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.

The dynamics and stability of collapsing gas columns, generated by a fast capillary discharge setup, are studied for obtaining soft x-ray amplification in highly ionized ions. Electron temperature and density measurements at the peak of the compression stage are used for tuning the discharge parameters. Once the needed conditions were achieved, strong amplification of the 3s-3p transition in Ne-like Ar ions at 469 A is observed. A gain coefficient of >0.75 cm(-1) and a beam divergence of <5 mrad are measured along plasma columns of <150 microm diameter and up to 165 mm length.

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Ab initio calculations of the rate coefficients for dielectronic recombination (DR) of ten ions along the Ni t isoelectronic sequence in the ground state (Mo', Ag' +, Xe +, Pr ', Gd +, Dy +, Ta4s+, Aus' At +, and U ) through the Cu-like 3d94ln'l' (n' =4,5) inner-shell excited configurations were performed using the HULLAc code package. Resonant and nonresonant radiative stabilizing transitions and decays to autoionizing levels followed by radiative cascades are included. Collisional transitions following electron capture are neglected. Nonresonant stabilizing transitions are found to enhance the DR rates, and may even dominate the process at low electron temperature. The remarkable difference between the isoelectronic trend of the rate coefficients for DR through 3d 4l4l ' and through 3d 4l 5 l ' is emphasized, The trend of DR through 3d 4l4l' shows irregularities at relatively low temperature due to the progressive closing of DR channels as Z increases. Thus, the DR coefficients cannot be reproduced or interpolated by a simple analytical formula.Even for the smooth contributions of the 3d 4l5l' configurations, a simplified model using configuration averaging for autoionization and radiative decays instead of level-by-level detailed computations is found to overestimate the DR rates by a factor of up to 2. PACS number(s): 34.80.Kw