MEASUREMENTS OF Fe XXIV LINE EMISSION : 3 ] 2 TRANSITIONS NEAR EXCITATION THRESHOLD

Using the Electron Beam Ion Trap facility at Lawrence Livermore National Laboratory, we have measured relative cross sections for Fe XXIV line emission at electron energies between 0.7 and 3.0 keV. The measurements include line formation by direct electron impact excitation (DE), radiative cascades, resonant excitation (RE), and dielectronic recombination (DR) satellites with captured electrons in n o 5 levels. Good agreement with R-matrix and distorted wave calculations is found. In collisionally ionized plasmas, at temperatures near where the ion abundance peaks keV), the RE contributions are (kT e D 1.7 found to be of the line emission, while the DR satellites contribute While good agreement [5% [10%. with state-of-the-art atomic physics calculations is found, there is less good agreement with existing spectral synthesis codes in common astrophysical use. For the Fe XXIV and 3p3@2 ] 2s1@2, 3p1@2 ] 2s1@2, transitions, the synthesis code MEKAL underestimates the emissivity in coronal equi3d5@2 ] 2p3@2 librium by D20% at temperatures near where the ion abundance peaks. In situations where the ionization balance is not solely determined by the electron temperature, RE and DR satellites may contribute a considerable fraction of the line emission. Subject headings : atomic data È atomic processes È line : formation È methods : laboratory È X-rays : general