Gold charge state distributions in highly ionized, low-density beam plasmas

Abstract

We present a systematic study of Au charge state distributions (CSDs) from low density, nonlocal thermodynamic equilibrium plasmas created in the Livermore electron beam ion traps (EBIT-I and EBIT-II). X-ray emission from Ni-like to Kr-like Au ions has been recorded from monoenergetic electron beam plasmas having E beam = 2.66, 2.92, 3.53 and 4.54 keV, and the CSDs of the beam plasmas have been inferred by fitting the collisionally excited line transitions and radiative recombination emission features with synthetic spectra. We have modeled the beam plasmas using a collisional-radiative code with various treatments of the atomic structure for the complex M-and N-shell ions, and find that only models with extensive doubly excited states can properly account for the dielectronic recombination (DR) channels that control the CSDs. This finding would be unremarkable for plasmas with thermal electron distributions, where many such states are sampled, and the importance of DR is well established. But in an EBIT source, the beam is resonant with only a subset of such states that have spectator electrons in orbitals with high principal quantum number n (8 ≤ n ≤ 20). The inclusion in the model of such states was also necessary to obtain agreement with observed stabilizing transitions in the x-ray spectra.

Highly charged ions in magnetic fusion plasmas: research opportunities and diagnostic necessities

“…Progress also arose from the realization that dielectronic recombination must be adequately taken into account. The dominating role of dielectronic recombination in determining the ionization balance of gold ions was recently demonstrated by May et al, who showed that even weak resonances, which merely involved spectator electrons in very high principal quantum numbers with n = 14-17, made a huge difference even in such simple plasmas as those produced by an electron beam in an ion trap [177]. Although the type of dielectronic resonances in this case was highly restricted to a small subset of the multitude of resonances possible in a Maxwellian plasma, there were still too many resonances to perform a full ab initio calculation that included all of the levels with spectators in n = 14-17 for the near-nickellike ionization stages of gold that were formed in the plasma.…”

“…The measurements were performed on the electron beam ion trap at Livermore. [177]. The sensitivity of the ionization balance calculations to how the atomic processes involving autoionizing levels are treated means that so far no fully adequate calculation exists.…”

State-selective influence of the Breit interaction on the angular distribution of emitted photons following dielectronic recombination

“…Besides this fundamental interest on the relativistic details of electron-electron interaction, investigations of the DR are also mandatory for modeling hot astrophysical and laboratory plasmas, as DR often dominates the recombination rates due to its high resonance strength [19][20][21]. Resonances can thus be used for temperature and density plasma diagnostics [22][23][24][25].…”

Atomic physics and ionization balance of high-Z Ions: Critical ingredients for characterizing and understanding high-temperature plasmas

“…Electrons in a beam with a nominal energy of 3.6 keV and a 50 eV spread will find lots of resonance to excite. [Figure adaptedfrom[52]. ]…”