Based on 20 years of solar investigations, the extreme-ultraviolet (EUV) spectrum in the range 50-500 Â has proved to be extraordinarily rich in providing information on the density, temperature, and element abundances of the plasma in the Sun's chromosphere and corona, especially when observed with high spectral resolution. With the advent of new instruments using multilayer optics and CCD detectors, similar measurements with a spectral resolution of about 10,000 become possible on large numbers of nearby stars, in contrast with lower resolution instruments now scheduled for space flight. In this paper we provide a description of the lines from the most abundant ions in this spectral region that have special diagnostic significance. These lines are from plasmas in the temperature range from 10 5 to 10 7 K. We demonstrate their utility for determining temperature, density, and abundances. We also describe an objective grating spectrometer, based on the Naval Research Laboratory Skylab slitless spectrograph, for making celestial observations in the EUV spectral region with high spectral resolution.

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Narrow (4A, & 5 A), intense quasicontinuum bands, appearing in emission spectra of highly ion-0 ized rare-earth elements between 70 and 100 A, previously observed in tokamaks and laser-produced plasmas, have been obtained from a low-inductance vacuum spark. The bands shift toward shorter wavelengths with increasing atomic number Z. Using the unresolved transition array model, these bands are identified as primarily 4d 4f tra-nsitions in Rht to Rbtlike ions, although the widths come out too large and the mean wavelengths are much too dependent on ionization stages. Detailed ab initio computations show that the interactions between the 4p 4d '4f and 4p 41 +' configurations are responsible for the narrowing and the superposition of the transition arrays for the different ionization stages of a given element, in agreement with experimental data.

Relativistic calculations of the dielectronic recombination ͑DR͒ cross section and rate coefficient for Ne-like tungsten (W 64ϩ ) in the ground state 2s 2 2p 6 are performed. The DR contributions of the most important Na-like doubly excited configuration complexes, namely, (2s2p) 7 3lnЈlЈ (nЈϭ3 -13) and (2s2 p) 7 4l4lЈ, are calculated by level-by-level computations. Collisional transitions following the initial electron capture are neglected. The total DR cross section is presented in the form of 5-eV-wide peaks for the purpose of revealing the detailed atomic level structure, and in the form of 50-eV-wide peaks for the purpose of comparison with planned electron-beam experiments. The (2s2 p) 7 3l3lЈ complex is found to provide the dominant DR channel. The sum of the DR contributions of the (2s2 p) 7 3lnЈlЈ configurations with nЈϾ13, evaluated by applying the complex-by-complex nЈ Ϫ3 extrapolation method, is found to represent at most 3% of the total DR rate coefficient. The DR contribution of (2s2p) 7 3lnЈlЈ configurations with lЈϾ5 is found to be negligible ͑about 0.1%͒. The total DR rate coefficient is fitted to a convenient analytical expression which reproduces the original data with an accuracy of about 2% or better within a very wide electron temperature range.

Using high-resolution x ray and extreme ultraviolet (EUV) spectrometry, the line emission of W 28+-W 50+ ions was measured at the Berlin Electron Beam Ion Trap (EBIT). Our study encompasses a wide range of wavelengths (5-800 Å) and includes the observation of electric and magnetic dipole lines. The results of our measurements are compared with predicted transition wavelengths from ab initio atomic structure calculations.

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