1. Emission Spectroscopy of Atmospheric-Pressure Ball Plasmoids: Higher Energy Reveals a Rich Chemistry
Scott E. Dubowsky, Amber N. Rose, Nick Glumac, Benjamin J. McCall
The 72nd International Symposium on Molecular Spectroscopy
University of Illinois at Urbana-Champaign
WE02: Spectroscopy as an Analytical Tool
Wednesday, 21 June

2. Outline Introduction and Motivation Ball plasmoid characteristics
Ball Lightning
Ball plasmoid characteristics
Adjusted parameters
Optical Emission Spectroscopy
Spectral Analysis and Fitting
Conclusions
Future Work
Tesla, N. Colorado Springs Notes: , Nolit, Beograd, Yugoslavia, 1978

3. Ball Lightning Sphere of light Difficult to observe in the field
Up to 1m diameter
Lasts many seconds
Difficult to observe in the field
One documented observation to date
Formation / Lifetime
Laboratory analogue: Ball plasmoid
Image courtesy of the Wikimedia Commons
Cen, J.; Yuan, P.; Xue, S. Phys. Rev. Lett. 2014, 112,
McNally, J. R. Preliminary Report on Ball Lightning, No. 3938; Oak Ridge National Laboratory: Oak Ridge, TN, 1966
Ter Haar, D. Phys. Scr. 1989, 39, 735
Endean, V.G. Nat. 1976, 273,
Wu, H. C. Sci. Rep. 2016, 6, 28263

4. Hardware

5. Discharge Characterization:
99 ms
105 ms
150 ms
200 ms
300 ms
10 cm
Pre-initiation
Switch closes
Current begins to flow (< cathode spot threshold)
Buildup
More current begins to flow
Plasma forms at electrode
Detachment
Plasma detaches from electrode and becomes plasmoid
Buoyancy
Detachment
Buildup
Switch
Closes
Pre- initiation

7. OES Experiment: Whole Plasmoid
Ocean Optics S2000: nm, 0.5 nm resolution
- better efficiency in the blue/violet/UV
Ocean Optics Jaz: nm, 0.7 nm resolution
- better efficiency in the red
- measurements made with optical fiber
Wavelength and intensity calibration with external sources
Internal trigger: continuously fire when signal across chip is > 2% dark spectrum
Spectrometer

8. OES Experiment: Height Resolved
Series of equal-sized holes oriented vertically
0-40 cm
“Field of view” of spectrometer (fiber) is larger than hole
Spectrometer

9. Spectral Assignment
Use NIST Atomic Spectral Database to assign atomic transitions
Use SPECAIR to “Auto-find” transitions
Extract appropriate molecular info from Exomol and HITRAN databases
Use literature values to get other spectroscopic constants not included in these databases
Generate molecular spectra with PGOPHER
Tennyson et al., J. Mol. Spec., 2016, 327,
Rothman et al. Journal of Quantitative Spectroscopy & Radiative Transfer, 2013, 130, 4
PGOPHER, a Program for Simulating Rotational Structure, C. M. Western, University of Bristol,

10. Example Spectra: Whole Plasmoid
Tungsten electrode, 7 kV

11. Example Spectra: Height Resolved
O I (5P→5S) Hβ
0 cm
(cathode level)
+10 cm OH

12. Example Spectra: Height Resolved
O I (5P→5S) Hβ
+20 cm OH
O I (5P→5S) Hβ
+40 cm

13. Electrode Composition

14. Molecular Fitting No emission 1 4540 ± 490 2 3680 ± 360 3 3700 ± 350 4#
Trot OH [K]
No emission 1
4540 ± 490 2
3680 ± 360 3
3700 ± 350 4
3670 ± 350 5
3790 ± 370

15. Temperature Fitting # Trot [K] 0407-18 Trot [K] 0407-19
No emission 1
7680 ± 900
5910 ±740 2
5820 ± 730
5730 ± 720
5570 ± 700 3
5130 ± 650
5270 ± 670
4870 ± 620 4
4630 ± 590
4110 ± 520
4070 ± 500 5
4040 ± 510

16. Blackbody Fitting
This is not exactly a blackbody curve, however, a Planck function can be fit to the spectra
Possible mechanisms of continuum emission:
Triatomics
Thermal radiation
Bremsstrahlung radiation

17. Summary
First time-resolved measurements of OH rotational temperature in ball plasmoids
High temperature early in discharge, decreases over time
First continuum fits of ball plasmoid emission spectra
Possible radiation processes:
Triatomic emission
Bremsstrahlung radiation
Thermal radiation

18. Future Directions Plasmoid X-band interferometer operating at ~9.2 GHz
X-Band horn antennas
Plasmoid
Amplifier
Function
Generator
“Magic” Tee
Frequency
Multipliers
Amplifiers
Splitter
(sum)
(difference)
Coax cable
Phase
Shifter
Attenuator
Amplifier
X-band interferometer operating at ~9.2 GHz
Measure phase shift and attenuation of beam induced by plasmoid
Calculate electron “column density” as a function of time
Deduce primary electron recombination mechanism?

19. Acknowledgements McCall Research Group Amber N. Rose
Prof. Nick Glumac – instrumentation and emission spectroscopy
Prof. J. Gary Eden – microwave interferometry