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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