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.
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
The consequences of tungsten (W) melting on divertor lifetime and plasma operation are high priority issues for ITER. Sustained and controlled W-melting experiment has been achieved for the first time in WEST on a poloidal sharp leading edge of an actively cooled ITER-like plasma facing unit (PFU). A series of dedicated high power steady state plasma discharges were performed to reach the melting point of tungsten. The leading edge was exposed to a parallel heat flux of about 100 MW.m−2 for up to 5 s providing a melt phase of about 2 s without noticeable impact of melting on plasma operation (radiated power and tungsten impurity content remained stable at constant input power) and no melt ejection were observed. The surface temperature of the MB was monitored by a high spatial resolution (0.1 mm/pixel) infrared camera viewing the melt zone from the top of the machine. The melting discharge was repeated three times resulting in about 6 s accumulated melting duration leading to material displacement from three similar pools. Cumulated on the overall sustained melting periods, this leads to excavation depth of about 230 μm followed by a re-solidified tungsten bump of 200 μm in the JxB direction.
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