ON THE STRUCTURE OF DIELECTRIC BARRIER SURFACE DIS-CHARGES

A. B. Saveliev and G. J. Pietsch

Electrical Engineering and Gas Discharge Technology, Aachen University of Technology,52056 Aachen, GERMANY

Abstract. Pure surface discharges on a dielectric layer are an efficient source for non-thermalplasmas, which can be used for e. g. surface treatment and light sources. The structure of such dis-charges has been investigated with an intensified CCD camera at pulsed and AC voltage. Inde-pendent on polarity a channel structure has been found. Its character depends on polarity and thelevel of the applied voltage to the surface electrode. The structures can be explained by field dis-tortions resulting from surface charges.

1. INTRODUCTION

Common dielectric barrier discharges (DBDs) appear in arrangements with a gas gap (volume dis-charge, VD) and consist in general of numerous microdischarges. If AC voltage is applied to an ar-rangement with e. g. a wire electrode on the surface of a dielectric plate and an extended metallicelectrode on the reverse side, surface discharges (SDs) appear. The properties of VDs and SDs differ.While the length of the discharge in VD arrangements is determined by the gas gap, the SDs propagatestepwise on the dielectric surface with rising voltage.

There exist already some information about the structure of SDs by measuring the residual chargedensity on the dielectric surface after the extinction of the discharge, mainly based on the dust figuremethod [1]. However, these results refer most often on the application of pulsed voltages and are notrepresentative for normal AC operation.

In the following the discharge structure in dry air at atmospheric pressure is investigated with an inten-sified CCD camera depending on polarity, voltage amplitude and shape (pulsed and AC voltages).

2. EXPERIMENTAL SET-UP

The structure of the SDs has been examined in an arrangement depicted in Fig. 1. The grounded elec-trode was transparent by using salted water in between two glass plates. The capacity of a glass plateis 3.8 pF / cm2, its thickness 2 mm and the relative permittivity r = 8.6. The surface electrode has adiameter of 2 mm. If high voltage is applied to the surface electrode, SDs appear on the glass surfaceperpendicular to the surface electrode. The structure of the discharge is observed with a CCD-cameratogether with a quick triggering image amplifier in the visible wavelength range. The voltage is meas-ured with a voltage divider 1:1000 and a digital oscillograph, the current by integrating it in a capacityof 1 µF.

If rising voltage is applied to the surface electrode, first discharge phenomena, a discharge pulse, ap-pears at breakdown voltage. This pulse lasts about 50 to 200 ns [1]. During its activity charge is col-lected on the glass surface, which causes a counter-field. The discharge extinguishes by the growingcounter-field in the above-mentioned time period. The surface charge distribution after the pulse influ-ences the appearance and development of the succeeding discharge pulse.

As the detection of the discharge structure by the dust figure method has some limitations, the residualcharge distribution after discharge extinction can be detected only. A sensitive CCD camera (withimage amplifier) and exposure times down to the nanosecond range has been used to identify the dis-

charge structure.

Transparent electrode

Glass

High voltage

Oscilloscope

Surface electrode

Charge measurement

Gas

CCD-Camera

FIGURE 1. Sketch of the experimental set-up.

The discharge was investigated with supply voltages of two kinds, pulsed voltages of both polarities(peak voltage Up = ± 20 to 25 kV, rising time ~ 150 µs) and voltage oscillation packets (Up up to~ 23 kV, frequency ~ 20 kHz, number of oscillations 100 to 300, repetition frequency between packets0.5 to 10 Hz). Applying oscillation packets to the arrangement it is possible to operate with relativehigh voltages without overloading the device.

In order to observe the discharge structure and its development on an uncharged surface, it wascleaned with a conductive brush from residual charges before applying pulsed voltages.

3. DISCHARGE STRUCTURE AT PULSED VOLTAGE

In Fig. 2 and 3 photographs of the discharge development at positive and negative polarity at the sur-face electrode on the dielectric is shown. The exposure time is 10 µs in each case, significant longerthan the duration of the discharge pulse (50 to 200 ns) and much shorter than the voltage rise time upto voltage peak (150 µs). The frames do not belong to a single voltage pulse; each picture belongs to adifferent pulse.

The dark stripe in the frames marks the position of the surface electrode. At both polarities the dis-charge shows a channel structure in contradiction to the findings in [1]. They propagate mainly per-pendicular to the surface electrode. In general the higher the applied voltage the longer the dischargechannel.

4 6.5 9.5 12 14 16 18 19.5 22.5 kV

2

1

0 cm

FIGURE 2. Development of the discharge structure at pulsed voltage and positive polarity of the surface elec-trode (dry air of atmospheric pressure, exposure time of each frame 10 µs).

At positive polarity of the surface electrode (Fig. 2) the discharge channels are rather thin. Their thick-ness and light intensity do not change significantly in the whole voltage range and in different dis-tances to the electrode (except of the region nearby the electrode). The length of the more intensiveradiation region near the electrode rises with voltage as well.

At negative polarity of the surface electrode (Fig. 3) the channels are shorter, much broader, moreexpanded at their tips and sometimes branching in comparison to the opposite polarity. They are nar-row nearby the cathode (surface electrode) and get thicker with distance from the surface electrode. Ingeneral the light intensity decreases with the channel length. However, in the higher voltage region(from about 15 to 18 kV) the channel structure changes. Their number decreases and they becomemuch stronger (see – 14 and – 21 kV) discharging a broad area far from the surface electrode (in adistance of more than 8 to 10 mm). If stronger channel appear it happens occasionally that the areanearer to the surface electrode is discharged by shorter channels (see – 17 and – 20 kV).

0.5 2 5 9 12.5 15.5 18 19.5 20.5 kV

2

1

0 cm

FIGURE 3. Development of the discharge structure at pulsed voltage and negative polarity of the surface elec-trode (dry air of atmospheric pressure, exposure time of each frame 10 µs).

In Fig. 4 the channel lengths depending on voltage are presented for voltage pulses of both polarities.Up to 15 kV their lengths at positive polarity have twice the value of those at negative polarity. In theupper voltage range a division of the lengths at negative polarity in two parts happens. The longer andstronger channels have lengths nearly comparable with the channels at positive polarity, while theshorter channels are hindered in their propagation by the stronger ones.

The longer channel length the more charge is deposited on the surface resulting in a stronger counter-field, i. e. the residual effective field strength is reduced.

0

5

10

15

20

25

300 5 10 15 20 25

0 -5 -10 -15 -20 -250

5

10

15

20

25

30

mm

U, kV

channel length positive polarity negative polarity negative polarity (intense)

U, kV

FIGURE 4. Length of the discharge channels depending on the value of the pulsed voltage for both polarities(at the surface electrode).

The mean value of the electrical field strength in the region, which is more or less dense covered bydischarges, has a nearly constant value [2]. The value depends on polarity. As the (absolute) meanvalue of the potential at the tip of this region is much smaller than at the surface electrode (appliedpotential), it can be approximately estimated from the gradients in Fig. 4. For the negative polarityfollows a mean field strength component in parallel to the surface of ~ 1.6 kV/mm and about half thevalue ~ 750 V/mm for the positive polarity at the surface electrode.

In Fig. 5 the dependence of the charge (per unit electrode length) on voltage for both polarities of volt-age pulses is given. As the discharged capacity of the dielectric is proportional to the extension of thedischarge area it is proportional to voltage amplitude. And as the charge is furthermore proportional tothe capacity times voltage, the charge is proportional to the square of the voltage. This behaviour isconfirmed by measurements (Fig. 5). For negative polarity the changing of the discharge structuredisturbs this behaviour (for voltages more than 18 kV). According to the different extensions of thedischarges on the surface for the voltage polarities, the charge is approximately two times larger forthe positive polarity.

0

50

100

150

200

2500 5 10 15 20 25 30

Negative voltage Experiment Q = A*U*U

A = -0.225 nC/kV2/cm

U, kV

Q, nC/cm

Positive voltage Experiment Q = A*U*U

A=0.418 nC/kV2/cm

U, kV

0 -5 -10 -15 -20 -25 -300

-50

-100

-150

-200

-250

Q, nC/cm

FIGURE 5. Charge-voltage behaviour of SDs in air (charge per unit electrode length, voltage pulses).

4. DISCHARGE STRUCTURE AT AC VOLTAGE

The discharge structure at AC voltage is represented in Fig. 6 and 7. Residual charges on the surfaceinfluence the propagation. In Fig. 6 the voltage amplitude is so low that no strong channels appear. Forthe two left frames the surface electrode is positive with respect to the surface potential, the othersbelong to the opposite polarity.

5 0 mm

0 25 50

U

t, µs

FIGURE 6. Channel structure at AC voltage of low amplitude (7.1 kV, exposure time 5 µs).

With higher voltage amplitude the appearance of strong channels determines the discharges structure(Fig. 7). Because of charges on the surface from preceding discharge activities at AC voltage, strongchannels (of both polarities) appear already at voltages of 12 to 14 kV. The number of strong channelsis independent on polarity. Their position is almost fixed on the surface because of the charge distri-bution and local heating effects.

From short-time exposures (up to 20 ns) it is found that at positive polarity most often several chan-nels appear at the same position on the surface one after the other. At negative polarity the strongchannels are continuously burning at the same position up to voltage peak. The branching of the strongchannels is more homogeneous at negative polarity.

5 0 mm

5 0 mm

0 25 50

U

t, µs

FIGURE 7. Channel structure at AC voltage of high amplitude (23 kV) for exposure times 25 and 5µs.

5. CONCLUSIONS

With the help of high speed and sensitive CCD photographs it has been found out that the dischargestructure of SDs consists of channels even at negative polarity of the surface electrode. Furthermore,the character of the channels as well as the optical appearance of the discharge changes at a certainlevel of the applied voltage at negative polarity.

At AC voltage supply the structure is mainly determined by surface charges from preceding dischargeactivities. From this results strong discharge channels at both polarities at elevated applied voltages.

REFERENCES

[1] Gibalov V. I., and Pietsch G. J. , J. Phys. D: Appl. Phys., 33 2618-36 (2000)[2] Saveliev A. B., Gibalov V. I., Pietsch G. J., and Saenko V. B., Efficiency of ozone synthesis in the surface

discharge arrangements. In: 14th Int. Sym. on Plasma Chemistry, Prague, 1999, pp. 2343-8.