effects of both UK-14,304 and guanfacine were blocked by intraperitoneal injections of yohimbine, a result indicating that both drugs induced their actions by activating α2-adrenoceptors. Both UK-14,304 and guanfacine were found to reduce the production of cyclic AMP (cAMP) in the spinal cord, as determined using an in vitro radioisotopic method. The cAMP inhibitory effects of both agonists were effectively blocked by yohimbine, but not by prazosin, a finding indicating the α2-adrenergic nature of the response. However, the cAMP inhibitory effect of UK-14,304 was not potentiated by pretreatment with DSP4, a finding in marked contrast with the strong potentiation of the antinociceptive action of UK-14,304 induced by the chronic depletion of endogenous noradrenaline. Moreover, intrathecal injections of forskolin, which increased the endogenous levels of spinal cord cAMP fivefold, did not modify the antinociceptive effects of UK-14,304 or guanfacine in neither normal nor DSP4-treated animals. It is suggested that there exist pharmacologically differing α2-adrenergic receptor pathways capable of mediating antinociceptive effects at the level of the spinal cord. The cAMP inhibitory actions of spinal cord α2-adrenoceptors appear not to be directly linked with the antinociceptive actions of these receptors.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

We report a theoretical study of the electronic structure of GaAs nanowhiskers, grown in the [111] direction with hexagonal cross section, based on a tight binding approach. It is shown that the band structure of the GaAs nanowhiskers shifts from a direct band gap to an indirect band gap when the lateral size of the nanowhiskers becomes smaller than a certain value. The effective masses of the electrons and holes are shown to increase with decreasing the nanowhisker lateral size. It is also shown that the light-hole states appear to be at the top of the valence bands. The electrical and optical properties of the nanowhiskers are discussed in terms of the results of the calculations. © 2002 American Institute of Physics.

Electronic Resource

1089-7666

AIP Digital Archive

Physics

The global asymptotic matching equations for multiple coupled resistive modes of arbitrary parity in a cylindrical plasma are derived. Three different variational principles are given for the outer region matching data, while the inner region analysis features a careful treatment of the symmetry-breaking effect of a gradient in the equilibrium current for a zero-β slab model. It is concluded that the usual constant-ψ result remains valid and constrains the matrix matching formalism. The dispersion relation is compared with initial value calculations of a double tearing mode when there are small relative rotation velocities between the rational surfaces. In treating differential rotation within the asymptotic matching formalism, flow is ignored in the outer region and is assumed to affect the inner response solely through a Doppler shift. It is shown that the relative rotation can have a strong stabilizing effect by making all but one rational surface effectively ideal.

Electronic Resource

1089-7666

AIP Digital Archive

Physics

The nonlinear evolution of tearing modes with multiple rational surfaces is discussed. It is demonstrated that, in the presence of small differential rotation, the nonlinear growth might be faster than exponential. This growth occurs as the rotation frequencies of the plasma at the different rational surfaces equilibrate.

Electronic Resource

1089-7666

AIP Digital Archive

Physics

A combined numerical and analytical study is presented of the linear stability problem for resistive tearing modes in the presence of perpendicular and parallel equilibrium flows. Detailed information is given about growth rates and the shape of the marginal stability curve. In the case of purely perpendicular flows, a region in parameter space is found where the growth rate is complex, and the region of instability is larger than previously recognized. An exact analytic solution is found for parallel flows when the plasma response is dominated by viscous forces.

Electronic Resource

1089-7666

AIP Digital Archive

Physics

The nonlinear evolution of tearing modes in the presence of sheared mass flow is studied as an initial value problem. Under certain conditions, when the mode is driven unstable primarily by the mass flow, the nonlinear evolution leads to a dynamic state in which the size and shape of the magnetic islands are oscillatory.

Electronic Resource

1089-7690

AIP Digital Archive

Physics

Chemistry and Pharmacology

We report observations of coherent elastic and rotationally inelastic scattering of N2, O2, and CH4 from a 10 K Cu(111) surface, kept clean by pulsed laser heating. The related sharp features in the measured angular distributions decrease drastically in intensity at elevated target temperatures. At low temperature rotational transitions reduce the elastic scattering probability by about an order of magnitude. This effect is weak for D2 at the impact conditions of concern. Quantum scattering calculations for N2 and D2 show that this difference is primarily caused by the large difference in rotational constants and the associated rotational transition energies of these molecules. © 2000 American Institute of Physics.

Electronic Resource

1089-7690

AIP Digital Archive

Physics

Chemistry and Pharmacology

We have performed first-principles total-energy calculations of low-dimensional sections of the electronically adiabatic potential energy surface (PES) that are relevant for the Eley–Rideal (ER) reaction of H atoms on a rigid Cu(111) surface. These calculations were performed within density-functional theory using a plane-wave and pseudopotential method and the generalized gradient approximation for the exchange-correlation energy. The calculated energy points for various configurations of one and two atoms on the Cu(111) surface were used to construct a model PES that can be used in ER reaction dynamics calculations. © 1999 American Institute of Physics.

Electronic Resource

1089-7690

AIP Digital Archive

Physics

Chemistry and Pharmacology

We have performed a detailed study of collision-induced desorption (CID) of Xe from Pt(111) by hyperthermal Ar atoms striking the surface. The study combines extensive molecular beam experiments with detailed molecular dynamics calculations in an effort to quantify fully and to understand in detail the dynamics of this model system. The calculations employ accurate semi-empirical potential energy functions for all relevant interatomic interactions. We have characterized the CID process with respect to incidence energy, Ei, angle, θi, and substrate surface temperature, Ts, for both the isolated adsorbate and the Xe/Pt(111) monolayer. The generally good agreement of the calculated results such as the desorption yield and desorbate energy and angular distributions with the experimental measurements demonstrates that the calculations capture the most important aspects of the CID processes. © 1997 American Institute of Physics.

Electronic Resource