Atomic Data Generation and Collisional Radiative Modeling of Ar II, Ar III, and Ne I for Laboratory and Astrophysical Plasmas

Abstract

Accurate knowledge of atomic processes plays a key role in modeling the emission in laboratory as well as in astrophysical plasmas. These processes are included in a collisional-radiative model and the results are compared with experimental measurements for Ar and Ne ions from the ASTRAL (Auburn Steady sTate Research fAciLity) experiment. The accuracy of our model depends upon the quality of the atomic data we use. Atomic data for near neutral systems present a challenge due to the low accuracy of perturbative methods for these systems. In order to improve our model we rely on non-perturbative methods such as R-Matrix and RMPS (R-Matrix with Pseudo-States) to include correlation in the collision cross-sections. These methods are computationally demanding, requiring supercomputing resources, and producing very accurate atomic collision data. For Ar+ and Ne, R-Matrix data was already available, however for Ar2+ we had to set up new R-Matrix calculations. To set up a new calculation we require good quality atomic structure. A new code (LAMDA) was developed to optimize the atomic structure for different ions in AUTOSTRUCTURE. The AUTOSTRUCTURE code was used and optimized by systematically adjusting the orbital scale factors with the help of a Singular Value Decomposition algorithm.

We then tested the quality of our newly optimized atomic structure by comparing the level or term energies, and line strengths from our optimized structure with those given by NIST. In the case of Ar+ we compared R-Matrix electron-impact excitation data against the results from a new RMPS calculation. The aim was to assess the effects of continuum-coupling effects on the atomic data and the resulting spectrum. We do our spectral modeling using the ADAS suite of codes. Our collisional-radiative formalism assumes that the excited levels are in quasi-static equilibrium with the ground and metastable populations. In our model we allow for Ne and Te variation along the line of sight by fitting our densities and temperature profiles with those measured within the experiment. The best results so far have been obtained by the fitting of the experimental temperature and density profiles with Gaussian and polynomial distribution functions. The line of sight effects were found to have a significant effect on the emission modeling. The relative emission rates were measured in the ASTRAL helicon plasma source. A spectrometer which features a 0.33 m Criss-Cross Scanning monochromator and a CCD camera is used for this study. ASTRAL produces bright intense Ar and Ne plasmas with ne = 1011 to 1013 cm−3 and Te = 2 to 10 eV. A series of 7 large coils produce an axial magnetic field up to 1.3 kGauss. A fractional helix antenna is used to introduce RF power up to 2 kWatt. Two RF compensated Langmuir probes are used to measure Te and Ne. In a series of experiment Ar II, Ar III, and Ne transitions are monitored as a function of Te, while Ne is kept nearly constant. Observations revealed that Te is by far the most significant parameter affecting the emission rate coefficients, thus confirming our predictions. The spectroscopy measurements are compared with those from our spectral modeling which in turn help us to compare the effectiveness of the new atomic data calculations with those from other calculations. It also shows some differences between the R-Matrix and the RMPS data due to continuum coupling effects for Ar II, and Ne. We believe that this is the first experimental observation of continuum-coupling effects.

We performed a new R-Matrix calculation for Ar2+. Emission from Ar2+ is seen in planetary nebulae, in H II regions, and from laboratory plasmas. Our calculation improved upon existing electron-impact excitation data for the 3p4 configuration of Ar2+ and calculated new data for the excited levels. Electron-impact excitation collision strengths were calculated using the R-Matrix intermediate-coupling (IC) frame-transformation method and the R-Matrix Breit-Pauli method. Excitation cross-sections are calculated between all levels of the configurations 3s2 3p4, 3s 3p5, 3p6, 3p5 3d, and 3s2 3p3 nl (3d ≤ nl ≤ 5s). Maxwellian effective collision strengths are generated from the collision strength data. Good agreement is found in the collision strengths calculated using the two R-Matrix methods. The effects of the new data on line ratio diagnostics were studied. The collision strengths are compared with literature values for transitions within the 3s2 3p4 configuration. The new data has a small effect on Te values obtained from the I(λ7135°A +λ7751°A)/I(λ5192°A) line ratio, and a larger effect on the Ne values obtained from the I(λ7135°A)/I(λ9μm) line ratio. The final effective collision strength data is archived online. Neon as well as Argon is a species of current interest in fusion TOKAMAK studies. It is used for radiative cooling of the divertor region and for disruption mitigation. It could also be useful as a spectral diagnostic if better atomic data were available. We present results from modeling emission line intensity for neutral neon by using Plane Wave Born, R-Matrix, and RMPS electronimpact excitation calculations. We benchmark our theoretical calculations against cross-section measurements, then against spectral measurements from ASTRAL.