摘要:

Cross sections for a variety of electron-ion collision phenomena are the backbone for understanding energy balance in high electron temperature plasmas. Such plasmas include such seemingly disparate objects such as the Io torus around Jupiter, solar and stellar atmospheres, the interstellar medium, and fusion devices. Several experimental approaches used with multiply-charged ions (MCIs) will be reviewed. These include measurement of excitation cross sections using the electron energy-loss method, measurement of ionic lifetimes using a Kingdon trap, and measurement of dissociative recombination cross sections using ion storage rings. New JPL results will be presented ofe-S2+inelastic scattering, relevant to the problem of ion density and radiated energy in the Io torus; and metastable-state lifetimes inC+,N+,andAr2+relevant to stellar absorption by the interstellar medium. ©1998 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.56569

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

Some recent measurements of excitation of multiply charged ions by electrons studied in beam-beam experiments are highlighted. The emphasis is on absolute total cross sections measured with the merged electron-ion beams energy-loss (MEIBEL) technique, although some results obtained with the crossed-beams fluorescence method are also presented. The MEIBEL technique allows the investigation of optically-allowed and forbidden transitions with sufficient energy resolution, typically about 0.2 eV, to resolve resonance structures in the cross sections. Results from the JILA/ORNL MEIBEL experiment on dipole-allowed transitions in several ions demonstrate the success of various theoretical methods in predicting cross sections in the absence of resonances. Comparisons of R-matrix calculations and measured cross sections for spin-forbidden transitions in Mg-likeSi2+andAr6+,however, show that further refinements to the theory are needed in order to more accurately predict cross sections involving significant contributions from dielectronic resonances and interactions between neighboring resonances. ©1998 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.56562

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

Quasars are among the most luminous objects in the universe, and the most distant for which we can obtain good spectroscopy. These spectra carry with them information about the formation of the first large structures in the universe and the first generations of star formation in newly born galaxies. The spectrum of a quasar has strong broad emission lines, formed by a non-equilibrium low-density photoionized gas. This spectrum can best be interpreted by reference to large-scale simulations of the full environment. This in turn is strongly coupled to many basic plasma physics questions. Here we discuss the current status of quasar research, numerical plasma simulations, and the atomic database. ©1998 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.56547

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

Many aspects of the Sun’s corona and wind are studied using data from the ultraviolet spectrum. Accurate atomic parameters are needed to interpret these data correctly, and a good understanding of the behaviors of atoms and ions in plasmas is essential to modeling the Sun’s atmosphere. Here I present two examples of studies being carried out using the Solar and Heliospheric Observatory (SOHO) extreme ultraviolet spectrographs. The first of these is the study of flows in the Sun’s chromosphere and corona. SOHO has provided new information concerning previous observations of the predominant down-flows in the Sun’s lower atmosphere. Accurate measurements of Doppler line shifts have been extended to the corona. It has also been found that the Doppler shifts vary over different parts of the Sun. The second study discussed involves the use of SOHO data to measure elemental abundances in coronal structures know as streamers, giving more information on the “FIP” effect—the observation that there is a relative deficit of elements with high first ionization potentials (FIPs) in the corona and solar wind. ©1998 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.56548

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

The physical conditions relating to the emission of x-rays from Jovian and cometary atmospheres and to supernova ejecta are briefly described. Emphasis is placed on elucidating the relevance and importance of atomic collision processes, the availability of data, and the outstanding data needs for modeling these environments. Some preliminary theoretical studies of electron capture for important collisions systems, involving molecular and atomic metal targets, are presented. ©1998 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.56563

出版商:AIP    

年代:1998

数据来源: AIP

ISSN:0094-243X    

DOI:10.1063/1.56570

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

The Quasimolecular Model using a Two Centre basis to describe the electronic emitting structure gives an alternative treatment of line broadening in dense and hot plasmas. Two codes are developed: IDEFIX for the radiative properties, QMSPECTRA (postprocessed to the first one) for the spectral line shapes. The observability of dense plasma effects (PPS, asymmetries and satellite features) in spectroscopic measurements is analysed within the proposed model and taking care of the eventual integrations over density gradients. ©1998 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.56571

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

The interaction of a high intensity laser pulse with a neutral gas or preformed plasma has been studied over a wide range of target and laser conditions. It was found that the propagation of 2ps laser pulses (&lgr;=1.054&mgr;m,P=5−10TW,I∼5×1014−1×1014−1×1018&hthinsp;Wcm−2)in neutral gases with atomic densities greater than 0.001 of critical was strongly influenced by ionisation induced refraction. Preformed density channels were effective in overcoming refraction but the channel length was found to be limited by ionization induced defocusing of the prepulse. ©1998 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.56549

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

The combination of accelerator technology and merged-beams techniques in storage ring experiments has opened up new experimental opportunities for electron-ion collision studies. In particular, charge changing electron-ion interactions at energies reaching from keV down to &mgr;eV with meV (and lower) electron beam temperatures have become accessible with ions of almost all charge states betweenH−andU92+.Examples of recombination and ionization measurements are presented and specific issues such as interference effects, high-resolution spectroscopy and rate enhancements in the presence of external fields are discussed. ©1998 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.56572

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

New results of computations of dielectronic recombination (DR) and excitation-autoionization (EA) are presented. The detailed calculations for these processes are performed using the HULLAC relativistic atomic code. Calculations of the DR rate coefficients have been performed for Ar-like Tungsten (W+56,ground3p6). Extrapolation methods for including captures onto very high autoionizing configurations are exposed. The EA calculations have been performed for ions along the GeI isoelectronic sequence (ground3d104s24p2). It is found that in this sequence the EA processes might enhance the direct ionization rates by a factor of up to 7. ©1998 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.56573

出版商:AIP    

年代:1998

数据来源: AIP