Session H02: Electron-atom and atom-atom collisions

Live

The problem of ‘runaway electrons’ in plasma kinetics modeling is under renewed scrutiny as efforts ramp up to model fusion plasma devices as the construction of ITER proceeds. The problem occurs when atomic impurities are injected into fusion plasmas to mitigate disruptions. The ionization of such impurities can occur through interaction with the bulk electron plasma component or from the relativistic runaway electrons. The angular distribution of the ejected electrons from such ionization events is not normally considered within coupled plasma simulation efforts and could lead to unphysical results of the bulk plasma properties and the electron energy distribution. To this end, we have calculated the triple differential cross sections for He, Ne, and Ar, using both the time-dependent close-coupling and distorted-wave approaches. We find that the largest cross sections occur when the scattered electron retains most of the available energy after ionization and is almost undeflected by the target. The angular distribution of the ejected electron, is strongly peaked at momentum transfer directions away from the scattered electron, in contrast with the isotropic distribution assumed in plasma simulations. We discuss our calculations and explore the consequences of these results.   

Live

Radiative charge transfer processes are of interest for astrophysical and for ultracold atom-ion collisional applications. More generally, radiative charge transfer rates represent lower limits to total charge transfer rates when other charge transfer mechanisms are unfavorable. Here, radiative charge transfer cross sections, emission spectra, and rate coefficients for collisions of an alpha particle and a hydrogen atom are reported. Applications of the data are discussed.   

Live

We report the most recent advances in the development of a novel source of spin-polarized electrons: the Rb Spin Filter[1]. Polarized electron beams are produced by driving an unpolarized beam of thermionically emitted electrons through a target cell containing a mixture of spin-polarized Rb vapor and a buffer gas- in this experiment either N2 or ethene. We have studied systematically the density of Rb which optimizes the transfer of the Rb valence electron spin to the unpolarized electron beam. The more Rb present in the system, the more likely a free electron is to collide with a polarized atom. On the other hand, more Rb in the system increases the effects of radiation trapping, reducing the overall polarization of the Rb[2]. We conducted these studies at buffer gas pressures in the sub-Torr regime. The experimental results are compared with Monte-Carlo simulations to better understand the dynamics of the system. [1]M. Pirbhai \textit{et. al.}, Phys. Rev. A \textbf{88}, 060701(R) (2013). [2]D. Tupa and L. W. Anderson, Phys. Rev. A \textbf{36}, 2142 (1987).   

Live

When a helium atom containing a $^6$He halo nucleus undergoes beta decay, the two atomic electrons become redistributed over all possible states of the daughter $^6$Li nucleus, including single- and double-electron emission (shake-off). The present study focuses on the probability for double electron emission to form Li$^{3+}$, where there is a substantial disagreement between theory [1] and experiment [2]. We use pseudospectral representations together with Stieltjes imaging to separate the ${\rm Li}^{3+} + 2e^-$ channel from the energetically overlapping ${\rm Li}^{2+} + e^-$ single ionization channel. We find that the formation of Li$^{3+}$ is strongly suppressed near threshold relative to Li$^{2+}$, thereby accounting for part of the disagreement with experiment. However, there still remains a substantial disagreement in the total probability. \newline [1] E. E. Schulhoff and G. W. F. Drake, Phys.\ Rev.\ A {\bf 92}, R050701 (2015). \newline [2] R. Hong et al., Phys.\ Rev.\ A {\bf96}, 053411 (2017).   

Live

The main objective of the present work is to investigate the vibrational excitation for Argon-Nitrogen mixed gaseous thermal plasma using Direct Simulation Monte Carlo (DSMC) simulations. Here, the harmonic oscillator model is applied to the vibrational mode with characteristic vibrational temperature $\theta_{v\thinspace }= $3371 \quad K corresponding to Nitrogen molecules, and vibrational excitation is integrated with the rotational excitation. It is assumed that there is only one vibrational mode associated with each diatomic Nitrogen molecule. The DSMC simulations are carried out for temperature dependent vibrational relaxation collision number $Z_{v} = (C_{1}/T^{y}$\textit{) exp (C}$_{2} T^{-1/3}) $with the constants $C_{1\thinspace }=$ 9.1 \quad and$_{\mathrm{\thinspace }}C_{2\thinspace }=$\textit{ 220, }and \quad for rotational relaxation collision number $Z_{r} =$\textit{ 7.5} for the Nitrogen molecule with viscosity temperature index $y =$\textit{ 0.75} (VHS model), $y =$\textit{ 1.0} (Maxwell model), and $y =$\textit{ 0.5} (HS model). An important finding is that the rotational mode \quad comes to the equilibrium value with the translational mode very quickly. However, the vibrational relaxation slows as the temperature decreases The DSMC simulation result ensures that the collision temperature dependent vibration rate is consistent with the principle of detailed balance. The vibrational distribution function sampled in the DSMC simulations is in excellent agreement (error within 2{\%}) with the Boltzmann distribution.   

On Demand

Collisions between lithium atoms and noble gas atoms occur in stellar atmospheres, as well as in buffer-gas filled spectroscopy cells in laser cooling experiments. These collisions broaden and shift the lithium absorption spectral features, and the Doppler-broadened spectra have been well characterized, especially at high noble gas pressures. Using saturated-absorption spectroscopy, we study the low-pressure shifts and broadening for lithium colliding with various noble gases in the regime most commonly employed in laser cooling experiments. These results can not only improve laboratory practice, but also can be used to constrain lithium-noble gas interatomic potentials.   

On Demand

Relativistic electrons generated in post-disruption tokamak discharges have the ability to cause significant damage to devices such as ITER. A primary disruption mitigation approach currently being considered for ITER is to inject large amounts of high-Z impurities, such as neon or argon. Interaction between relativistic electrons and high-Z impurities can modulate the electron distribution function and the plasma cooling rate, so it is crucial to understand the ion charge state distribution and radiative power loss. In order to generate greater understanding of these properties, we have extended upon the popular FLYCHK collisional-radiative model to accommodate relativistic effects of inelastic electron impact cross-sections. It is shown that significant differences in ion charge state and radiative losses are produced by including these relativistic effects, when compared to results produced without these effects using a thermal Maxwellian electron distribution only, as commonly done in the fusion community. Unique spectral signatures of relativistic electrons are analyzed, offering options for possible diagnostic methods in future. This work highlights the importance of accurate atomic data in improving predictive capabilities of these complex plasmas in fusion science.   

On Demand

Radiative double electron capture occurs when a highly charged projectile captures two electrons from a target atom and simultaneously emits a photon. RDEC can be considered the time inverse of double photoionization and hence can be used to study electron correlation. Previous results for RDEC have been reported for thin-foil carbon$^{\mathrm{1}}$ (by incident $\sim $2 MeV/u O$^{\mathrm{8+}}$ and F$^{\mathrm{9+}}$ ions) and gas$^{\mathrm{2}}$ (by $\sim $2 MeV/u F$^{\mathrm{9,8+}}$ ions) targets. Here, RDEC measurements for $\sim $2 MeV/u F$^{\mathrm{9,8+}}$ and O$^{\mathrm{8,7+}}$ ions in collisions with thin-foil carbon were conducted and compared with previous carbon and gas target results. Measurements for the one-electron projectiles F$^{\mathrm{8+}}$ and O$^{\mathrm{7+}}$ with the C-foil target avoid transfer of both electrons to the projectile K shell. Preliminary cross sections obtained from the present data for bare ions on carbon generally agree with previous bare ion results for the solid and gas targets. However, the new measurements show the interesting result that the cross sections for the one-electron projectiles on carbon are nearly the same as those for the bare ions, in significant contrast with the previous results for gas targets. *Supported in part by NSF $^{\mathrm{1}}$ A. Simon et al., \textit{PRL} \textbf{104}, 123001 (2010) $^{\mathrm{2}}$ P. N. S. Kumara et al., \textit{Nucl. Instrum. Methods Phys. Res.} B \textbf{408} 174-177(2017)   

Opacity calculations for low temperature Xe for lithography applications

Strong emission from plasmas of mid-Z elements, such as tin and xenon, in the 11-14 nm wavelength has long been acknowledged as a powerful source of EUV light with significant applications for lithography. We employ the Los Alamos suite of atomic physics codes and plasma kinetics modeling code ATOMIC to compute the LTE emissivity of Xe in plasma regimes of interest to EUV lithography applications. In opacity calculations, large numbers of configurations are necessary to ensure a converged partition function. Full configuration interaction (CI) calculations for many configurations quickly becomes computationally prohibitive. Instead, we use a model where full CI is utilized for the most important transitions, while intermediate-coupling is used for all other levels. We have performed investigations into the complex atomic structure of relevant ion states (from 5 times ionized to 20 times ionized) to model the low temperature (\textless 50 eV) opacity in Xe. We present opacities generated at temperatures and densities related to lithography applications. Our preliminary results indicate that our models are in good agreement with transmission measurements from laser-produced Xe plasmas.