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 http://gallerypick.com/university-of-illinois-at-urbana-champaign-images/

Outline Introduction and Motivation Ball plasmoid characteristics Ball Lightning Ball plasmoid characteristics Adjusted parameters Optical Emission Spectroscopy Spectral Analysis and Fitting Conclusions Future Work http://drevol.com/en/blog/Nikola-Tesla-158-years-birthday-009 Tesla, N. Colorado Springs Notes: 1899-1900, Nolit, Beograd, Yugoslavia, 1978

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, 035001 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, 753-755 Wu, H. C. Sci. Rep. 2016, 6, 28263

Hardware

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

OES Experiment: Whole Plasmoid Ocean Optics S2000: 200-500 nm, 0.5 nm resolution - better efficiency in the blue/violet/UV Ocean Optics Jaz: 250-800 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

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

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 http://www.bristoldynamics.com/activity/laser-spectroscopy-and-pgopher/ https://www.nist.gov/ Tennyson et al., J. Mol. Spec., 2016, 327, 73-94. Rothman et al. Journal of Quantitative Spectroscopy & Radiative Transfer, 2013, 130, 4 http://www.spectralfit.com/ PGOPHER, a Program for Simulating Rotational Structure, C. M. Western, University of Bristol, http://pgopher.chm.bris.ac.uk

Example Spectra: Whole Plasmoid Tungsten electrode, 7 kV

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

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

Electrode Composition

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

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

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

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

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?

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