21 June 2008

Spectroscopic Investigation of an Asymmetric Surface Mode Dielectric Barrier Discharge

Scott A. Stanfield1 and James Menart2

Wright State University, Dayton, Ohio, 45435

Charles DeJoseph Jr.3

Retired from Air Force Research Laboratory, Wright-Patterson AFB, Dayton, Ohio, 45433

Brian Sands4

UES, Inc.

Spatially resolved relative concentrations of N2(C3πu) and N2+(B2Σ ) along with their

respective rotational temperatures have been obtained for the positive and negative voltage half-cycles for an asymmetric surface mode dielectric barrier discharge (SDBD) operating at 3000 Hz using optical emission spectroscopy. Results show that rotational temperatures for the excited neutral remain constant for both half cycles and have a maximum value of 350 K at the inner electrode gap. The rotational temperature of the excited neutrals during the negative half-cycle is observed to increase in the induced flow direction whereas it decreases during the positive half cycle. The rotational temperature of the excited ions is found to be a maximum of 250 K higher than the excited neutrals. It was also noted that as the rotational temperature of the excited neutrals increases during the negative half-cycle, the ions drop off sharply indicating possible energy transfer between the two species. The asymmetry of the discharge was also found to have a dramatic effect on the relative concentrations. It was found that the relative concentration of N

+

u

2(C3) was a factor of two greater during the negative half-cycle than the positive half-cycle. The relative concentration of N2

+(B2) was found to have the exact opposite behavior. During the positive half-cycle the relative concentration of N2

+(B2) was a factor of three greater than during the negative half-cycle. The relative concentration of N2(C3) was greater than N2

+(B2) for all cases. Finally, the relative concentration of N2

+(B2) was found to increase to a maximum value in the induced flow direction for the positive half-cycle.

I. Introduction SYMMETRIC surface mode dielectric barrier discharges (SDBD) have demonstrated potential use as flow control devices capable of altering the gas flow around them through momentum coupling1. This simple

promising concept has spurred an abundance of work needed in developing a quantitative and qualitative understanding necessary for the applications of SDBDs to flow control. It has been shown that SDBDs are capable of suppressing flow separation,2-5 controlling the boundary layer,6 aiding lift enhancement and roll-control,7 controlling the wake of a circular cylinder,8, 9 reducing drag,10 and influencing lift and controlling flow separation for UAVs.11 Modeling work on the fundamental operation of a DBD has also been done.12-21 Other studies have shown the effect of air SDBD discharges on the electrode surface,22 studied the chemical kinetics,23,24 measured the induced velocity,25-27 measured the thrust produced,28, 29 studied the effects of operating frequency on efficiency,30 and studied the fundamental operations of the device.1, 31-33

A

1PhD Candidate, Department of Mechanical and Materials Engineering, Stanfield.3@wright.edu, Member AIAA. 2Associate Professor, Department of Mechanical and Materials Engineering, james.menart@wright.edu, Associate Fellow AIAA. 3 Principle Research Physicist, Retired from Air Force Research Laboratory. 4 Research scientist.

1

39th Plasmadynamics and Lasers Conference<BR>23 - 26 June 2008, Seattle, Washington

AIAA 2008-3780

Copyright © 2008 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.

In this paper the asymmetry associated with the two different voltage half-cycles of a sinusoidal-driven SDBD (positive and negative voltage) are investigated by means of emission spectroscopy with an optical chopper. In particular, emission spectroscopy has been used to resolve rotational temperatures and relative concentrations of N2(C3πu) and N2

+( B2Σ ) as a function of voltage amplitude and voltage half-cycle. It was shown previously+u

34 that the rotational temperature obtained from the N2(C3πu→B3πg) electronic band system within an SDBD is smaller than the rotational temperature obtained from the N2

+(B2Σ +u →X Σ ) electronic band system. In addition, it was

shown that the average difference in temperature between the two species at 8000 volts was 120 K and increased to 205 K at 13000 volts. These measurements were averaged over many positive and negative voltage half-cycles washing out individual contributions from each half-cycle. It has been shown by Enloe et al. in reference 35, Fig. 19 that the structure of the discharge is dramatically different for each voltage half-cycle. During the negative half-cycle (exposed electrode is the cathode) the attachment points of the discharge to the exposed electrode appear as cathode spots. The discharge then propagates downstream from the cathode spot in a negative streamer . During the positive half-cycle (exposed electrode is the anode) the discharge regime is a positive streamer. The asymmetry present between the two voltage half-cycles is expected to have an effect on the mechanisms responsible for the temperature difference.

2 +u

36

Stefanovic et al.37 found the same deviation in rotational temperature between N2 ( )gu BC Π→Π 33 and N2

+ ( )++ Σ→Σ gu XB 22 for a parallel plate dielectric barrier discharge (PDBD) operating in dry air. The mechanism responsible according to Stefanovic is related to the populating kinetics. Stefanovic argued that the lifetime of

( )++ ΣuBN 22

species is around 0.1 ns and since the main production mechanism is

( ) ( ) eBNeXN ug 222

12 +Σ→+Σ +++ , (1)

the spontaneous emission of N2

+ ( )++ Σ→Σ gu XB 22 occurs on a similar time scale as the lifetime, therefore only

occurring during the active time of the microdischarge. The main production mechanism of N2( ) identified by Stefanovic occurs from

uC Π3

( ) ( ) ( ) ( )+++ Σ+Π→Σ+Σ guuu XNCNANAN 1

23

23

23

2 . (2)

It should be mentioned that other sources indicate direct electron excitation produces the N2( ) state for this type of discharge.

uC Π338 According to Stefanovic, the production of the metastable species ( )+ΣuAN 3

2 is predominantly

produced by recombination reactions effectively delaying the emission of the transition of Ngu BC Π→Π 33 2. This

means that the emissions of these two electronic band systems occur at different times within the discharge. They performed a computational study to see if the gas temperature can change during this time and found that after the current pulse a rapid jump in temperature and pressure occurred. They show that the temperature change causes the gas to expand radially outward from the microdischarge on a time scale of 0.1-1 μs. This expansion process causes the gas temperature to drop. Spontaneous emission from the N2

+ ( )++ Σ→Σ gu XB 22 electronic band system occurs predominantly before this expansion process when the gas temperature is hot; whereas, the N2( ) band system lags behind and emits after the expansion when the gas temperature is cooler. One problem with this explanation comes from Kozlov et al.

gu BC Π→Π 33

39 who shows that the spontaneous emission from N2+ ( )++ Σ→Σ gu XB 22 lags the

emission from N2( ) within an atmospheric pressure air PDBD by several nanoseconds. This does not appear to be enough time for the microdischarge region to cool noticeably. Also the emission of both transitions occurs at the same time current is flowing through the microdischarge. This would indicate that the microdischarge region is still heating when emission from N

gu BC Π→Π 33

2( ) is occurring. gu BC Π−Π 33

2

II. Dielectric Barrier Discharge, Test Facilities and Equipment

A. Dielectric Barrier Discharge The SDBD used in this work is shown in Figure 1 and consists of two electrodes; one exposed and one

encapsulated beneath a dielectric material. The electrode material is copper and the dielectric material is 1 mm thick glass. The width of the encapsulated electrode is 10.7 mm and the width of the exposed electrode is 4.4 mm. The region where the electrodes overlap in the spanwise direction is rather short, 38.1 mm. The coordinate system used in this work in shown in Figure 1. The induced flow is in the positive y-direction and the spanwise direction is the x-direction.

A discharge is established over the encapsulated electrode by placing an oscillating voltage between the two electrodes. In this work the applied voltages are a 12,300 and 14,100 volt, peak-to-peak sinusoid with a frequency of 3000 Hz. The encapsulated electrode is grounded.

38.1 10.7

2.0

4.4

4.4

X

Y

Figure 1. Electrodes used in this work shown without a DBD and with a DBD. All dimensions are in mm.

B. Collection Optics and Spectroscopic System Figure 2 shows the optical collection system which channels the emitted radiation from the SDBD to the spectrometer. The emitted radiation from the SDBD is collected by a fused silica bi-convex lens with an effective focal length of 35 mm. The collected radiation is focused onto a fiber optic bundle. An aperture is adjusted at the focal plane so that the opening is smaller than the slit of the chopper blade. The aperture diameter was set at 2.0 mm. The purpose of the optical chopper is to block radiation from one voltage half-cycle, only allowing radiation from the other voltage half-cycle to reach the spectrometer. Verification of this technique is given in the experimental verification section.

3

2f 2f

Plasma Bi-Convex Lens Aperture Chopper Fiber Bundle

A BCollection Volume

Figure 2. Optical collection system.

The fiber optic bundle channels the collected radiation into an Acton 2756 spectrometer where it is diffracted and focused onto an Andor DU 440-BU CCD camera. The fiber has a round ferrule on the collection end and a slit pattern on the exit end designed to match the entrance slit of the spectrometer. The spectrometer has a resolution of 0.015 nm at 435.8 nm using a 2400 grooves per mm grating. The spectral response of the system was calibrated using a Newport 63976 tungsten halogen lamp. The slit height and width for all measurements presented are 4 mm and 15 μm respectively. The experimental set-up is shown in Figure 3. The model is mounted vertically on a xy translating stage. The translating stage consists of two Newport M-443 series linear stages connected together with a Newport 360-90 bracket. The desired positioning is obtained in both directions by adjusting a Newport SM-50 micrometer which has a maximum travel of 50.8 mm. The experiment is housed within a box lined with black flocked paper to reduce the signal contribution from the background lighting.

Figure 3. The left figure shows the overall experimental set-up. The right figure shows the optics and other

components housed within the box. The spatial resolution (i.e. the dimensions of the emitting volume collected) was obtained by shining an expanded laser beam through the backside of the optical fiber. The collection volume, shown in Figure 2, has dimensions of A

1.0 mm and B ≈ 1.7 mm. The collection volume is essential for interpreting results because every measurement made represents the averaged value of the measured quantity over the emitting region within the collection volume. ≈

4

C. Voltage and Current Measurement Technique

The voltage output from an SI model 300 CD optical chopper is used to trigger an Agilent 33220A function generator. This voltage waveform enters a Crown CE1000 power supply via a BNC connection. The function generator is capable of many different waveforms including those specified by the user and has a maximum peak-to-peak voltage output of 5 volts. The power supply voltage output is further amplified with a Corona Magnetics Inc CMI-5012 step-up transformer. The transformer is directly attached to the SDBD and is capable of delivering up to 20kV at 5 kHz. The voltage waveform is measured using a Tektronix P6015A high voltage probe and is connected to a Tektronix TDS 3034B oscilloscope with a BNC cable. The high voltage probe is capable of measuring peak-to-peak voltages up to 20kV and has a bandwidth of 75 MHz. The discharge current is obtained by measuring the voltage drop across a 100 Ohm resister with a Tektronix P6139A probe. Both probes are designed for the oscilloscope and are properly compensated. Example voltage and current profiles are given in Figures 4 and 5. An explanation of the chopper voltage is given in the next section.

-8

-6

-4

-2

0

2

4

6

8

-200.00 -100.00 0.00 100.00 200.00

-0.06

-0.04

-0.02

0

0.02

0.04

0.06Voltage (kV)Chopper (V)Current (A)

Time (microseconds)

Cur

rent

(A)

Vol

tage

(V)

Figure 4. Voltage and current characteristics of the SDBD. Chopper is blocking the positive half-cycle.

Time (microseconds)

Vol

tage

(V)

Cur

rent

(A)

-8

-6

-4

-2

0

2

4

6

8

-200.00 -150.00 -100.00 -50.00 0.00 50.00 100.00 150.00 200.00

-0.06

-0.04

-0.02

0

0.02

0.04

0.06

Voltage (kV)Choppe (V)rCurrent (A)

Figure 5. Voltage and current characteristics of the SDBD. Chopper is blocking the negative half-cycle.

D. Experimental Verification The optical chopper experiment was verified by comparing the measured spectra of both voltage half-cycles with

spectra measured with a PI Max ICCD camera. In this experiment the ICCD was triggered with the Agilent function generator. The ICCD has a minimum offset of 26 nanoseconds which is much shorter than the 60 microseconds

5

current is delivered. Also the radiative lifetime of the excited states in an atmospheric pressure air discharge is on the order of 1 nanosecond.39 Three regions within the discharge spectra were measured with the ICCD camera as indicated in Figure 6. Region 1 corresponds to the positive half-cycle, region 3 corresponds to the negative half-cycle and region 2 corresponds to the region between the two half-cycles. The spectra obtained for these three regions are shown in Figure 7. The spectra obtained for region 2 indicates that the phase correlation is correct for the current, voltage and spectra data. The region 2 spectra lie on the horizontal axis because the emission is essentially zero. There is no significant emission when there is no current through the discharge. The asymmetry of the spectra for the two half-cycles is also apparent. The negative half-cycle is characterized by lower intensities for all emitting species. The spectra from the positive half-cycle are dominated by the N2

+ ( )++ Σ→Σ gu XB 22 electronic band system around the band head of this system. The intensity of the N2( ) electronic band system is also greater for this half-cycle.

gu BC Π→Π 33

The voltage output of the chopper was used to determine which voltage half-cycle was blocked. A chopper voltage of 0 in Figures 4 and 5 means the chopper is blocking that portion of the discharge. The spectra from both voltage half-cycles measured with the optical chopper is overlaid in Figure 8. The same qualitative tendencies observed with the ICCD are present. In Figure 9, a comparison between spectra obtained for the chopper blade in an open and closed position is compared. This Figure shows that no signal is obtained for the closed position whereas a signal of 160,000 counts is obtained for the open position. It was concluded that the optical chopper is functioning as intended from these two figures.

-10

-8

-6

-4

-2

0

2

4

6

8

10

-200.00 -100.00 0.00 100.00 200.00

-6

-4

-2

0

2

4

6

VoltageCurrent

Time (microseconds)

Vol

tage

(V)

Region 1 Region 2 Region 3

Figure 6. Voltage and current characteristics of the SDBD for ICCD.

6

00.0020.0040.0060.0080.01

0.0120.0140.0160.0180.02

382.9 384.9 386.9 388.9 390.9 392.9 394.9

Positive Half-CycleNegative Half-CycleBetween Two Cycles

0-0

1-13-6

2-5

Wavelength (nm)

Inte

nsity

0-2

0

0.002

0.004

0.006

0.008

0.01

0.012

370 372 374 376 378 380 382

Negative Half-CyclePositive Half-Cycle

0-2

1-3

2-4

Wavelength (nm)

Inte

nsity

Figure 7: Top: spectra for regions 1-3 with an ICCD. Bottom: spectra measured in regions 1 plus 3 with an ICCD . Vibrational transitions marked with red numbers indicate vibrational bands for N2

+ ( )++ Σ→Σ gu XB 22 electronic band system and vibrational transitions marked with blue numbers indicate vibrational bands for N2( ).

gu BC Π→Π 33

0

20000

40000

60000

80000

100000

120000

140000

160000

180000

385.1 387.1 389.1 391.1 393.1

Positive Half-CycleNegative Half-Cycle

Wavelength (nm)

Inte

nsity

Figure 8: Comparison of the spectra measured with the optical chopper for both voltage half-cycles.

7

-2000

0

2000

4000

6000

8000

10000

12000

14000

16000

320 325 330 335 340 345 350

Chopper Open

Chopper Closed

Figure 9: Comparison of the spectra measured with the optical chopper in the open and closed position.

III. Spectra Reduction Technique

A line-by-line code called N2SPECFIT was used to fit the measured spectra. The fitting parameters determined by the code are the upper state vibrational number density of each band and the rotational temperature of each electronic band system. The code assumes the rotational energy levels of each separate electronic band system are described with a Boltzmann distribution; however, no assumption about the vibrational distribution is made. The translational temperature is inferred from the measured rotational temperature by assuming the two are in equilibrium. This is a valid assumption at the temperatures and pressure involved in this work because the rotational energy levels are closely spaced and allow for rapid energy transfer between the two energy modes.40 The vibrational upper state densities and rotational temperatures are obtained from a two temperature fit by numerically comparing the measured spectra of the 0-1, 0-2, 1-3, 2-5 and 3-7 vibrational transitions of N2 and the 0-0 and 1-1 vibrational transitions of N2

+ to a calculated spectrum. The comparison was done quantitatively using the statistical parameter , which was minimized iteratively by adjusting the fitting parameters.2

Rχ41,42 The upper state electronic

number density is obtained by summing up the populated upper state vibrational number densities. Figure 10a shows the fit to measured spectra from the positive half-cycle and Figure 10b shows the fit for spectra from the negative half-cycle.

Wavelength (nm) Wavelength (nm)

020000400006000080000

100000120000140000160000180000200000

385.1 387.1 389.1 391.1 393.1

Y(MEASURED)Y(FIT)

0100002000030000400005000060000700008000090000

100000

385.1 387.1 389.1 391.1 393.1

Y(MEASURED)

Y(FIT)

Inte

nsity

Inte

nsity

0-0

1-12-53-6

0-0

2-5

3-6

1-1

a. b.

Figure 10: Figure 11a. shows the measured and fit spectra for the positive half-cycle and Figure 11b. shows the measured and fit spectra for the negative half-cycle. Vibrational transitions marked with red numbers indicate vibrational bands for N2

+ ( )++ Σ→Σ gu XB 22 electronic band system and vibrational transitions marked with blue numbers indicate vibrational bands for N2( ).

gu BC Π→Π 33

8

IV. Results The SDBD was run for 20 minutes for each voltage setting (12.3 kV and 14.1 kV) before any data was taken to

ensure a quasi-steady operational state. The greatest upper state electronic number density measured was used to normalize all the concentration data. This included the concentration data measured for both voltage settings and both electronic band systems. All data was taken with respect to the coordinate system defined in Figure 1. The region of interest within the SDBD was limited to the interior region to eliminate end effects. The optical line of sight for all measurements was in the z-direction of Figure 4 (going into the page). Error bars are left off all figures to improve visual clarity, but are reported in the description of each figure.

In Figure 11 relative concentrations are reported for N2(C3) along the gap at y = 0.0 for both voltage half-cycles and two different peak-to-peak voltages. The figure shows the relative concentration of the excited neutral is greatest for the negative half-cycle by a factor of 2 approximately. The figure also shows that the relative concentration of the excited neutral state increases with increasing voltage as expected. Error bars for all N2(C3) concentrations reported have a range of 3.0-4.5%. This was determined from the statistical parameter . ± 2

Rχ

0.20

0.40

0.60

0.80

1.00

1.20

1.40

0.5 0.7 0.9 1.1 1.3 1.5 1.7 1.9

12.3 kV (Positive Half-Cycle)12.3 kV (Negative Half-Cycle)14.1 kV (Positive Half-Cycle)14.1 kV (Negative Half-Cycle)

X (mm)

Rel

ativ

e C

once

ntra

tion

N2(C3) Concentration at y = 0.0

Figure 11: Relative concentrations of N2(C3) along the gap between the two electrodes

Figure 12 gives the relative concentrations of N2

+(B2) along the gap at y = 0.0 for both voltage half-cycles and two different peak-to-peak voltages. The figure shows the relative concentration of the excited ion is greatest for the positive half-cycle by approximately a factor of 3. It is again observed that the relative concentration increases with increasing voltage. This trend is exactly opposite of the excited neutrals. Error bars for all N2

+(B2) concentrations reported have a range of 4.0-6.0%. ±

9

X (mm)

Rel

ativ

e C

once

ntra

tion

N2+(B2) Concentration at y = 0.0

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.5 1 1.5 2

12.3 kV (Positive Half-Cycle)12.3 kV Negative Half-Cycle14.1 kV (Positive Half-Cycle)14.1 kV (Negative Half-Cycle)

Figure 12: Relative concentrations of N2

+(B2) along the gap between the two electrodes.

Figure 13 gives the rotational temperatures obtained from the N2( ) electronic band system along the gap at y = 0.0 for both voltage half-cycles and both peak-to-peak voltages. The maximum temperature obtained is approximately 350 K and corresponds to the negative half-cycle at 14.1 kV. Error bars for all rotational temperatures obtained for the N

gu BC Π→Π 33

2( ) electronic band system reported have a range of 7-15 K. It is therefore noted that the rotational temperature of the neutrals for both half-cycles and both voltages are the same within experimental accuracy.

gu BC Π→Π 33 ±

X (mm)

Rot

atio

nal T

empe

ratu

re (K

)

Rotational Temperature of N2(C3) at y = 0.0

325

335

345

355

365

375

0.5 1 1.5 2

12.3 kV (Positive Half-Cycle)12.3 kV (Negative Half-Cycle)14.1 kV (Positive Half-Cycle)14.1 kV (Negative Half-Cycle)

Figure 13: Rotational temperatures obtained from N2(C3) to N2(B3) transition along the gap between the

electrodes.

Figure 14 gives the rotational temperatures obtained from the N2+ ( )++ Σ→Σ gu XB 22 electronic band system along

the gap at y = 0.0 for both voltage half-cycles and both peak-to-peak voltages. It is immediately noted that the rotational temperature of the excited ion is around 200 K greater for the negative half-cycle than the positive half-cycle. It also noted that the rotational temperature of the excited ions is around 250 K greater than the rotational temperature of the excited neutrals for the negative half-cycle and 120 K greater for the positive half-cycle. The good agreement of the fits for both electronic band systems (see Figure 10) indicates that the rotational energy levels of both band systems are well described by their own separate Boltzmann distributions. Error bars for all rotational temperatures obtained for the N2

+ ( )++ Σ→Σ gu XB 22 electronic band system reported have a range of 15-25 K. ±

10

400

450

500

550

600

650

700

0.5 1 1.5 2

12.3 kV (Positive Half-Cycle)12.3 kV (Negative Half-Cycle)14.1 kV (Positive Half-Cycle)14.1 kV (Negative Half-Cycle)

Rot

atio

nal T

empe

ratu

re (K

)

X (mm)

Rotational Temperature of N2+(B2) at y = 0.0

Figure 14: Rotational temperatures obtained from N2

+(B2) to N2+(X2) transition along the gap between the

electrodes. Figures 11-14 were along the gap at y = 0.0. The next sets of figures show the dependence of the relative

concentrations and rotational temperatures in the y-direction. In Figure 15, the relative concentrations of N2(C3) are given for both voltage half-cycles and for both applied peak-to-peak voltages. The results show the largest concentration occurs within the gap at y = 0.0 and decreases in the positive y-direction. At y = 0.0 the concentration of the negative half-cycle is greater than the positive half-cycle; but, as you go in the y-direction it appears to reverse. That is at y > 0.5 mm the concentrations of the positive half-cycle become greater than the negative half-cycle.

0.20

0.40

0.60

0.80

1.00

0 0.2 0.4 0.6 0.8 1 1.2 1.4

12.3 kV (Positive Half-Cycle)12.3 kV (Negative Half-Cycle)14.1 kV (Positive Half-Cycle)14.1 kV (Negative Half-Cycle)

N2(C3) Concentration

Rel

ativ

e C

once

ntra

tion

y (mm) Figure 15: Relative concentrations of N2(C3) in the y-direction.

In Figure 16 the relative concentrations of N2

+(B2) are given for both voltage half-cycles and both applied peak-to-peak voltages. The relative concentrations are seen, for both applied voltages, to decrease in the y-direction for the negative half-cycle. During the positive half-cycle the relative concentrations are observed to increase until y 0.5 mm and then decrease. ≈

11

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0 0.5 1 1.5

12.3 kV (Positive Half-Cycle)12.3 kV Negative Half-Cycle14.1 kV (Positive Half-Cycle)14.1 kV (Negative Half-Cycle)

N2+(B2) Concentration

Rel

ativ

e C

once

ntra

tion

y (mm) Figure 16: Relative concentrations of N2

+(B2) in the y-direction.

In Figure 17, rotational temperatures for the excited neutrals are obtained for both voltage half-cycles and both applied peak-to-peak voltages. The rotational temperatures are at a maximum in the gap at y = 0.0 and are observed to decrease in the y-direction for the positive half-cycle. During the negative half-cycle, the temperature begins to decreasing in the y-direction until y 0.5 mm and then increases. ≈

325

330

335

340

345

350

355

360

365

370

0 0.5 1 1.5 2

12.3 kV (Positive Half-Cycle)12.3 kV (Negative Half-Cycle)14.1 kV (Positive Half-Cycle)14.1 kV (Negative Half-Cycle)

Rotational Temperature of N2(C3)

Rot

atio

nal T

empe

ratu

re (K

)

y (mm) Figure 17: Rotational temperatures obtained from N2(C3) to N2(B3) in the y-direction.

Figure 18 shows the rotational temperatures for the excited ions decreasing rapidly in the y-direction for the

negative half-cycle, but remains relatively constant during the positive half-cycle. It is also noted that it appears the ion temperature of the negative half-cycle decreases below the ion temperature of the positive half-cycle at y≈ 1.0 mm. Data points at greater y-coordinates could not be obtained because of signal drop off.

12

400

450

500

550

600

650

700

0 0.5 1 1.5 2

12.3 kV (Positive Half-Cycle)12.3 kV (Negative Half-Cycle)14.1 kV (Positive Half-Cycle)14.1 kV (Negative Half-Cycle)

Rotational Temperature of N2+(B2)

Rot

atio

nal T

empe

ratu

re (K

)

y (mm) Figure 18: Rotational temperatures obtained from N2

+(B2) to N2+(X2) in the y-direction.

V. Discussion of Results The results obtained clearly demonstrate the asymmetry of the SDBD. Most puzzling is the mechanism

responsible for the deviation in rotational temperature of the excited ion and excited neutral. In this section possible mechanisms for the measured behavior are discussed. These mechanisms are only given as potential possibilities and require more experiments to prove or disprove. The positive voltage half-cycle has been shown by Enloe et al.35 to be composed of positive streamers. These streamers are characterized as having an ionization wave sweep across the encapsulated electrode setting up a positively charged head with a tail. This positive sheath is capable of picking up energy from the applied electric field raising the temperature of the particles within this small region. Emission from the excited ion occurs predominantly in the positive sheath where the temperature is at a maximum. Emission from the excited neutral occurs from the positive sheath as well as from the tail of the streamer. The tail is characterized with a lower temperature. The spatial resolution was large enough such that each measurement has emission from both regions resulting in an average temperature for the ions to be greater than the average temperature of the neutrals.

The discharge during the negative voltage half-cycle is characterized with a number of cathode spots at y = 0.0 which propagates outward with a fan-like, diffuse glow pattern. It was again observed that the rotational temperatures of the excited ions were greater than the exited neutrals. The proposed mechanism for this disjoint in heavy particle temperatures is that emission from the excited positive ion occurs predominantly within the cathode spot region, which has an elevated temperature. This temperature could be higher than what our measurements show due to the averaging done over the spot size which is considerably larger than the cathode spot. Radiation from the excited neutral state occurs within this region, as well as the entire fan-like glow region. The fan-like glow region is characterized with a temperature not much higher than the background gas. Figure 19 illustrates the proposed mechanism. As mentioned, the measured temperature is an averaged value obtained over the emitting regions within the collection volume. In this case the excited positive ion emission is located predominantly in a region characterized by an elevated temperature whereas the neutral is averaged over the hot region (cathode spot) and the cooler fan-like glow region resulting in a lower temperature.

A number of interesting results on the relative concentration of N2(C3) and N2+(B2) are presented in the previous

section. All the concentration plots presented indicate that the concentration of N2(C3) is higher than the concentration of N2

+(B2). This is reasonable because the energy required to produce N2+(B2) is much greater than

that needed to produce N2(C3). The concentration plots also show that higher N2(C3) concentrations are obtained during the negative half-cycle than the positive half-cycle in a region close to the edge of the exposed electrode. In an contrary fashion, higher N2

+(B2) concentrations are obtained during the positive portion of the cycle than the negative portion of the cycle in a region close to the exposed electrode. This illustrates the asymmetry of the SDBD discharge and indicates that different mechanisms are at work for each half-cycle. As one moves away from the edge of the exposed electrode this behavior appears to be switching. This is most clearly shown in Figure 15 where the 14.1 kV results show a crossing of the negative half-cycle concentrations with the positive half-cycle concentrations for N2(C3). This may be due to the way the positive streamers propagate in to the discharge region. It is believed that

13

during the positive half-cycle the positive streamers propagate further in the y-direction than the fan like cathode jets during the negative half-cycle. It may be that similar behavior occurs in the N2

+(B2) concentrations, but data would have to be taken at larger y-locations. This cannot be done at this time because of low signal strength.

Exposed Electrode

Encapsulated Electrode Collection Volume

Cathode Spot

Fan-Like GlowY

X

Figure 19: Qualitative schematic of proposed mechanism for the measured rotational temperatures of the excited

neutral and excited ion for the negative half-cycle.

VI. Conclusion The relative concentrations of N2(C3), N2

+(B2) and rotational temperatures of N2 and N2+ are obtained as a

function of voltage half-cycle (positive and negative half-cycle) and peak-to-peak voltages of 12.3 and 14.1 kV for an asymmetric surface mode dielectric barrier discharge operated with a 3 kHz sinusoid in atmospheric air. The following asymmetric properties of the spectra from an SDBD were observed:

1. The rotational temperature of N2+ during the negative half-cycle is approximately 200 K greater than

during the positive half-cycle. 2. The rotational temperature of N2 during the negative half-cycle decreases in the induced flow direction and

then increases. The rotational temperature of N2+ does the exact opposite. It increases up to the same

inflection point as the neutral and then decreases. This might indicate energy transfer between the two species.

3. The rotational temperature of N2 within the inner electrode gap is around 350 K for both half-cycles. 4. For every location within the discharge during all voltage half-cycles the rotational temperature of N2

+ was found to be significantly higher than the rotational temperature of N2. It was noted that the spectral fits of the measured electronic band systems indicate the rotational energy levels of both systems are described with a Boltzmann distribution. Possible mechanisms dependent on the specific structure of each voltage half-cycle were given.

5. It was observed that the relative concentration of N2+(B2) increased to a maximum in the flow direction

and then decreased during the positive voltage half-cycle. The relative concentration decreased during the negative voltage half-cycle.

6. The relative concentration of N2+(B2) was approximately a factor of three greater during the positive

voltage half-cycle than during the negative voltage half-cycle in the inner electrode gap. 7. The relative concentration of N2(C3) was approximately a factor of two greater during the negative half-

cycle than during the positive voltage half-cycle in the inner electrode gap. 8. The relative concentration of N2(C3) during the negative voltage half-cycle was greater than the relative

concentration during the positive half cycle initially but inverted further downstream. Which might

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indicate that the energy available to excite N2(C3) damps out more rapidly in the induced flow direction for the negative voltage half-cycle than the positive voltage half-cycle.

Acknowledgments This work was carried out in the CAPEC Lab (Center for Advanced Power and Energy Conversion) at Wright

State University. CAPEC is a joint collaboration between AFRL at Wright Patterson Air Force Base and Wright State University. This work was funded through the Air Force Office of Scientific Research and through the Dayton Area Graduate Studies DAGSI/AFRL program. In addition equipment support was obtained from Wright Patterson Air Force Base with the help of Dr. Roger Kimmel. The authors are very grateful for this support and help in carrying out this project.

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