1. "Point-to-point nanosecond pulse "diffuse filament" discharges for studies of energy transfer and nonequilibrium chemical reaction mechanisms in molecular plasmas D. Burnette, S. Bowman, K. Frederickson, B. Goldberg, S. Lanier, A. Montello, M. Nishihara, A. Roettgen, Z. Yin, I.V. Adamovich, and W.R. Lempert

2. Objective / Outline Main thrust: isolating molecular energy transfer processes and reaction pathways in nonequilibrium, high-pressure, highly transient molecular plasmas Previous results in plane-to-plane nsec pulse discharges Point-to-point nsec pulse discharges to achieve high specific energy loadings Vibrational level populations and temperature measurements by psec CARS: results for nitrogen and air CARS and spontaneous Raman spectroscopy results analysis, comparison with kinetic modeling: do we understand kinetics mechanisms involved? Overview of absolute species concentrations measurements in reacting air plasmas: NO LIF, O and N atom TALIF Path to measuring both reactant and product species in vibrationally enhanced plasma chemical reactions Electric field measurements of in nsec pulse discharges by 4-wave mixing Work in progress: development of electron density measurements by Thomson scattering

3. Previous results: repetitive nanosecond pulse discharge in plane-to-plane geometry (6.3 cm x 2.2 xm x 1 cm, ~25 kV, ~10 nsec pulses, pulse rep rate 10 kHz) Side view (0.5 nsec gate) Pulse #1 Pulse #10Pulse #100 T 0 =300 K P=60 torr T 0 =300 K P=120 torr Air T 0 =500 K P=200 torr End view (50 nsec gate) Air, T 0 =300 K, P=50 torr, Pulse #10 Preheating enhances diffusion and thermal conductivity, greatly improves plasma uniformity Instability onset is not controlled by reduced electric field, E/N

4. RHS terms represent vibrational quantum state change by the following processes: El. Imp.: inelastic electron impact processes by free electrons VT: vibration-to-translation/rotation energy relaxation VV: vibration-to-vibration energy exchange VE: electronic-vibration energy transfer during collisional quenching V-Chem: vibrational – chemistry coupling for vibrationally enhanced reactions such as N 2 (v) + O → NO + N, O 2 (v) + N → NO + O Rotational and translational modes are in equilibrium at a gas kinetic temperature Single vibrational quantum change processes dominate at low temperatures involved Significant body of theory and experimental validation data for the rates used Master equation coupled to Boltzmann equation for electron energy distribution, species concentrations equations, and quasi-1-D compressible flow equation Nonequilibrium air plasma chemistry, excited electronic states kinetics are included 0-D, with correction for diffusion E/N, n e waveforms predicted by separate nsec pulse, plane-to-plane discharge model Kinetic modeling: coupled master equation / Boltzmann equation model of nonequilibrium air plasma

5. Excitation of air by nsec pulse discharge burst: Psec CARS measurements, master equation modeling Dry air, P=100 torr, T 0 =300 K, burst of 100-150 pulses, repetition rate 10 kHz T rot, T v (N 2 ) measurements by psec CARS ~40% of coupled energy is loaded into N 2 vibrational mode Model predicts N 2 (X,v) populations, electron impact / vibrationally stimulated reaction products Vibrational relaxation mainly by O atoms, ozone Specific energy loading fairly low, ~1 meV/molecule/pulse Temperature rise ~ 1 K/pulse, T v (N 2 ) remains fairly low

6. New Test Bed: Diffuse Filament Nsec Pulse Discharge between two bare metal spherical electrodes Use of small (a few mm diameter), bare spherical electrodes increases power loading (up to ~1 eV/molecule/pulse at P=100 Torr, coupled pulse energy up to ~10-20 mJ) AND creates plasma large enough to be easily probed by CARS, spontaneous Raman spectroscopy, and two-photon absorption LIF (TALIF) 10 mm 2 mm 4 mm Air, single-pulse nsec pulse discharge: 100 ns gate, during the pulse (left) and 1 μsec after the pulse (right)

7. Air, single-pulse nsec pulse discharge in point-to-point geometry: schlieren images and waveforms Compression waves formed by “rapid” heating, on sub- acoustic time scale From known initial temperature & pressure, voltage, current, and filament diameter → Reduced electric field (E/N) and electron density (n e ) for kinetic modeling

8. Psec BOX-CARS using broadband dye mixture: good spatial and time resolution, access to multiple vibrational levels Interrogation volume CARS signal beam Folded Box-CARS 95% of signal generated over ~0.5 mm Ekspla Nd:YAG laser - ~150 psec pulses, 125 mJ per pulse max @ 532 nm Modeless Psec Dye Laser -Broadband ~592-610 nm FWHM, ~7-10% conversion -Broadband Pyrromethene Dye Mixture (*S. Tedder, et al, 2011) Spectral Resolution ~0.4 cm -1

9. t = 200 nsec (zoomed) v=0-9 are detected Typical psec CARS Spectra, 100 torr N 2 (Normalized to v=0, corrected for dye laser spectral profile) v=0 ↓ v=3 ↓ v=6 ↓ v=9 ↓ 100 laser “shot” averaged spectra vs. time after rising edge of current pulse Vibrational level populations inference: least squares fitting to Voigt line shape t = 200 nsec

10. t = 100 μsec t = 200 nsec Direct evidence of additional N 2 vibrational excitation after discharge pulse Square root of integrated band CARS signal proportional to difference in vibrational level populations, n v –n v+1

11. N 2 “first level” vibrational temperature distribution 100 μs after the pulse Spatial resolution: “1 st level” T v (N 2 ) measurements, 100 torr N 2 at a lower pulse energy (5 mJ/pulse) 10 mm 2 mm N 2, single-pulse nsec pulse discharge in point-to-point geometry, 100 ns gate

12. Rotational Temperature Measurements 100-shot accumulation spectrum in “cold” 100 torr air, with Sandia CARSFIT best fit synthetic spectrum. Histogram plot from measuring and fitting 80 such spectra. 95% confidence interval ~ ± 9 K.

13. CARS results summary: time-resolved N 2 vibrational populations in nitrogen Significant vibrational excitation after the pulse (~100 nsec long), followed by eventual relaxation Energy appears to come to N 2 (v=1-8) from an “internal storage”, not from electron impact during the discharge Increase in both “first level” N 2 vibrational temperature, T v01, and total number of quanta per molecule, Q

14. CARS results summary: time-resolved N 2 vibrational populations in air Significant vibrational excitation after the pulse (~100 nsec long), followed by eventual relaxation Energy appears to come to N 2 (v=1-8) from an “internal storage”, not from electron impact during the discharge Increase in both “first level” N 2 vibrational temperature, T v01, and total number of quanta per molecule, Q

15. Air, experiment vs. model: number of vibrational quanta per molecule Average number of vibrational quanta per molecule (N quanta ): Significant increase of number of vib. quanta per molecule ~1-10 μs after the pulse (by ~60%) At variance with the model, which predicts N quanta =const after the pulse (V-V exchange conserves quanta)

16. (1)Initial appearance and growth of all vibrational levels observed (Δt ~ 100 nsec – 1 μsec) (2)Steady growth of vibrational levels detected (Δt ~ 1 μsec – 100 μsec) (3)Vibrational energy decay: V-T relaxation (by O atoms) and diffusion (Δt ~ 100 μsec – 10 msec) Air, experiment vs. model: N 2 (X,v) vibrational level populations The VDF evolution can be divided into 3 phases: As time evolves, v=0, 1 level populations are well predicted by model; higher level populations are significantly underestimated. T v01 (N 2 ) is not a good metric.

17. Average number of vibrational quanta per molecule (N quanta ): Significant increase of number of vib. quanta per molecule ~1-10 μs after the pulse (by more than a factor of 2) At variance with the model, which predicts N quanta =const after the pulse (V-V exchange conserves quanta) N 2, experiment vs. model: number of vibrational quanta per molecule

18. N 2, experiment vs. model: N 2 (X,v) vibrational level populations

19. T v01 (N 2 ) rise is primarily due to N 2 –N 2 V-V exchange during relaxation: v=0, w → v=1, w-1 N 2, experiment vs. model: T v01 (N 2 ) and T

20. Both in N 2 and air, model overpredicts “rapid” heating, likely N 2 (A,B,C,a) + M → N 2 (X,v) + M (E-V processes) In air, also model underpredicts “slow” heating (absent in N 2 ), likely V-T relaxation by O: N 2 (X,v) + O → N 2 (X,v-1) + O and O atom recombination: O + O + M → O 2 + M CARS results summary: time-resolved rotational/ translational temperature in nitrogen vs. air

21. Where is the energy coming from? Effect of possible electronic-to-vibrational energy coupling in N 2 30% energy into N 2 (X,v) during N 2 *( A,B,C,a) quenching, e.g. α – adjustable parameter controlling energy into vibrational mode, Better agreement of N 2 vibrational and rotational temperature with the data

22. N 2 (v=1-8) rise at t ~ 0.1-1 μsec overpredicted, but better agreement at quasi steady state Where is the energy coming from? Effect of possible electronic-to-vibrational energy coupling in N 2

23. 5 μs delay Spontaneous Raman spectroscopy: consistent with psec CARS results (nitrogen, N 2 (v=0-12), P=100 torr) Signal collection region (2.75 mm) Integrated band Raman signal proportional to vibrational level population, n v More vibrational levels detected Effect of vibrational quanta rise after discharge pulse observed again

24. Spontaneous Raman results in N 2 vs. “baseline” kinetic model predictions Again, N 2 (v=2-5) rise at t ~ 1-10 μsec is not reproduced by the model Model clearly missing electron impact excitation processes of N 2 (v>8) Can electron impact excitation processes, N 2 (v) + e → N 2 (w) + e (v≥0, w>8), be the key?

25. Spontaneous Raman results in N 2 vs. modified model predictions (30% of energy defect to N 2 (X,v) during E-V transfer) Better agreement with data at t ~ 1 μsec – 1 msec

26. Previous work: NO LIF Previous and current work: calibrated N and O TALIF N 2, 124 torr, point-to-point, ~60 meV/molecule Air, 40 torr, plane-to-plane, ~6 meV/molecule (21 pulses) Air, 60 torr, plane-to-plane, ~0.3 meV/molecule O, NO O N [NO] prediction including reaction N 2 (X,v≥12) + O( 3 P)  NO + N Psec CARS (spontaneous Raman), NO LIF, and calibrated TALIF diagnostics: Reactants, products, and temperature can be measured for the same experimental conditions, state-specific reactant rates can be inferred.

27. Hydrogen, P=100 torr, L=10 mm gap Line: high voltage probe Symbols: CARS Field in the range of the 1-10 kV/cm has been measured (averaging over 128 discharge pulses) Sub-nsec resolution electric field measurement in hydrogen by CARS-like 4-wave mixing

28. Thomson Scattering for electron density measurements Triple Grating Spectrometer Schematic*,** Mask ↓ * Patterned after Y. Noguchi, et al., Jpn. J. Appl. Phys 40, 2001 ** Acknowledgement: U. Czarnetzki, Ruhr-University Bochum

29. Summary Time-resolved, spatially resolved T, T v (N 2 ), and N 2 (X,v=0-12) population measurements (psec CARS, spontaneous Raman) in high energy loading nsec pulse discharges in air and nitrogen Results suggest coupling between electronic and vibrational mode energies in N 2 Time-resolved, spatially resolved measurements of absolute species concentrations in reacting air plasmas: NO (LIF), O and N atoms (TALIF) Results demonstrate feasibility of measuring both reactant and product species in vibrationally enhanced plasma chemical reactions, N 2 (X,v≥12) + O( 3 P)  NO + N, inference of state-specific reaction rates Electric field measurements (4-wave mixing / CARS) in nsec pulse discharges in H 2 Good progress on Thomson scattering diagnostics for electron density measurements Comprehensive set of optical diagnostics for characterization of nsec pulse discharges in molecular gases