Fundamental Mechanisms, Predictive Modeling,

and Novel Aerospace Applications

of Plasma Assisted Combustion

Walter Lempert, Igor Adamovich, J. William Rich, and Jeffrey Sutton

3rd MURI Annual Review Meeting

November 6-7, 2012

PAC MURI 3-year milestone:

Overview of goals and approaches

Approaches

•Experimental Platform I. Plane-to-plane, high repetition rate nsec pulse discharge: large-

volume, premixed plasma chemical fuel oxidation and ignition at near-0-D conditions.

•Experimental Platform II. 1-D low-pressure premixed flame: effect of nonequlibrium plasmas

on premixed combustion chemistry

•Experimental Platform III. Point-to-point, single-pulse nsec pulse discharge: kinetics of energy

transfer among excited species (electronic and vibrational) and radicals at high energy loadings

per molecule

•Kinetic modeling. Integrated model of electric discharge, plasma kinetics / chemistry, and

“conventional” hydrogen / hydrocarbon chemistry mechanism

Goals

•Obtain quantitative, time- and space-resolved data in well-characterized plasma assisted

combustion experiments: temperature, species number densities, vibrational state populations,

electric field, electron density

•Quantify the effect of plasma generated species – radicals and excited states – on fuel oxidation,

ignition, combustion, and flameholding

•Elucidate detailed kinetic mechanisms, develop predictive kinetic models of nonequilibrium

plasma assisted combustion processes, assess and validate the models

Task 1: Low-to-Moderate (T=300-800 K) temperature, spatial and time-

dependent radical species concentration and temperature

measurements in nanosecond pulse plasmas in a variety of fuel-air

mixtures pressures (P=0.1 - 5 atm), and equivalence ratios (φ~0.1-3.0)

Goal: Generate an extensive set of experimental data on radical species

concentrations and temperature rise; elucidate kinetic mechanisms of

low-temperature plasma chemical fuel oxidation and ignition using

kinetic modeling. Bridge the gap between room-temperature data

(low-pressure gas discharges) and high-temperature data (shock tubes)

Thrust 1. Experimental studies of nonequilibrium air-fuel

plasma kinetics using advanced non-intrusive diagnostics

Experimental Platform I: High-temperature nsec pulse

discharge cell for temperature and species measurements

• H2-air, C2H4-air, CH4-air, and C3H8-air at T0= 100-300 C, P=50-500 torr, ϕ=0.03-1.2

• Flow velocity u=40 cm/sec; residence time in the discharge ~0.1 sec

• Pulse repetition rate (20-25 kV peak, ~10-50 nsec), ν=10-40 kHz

• Ample optical access (LIF, TALIF, CARS) for species and temperature measurements

• OH LIF absolute calibration: adiabatic burner in Hencken burner, more recently Rayleigh scattering

• Discharge dimensions 1 cm x 2 cm x 6 cm

• Right angle quartz prism 6 cm long

provides optical access to discharge

Accurate measurements of instantaneous

discharge power and coupled pulse energy

• Custom, calibrated, high bandwidth (~1 GHz), low inductance voltage and current probes

• Noise cancellation: common mode voltage and current cancelled using two pairs of identical

probes, connected symmetrically

• Stray phase shift between voltage and current waveforms determined at “no-breakdown“

(high pressure) conditions

Air, 40torr

Pulse waveforms and coupled energy:

CPT and FID pulsers

CPT Pulser FID Pulser

Air, 40torr, pulse #50 in a 40 kHz, 200-pulse burst Air, 40torr, pulse #50 pulse in a 10 kHz, 125-pulse burst

reflected pulses

Analytic nsec discharge model: coupled energy proportional to # density if plasma is uniform (AIAA 2012-3093) Pulse energy remains constant during the burst until significant heating occurs

Plasma images: Air, T0=300 K, P=50 torr 25 kV, ~10 nsec pulses, ν=10 kHz, pulse #10 in burst, 0.5 nsec gate

200C, 104torr, Pulse #160

With discharge

Outline: 22 mm x 10 mm channel cross section

No discharge, illumination by a mercury lamp

1 nsec 2 nsec 3 nsec 4 nsec 5 nsec

6 nsec 7 nsec 9 nsec 14 nsec 24 nsec

Side view

End view

Diffuse, volume-filling plasma, no sign of instabilities

AIAA Paper 2009-0688: slow instability development over ~100 pulses (~10 msec),

indication of ionization – heating instability

Preheating enhances diffusion and thermal conductivity, greatly improves plasma uniformity

Instability onset is not controlled by reduced electric field, E/N

Plasma images: effect of pressure and temperature 25 kV, ~10 nsec pulses, ν=10 kHz, 50 nsec gate

Pulse #1 Pulse #10 Pulse #100

T0=300 K

P=60 torr

T0=300 K

P=120 torr

Air

T0=500 K

P=200 torr

Same

number

density

Plasma images at high pressures: H2-air, C2H4-air, CH4-air

25 kV, ~10 nsec pulses, 50 nsec gate

Diffuse, volume filling, mildly preheated plasmas at pressures up to hundreds of Torr

Used for quantitative measurements of temperature and radical species concentrations

H2 – air, ϕ=0.3 T0=300 K, P=100 torr

Pulse #1 Pulse #10 Pulse #100

C2H4 – air, ϕ=0.3 T0=300 K, P=100 torr

Pulse #10

Plasma / Laser Diagnostics Timing

DISCHARGE

Burst

T=100 msec

Time axis Q-Switching

Timing

T=100 msec

Δt Δt Δt

Synchronization Scheme

LIF pump, CARS pump / probe laser timing Δt:

From 2 μsec after last pulse in the burst (closest to the pulse without EMI noise

affecting data), to 5 msec after last pulse in the burst

Experimental conditions:

T0=500 K, P=100 torr, ν=10 kHz, ϕ=0.01-0.5, 50-150 pulses in a burst

Fuels: H2, C2H4, CH4, C3H8

Linear Regime:

Quantum Efficiency:

Boltzmann Fit:

Calibration:

JBOHcf EBfnVS 12

1110

001001 )/(

QAV

QAVAAJ

kT

BVnln

kT

E

)1J2(B

E/Sln cOHJ

J12

f

η: Optical Efficiency

Vc: Collection Volume

fB: Boltzmann Distribution

E: Laser energy fluence

Assuming same collection efficiency for

(0,0) and (1,1) bands

For temperature inference

C2H4-air Hencken burner flame with

equilibrium [OH] calculated by STANJAN

PACB

FlameB

PAC

Flame

Flame

PACFlameOHPACOH

f

f

S

Snn

,

,

,,

Excitation Transitions:

Least squares Voigt fitting to experimental spectra

Transition

Temperature P1(1) Q1(2) Q1(3) Q1(4) Q1(5)

[OH] Q1(3)

A2Σ+ ← X2Π (v'=1, v"=0)

Extract

Linear Fit to Integrated Spectra

Temperature Measurements A

Multi-Line OH LIF Thermometry

0.75 m

Spectrometer

Short-Pass

Filter

100mm Lens

Beam Dump

Dichroic

Mirrors

Camera

Relay Lens

Magnification

System

Test section

Temperature Measurements B

Psec USED-CARS

Nd:YAG

Broadband

Dye Laser

Pump / Probe

beam

In Stokes beam

CARS signal

Out

Phase-matching

ASk

Sk

Pk

Pk

• Ekspla Nd:YAG laser: 10 Hz, ~150 psec pulses, 125 mJ per pulse max @ 532 nm

• Modeless Psec Dye Laser (Pump / Probe): Broadband ~ 601-608 nm FWHM, ~ 6% conversion efficiency

2315 2320 2325 23300

50

100

150

200

250

Raman Shift [cm-1

]

Sq

rt(I

nt)

[a

u]

2315 2320 2325 2330

Raman Shift [cm-1

]

Data

Tfit

= 486K

Data

Tfit

= 310K

Typical psec CARS spectra

and best fit Trot in air and H2-air

100-shot accumulation spectrum

in 100 torr air, T0 = 500 K,

95% confidence interval ~15 K

2290 2300 2310 2320 2330 23400

5

10

15

20

25

30

Raman Shift [cm-1

]

Sq

rt[I

nt]

[a

.u.]

Experiment

Tfit

= 1250 K

100-shot accumulation spectrum

in 92 torr H2 –air during ignition, Tpeak= 1250 K

Also yields N2(v=1) level population, N2 vibrational temperature

Comparison of multi-line OH LIF and psec CARS

temperature measurement

Temperature after ν =10 kHz, 50-pulse burst, T0=500 K, P=100 torr

Temperature after the discharge burst:

CARS matches OH LIF thermometry

ΔT=50-80 K after 50-pulse discharge burst

Temperature remains nearly constant for up to 1 ms after discharge burst

Model slightly overpredicts experimental results

C2H4-air

ϕ=0.09 2 μs after burst

[OH], temperature OH LIF (Proc. Comb. Symp. 2013):

[OH] predicted by kinetic model agrees well with experiment

Measured (OH LIF) peak temperature ~200-300 K higher

than predicted

T0=500 K, P=80 torr, ϕ=0.4, v=10 kHz, 115 pulses

Psec CARS temperature measurements

during plasma assisted ignition of H2-air

Temperature measured by psec CARS:

Time-resolved temperature in excellent agreement with kinetic

model prediction from previous work (Proc. Comb. Symp. 2013)

Model overpredicts temperature by ~50 K at short time delays

after the burst (~1 msec)

CARS measurements agree with OH LIF at short time delays but

yield lower temperature during ignition

T0=500 K, P=92 torr, ϕ=0.4, v=10 kHz, 120 pulses

Difference Between CARS and OH LIF Measurements During Ignition

• Burst-to-burst ignition delay reproducibility over long time is not very good; OH LIF

thermometry requires long accumulation time, more susceptible to systematic error

• Prior to ignition, data reproducibility is good, OH LIF is in much better agreement

with CARS

When burst size reduced to 99 pulses,

ignition stops to occur

Threshold ignition temperature Tf~700 K

No overshoot in non-ignition cases,

temperature decays to baseline T0=500 K

Psec CARS N2 vibrational temperature measurements:

vibrational excitation is not a factor at these conditions

Tv(N2) remains low:

Air, T0=300 K: Tv=1200-1300 K for 100-150 pulses

Air, T0=500 K: Tv=850-1050 K for 50-100 pulses

H2-air: Tv~600-700 K , close to detection limit

• Dry air, P=100 torr, T0=300 K, burst of 40-150 pulses, repetition rate 10 kHz (10 pulses/msec)

• Trot, Tv(N2) measurements: psec CARS

• Model: coupled nsec pulse discharge / master equation / Boltzmann equation / air chemistry

to predict N2(v) populations, electron impact / chemical reaction products

• E/N after breakdown quite low: ~40% of coupled energy is loaded into N2 vibrational mode

• Vibrational relaxation mainly by O atoms, ozone

• Calibrate OH LIF signal from a Hencken flame using Rayleigh scattering, validate

calibration procedure using absolute [OH] measurements by absorption

• Use Rayleigh scattering and OH LIF from the discharge cell in the furnace (same

optical system for both data sets) for absolute [OH] measurements

Rayleigh scattering at 307.5 nm in N2

)PE(DS RayleighRayleigh T=2173±36 K

OH A-X (0,0) excitation 306.8-307.1 nm

11 main R-branch transitions

CH4-air flame

Hencken burner, OH LIF calibrated by Rayleigh scattering: NOH = 7.06±0.32·1015 cm-3

Measured by OH absorption: NOH = 7.70±0.63·1015 cm-3

Improved absolute [OH] calibration

using Rayleigh scattering* * Collaboration with Cam Carter (AFRL Propulsion Directorate)

Absolute OH measurements after a 50-pulse burst:

H2-air, CH4-air, T0=500 K, P=100 torr

• Very lean mixtures, ϕ=0.015-0.24

• H2-air: [OH] decays within ~ 1 msec, weak dependence of the equivalence ratio

• CH4-air: [OH] decays within ~ 0.1-1.0 msec, peak / decay time reduced as ϕ is increased

• No significant transient rise after the last pulse

T T

Absolute OH measurements after a 50-pulse burst:

C2H4-air, C3H8-air, T0=500 K, P=100 torr

• Very lean mixtures, ϕ=0.04-0.36

• [OH] decays within ~ 20-100 μsec, peak / decay time reduced significantly as ϕ is increased

• Significant transient rise after the last pulse

T T

High-pressure (~1 bar) PAC experiments underway:

use of liquid metal electrodes

Discharge test with salt water electrodes

(cells on top and bottom of the channel)

P=40 torr, air, ν=10 kHz

Discharge channel, electrode cells filled

with liquid metal (indium alloy)

New high-pressure discharge channel, with electrode cells and preheating coil shown, gap L=5 mm

• Electrodes are fully encapsulated in quartz cells

• No corona outside the channel, no damage of dielectric layers

• No discharge pulse energy reduction and / or “drifting”

Goal: Examine the effects of non-equilibrium plasma on radical species concentrations in a 1D

low-pressure flame/plasma chamber

FACILITY BURNER

CONFIGURATION

HVE

Plasma off Plasma ON

Plasma ON

DIRECTLY-COUPLED PLASMA/FLAME

“Belke” Pulser FID Pulser

TWO PLASMA DISCHARGE GENERATORS

McKenna burner; Low-pressure 1D flame (20-30 Torr)

Temperature and species vary only with distance above burner

Porous HV electrode (40 mm above burner) – no flow field disturbance (burner is grounded)

Peak voltage = 3 kV

Duration (FWHM) = 170 ns

Coupled energy < 1 mJ/pulse

Peak voltage = -14 kV

Duration (FWHM) = 7 ns

Coupled energy ~ 3 mJ/pulse

Primary pulser for reported results

Experimental Platform II: 1-D low-pressure premixed flame

coupled with nonequilibrium plasma

Spatially-Resolved OH LIF Measurements

First target species is OH

Plasma effects are the greatest in the “preheat” zone (closest to burner)

Planned temperature measurements will put relative LIF signals on an absolute scale

CxHy/O2/N2 Flames

CxHy/O2/Ar Flames

200-300%

increase

Time-Resolved OH LIF Measurements

HVE

Initial decay of “plasma-enhanced” OH is 20-30 ms

Mechanism underlying OH “oscillation” is currently unknown

Plasma off Plasma ON

Ongoing / near future work

• Place relative OH LIF measurements on absolute scale using temperature measurements and OH

absorption measurement calibration

• Compare OH measurements with 1D burner-stabilized flame model (ChemKin) using measured

temperature profiles with and without plasma – indirect assessment of thermal vs. kinetic contributions

• Assess OH generation under stable premixed combusting conditions as a function of

• Initiate O atom measurements in same series of premixed flames.

Task 8: Development and validation of a predictive kinetic model of non-

equilibrium plasma fuel oxidation and ignition, using

experimental results of Thrust 1

Goal: Identify key mechanisms, reaction, and rates of plasma chemical

fuel oxidation processes for a wide range of fuels, pressures,

temperatures, and equivalence ratios. This is absolutely essential

to predictive capability of the model.

Thrust 2. Kinetic model development and validation

Chemistry Model

Popov H2-O2 chemistry mechanism

GRI Mech 3.0, USC/Wang, or Konnov

mechanism for hydrocarbons

1 Calculated by the Boltzmann solver from the experimental cross sections 2 Sum of electronic excitation cross sections (b3Σ, b1Σ, c3Π, a3Σ, c1Π, and d3Π)

3 Three dissociation channels into C2H3 + H, C2H2 + H2, and C2H2 + H + H.

Dominant Radical Species Generation Processes in the Plasma

Air

H

yd

rog

en

Hy

dro

carb

on

s

Process Rate, cm3/s

A1 N2 + e- = N2(A3Σ, B3Π, C3Π, a'1Σ) + e- σ1

A2 N2 + e- = N(4S) + N(4S) + e- σ

A3 O2 + e- = O(3P) + O(3P,1D) + e- σ

A4 N2(C3Π) + O2 = N2 (B

3Π ) + O2 3.0∙10-10

A5 N2(a'1Σ) + O2 = N2 (B3Π) + O2 2.8∙10-11

A6 N2(B3Π) + O2 = N2 (A

3Σ) + O2 3.0∙10-10

A7 N2(A3Σ) + O2 = N2 + O + O 2.5∙10-12

H1 H2 + e- = H + H + e- σ2

H2 N2(a'1Σ) + H2 = N2 + H + H 2.6∙10-11

H2 N2(B3Π) + H2 = N2(A

3Σ) + H2 2.5∙10-11

H4 N2(A3Σ) + H2 = N2 + H + H 4.4∙10-10 exp(–3170/T)

H5 O(1D) + H2 = H + OH 1.1∙10-10

M1 CH4 + e- = CH3 + H + e- σ

M2 N2(A3Σ) + CH4 = N2 + CH3 + H 1.2∙10-10 exp(–3500/T)

M3 N2(B3Π) + CH4 = N2 + CH3 + H 3.0∙10-10

M4 N2(C3Π) + CH4 = N2 + CH3 + H 5.0∙10-10

M5 N2(a'1Σ) + CH4 = N2 + CH3 + H 3.0∙10-10

E1 C2H4 + e- = products3 + e- σ

E2 N2(A3Σ) + C2H4 = N2 + C2H3 + H 9.7∙10-11

E3 N2(B3Π) + C2H4 = N2 + C2H3 + H 3.0∙10-10

E4 N2(C3Π) + C2H4 = N2 + C2H3 + H 3.0∙10-10

E5 N2(a'1Σ) + C2H4 = N2 + C2H3 + H 4.0∙10-10

P1 N2(A3Σ) + C3H8 = N2 + C3H6 + H2 1.2∙10-12

P2 N2(B3Π) + C3H8 = N2 + C3H6 + H2 3.0∙10-10

P3 N2(C3Π) + C3H8 = N2 + C3H6 + H2 3.0∙10-10

P4 N2(a'1Σ) + C3H8 = N2 + C3H6 + H2 4.0∙10-10

Governing Equations

Two-term expansion Boltzmann

equation for plasma electrons

Species concentrations

Energy equation for centerline

temperature

Heat transfer to the walls

Quasi-one-dimensional flow

equations

Coupled Pulse Energy (AIAA Paper 2012-3093)

Independent of pulse repetition rate

Remains constant during pulse burst

Proportional to pressure

Validated by experimental data from

CARS, O atom TALIF and OH LIF

Low-temperature H2-air and CxHy-air

plasma chemistry kinetic model

• Experimental [OH]: axis on the left, predicted [OH]: axis on the right

• Predicted temperature in agreement with the data (temperature data points are circled)

• Predicted quasi-steady-state [OH] ~ 40% higher than measured – systematic uncertainty in

the calibration or model deficiency?

• Predicted [OH] decay rate agrees very well with the data

• Slow decay rate between the pulses (0.1 msec at 10 kHz), thus no transient overshoot caused

by individual pulses

Comparison with OH measurements after a 50-pulse

burst, T0=500 K, P=100 torr : H2-air

Konnov

Comparison with OH measurements after a 50-pulse

burst, T0=500 K, P=100 torr: CH4-air

• All three mechanisms predict similar

OH decay rates, although GRI and

UCS overpredict absolute [OH] by

~60%

• Konnov’s low-temperature mechanism

predictions for absolute [OH] is closest

to the data

• OH is reduced as ϕ is increased: more

rapid OH consumption in reactions

with fuel

GRI 3.0 USC/Wang

Konnov

Comparison with OH measurements after a 50-pulse

burst: T0=500 K, P=100 torr, C2H4-air

GRI 3.0 USC/Wang

• Much faster OH decay than in H2, CH4

(at the same primary radical production

rate in lean mixtures)

• GRI overpredicts absolute [OH] ,

underpredicts OH decay rate by a

factor of ~3

• USC performs somewhat better (factor

of ~2 difference)

• Konnov’s mechanism predictions for

absolute [OH] and decay rate closest to

the data

• No chain branching (T is too low)

Konnov

Comparison with OH measurements after a 50-pulse

burst: T0=500 K, P=100 torr, C3H8-air

GRI 3.0 USC/Wang

• GRI overpredicts absolute [OH],

underpredicts OH decay rate by a

factor of ~3

• USC is actually performs worse (factor

of ~6 difference in [OH])

• Konnov’s mechanism predictions does

somewhat better (factor of ~2 difference

in [OH], decay rate)

• None of the 3 mechanisms is accurate:

need to update / validate low-T reaction

mechanism for complex fuels (C3 and

higher)

Thrust 3. Experimental and modeling studies

of fundamental nonequilibrium discharge processes

Task 10: Characterization and modeling of nsec pulse discharges

Goal: Prediction of E/N and electron density in the plasma, individual

pulse energy coupled to the plasma, energy partition, and their

scaling with pressure, temperature, pulse waveform, and mixture

composition

Motivation: CARS and O atom TALIF measurements

in point-to-plane nsec discharge in air, fuel-air at 1 bar*

Streamer ~ 0.25 mm dia x 8 mm length.

28 kV, 12 nsec FWHM, E/N ~200 Td

N2 (v=1, 2) CARS signal rise after the pulse

2270 2280 2290 2300 2310 2320 2330 2340

=2=1

Inte

ns

ity

(A

.U.)

Raman Shift (cm-1)

=0

1E-9 1E-8 1E-7 1E-6 1E-5 1E-4 1E-3

0

5

10

N2(X

1

g

+)

Re

lati

ve

Po

pu

lati

on

(A

.U.)

Time after Current Rise (s)

N2(X

1

g

+) =1

N2(X

1

g

+) =2

* with S. Pendleton, M. Gundersen: USC, Cam Carter: AFRL / Propulsion Directorate

Typical N2 CARS spectrum

O atom concentration (TALIF) after the pulse

Experimental Platform III:

Point-to-point, “diffuse filament” single-pulse nsec discharge

•Stable “diffuse” filament discharges (~2-3 mm in diameter) in air and N2 at P=0.1-0.2 atm

•High energy loading per molecule (up to ~ 0.2-0.3 eV/molecule), significant estimated N2

vibrational excitation, Tv(N2) ~ 3000 K, Trot ~ 500 K

•Spatially resolved, time-resolved psec CARS and spontaneous Raman measurements:

Trot, N2(v=0-12) vibrational levels, electric field)

•“Test bed” to study possible coupling between vibrational modes and radical species in

high energy loading fuel-air plasmas

•Can N2(X,v) molecules affect radicals kinetics (HO2 and OH), as suggested recently?

10

mm

2 mm

Air, P=100 torr, 16 mJ/pulse

Psec BOX-CARS with broadband dye mixture:

good spatial resolution, access to high vibrational levels

Very Broadband Pyrromethene

Dye Mixture (*S. Tedder, et al, 2011)

-0.4 -0.2 0 0.2 0.40

0.5

1

1.5

2

2.5

3

Distance [mm]

Inte

gra

ted

Sig

na

l In

ten

sit

y [

au

]

95% region

Data

Gaussian

Fit

Interrogation

volume

CARS signal

beam Folded Box-CARS

95% of signal generated over ~0.5 mm

Typical Psec CARS Spectra, 100 Torr N2 (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

9

0v

vvfQ 10

01ln nn

T vv

•Strong vibrational disequilibrium, up to Tv01(N2) = 3000 K (Q=0.8 vibrational quanta per

molecule), moderate translational/rotational mode temperatureTrot = 300-800 K

•N2(X) vibrational levels up to v=9 are detected

•First level vibrational temperature keeps rising after the discharge pulse: V-V energy

transfer from higher vibrational levels

•Gradual relaxation on long time scale: V-T relaxation, diffusion out of the filament region

Results: psec CARS measurements in air

Air, P=100 torr, 16 mJ/pulse

10

mm

2 mm

N2, P=100 torr,

5 mJ/pulse, 100 μs delay

Results: psec CARS measurements in N2

•Results are similar to air; lower translational/rotational mode temperature, Trot = 300-500 K

•Vibrational quanta per molecule keeps increasing after the pulse, from Q=0.3 to 0.8 *

•Evidence of coupling between N2 electronic levels and vibrational mode of ground electronic state

* First detected by Devyatov, Sov. Phys. JETP, 1986

N2, P=100 torr,

15 mJ/pulse

“Rapid” and “Slow” Heating in Air and Nitrogen

• “Rapid” heating in N2 and air:

Likely N2(A) + N2(A) → N2(B,v) + N2(X,v) (E-V processes)

N2(A,B,C,a) + O2 → N2(X,v) + O + O

• “Slow” heating in air (nearly absent in N2),

Likely V-T relaxation by O atoms: N2(X,v) + O → N2(X,v-1) + O

Compression waves generated by the filament

at 1-10 μsec (frames are 1 μsec apart)

5 μs delay

Spontaneous Raman results in nitrogen: N2(v=0-12),

consistent with psec CARS measurements

Cathode sheath dynamics: plane-to-plane, nsec pulse, double dielectric barrier discharge

Air, T=300 K, P=30 torr, L=10 mm gap, 25 kV, 10 nsec FWHM pulses,

10 kHz pulse repetition rate

0.5 nsec camera gate, pulse #10

Bottom electrode is negative polarity

Before

breakdown 0 ns

1.7 ns Sheath collapse on

sub-nsec time scale

4 ns Sheath expands

again

25 ns Emission decays

1.4 ns Volume breakdown

outside the sheath

8 ns Persists during the

rest of the pulse

50 nsec camera gate

#10

#30

#50

Sheath formation / collapse / expansion

timing reproduced extremely well

1-D analytic model of plane-to-plane nsec pulse discharge:

estimate of sheath thickness, field in the sheath

1 nsec

2 nsec

3 nsec

4 nsec

5 nsec

6 nsec

7 nsec

9 nsec

14 nsec

24 nsec

50 nsec gate

Pulse #10, 0.5 nsec camera gate, end view

mmn

n

El

wall

plasma

s

s 1~ln)(

1

cmkVtEl

l

lL

E applied

s

s /100~)(2

2

torrcmVpEs /10~/ 3

L. Tsendin, PSST, 2009

I. Adamovich, Phys. Plasmas, 2009

1100~/

exp)(

cm

PE

BAE

s

s

L

l,ε ls

plasma

sheath

cathode

anode

E

Sheath thickness, field in the sheath predicted

by 1-D analytic model: consistent with sheath images

• Sheath expansion due to very low electron emission from dielectric wall, ionization coefficient

reduction at very high fields

• Sheath ~ 1 mm thick, ~ 5 cm long sheath, with peak field of ~ 100 kV/cm sustained for ~ 10 ns

• Implication: field measurements in plane-to-plane nsec discharge sheath by psec collinear CARS

N2, P=30 torr, T=300 K, 1 cm gap, 25 kV peak, 10 nsec FWHM pulse

10 mm gap

Sub-Nsec Resolution Electric Field Measurement

by CARS-like Four Wave Mixing

Energy Level Diagram

for E-Field CARS

ωS

ωp

IR CARS

ωAS

ωp

IR IR Pump Stokes External

CARS CARS Pump Stokes Pump

ExternalIR IR

CARS CARS Pump

IR PumpCARS

External

IR CARS

E E E E

E E E E

EE*

E E

I IE

I

*V.P. Gavrilenko, JETP Lett. 1992

T. Ito. J. Phys D:Appl. Phys. 2010

• “E-Field CARS” is a 4-wave mixing process

• CARS probe beam is replaced by an external electric field, which is at essentially zero frequency. This creates an IR “CARS” signal at the vibrational frequency.

• The physical origin of this signal is the dipole induced by the external field

• Phase Matching for E-Field CARS is collinear

Initial Electric Field Measurement Results:

“Diffuse Filament” Point-to-Point Discharge in Hydrogen

Hydrogen, P=100 torr, L=10 mm gap -0.2 0 0.2 0.4 0.6

2

4

6

8

10

secondsE

-Fie

ld R

esults (

kV

/cm

)

Line: high voltage probe

Symbols: CARS

•Field in the range of the 1-10 kV/cm has been measured (averaging over 128 discharge pulses)

•Higher peak field in dielectric barrier sheath (tens of kV/cm?), in plane-to-plane discharge

geometry a few cm long is expected increase signal to noise considerably, in spite of low

pressure (P~30-40 torr)

MURI Teams

•Penn State (Nick Tsolas and Rich Yetter): Species concentrations measurements in a high-

temperature, nonequilibrium plasma flow reactor / kinetic mechanism validation:

complementary to OSU Platform I experiment

•Georgia Tech (Sharath Nagaraja and Vigor Yang): Kinetic modeling of repetitive nanosecond

pulse discharges in air and fuel-air mixtures (modeling OSU Platform I and II experiments)

•Princeton (Mikhail Shneider): Non-simplistic kinetic modeling of dielectric barrier plasma

sheaths at very high peak electric field values

Collaboration:

The whole is greater than the sum of the parts

Outside MURI

•AFRL (Cam Carter): Absolute OH LIF calibration by Rayleigh scattering

•USC (Scott Pendleton, Martin Gundersen): N2 vibrational excitation and absolute [O] TALIF

measurements in 1 bar nsec pulse discharges in air and fuel-air (CH4, C2H4, C3H8)

•AFRL (John Poggie): Kinetic modeling of nsec pulse discharges, with emphasis on high-speed

flow control

•Moscow State University (Nikolay Popov): Kinetic modeling of rapid heating in high energy

loading, nsec pulse discharges (OSU Platform III experiment)

•High Temperature Institute, Russian Academy of Sciences, IVTAN (Sergey Leonov): pulsed

electric discharges for flow mixing enhancement, kinetics of vibrational energy transfer

• Stable, diffuse plasma generation in preheated, high-pressure fuel-air mixtures:

T0=500 K, up to P=500 torr, H2 - air, CH4 - air, C2H4 - air, C3H8 - air

•Time-resolved temperature measurements (OH LIF, psec CARS) and absolute [OH]

measurements (OH LIF) measurements in H2 - air, CH4 - air, C2H4 - air, and C3H8 – air,

in a well characterized nsec pulse plasmas, over a wide range of equivalence ratios

•Spatially- and time-resolved relative [OH] measurements in lean low-pressure flames , with

nsec pulse plasmas coupled directly to reaction zone: CH4 – O2 – N2, C2H4 – O2 – N2, CH4 –

O2 – Ar, C2H4 – O2 – Ar

•Time-resolved T, Tv(N2), and N2 (X,v=0-12) population measurements (psec CARS,

spontaneous Raman) in high energy loading nsec pulse discharges in air and nitrogen

•Evidence of coupling between electronic and vibrational mode energies in N2

•Electric field measurements (4-wave mixing / CARS) in nsec pulse discharges in H2

•O atom measurements (TALIF) in nsec pulse discharges in air, O2-Ar, H2-O2-Ar, H2-air,

C2H4-air

•Time-resolved temperature measurements (purely rotational CARS) in air, H2-air, C2H4-air

•Nsec pulse discharge model, H2 / CxHy – air plasma chemistry kinetic model are being

developed, updated, and validated based on these experimental data, used by MURI team

PAC MURI 3-year milestone: main achievements

PAC MURI 3-year milestone: plan for Years 4 and 5

• Stable plasma generation in preheated, high-pressure fuel-air mixtures: T0=500-600 K, P=1 bar,

H2 - air, CH4 - air, C2H4 - air, C3H8 - air

•Time-resolved temperature measurements (psec CARS), absolute [OH] measurements (OH LIF),

and absolute [H] measurements (TALIF) in H2, CH4, C2H4, and C3H8-air, over a wide range of

equivalence ratios

•Spatially- and time-resolved temperature and absolute [OH] measurements in lean low-pressure

flames , with nsec pulse plasmas coupled directly to reaction zone: CH4 and C2H4 flames

•Time-resolved T, Tv(N2), and N2 (X,v=0-12) population measurements (psec CARS, spontaneous

Raman) in high energy loading nsec pulse discharges in H2 - air and CxHy - air

•Explore possible effect of N2 (X,v=0-12) on OH decay kinetics in nonequilibrium plasmas

•Spatially resolved electric field measurements (4-wave mixing / CARS) in nsec pulse discharge,

sheath in H2 - air

•O and N atom measurements (TALIF) in nsec pulse discharges in air, H2-air, C2H4-air

•Time-resolved temperature measurements (purely rotational CARS) in air, H2-air, C2H4-air

•Electron density measurements (Thomson scattering)

•Further discharge / fuel-air plasma chemistry model development and validation over a wider

pressure, equivalence ratio range, and for a larger set of fuels; reduced mechanism development

Pulse #10

Cathode sheath dynamics An image with a larger field of view (air, P=30 torr)

Pulse #10

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

Backup

1-D analytic model of plane-to-plane nsec pulse

discharge: predicts sheath thickness, field in the sheath

L

l,ε

ls

plasma

sheath

cathode anode

E

wall

plasma

s

sn

n

El ln

)(

1

)(2

2

tEl

l

lL

E applied

s

s

86 88 90 92 94

0.0E+0

5.0E+11

1.0E+12

1.5E+12

2.0E+12

Electron density, cm-3

Time, nsec

numerical model

analytic solution

70 75 80 85 90 95 100

Plasma electric field, kV/cm

Time, nsec

numerical model

analytic solution

Vapp/(L+2l/)

0

2

4

6

8

10

12

14

16

18

20

86 88 90 92 94

Sheath electric field, kV/cm

Time, nsec

numerical model

analytic solution

0

50

100

150

200

84 86 88 90 92 94

0.0

0.1

0.2

0.3

0.4

Sheath boundary location, cm

Time, nsec

numerical model

analytic solution

Model assumes local ionization – is it applicable at very strong fields (E/P>103 V/cm·Torr)?

PE

BAE

s

s/

exp)(

N2, T=300 K, P=60 torr, electrode gap L=1 cm

Very good agreement with numerical calculations Adamovich et al., Phys. Plasmas, 2009

Backup

PIC modeling of plane-to-plane breakdown

with non-local (runaway) ionization included

Simple estimate of

runaway field from

electron energy equation: Drag force, I≈80 eV for N2 Runaway field for N2

More accurate estimate

(new electrons are born

with low energies):

Majority of electrons in Townsend regime even at

E>Ecr

Fraction of runaway electrons remains small

• PIC model of breakdown at high fields

• Experimental cross sections for nitrogen

• Townsend ionization mode exists at E>Ecr

• Runaway electrons dominate only in short

gaps, such as d ~ 1 / αion(E/P). In long gaps,

Townsend mode becomes dominant.

• Ionization coefficient α starts to decrease

at E > Emax ≈ 1500 V/cm·Torr (σion drops)

• Good agreement with experiments

• In N2, at ~30 torr, ~1 mm sheath, runaway

ionization would dominate at E ~ 106 V/cm

Tarasenko, Yakovlenko, Physics – Uspekhi, 2004

Backup