Numerical and Experimental Study of Atmospheric Pressure Glows in Helium

8/10/2019 Numerical and Experimental Study of Atmospheric Pressure Glows in Helium

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Numerical and Experimental Study of Atmospheric PressureGlows in Helium

P. Zhang, C. Anderson, J. Heberlein, U. Kortshagen

Department of Mechanical Engineering, University of Minnesota111 Church St. S.E.

Minneapolis, Minnesota 55455 USA

ABSTRACT . In this study, we investigate atmospheric pressure glow (APG) discharges in

helium, comparing numerical simulations with experimental data. Our two-dimensional fluidmodel includes the dielectric barriers and the discharge gap in the simulation domain. Wecompare numerical results to space and time-resolved optical emission spectroscopy (OES)measurements. The emission lines from He I (3s 3S - 2p 3P: = 706 nm) and N 2 (C

3 u - B3 g:

= 337 nm) are used to show qualitatively the distributions of electrons at various thresholdenergies. The relative distribution of He (2 3S) within the gas gap is mapped by the observationof the emission from N 2

+ ((0,0) B 2u+ - X 2g

+: = 391 nm), which is produced throughPenning ionization involving He (2 3S). Both numerical and experimental results show thatthe breakdown first appears at the center of the gap, followed by the axial and radial

propagation of the ionization wave. Additionally, the influence of increasing the drivingfrequency manifests in a shift from a non-uniform discharge to a uniform glow discharge.This transition is attributed to the increased density of seed electrons remaining in thedischarge gap before the subsequent breakdown. The effects of nitrogen impurities and thePenning ionization are also discussed.

INTRODUCTIONThe dielectric barrier discharge (DBD) is an atmospheric pressure discharge in which an

insulating layer covers one or both of two parallel plate or coaxial electrodes in the dischargesystem [1]. DBDs exhibit a filamentary structure, characterized by individual microdischarges.This behavior is highly transient and non-uniform over the electrode surface, hence high-

pressure DBDs are not suited for application that require good spatial plasma uniformity. Firstdemonstrated in the late 1960s [2], the atmospheric pressure glow (APG) discharge hasattracted great attention due to its diffuse, transversely uniform structure. Already APGs havefound applications in thin film deposition [3], VLSI processing [4], and biological sterilizationor decontamination [5].

Despite their uniform appearance, two-dimensional effects have been observed in APGs inhelium [6]. Using high-speed camera images, such effects as a radial spreading of theionization front and a secondary breakdown observed at the electrode edges were reported. Inorder to study the discharge behavior in the axial direction, Tochikubo et al. [7] used time-resolved spectroscopic measurements of He I and N 2 to show the distributions of high and lowenergy electrons, respectively. For this work we use a similar measurement technique to studythe both the axial and radial behavior of the discharge emission species.


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Numerical simulation is another tool for interpreting the underlying physics of APGs. Inorder to identify the conditions essential to create a uniform glow discharge, and to investigatethe mechanism of transition to the filamentary mode, two-dimensional simulations of thedischarge initiation and comparisons with experimental results are necessary.

In the current paper, we demonstrate results of two-dimensionally resolved time-dependent

spectroscopic measurements as well as the two-dimensional numerical simulation results. The paper is organized as follows: Section 2 describes the experimental set-up and the two-dimensional model, results and their interpretation are also presented. Section 3 summarizesthe main conclusions.

PROCEDURES AND RESULTS DISCUSSION Experimental set-up

Figure 1. Experimental apparatus.

A general schematic of the apparatus used in this study is shown in Figure 1. The details ofthe discharge chamber can be found in reference [6]. For the spectroscopic measurements, thesystem consists primarily of a .5 meter focal length spectrometer (SpectraPro 500, ActonResearch Corporation) and a high-speed intensified CCD camera (PIMAX, Roper Scientific).The CCD is synchronized with the electrical signal from the discharge such that the currentmaximum is defined as 0.0 s in time. The electrical signal is shown in Figure 2 noting thatthe discharge current follows a similar periodic behavior as has been observed by others (seereference [8]); a single current pulse is observed during each half cycle of applied voltage.

The emission from the discharge gap was focused onto the entrance slit of the spectrometerusing two plano-convex lenses (f L 35 cm and 45 cm). Since the slit is vertically oriented, thisfocusing system allows a time sequence measurement of the discharge in the axial direction ata single radial location. By scanning incrementally across the radial direction, the two-dimensional behavior of a desired spectral emission line can be recorded. These images werethen converted to numerical ASCII format, and combined into sequential animations usingsimple C commands. Prior to recording these images, the discharge is allowed to run for anappropriate length of time to clean the dielectric surfaces of any contaminants, as well aseliminate any long-range transient behavior. During the measurement process, great care wastaken to ensure that the synchronization between the CCD and discharge current remainedaccurate and constant.


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Fig. 2 Electrical signal from APG.

Two-dimensional fluid model

-3 -2 -1 0 1 2 3

Gap

Dielectric Plates

electrode

z

r

Figure 3. Numerical simulation domain.

A two-dimensional fluid model has been developed for the simulation of APGs. Thesimulation domain includes two dielectric barriers and the discharge gap as shown in Fig. 2.The electrodes with 2-cm radius are embedded in dielectrics. The gap is filled withatmospheric-pressure helium. Assuming azimuthal symmetry, cylindrical coordinates (r-z) areused.

The model involves the self-consistent solution of Poisson equation for electric field and thecontinuity and momentum equation for all the species. The momentum equations aresimplified by using the drift-diffusion approximation. Due to the small leak rate of theexperimental system, the known effects of nitrogen impurities must be considered in thesimulation of a helium discharge. The following species are then used for the model: electrons(e -), atomic and molecular helium ions (He +, He 2

+), helium metastables (He *), and molecularnitrogen ions (N 2+). Here helium excited states of 2 3S and 2 1S are lumped into one single statefor simplification (denoted He *). The reactions and the corresponding rate coefficients are thesame as in the ref [9].

The secondary electron emission at the dielectric surface due to ion bombardment isregarded as the main source for self-sustaining the glow discharge. The space chargedeposited on the plates is obtained from the drift-diffusion flux to the dielectrics. The surfacerecombination coefficient of the electron and ions is set to 1 for simplification.

The transport terms in the continuity equations are discretized using the Scharfetter-Gummel exponential scheme [10] on a set of non-uniform meshes. To increase the time-stepping efficiency, an adaptive time step and a semi-implicit time integration scheme [11] are


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used. All the numerical simulations were implemented on an IBM Power4 supercomputer atthe University of Minnesota Supercomputing Institute. Simulations were run until a periodicsolution was reached, usually occurring after 8-10 AC cycles.

Experiment results

The applied signal voltage and frequency for the case presented was 1.9 kV at 15 kHz. Thedielectric plates used were 0.635 mm thick alumina, separated by a fixed gap distance of 6.35mm. Figures 4-6 show the two-dimensional spectral emissions from the three lines chosen forthis study; namely He I (706 nm), N 2 (337 nm), and N 2

+ (391 nm).From He I emission (Figure 4), with its high threshold energy (24.8 eV), we can observe

qualitatively the regions of high electric field strength in the gap. This line emission alsoindicates the instantaneous production rate of He* metastable states during the discharge pulse.It is apparent that the initiation of the cathode layer is in the center region, and propagatesradially outward with time. This sheath quickly collapses in the center, so the profile takes ona ring-like structure as it ultimately decays. By 3.0 s after the current maximum, the sheathhas almost completely vanished.

In order to observe the distribution of lower energy electrons throughout the gap region, wehave measured the emission from N 2 (Figure 5). In addition to the intensity seen near themomentary cathode, there is significant intensity in the positive column region where theelectric field is much weaker.

With the known effect of Penning ionization in this system, we can observe the relativedistribution of He* metastables from the emission of N 2+ at 391 nm (Figure 6). Again the

profile shows that the emission is initiated in the center region, and spreads radially outwardin time, as with He I at 706 nm. However, in this case the emission remains strong in thecenter, as well as showing significant intensity further into the gap region. In this way, theemission from N 2+ shows the cumulative effect of the production of He* metastables.

Figure 4. He I line emission at 706 nm. Figure 5. N 2 line emission at 337 nm.


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Figure 6. N 2+ line emission at 391 nm.

Model results As an initial condition for the model, the electron density is set equal to 10 6 cm -3, and is

distributed uniformly in the gap. It takes several AC cycles to build up the electron densityand to initiate the gas breakdown. In this way, the influence of the assumed initial electrondensity can be reduced. Generally, 8-10 voltage cycles are required to reach a steady statecondition. The two-dimensional plots presented here are all obtained under steady state.

The evolution of the electron density profile (Figure 7) mirrors the ionization processes inthe discharge. The breakdown first appears in the center of the gap region, followed by thefast axially propagation. The radial movement of the ionization wave becomes more

pronounced close to the dielectrics, where a sheath rapidly develops.

G a p [ c m ]

0

0.2

0.4

G a p [ c m ]

0

0.2

0.4

0.6

G a p [ c m

]

0

0.2

0.4

0.6

Radius [cm]

G a p [ c m ]

0 0.5 1 1.5 2 2.50

0.2

0.4

0.6

5.0E+09 9.0E+09 1.6E+10 2.9E+10 5.1E+10 9.2E+10 1.6E+11

ne

G a p [ c m ]

0

0.2

0.4

G a p [ c m ]

0

0.2

0.4

0.6

G a p [ c m ]

0

0.2

0.4

0.6

Radius [cm]

G a p [ c m ]

0 0.5 1 1.5 2 2.50

0.2

0.4

0.6

1.0E+06 8.4E+06 7.0E+07 5.8E+08 4.9E+09 4.1E+10 3.4E+11 2.9E+12 2.4E+13 2.0E+14

Excitation rate of He *

Figure 7. Electron density evolution. Figure 8. Generation rate of He *.

The helium metastable production rate is shown in Figure 8. Before the breakdown, thehelium metastables generation is low and the peak is located some distance away from thecathode. After the gas breakdown, due to the formation of sheath, helium generation rate isincreased and the peak moves to the cathode. The radial propagation of the distribution is also


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observed. The result is qualitatively consistent with the experiment. Figure 9 shows theelectron temperature distribution during these four sequences. The collapse of the cathodelayer and the formation of the sheath can be clearly seen. It should be pointed out that thelocal maximum at the edge of the electrodes in both figures are mainly due to the edge effectof the finite length of the electrodes.

G a p [ c m ]

0

0.2

0.4

0.6

G a p [ c m ]

0

0.2

0.4

G a p [ c m ]

0

0.2

0.4

0.6

Radius [cm]

G a p [ c m ]

0 0.5 1 1.5 2 2.50

0.2

0.4

0.6

5.0E-01 7.2E-01 1.0E+00 1.5E+00 2.1E+00 3.1E+00 4.4E+00

Te

Figure 9. Electron temperature distribution.

The existence of nitrogen impurities has a large effect on the discharge behavior, manifestedin a decrease in the gas breakdown voltage due to Penning ionization. As pointed out inreference [8], high pre-ionization at a low electric field through Penning ionization isimportant for obtaining a glow discharge. To study this effect of pre-ionization, weintentionally increased the recombination coefficient of the nitrogen molecular ions to =5x10 -6 cm 3 s -1. Since the recombination process is dominant during post-glow, using a higherrecombination rate will decrease the electron density in the gap before the next breakdown, i.e.decrease the pre-ionization level. The result is shown in Figure 10; a filamentary dischargeappears as seen in Figure 10(a). Figure 10(b) shows the non-uniform distribution of thesurface charge on the powered dielectric.

For a filamentary discharge, if we increase the driving frequency, holding all other parameters constant, the surface charge distribution becomes uniform, and the dischargeappears diffuse once again (see Figure 11). Two reasons for this are: (1) the pre-ionizationlevel is increased because of the reduced time for recombination during discharge pulses. As aresult the seed electron density before the breakdown in this case increases from less than 10 9 to 10 10 cm -3. (2) The number of filaments increases with the driving frequency. Theoverlapping of these filaments results in a uniform discharge, which we will discuss elsewhere.


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Radius [cm]

[ C / c m

2 ]

0 0.5 1 1.5 2 2.5 3

-1E-08

-8E-09

-6E-09

-4E-09

-2E-09

0

2E-09

0 s

1.5 s

3.0 s

4.5 s

(a) (b)

Figure 10. (a) Electron density profile evolution and (b) surface charge density on the powered dielectric. = 5x10 -6 cm 3 s -1, f = 15 kHz for both plots.

Radius [cm]

G a p [ c m ]

0 0.5 1 1.5 2 2.50

0.2

0.4

0.6

ne: 5.0E+09 7.5E+09 1.1E+10 1.7E+10 2.6E+10 3.9E+10 5.8E+10 8.8E+10 1.3E+11 2.0E+11

36 s

G a p [ c m ]

0

0.2

0.4

0.6

18 s

G a p [ c m ]

0

0.2

0.4

0.6

9 s

G a p [ c m ]

0

0.2

0.4

0.6

0 s

Radius [cm]

[ C / c m

2 ]

0 0.5 1 1.5 2 2.5 3-1.5E-08

-1E-08

-5E-09

0

5E-09

1E-08

1.5E-08

0 s

6 s

12 s

18 s

(a) (b)

Figure 11. (a) Electron density profile evolution and (b) surface charge density on the powered dielectric. = 5x10 -6 cm 3 s-1, f = 25 kHz for both plots.

CONCLUSIONSWe have studied an APG in helium using both experimental and numerical simulation

methods. From the OES measurements, we have seen qualitatively the distributions of highand low energy electrons, as well as helium metastable atoms. The results show evidence ofPenning ionization of N 2. Perhaps most significant is the observation of a ring-like cathodelayer, initiating at the center and moving outwards in time. A similar phenomenon is seenfrom the numerical simulations. Further results from the model show that sufficient pre-ionization between half cycles is necessary in sustaining a uniform discharge. This is provided

primarily by helium metastables, owing to their slow decay and long diffusion time. The pre-


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ionization level is also affected by the driving frequency. Increasing the frequency favors atransition from a filamentary to homogenous glow discharge.

ACKOWNLEDGEMENTSThis work is supported by the Department of Energy under grant DE-FG02-00ER54583 and

by the University of Minnesota Supercomputing Institute.

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Plasma Sci. 30, 1400-1408.

[3] Yokoyama, T., Kogoma, M., Kanazawa, S., Moriwaki, T., and Okazaki, S. (1990). The

improvement of the atmospheric-pressure glow plasma method and the deposition of organic

films . J. Phys. D: Appl. Phys. 23, 374-377.[4] Sawada, Y., Tamaru, H., Kogoma, M., Kawase, M., and Hasimoto, K. (1996). The reduction

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discharge . J. Phys. D: Appl. Phys. 29, 2539-2544.

[5] Laroussi, M., Sayler, G.S., Glasscock, B.B., McCurdy, B., Pearce, M.E., Bright, N.G., and

Malott, C.M. (1999). Images of biological samples undergoing sterilization by a glow

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[6] Mangolini, L., Orlov, K., Kortshagen, U., Heberlein, J., and Kogelschatz, U. (2002). Radial

structure of a low-frequency atmospheric-pressure glow discharge in helium . Appl. Phys. Lett.

80, 1722-1724.[7] Tochikubo, F., Chiba, T., and Watanabe, T. (1999). Structure of low-frequency helium glow

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[8] Massines, F., Rabehi, A., Decomps, P., Gadri, R.B., Segur, P., and Mayoux, C. (1998).

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[9] Mangolini, L., Anderson, C., Heberlein, J., and Kortshagen, U. (2004). Effects of current

limitation through the dielectric in atmospheric pressure glows in helium . J. Phys. D: Appl.

Phys. 37(7), 1021-1030.[10] Scharfetter, D.L. and Gummel, H.K. (1969). Large signal analysis of a Silicon Read diode

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Numerical and Experimental Study of Atmospheric Pressure Glows in Helium