Showing papers in "Physica Scripta in 1995"

Ion-neutral collisions in a 22-pole trap at very low energies

Abstract: This contribution concentrates on the use of an ion trapping technique which has been developed for studying collision processes and chemical reactions at low temperatures and which utilizes inhomogeneous radio frequency fields Special emphasis is put onto recent progress achieved with a cryo cooled 22-pole ion trap Experimental results, measured at a nominal temperature of 10 K, are presented for radiative association of CH3+ and C2H2+ with H2 Another group of results discussed in this paper deals with the dynamics of the growth of hydrogen clusters starting from H3+ In all cases, the influence of H2 rotation was examined by utilizing p-H2 and n-H2 Also the process of isotope fractionation in the presence of HD and D2 is briefly mentioned

Cooling methods in ion traps

Abstract: In many experiments utilizing ion traps, the ions must be cooled in order to increase the precision and accuracy of the measurements. Laser cooling is very effective when it can be applied, but it can only be used with a few kinds of ions, since it depends critically on the details of the electronic level structure. Other methods, such as resistive cooling, active-feedback cooling, collisional cooling, radiofrequency side-band cooling, or sympathetic laser cooling, can be applied to many kinds of ions. Progress in cooling of trapped ions has made possible improved measurements of mass ratios and atomic spectra and the observation of new phenomena, such as the formation of ordered Coulomb "crystals" of ions.

Recent experimental achievements with negative ions

Abstract: Experimental results obtained about the atomic physics of negative ions since the Physica Scripta 1991 review are presented. Most new data concern the metastable states and resonances of He− and alkaline-earth negative ions, and the ions of the C− column. A few photodetachment experiments were also performed on ions submitted to external fields, and we have now a better understanding of the response of negative ions to microwaves and ponderomotive threshold shifting. A great progress was made in the study of multiphoton detachment processes; 1991 was the year of discovery of excess-photon detachment. Every year brings new precision detachment threshold measurements about, but still relying on the 1985 measurement numerical reasons make it necessary to revise the electron affinity of oxygen: EA(O) = 11 784.648(6)cm−1, or 1.4 611 107(17)eV.

Averaging correlated data

Abstract: The problem of averaging strongly correlated data is addressed for the case that the exact correlation pattern is unknown. A procedure is proposed to estimate the effective size of the correlations from the data themselves and to take them properly into account when forming the average. The properties of the procedure are illustrated by using it for averaging measurements of the strong coupling constant and QCD colour-factor ratios.

The oxygen coordination of metal ions in phosphate and silicate glasses studied by a combination of x-ray and neutron diffraction

Abstract: A combination of results from x-ray and neutron diffraction is used to obtain structural information about the metal-oxygen coordination shell in oxide glasses. Two ways to extract structural parameters of the Me-O coordination are presented. The first variant is a direct combination of both distance correlation functions which are considered simultanously in a least-squares fit procedure. On the other hand a suitable difference of the two structure factors is introduced, which do not contain any O-O correlation. The corresponding distance correlation function directly shows the Me-O peak. The samples are metaphosphate glasses with Me = Al, Zn, Mg, Ca, Ba and Na and two sodium silicate glasses (76.5 and 67 mol% silicon dioxide). Four oxygens are found in contact to the Mg ion. But two additional, more distant positions are detected. Thus, the sum of all oxygen atoms in the coordination sphere is 6 rather than 4. The Zn cation is located in a real ZnO4-tetrahedron. The number of oxygens in the environment of the Na ion is of about five both in the metaphosphate glass and in the silicate glasses. But a surprising result is a splitting observed for the Na-O distance peak in case of silicate glasses.

Theory of shear Alfvén waves in toroidal plasmas

Abstract: Fundamental wave and stability properties of shear Alfven waves in nonuniform axisymmetric toroidal plasmas are reviewed. Using a slab model with nonuniform density, concepts such as continuous spectrum, resonant absorption, and linear mode conversion to kinetic Alfven wave are first discussed. We then analyze the formation of frequency gaps in the continuous spectrum due to the finite toroidicity-induced coupling between the poloidal harmonics in an axisymmetric torus. Existence of discrete eigenmodes both inside the frequency gap and the continuous spectra are further demonstrated. Kinetic excitations of collective toroidal shear Alfven instabilities via resonances with energetic particles are also reviewed; first in the one-dimensional radially local limit and then with a full two-dimensional global analysis. Effects due to the non-perturbative nature of the energetic-particle drive and the global extent of the eigenmodes are emphasized.

Efficient symplectic algorithms for numerical simulations of Hamiltonian flows

Abstract: A quite general approach to numerical simulations of Hamiltonian flows is presented, which is suitable to the development of efficient symplectic algorithms. Explicit schemes up to fourth order are worked out. These algorithms show a very good performance if implemented in typical molecular dynamics problems, i.e. in long-time simulations of Hamiltonian systems with a large number of degrees of freedom and steep potential functions.

A model for calculating icrh power deposition and velocity distribution

Abstract: During intense ICRH the velocity distribution of the resonating ions becomes strongly distorted. The power deposition and the velocity distribution depend on each other making it necessary to calculate a power deposition which is consistent with the velocity distribution. A method for calculating such power depositions is presented. A code called PION has been developed based on this method. The code uses simplified models of the power deposition and velocity distribution.

Two-photon two-color diode laser spectroscopy of the Rb 5D5/2 state

Abstract: We report on spectroscopic measurements of the Rb 5D5/2 state performed using two-photon excitation with a resonant intermediate state. With the two beams (5S1/2 → 5P3/2 at 780 nm and 5P3/2 → 5D5/2 at 776 nm) counterpropagating, the spectra are Doppler-free and the signals are very robust, even in a room temperature vapor. This is due to the near match of the transition wavelengths. The 5D5/2 hyperfine constants have been measured and compared to previous determinations. We have also performed the excitation in a sample of laser-cooled atoms and compare these results to those obtained at room temperature. The fact that the 5P3/2 → 5D5/2 transition is narrow, robust, diode-laser accessible, and relatively immune to AC-Stark shifts, may make it a promising candidate for a frequency reference at 776 nm.

Conservation laws and connection formulae for resonant MHD waves

Abstract: This paper reviews a recent development in the theory of resonant MHD waves in non-uniform plasmas. An asymptotic analysis of the equations for MHD waves in plasmas with high magnetic Reynolds numbers has shown that resonant slow and Alfven waves obey conservation laws and jump conditions across the dissipative layer. These conservation laws specify the dominant dynamics of the resonant MHD waves. In combination with the jump conditions they enable us to understand the basic physics of resonant MHD waves and also help us with the interpretation of results of large scale numerical simulations in resistive MHD. They also can be used to design an accurate and computationally simple methods for numerical studying resonant MHD waves in non-uniform plasmas. Conservation laws and jump conditions for resonant MHD waves are first discussed in linear MHD for 1-dimensional equilibrium states. Subsequently the generalization of these results to 2-dimensional equilibrium state in linear MHD and to nonlinear MHD is reviewed. The interaction of sound waves with an inhomogeneous plasma is discussed as an application of the theory. Firstly the results of linear theory are used to consider the interaction of sound waves with 1-dimensional magnetic tubes. The phenomenon of total resonant absorption is discussed. Secondly the nonlinear theory of cusp dissipative layers is used to study the interaction of sound waves with 1-dimensional inhomogeneous plasmas in planar geometry. New effects that owe their existence to nonlinearity in the cusp dissipative layer are reviewed.

Nonlinear Alfvén waves

Abstract: Some important nonlinear effects involving Alfven waves in plasmas are presented For illustrative purposes, we start with small amplitude Alfven waves and their relation with other low-frequency plasma modes We then show that Alfven and magnetosonic waves can be nonlinearly excited by a high-frequency external pump wave Finite-amplitude Alfven waves can either interact with the background plasma or with themselves, giving rise to a number of nonlinear phenomena such as wave- amplitude modulation or filamentation, density profile modification, as well as self-organization in vortical structures The nonlinear effects that are described here are of relevance to the large-amplitude disturbances which are frequently observed in laboratory and space plasmas

Extended mean field description of deformed states in neutron deficient Cd- and Sn-nuclei

Abstract: Extensions to the standard total routhian surface calculations are presented. The particle number symmetry is approximately restored using the Lipkin-Nogami method to avoid the well known problems of sharp pairing phase transitions, that are predicted by mean-field calculations in the vicinity of closed shells or in the high-spin regime. Secondly, the quadrupole pairing interaction is added to the standard seniority pairing to account for an improved treatment of particle-particle correlations. The method is applied to the analysis of collective rotational bands found in neutron deficient Cd, In, Sn, Sb and Te isotopes.

Fluorescence Lidar Monitoring of Vegetation Status

Abstract: Remote laser-induced fluorescence monitoring can complement passive reflectance measurements on vegetation. Vegetation classification and status assessment is normally performed by air- or spaceborne multispectral scanners. An increased research activity in the field of vegetation monitoring is motivated by the abundant stressed forests of Europe. We have used a mobile lidar system for remote studies of laser-induced fluorescence in vegetation. Point monitoring of fluorescence spectra is performed, but also multi-spectral imaging using a newly designed measurement system. Swedish experience in several field tests being part of the European LASFLEUR project is reported. Collaboration with expertise in plant physiology is stressed.

Parametric excitation of vacuum by use of femtosecond laser pulses

Abstract: We discuss the method of parametric excitation of electromagnetic waves in a cavity by creating a dense plasma layer (which represents the mobile wall of the cavity) due to irradiation of a semiconductor film with femtosecond laser pulses. The effect is analysed for various initial states including an initial vacuum state. The state of the field produced by this method from an initial vacuum state is different from an electromagnetic field of another origin because only even quantum levels are populated (similar to Schrodinger cat states) and specific distributions of wave packets in phase space takes place. Besides, the parametrically excited field has a specific angle distribution and appears in shorter time scales than the ones required for appearance of radiation of other origin. The instantaneous and adiabatic slow excitation of eigenmodes of the cavity are described. Estimations for experimental detection of the effect are made.

Cross-Sections for electron-impact excitation of Ca I and Ca II

Abstract: 143 line emission cross-sections of Ca I and 15 line emission cross-sections of Ca II have been measured at an electron energy of 30eV. The optical excitation functions (OEF) are recorded in a range of electron energy from 0 to 200eV: 59 OEF for Ca I and 5 OEF for Ca II. The investigated lines are located in a spectrum region 227-745 nm. For the cross-sections of 5 singlet series and 10 triplet ones of Ca I, consideration has been given to series relationships that are characterized by the power relationship between the cross-section value and the principal quantum number of the upper level. Deviations from the power relationship take place in the perturbed Ca I series. The perturbations manifest themselves also by changing the OEF shape.

Closed shells at drip-line nuclei

Abstract: The shell structure of magic nuclei far from stability is discussed in terms of the self-consistent spherical Hartree-Fock-Bogoliubov theory. In particular, the sensitivity of the shell-gap sizes and the two-neutron separation energies to the choice of particle-hole and particle-particle components of the effective interaction is investigated.

QCD results from the study of hadronic Z-decays

Abstract: The high statistics data collected from hadronic Z-decays have lead to remarkable progress in the understanding of the dynamics of strong interactions. In addition to precision measurements of the strong coupling constant, the colour factors defining the gauge structure of QCD were determined with large accuracy. They are found to agree with the expectation for an unbroken SU(3) symmetry, thus establishing unambiguously the non-abelian nature of QCD. Higher order effects in many cases are known to next-to-leading logarithmic precision. Coherence effects as predicted in the framework of the modified leading-log approximation and local parton-hadron duality are experimentally well established. Studies of the hadronization process favour a chain-like production mechanism which bridges the gap between perturbative QCD as described by coherent parton shower models and the final state hadrons.

New Skyrme effective forces for supernovae and neutron rich nuclei

Abstract: New Skyrme-like effective interactions are proposed suitable for neutron stars, supernovae and neutron-rich nuclei. The parameters of the force are adjusted to the properties of the symmetric infinite nuclear matter, with an additional constraint on the low and high density neutron matter equation of state. Preliminary Hartree-Fock plus BCS calculations along series of isotopes are shown. They seem to solve the problem of large discrepancies observed with other parametrizations.

Trapped metal cluster ions

Abstract: An overview is given of experiments with stored metal cluster ions in a Penning trap system. The setup allows axial injection of clusters produced in an external source and a time-of-flight mass analysis of the reaction products after axial ejection. The system's options include the selection of stored ions, the manipulation of their orbits, addition of reactant and buffer gases and axial optical access for laser spectroscopic studies. As described by various examples, investigations have been made with respect to the development of trapping techniques and the characterization of metal clusters in terms of their physical and chemical properties.

Overview of the electron beam ion trap program at NIST

Abstract: This paper surveys the ongoing physics experiments at the Electron Beam Ion Trap (EBIT) facility at NIST, with particular attention paid to the underlying physical principles involved. In addition, some new data on the performance of our EBIT are presented, including results related to the determination of the trap width, ion temperature, and number of highly charged ions in the trap.

Accurate atomic mass measurements from Penning trap mass comparisons of individual ions

Abstract: We report measurements of mass ratios of 20 pairs of molecular ions with a single ion Penning trap mass spectrometer having an accuracy exceeding one part in 1010. The dominant source of error is random magnetic field fluctuations which cause a 2.6 × 10−10 rms scatter in measurements of the cyclotron frequency. Robust statistical analysis of the data ensures that nongaussian outliers are weighted less heavily in a smooth and consistent manner. Systematic errors are estimated to be 2 × 10−11 or below for doublet mass comparisons. The ratios form an overdetermined set, such that the atomic masses of nine isotopes can be derived from at least two independent groups of ion mass ratios, providing many consistency checks for systematic errors at the 10−10 level. At this level of precision, certain mass measurements have important implications in fundamental metrology. Results presented here are essential for defining a practical atomic standard of mass, for calibrating γ-ray wavelengths and for determining the molar Planck constant and the fine structure constant.

Transition probabilities for some spectral lines of singly ionised nitrogen

Abstract: Sophisticated configuration interaction wave functions have been used to obtain transition probabilities for transitions between the fine structure levels of the thirty-two lowest lying 2s22p2, 2s2p3, 2s22p3s, 2s22p3p, 2s22p3d, 2s22p4s, 2s22p4p states together with the 2s2p23s state of singly ionised nitrogen. The calculations were performed using the CIV3 code and small empirical adjustments were introduced to the diagonal Hamiltonian matrix elements in order to achieve accurate energy splittings between the levels and thereby the CI mixing coefficients. The present results are believed to be the most accurate available. Significant differences are found when the present lifetimes of the more highly excited states are compared with recent experimental data.

MBPT calculation of energy spectra and E1 transition probabilities for boron isoelectronic sequence

Abstract: The energy spectra and the electric dipole transitions in the boron isoelectronic sequence (Z = 8 - 26) between the levels of the 1s22s22p, 1s22s2p2 and 1s22p3 configurations were considered. The stationary second-order many-body perturbation theory (MBPT) was used to account for the electron correlations. The relativistic corrections were included in the Breit-Pauli approximation.

Transition probabilities of some Si II lines obtained by laser produced plasma emission

Abstract: The absolute transition probabilities for 28 Si II spectral lines have been determined by measurement of emission line intensities from laser-produced plasmas of Si in Ar and Kr atmospheres. The studied plasma has a temperature of about 2 × 104 K and 1017 cm−3 electron density. The local thermodynamic equilibrium conditions and plasma homogeneity have been checked. The results are compared with the available experimental and theoretical data and with present Hartree-Fock calculations in LS coupling.

Electron acceleration with kinetic Alfvén waves

Abstract: It has been studied the quantitative modification of the electron runaway flux due to a spectrum of kinetic Alfven waves (kAw), using the quasilinear (waves) and Fokker-Planck (collisions) equations. It is shown that these modes enhance the runaway production rate via their Cherenkov dissipation. The runaway flux for narrow and broad band spectrum for low and high phase velocities is calculated, and it is found as a general feature that its enhancement is the larger the weaker the background electric field, while for its absolute enhancement it is just the opposite. In some cases, the runaway production rate is found to be enhanced by many orders of magnitude over that without kinetic Alfven waves. It has also been discussed the consequences of this study for cosmic plasmas, namely: amplification of nonthermal emissions from stellar atmospheres, and planetary magnetospheres, (radio frequency waves, and x-ray), life extension of stellar radio bursts, enhancement of Langmuir turbulence, depletion of thermal population in extragalactic jets, and enhancement of auroral electrons precipitation.

On the motion of a charged particle in a magnetic field

Abstract: Alfven's results for the charged-particle drift motions in a non-uniform magnetic field have been generalized

Turbulence in the SOL of ASDEX and W7-AS

Abstract: Electrostatic fluctuations have been measured in the scrape-off layer of ASDEX and W7-AS by Langmuir probes and by observation of H? light and light emitted from a fast Li atom beam with high spatial and temporal resolution. It was demonstrated that these fluctuations contribute a significant, if not dominant, fraction of the anomalous radial particle transport. The basic properties of the fluctuations are the same in both experiments. A model for an instability mechanism specific to the SOL is presented including density, temperature and electric potential fluctuations. From this model mixing length estimates for the radial transport and resulting density and pressure gradients in the SOL are derived and compared to measurements in the mid-plane and in the divertor of ASDEX. In spite of several simplifications in the model a quantitative agreement up to factors of 1-3 and a qualitative agreement for variations of discharge parameters is achieved between the model predictions and the measurements. Data from poloidal multi-pin probe arrays are decomposed into a sum of spatial-temporal events by means of a fitting procedure. Centres of selected events serve as reference points for the analysis of the dynamics in their surroundings. Averaging shows that positive and negative potential events appear mostly in pairs with the E x B drift in between directed radially outward.

Precision mass measurements in the UW-PTMS and the electron's "atomic mass"

Abstract: The Penning trap mass spectrometer at the University of Washington is described along with the relevant detection mechanisms and the systematic shifts associated with the finite energies in the normal modes of the trapped particle. The cyclotron frequency for the particle-of-interest is compared with the corresponding frequency of a single trapped carbon ion (usually C6+). Upon correcting for lost electrons and their binding energies, the relative mass ratio then becomes the atomic mass for the particle-of-interest. As a recent example, the value of the electron's "atomic mass" has been measured to be Me = 0.000 548 579 911 7(17) u and the corresponding proton–electron mass ratio becomes mp/me = 1836.152 664 6 (58).

Spectroscopic investigations of the short wavelength x-ray spectra from X-pinch plasmas

Abstract: Investigations of X-pinch plasma structure and generation of an accelerated electron beam in the X-pinch using the methods of x-ray emission spectroscopy are discussed. The diagnostics allowed to make an investigation of the x-ray radiation with high spectral and spatial resolution over the spectral range 0.5 to more than 10 A from a variety of wire materials.

Reverse monte carlo simulation of amorphous Ni80P20 and Ni81B19

Abstract: The structure of the metallic glasses Ni80P20 and Ni81B19 was simulated by means of the Reverse Monte Carlo method. The starting configurations for the RMC runs were created by a static version of a Molecular Dynamics algorithm. As basis for the fitting procedure the partial pair correlation functions, as established by neutron diffraction - isotopic substitution experiments, were used, and almost perfect agreement was achieved. From the three-dimensional RMC clusters of 1500 atoms structural properties were deduced, which are not accessible from experimental pair correlation functions. A detailed analysis for the occurrence of distorted trigonal prisms was performed. The degree of trigonal prismatic ordering in the metallic glasses does not exceed substantially that in statistical reference clusters, where the Ni atoms are distributed randomly around the P or B atoms.