72 nd International Symposium on Molecular Spectroscopy June

72 nd International Symposium on Molecular Spectroscopy June 19 -23, 2017 - Champaign-Urbana, Illinois THE OHIO STATE UNIVERSITY Non-Equilibrium Thermodynamics Laboratory ELECTRIC FIELD MEASUREMENTS IN NS PULSE DISCHARGES BY 4 -WAVE MIXING Marien Simeni, Edmond Baratte*, Kraig Frederickson, and Igor V. Adamovich Department of Mechanical & Aerospace Engineering The Ohio State University *Ecole Centrale Paris

Motivation and Objectives • Time-resolved electric field in ns pulse discharges controls electron energy, energy partition over molecular modes, species generation, charge transport • Discharge energy partition and the range of excited species and radicals produced in the plasma are controlled by reduced electric field, E/N • Accurate, non-intrusive field measurements are needed • Knowledge of the electric field in high pressure nonequilibrium plasmas and flames is critical for quantitative insight into kinetics, predictive capability of plasma assisted-combustion models • Present approach: electric field measurements by ps 4 wave mixing (4) N 2 vibrational excitation (5, 6) electronic excitation, dissociation, (7) ionization · Energy partition in air plasma vs. reduced electric field (E/N)

ELECTRIC FIELD MEASUREMENTS: 4 -WAVE MIXING COMPARISON WITH “CONVENTIONAL” CARS • Measurements of absolute value of E field • Pump, CARS and IR intensities need to be measured λ= 4. 3 μm (N 2)

PS 4 -WAVE MIXING EXPERIMENTAL APPARATUS • Use of stimulated Raman cell instead of a dye laser • Raman cell filled with 15 bar N 2 -He mixture • CARS, IR and Pump signals are measured

ELECTRIC DISCHARGE AND FLAME SETUP d = 1 mm Laser beam • Plane stainless steel electrodes in quartz sleeves • Bunsen burner: 2 slm hydrogen diffusion flame h = 2 cm • Flame temperature T=1300 ± 50 K • Ns pulse discharge (peak voltage 20 k. V, pulse duration ~10 ns) operated at 10 Hz Top view 5

ELECTRIC DISCHARGE AND FLAME SETUP Discharge in air, single-shot ICCD image, top view, 100 ns gate Side view of the flame Top view of the flame, d = 1 mm Discharge in flame, single-shot ICCD image, top view, 100 ns gate 6

COLLAGE OF 1 NS GATE PLASMA IMAGES IN FLAME -1 ns 0. 5 ns • Plasma Images in the flame • 1 ns gate, 50 accumulations 1. 5 ns 2. 5 ns 3. 5 ns 4. 5 ns 5, 5 ns 6, 5 ns • Breakdown occurs at t=0 • Illustrates discharge development • Emission decays after 6. 5 ns 7

4 -WAVE MIXING IR SIGNALS IN AIR AND HYDROGEN FLAME IR signal in air at breakdown IR signal in the flame is much lower than in air: • High temperature: lower number density • N 2 mole fraction in combustion product mixture is lower • Significant spread of N 2 molecules over rotational levels 8

4 -WAVE MIXING ELECTRIC FIELD MEASUREMENTS IN AIR AND IN HYDROGEN FLAME Time resolved electric field in air Time resolved electric field in hydrogen diffusion flame • Electric field in ns pulse in room air peaks at 120 k. V/cm • Electric field in ns pulse in the flame peaks at 70 k. V/cm • Detection limit in air ~5 k. V/cm, in the flame ~ 40 k. V/cm 9

CARS MEASUREMENTS Flame, dye laser Air, Raman cell CARS spectrum of the flame CARS spectra of room air and flame • Flame temperature measured by broadband CARS • Spreading of signal over N 2 rotational levels (~ 30 -50 cm-1) • T= 1300 ± 50 K • 4 -wave mixing bandwidth only ~ 5 cm-1 10

SUMMARY • Electric field in ns pulse discharges in ambient air and hydrogen diffusion flame is measured by ps 4 -wave mixing, using N 2 as test species • Breakdown field in air, 120 k. V/cm, considerably exceeds DC breakdown threshold, approximately 30 k. V/cm, due to short voltage rise time (~ 5 ns) • After breakdown in air, field decreases to ~15 k. V/cm over 3 ns, due to charge separation, plasma self-shielding • Sensitivity limit of the diagnostics in atmospheric pressure air is 3 -4 k. V/cm • In the flame (T = 1300 K), peak electric field is ~70 k. V/cm; sensitivity limit is ~30 k. V/cm • 4 -wave mixing signal in the flame is very weak, due to lower density, lower N 2 fraction in combustion product mixture, and wider spread of N 2 over rotational levels • Measurements of lower electric fields in flames would require the use of a broadband picosecond dye laser 11

ACKNOWLEDGMENTS DOE PSAAP-2 Center “Exascale Simulation of Plasma-Coupled Combustion” NSF “Fundamental Studies of Accelerated Low Temperature Combustion Kinetics by Nonequilibrium Plasmas” 12