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

##### Date

2009-07-16##### Author

Munoz Burgos, Jorge

##### Type of Degree

dissertation##### Department

Physics##### Metadata

Show full item record##### 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.

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