Abstract
The detection of a gravitational-wave signal and subsequent electromagnetic transient from a neutron star merger in 2017 is consistent with expectations of neutron star mergers as an
r
...-process element production site. Within the first few days post-merger, the kilonova spectra are consistent with a blackbody illuminating a mix of heavy,
r
-process elements. With increasing time, the kilonova transitions to the non-LTE regime where the level populations and ionization balance are determined by both collisional and photoprocesses. Detailed cross section data for electron-impact processes involving the relevant species are often not available. In such circumstances, it is reasonable to use approximate methods as baseline data for use in spectral modeling, and it is useful to evaluate the accuracy of such methods against more sophisticated collision calculations when possible. We describe new calculations of the electron-impact excitation cross sections of Pt
i
–
iIi
using the DARC
R
-matrix codes. Using collisional-radiative models, we show that, at plasma conditions expected in kilonovae, the expressions of van Regemorter and Axelrod are insufficient for producing electron-impact excitation data for complex, heavy species such as the low charge states of Pt. Through comparisons with data generated with the relativistic distorted wave approach, as implemented in the Flexible Atomic Code, we show the distorted wave method produces cross section data that, when incorporated into spectral models, predicts strong spectral feature distributions similar in intensity to those from models built on data computed with the
R
-matrix approach for the considered ions and plasma conditions.
ABSTRACT
Neutron binary star mergers have long been proposed as sufficiently neutron rich environments that could support the synthesis of rapid neutron capture elements (r-process elements) such as ...gold. However, the literature reveals that beyond neutral and singly ionized systems, there is an incompleteness of atomic data for the remaining ion stages of importance for mergers. In this work, we report on relativistic atomic structure calculations for Au i–Au iii using the grasp0 codes. Comparisons to calculations using the Flexible Atomic Code suggest uncertainties on average of 9.2 per cent, 5.7 per cent, and 3.8 per cent for Au i–Au iii level energies. Agreement around ∼50 per cent is achieved between our computed A-values and those in the literature, where available. Using the grasp0 structure of Au i, we calculated electron-impact excitation rate coefficients and use a collisional-radiative model to explore the excitation dynamics and line ratio diagnostics possible in neutron star merger environments. We find that proper accounting of metastable populations is critical for extracting useful information from ultraviolet–visible line ratio diagnostics of Au i. As a test of our data, we applied our electron-impact data to study a gold hollow cathode spectrum in the literature and diagnosed the plasma conditions as Te = 3.1 ± 1.2 eV and $n_\textrm {e} = 2.7^{+1.3}_{-0.9}\times 10^{13}$ cm−3.
Long-lived metastable states in neutral tungsten are shown to potentially impact measurements of tungsten erosion from plasma facing components. Time-dependent collisional radiative modeling of ...neutral tungsten is used to analyze the role of these states in tungsten emission and ionization. The large number of non-quasistatic atomic states in neutral tungsten can take on the order of milliseconds to reach equilibrium, depending on plasma conditions, causing erosion measurements to be affected by the metastable populations. Previous measurements using the 400.88 nm tungsten emission line could be affected by these non-quasistatic metastable effects. Therefore, a scheme for measuring the relative metastable fractions is proposed through simultaneous observation of multiple ultraviolet spectral lines of neutral W. The accuracy of gross erosion measurements could potentially be increased by inclusion of these previously unconsidered metastable effects.
We report the first unambiguous observational evidence of Rydberg Enhanced Recombination (RER), a potentially important recombination mechanism that has hitherto been unexplored in low-temperature ...photoionized plasmas. RER shares similarities with dielectronic recombination, with the difference that the electron is captured into a highly excited state below the ionization threshold-rather than above the threshold-of the recombining ion. We predict transitions of carbon and oxygen ions that are formed via the RER process, and their relative strengths with collisional-radiative spectral models. Optical C ii RER features are detected in published high-resolution spectra of eight planetary nebulae, and a C iii transition has been found in the ultraviolet spectrum in a symbiotic star system. The relative intensities of these lines are consistent with their production by this recombination mechanism. Because RER has not previously been accounted for in photoionized plasmas, its inclusion in models can significantly impact the predicted ionization balance and hence abundance calculations of important astrophysical species. Calculations for C + suggest that the enhancement in the total recombination rate can amount to a factor of 2.2 at 8100 K, increasing to 7.5 at T = 3500 K. These results demonstrate the importance of including RER in models of photoionized astrophysical plasmas and in elemental abundance determinations.
Electron-impact single and double ionization of W Pindzol, M. S.; Loch, S. D.
The European physical journal. D, Atomic, molecular, and optical physics,
04/2019, Volume:
73, Issue:
4
Journal Article
Peer reviewed
Electron-impact single and double ionization cross sections for the W
+
atomic ion are calculated and compared with crossed-beams measurements. The single ionization cross sections include ...contributions from outer subshell direct ionization and excitation-autoionization. Agreement with crossed-beams measurements is improved when non-perturbative time-dependent close-coupling calculations are used instead of perturbative distorted-wave calculations for the direct ionization of the 6
s
and 5
d
outer subshells. The double ionization cross sections include contributions from outer subshell double direct ionization, inner subshell direct ionization, and excitation-autoionization. The double ionization cross sections are an order of magnitude smaller than the single ionization cross sections and in reasonable agreement with crossed-beams measurements.
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DOBA, EMUNI, FIS, FZAB, GEOZS, GIS, IJS, IMTLJ, IZUM, KILJ, KISLJ, MFDPS, NLZOH, NUK, OBVAL, OILJ, PILJ, PNG, SAZU, SBCE, SBJE, SBMB, SBNM, SIK, UILJ, UKNU, UL, UM, UPUK, VKSCE, ZAGLJ
Abstract
Fundamental mechanisms governing the erosion and prompt re-deposition of tungsten impurities in tokamak divertors are identified and analyzed to inform the lifetime of tungsten plasma-facing ...components in ITER and other future devices. Various experiments conducted at DIII-D to benchmark predictive models are presented, leveraging the DiMES removable sample exposure probe capability and the Metal Rings Campaign, in which toroidally symmetric rows of tungsten-coated tiles were installed in the DIII-D divertor. In tokamak divertors, the width of the electric sheath is of the order of the main ion Larmor radius, and a vast majority of sputtered tungsten impurities are typically ionized within the sheath. Therefore, W prompt redeposition is mainly governed by the ratio of the characteristic ionization mean-free path of neutral tungsten to the width of the sheath. In-situ monitoring of the prompt redeposition of tungsten impurities in divertors is demonstrated via the use of WII/WI line ratios and the ionizations/photon (S/XB) method in L-mode discharges. Even with this relatively limited set of emission measurements, net erosion measurements were found to be a consistent upper bound to an analytic scaling based on the ratio of the W ionization length,
λ
iz
,
and the width of the magnetic sheath rather than the ratio of
λ
iz
and the W
+
gyro-radius. In the far-scrape-off layer (SOL) of the ITER divertor, however, it is calculated that the measurement of photon emissions associated with the ionization of tungsten impurities up to
W
5
+
may be required. Finally, W deposition patterns on DiMES collector probes, interpreted via DIVIMP-WallDYN modelling, reveal the key roles of progressive W erosion/re-deposition staps and E × B drifts in regulating long-range high-Z material migration.
Abstract
Fe ii emission lines are observed from nearly all classes of astronomical objects over a wide spectral range from the infrared to the ultraviolet. To meaningfully interpret these lines, ...reliable atomic data are necessary. In the work presented here we focused on low-lying fine-structure transitions, within the ground term, due to electron impact. We provide effective collision strengths together with estimated uncertainties as functions of temperature of astrophysical importance (10−100 000 K). Due to the importance of fine-structure transitions within the ground term, the focus of this work is on obtaining accurate rate coefficients at the lower end of this temperature range, for applications in low-temperature environments such as the interstellar medium. We performed three different flavours of scattering calculations: (i) an intermediate coupling frame transformation (icft) R-matrix method, (ii) a Breit–Pauli R-matrix (bprm) method, and (iii) a Dirac Atomic R-matrix Code (darc). The icft and bprm calculations involved three different autostructure target models each. The darc calculation was based on a reliable 20 configuration, 6069 level atomic structure model. Good agreement was found with our bprm and darc collision results compared to previous R-matrix calculations. We present a set of recommended effective collision strengths for the low-lying forbidden transitions together with associated uncertainty estimates.
The recent detection of a neutron star merger by the LIGO collaboration has renewed interest in laboratory studies of r-process elements. Accurate modeling and interpretation of the electromagnetic ...transients following the mergers requires computationally expensive calculations of both the structure and opacity of all trans-iron elements. To date, the necessary atomic data to benchmark structure codes are incomplete or, in some cases, absent entirely. Within the available laboratory studies, the literature on Au i and Au ii provides incomplete reports of the emission lines and level structures. We present a new study of Au i and Au ii lines and levels by exposing a solid gold target to plasma in the Compact Toroidal Hybrid (CTH) experiment at Auburn University. A wavelength range from 187 to 800 nm was studied. In Au i, 86 lines are observed, 43 of which are unreported in the literature, and the energies of 18 levels and 16 of the 18 known levels are corroborated by a least-squares level energy optimization. In Au ii, 76 emission lines are observed, and 51 of the lines are unreported in the literature. For both Au i and Au ii, the new lines predominantly originate from the most energetic of the known levels, and over half of the new Au ii lines have wavelengths longer than 300 nm. For the estimated electron parameters of CTH plasmas at the gold target (ne ∼ 1012 cm−3, Te ∼ 10 eV), two-electron transitions are similar in intensity to LS-allowed one-electron transitions.
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