PLASMATRON OPERATION MODES WITH HELIUM ATMOSPHERIC PRESSURE DIELECTRIC BARRIER DISCHARGE

Assoc.prof. Peter Dineff Ph.D., assoc. prof. Dilyana Gospodinova Ph.D, M.Sc.Eng. Momchil Shopov

Faculty of electrical engineering– Technical University, Sofia, Bulgaria

Abstract: A dielectric barrier discharge is an example of atmospheric pressure non-equilibrium plasma source widely used in industrial applications. There is a great increasing interest in atmospheric pressure discharges (APD) because they can be used for a wide range of technological applications without the need of vacuum systems. As a rule, helium is used as working gas in atmospheric pressure glow and dielectric barrier discharges (DBDs) at atmospheric pressure. The helium atmospheric pressure DBD generated in co-planar diode direct plasmatron was examined by monitoring the discharge electrode's current and voltage. The discharge generated in helium behaves differ-ently as compared to the one generated in air. The single discharge duration and the time between consecutive discharges are longer be-cause there is a different discharge evolution mechanism. The helium metastables concentration decay determines the discharge regime. The properties of these helium AP-DBD discharges in co-planar diode plasmatron were compared to the properties of air AP-DBD discharges in such plasmatron.

Keywords: co-planar diode plasmatron, dielectric barrier discharge, atmospheric pressure, helium, surface plasma-chemical modification, homogenous (type 2), glow, surface treatment, stepped ionization, non-equilibrium plasma.

1. Introduction The study of the dielectric barrier discharge (DBD) through

experimentally obtained external static, or volt-ampere, charac-teristic is a new engineering approach, introduced for discharge behaviour study in elementary gas – oxygen (Y. Filipov, Y. Emelyanov, 1957). The external static characteristic method was then successfully applied to the study of DBD's behaviour in air at atmospheric pressure (P. Dineff, D. Gospodinova, 2001).

The DBD's external static characteristic allows determining the different burning operation modes. The operation modes, thus determined, do not correspond to the transition from one elemen-tary process to another. It is established that the average value of the discharge current, respectively its density, is not specified as a quantitative measure of the elementary processes (dissociation, ionization, recombination and chemical conversion).

The discharge real power, and more often the real power sur-face density used, is seen as an essential characteristic of the discharge. This lays down the overall approach for determining the DBD's burning operation modes, which should be based on the discharge technological characteristic "real power - voltage". The complete or more precisely, the adequate differentiation between the operation modes of the helium atmospheric pressure discharge burning in co-planar diode plasmatron, fig. 1, can be realized only after studying its technological characteristics, obtained by calculations based on the external (volt-ampere) discharge characteristics, [5].

THE PURPOSE of this work is on the one hand to present the way the DBD burning operation modes are determined, and on the other hand to determine these modes for half-opened type plasmatron, [4], working with constant helium flow at atmos-pheric pressure: D = 0.70 dm3/min = const.

This helium flow is experimentally determined by prelimi-nary (screening) tests, showing the minimum gas expenditure at steady discharge burning. The main advantages of the plasmatron are the minimum gas and electricity expenditure at industrial frequency (50 Hz), fig. 2.

2. Determining the operation mode's real power critical parameters

The active power critical voltage (Ucrp,0; pS = 0) of the first operation mode coincides in the value with the current critical voltage (Ucr,0; Icr,0), or Ucrp,0 =Ucr,0, fig. 2а.

Fig. 1. Co-planar glass barrier (εr = 10) diode plasmatron,

producing non-equilibrium helium plasma at atmospheric pres-sure - half-opened plasma technology system.

HV-E – high voltage electrode; GE – grounded electrode; DB – dielectric (glass) barrier; B – a bottle of helium; М – pres-sure gauge ; RM – pressure reducing valve; MV – electromag-netic valve; С – insulation cage.

For all the other operation modes, however, it is not the same – both critical voltages differ considerably: Ucrp,0 ≠Ucr,0. The power critical voltage Ucrp,i is greater than the current critical voltage Ucr,i, fig. 3b.

This difference Ucr = Ucrp - Ucr can be determined in the following way using the current critical parameters and the dis-charge external static characteristic:

- from the fact that the critical point ( crpcrp IU , ) belongs to both operation modes, it follows that, fig. 3:

)()( 2,2,1,1,2,1, crcrpbrcrcrpbraa IIUIIUPP (1)

where Ubr,1 and Ubr,2 are the corresponding burning voltages of the studied DBD for each of the two operation modes ;

- for critical value Icrp is obtained, fig. 3:

)( 2,2,1,2,

1,1,2,2,crcrpcr

brbr

crbrcrbrcrp III

UUIUIU

I

; (2)

RM

He

He He He GE

Earth

НV

b d

MV

M

C p1

p0

p0

p0

p0 Въздух Въздух

B

M

62

Fig. 2. General appearance of barrier discharge at URMS = 10, 15 and 20 kV (50 Hz) in co-planar diode plasmatron with glass barrier at helium flow D = 0.7 dm3/min and atmospheric pres-sure (1013.25 hPa or mbar; 760 Torr). The reflection of the cage and the discharge on the glass barrier can be seen.

- for the difference between the two values current and power I is obtained, fig. 3b:

)()(

Δ1,2,

1,2,1,2,

brbr

crcrbrcrcrp UU

IIUIII

; (3)

- for the corresponding difference U is obtained, fig. 3b:

)()(

Δ1,2,2

1,2,1,2,

brbr

crcrbrcrcrp UUB

IIUUUU

; (4)

- the critical value is expressed through the burning voltage Ubr,2 and the current critical parameters Icr,1 and Icr,2:

22

2

2,

1,2,2

1,2,1,)()(

BA

BI

UUBIIU

U cr

brbr

crcrbrcrp

. (5)

The power critical voltage Ucrp separates the two operation modes, as to each of them correspond with different elementary

Fig. 3. Change of discharge current IAVG and real power surface density pa at discharge ignition (а) and transition from one burn-ing mode to another (b). The power critical voltage Ucrp,2 sets the beginning of a new burning mode at new and substantially differ-ent elementary processes.

processes in the discharge volume. It is greater than the current critical voltage Ucr,2, fig. 3:

2,1,2,2

1,2,1,)()(

crbrbr

crcrbrcrp U

UUBIIU

U

. (6)

3. Determining the operation modes The operation modes of AP-DBD burning in helium flow at-

mospheric pressure are determined according to the known methods, [5], after getting the external static characteristic and identifying the critical parameters of ignition and transition from one operation mode to another, as well as the burning voltage, typical for it.

The established dependence between the active power surface density pS and the burning voltage Ubr is used:

),( crAVGbra

S IIUSPp (7)

Dis

char

ge c

urre

nt I A

VG

Dis

char

ge c

urre

nt

ток

I

Ubr,1

Ucr,1 = Ucrpa,1 Icr

URMS

IAVG

Ucr,1

IAVG = B1 URMS + A

pa = Pa/S = D1 URMS + C1

A

Voltage URMS

Real

pow

er su

rface

den

sity

, pa

Ubr,2 Ucr,2

Voltage URMS

Real

pow

er su

rface

den

sity

, p a

Ucr,1 Ucrpa,2

Ucr,2 ≠ Ucrpa,2

IAVG

IAVG = B2 URMS + A2

pa = Pa/S = D2 URMS + C2

A

URMS

B

pa

pa

a)

b)

Icr

Icrp

10 kV

15 kV

20 kV

63

Table 1. Helium AP-DBD basic (Di, Ci) parameters and current (Ucr,i;

i = 0, 1, 2, ...) and power (pScr,i, i = 0, 1, 2, ...) critical parameters at small discharge gaps d.

Discharge gap d = 3 mm D1, W/(m2.kV) C1, W/m2 R2, - 0.8229 -1.3321 0.9504 D2, W/(m2.kV) C2, W/m2 R2, - 55.74 -292.69 0.9999 Ucr,0 = 1.619 kV pScr,0 = 0 W/m2 Ucrp,1 = 5.305 kV pScr,1 = 3.034 W/m2 Discharge gap d = 6 mm D1, W/(m2.kV) C1, W/m2 R2, - 4.3956 -10.893 0.9999 D2, W/(m2.kV) C2, W/m2 R2, - 58.084 -348.05 0.9999 Ucr,0 = 2.478 kV pScr,0 = 0 W/m2 Ucrp,1 = 6.280 kV pScr,1 = 16.711 W/m2 Discharge gap d = 9 mm D1, W/(m2.kV) C1, W/m2 R2, - 2.3864 -5.2206 0.9999 D2, W/(m2.kV) C2, W/m2 R2, - 50.961 -302,1 0.9999 D3, W/(m2.kV) C3, W/m2 R2, - 115.6 -1751,2 0.9999 Ucr,0 = 2.188 kV pScr,0 = 0 W/m2 Ucrp,1 = 6.112 kV pScr,1 = 9.365W/m2 Ucrp,2 = 22.418 kV pScr,2 = 840.362 W/m2

where S is the active electrode surface. For each of the studied working gaps with dimensions d = 3,

6, 9, 12, 15, 21 mm, are set the operation modes and their pa-rameters (D – slope, С – intercept), the power critical parameters (Ucrp,i and Icrp,i) and the corresponding burning voltage Ubr,i , Table 1 and 2.

Fig. 4. Changes real power surface density pS at discharge

ignition (working gap dimension d = 3 mm and glass barrier thickness b = 3 mm) – technological characteristic with two operation modes (В).

Technological characteristics with two or three operation modes are shown respectively on fig. 4 and 5. The non-operating mode, for which it is assumed that there predominates the capaci-tive nature of the electric current, passing through the discharge gap, is not shown, fig. 4 and 5.

Technological characteristics with two operation modes (А and В) are established for small (3 and 6 mm) and big (21 mm) working gaps. Technological characteristics with three operation

Table 2. Helium AP-DBD basic (Di, Ci) parameters and current (Ucr,i;

i = 0, 1, 2, ...) and power (pScr,i, i = 0, 1, 2, ...) critical parameters at large discharge gaps d.

Discharge gap d = 12 mm D1, W/(m2.kV) C1, W/m2 R2, - 4.9883 -12.307 0.9999 D2, W/(m2.kV) C2, W/m2 R2, - 48.416 -309.28 0.9999 D3, W/(m2.kV) C3, W/m2 R2, - 103.55 -1601.8 0.9999 Ucr,0 = 2.467 kV pScr,0 = 0 W/m2 Ucrp,1 = 6.838 kV pScr,1 = 21.805 W/m2 Ucrp,2 = 23.443 kV pScr,2 = 825.748W/m2 Discharge gap d = 15 mm D1, W/(m2.kV) C1, W/m2 R2, - 6.1772 -21.425 0.9999 D2, W/(m2.kV) C2, W/m2 R2, - 56.201 -334.53 0.9999 D3, W/(m2.kV) C3, W/m2 R2, - 144.59 -2635.6 0.9999 Ucr,0 = 3.468 kV pScr,0 = 0 W/m2 Ucrp,1 = 6.259 kV pScr,1 = 17.239 W/m2 Ucrp,2 = 26.033 kV pScr,2 = 1128.576W/m2 Discharge gap d = 21 mm D1, W/(m2.kV) C1, W/m2 R2, - 24.306 -142.14 0.9999 D2, W/(m2.kV) C2, W/m2 R2, - 63.132 -525.87 0.9999 Ucr,0 = 5.849 kV pScr,0 = 0 W/m2 Ucrp,1 = 9.883 kV pScr,1 = 98.084 W/m2

modes (А, В and С) appear for typical discharge gap sizes (d = 9, 12 and 15 mm), Table 1 and 2, fig. 4 and 5.

The three operation modes are shown on fig. 6 for each of the studied discharge gaps. These technological characteristics allow monitoring of the plasma-chemical surface processing result, choosing beforehand one of the three operation modes.

Fig. 5. Changes real power surface density pS at transition

from one operation mode to another (discharge gap size d = 21 mm and glass barrier thickness b = 3 mm) - technological characteristic with three operation modes (В).

An interesting discharge behaviour was observed at discharge gap size d = 6 mm, fig. 6b. The maximum value of the specific power pS = 1 400 W/m2 is realised at voltages around 17 kV (RMS), while for the small (3 mm) and very big working gaps (21 mm) this value gets closer, but already at voltages of about 30 kV (RMS).

0

200

400

600

800

1000

1200

1400

0 5 10 15 20 25 30 Voltage URMS, kV

0 50 100 150 200 250

0 2 4 6 8 10 12

A

A

B

B

C

b = 3 mm d = 21 mm

Real

pow

er su

rface

den

sity

p S, W

/m2

Ucrp,0

Ucrp,0

Ucrp,1

Ucrp,1

C

0

200

400

600 800

1000

1200 1400

1600

0 5 10 15 20 25 30 Voltage URMS, kV

Real

pow

er su

rface

den

sity

p S, W

/m2

0

10

20

30

40

0 1 2 3 4 5 6 7

A

A

B

b = 3 mm d = 3 mm

Ucrp,0

Ucrp,0

64

4. Burning voltage The helium atmospheric pressure AP-DBD burning voltage

Ubr,i, where i is the operation mode index, appears as the dis-charge's main characteristic – AP-DBD burns at constant value of the burning voltage, [5].

a)

b)

c)

Fig. 6. Helium atmospheric pressure barrier discharge burn-ing operation modes (a, b, c), presented by the technological characteristic „real power surface density pS – voltage URMS” at constant discharge gap size d.

The ignition (for the first operation mode) and burning volt-age (for the rest of the operation modes) increase fairly rapidly with the increase in the working gap size d, fig. 6.

The current and power critical voltages coincide only for the first operation mode Ucr,1 = Ucrp,1, while for the second and for the third operation mode the difference between them increases and becomes significant, fig. 7 and 8.

5. Barrier discharge development stages The change in the working gap size d, at constant pressure р0

and constant voltage URMS on the electrode system, breaks the similarity of the barrier discharges, as p1 d1 ≠ p2 d2 ≠ a = const (p1 = p2 = p0 = const). The discharges, burning at different dis-charge gap sizes d, are not similar, which means that they will ignite and burn in a different way.

Fig. 7. Ignition and burning voltage changes Ubr,i of helium barrier discharge at atmospheric pressure p0 at different opera-tion modes depending on the discharge gap size d.

In spite of the fact, that the characteristic „specific real power pS –discharge gap size d”, at constant value of the voltage URMS = const, is determined by calculations based on the AP-DBD discharge external static characteristic, it shows the differ-ent discharge burning stages as a sequence of typical peaks, which manifest themselves with the increase in the discharge gap size, fig. 9, 10, 11 and 12.

Fig. 8. Current Ucr,i and real power Ucrp,i critical voltages,

characterizing the different operation modes A, B and C of the helium barrier discharge at atmospheric pressure p0 and room temperature T0.

The barrier discharge, burning in helium at atmospheric pres-sure and at different voltage values - URMS = const, has three pronounced characteristic peaks – A, B and C. It can be easily established by comparing the respective technological character-istics, that AP-DBD, burning in helium, fig. 9, 10, 11 and 12, and AP-DBD, burning in air, fig. 13, pass, although in a different way, through the three known discharge development stages – the avalanche stage, the cathode directed (negative) streamers stage and the anode directed (positive) streamers stage, [5].

The noted differences are due mainly to the fact, that in the studied case of AP-DBD the discharge development is realised in an atomic inert gas – helium (99.5 weight percent), while in the

Curr

ent U

cr,i a

nd p

ower

Ucr

pa,i c

ritic

al

volta

ges,

kV

Discharge gap d, mm

0

5

10

15

20

25

30

0 3 6 9 12 15 18 21

Ucr,1

Ucr,2

Ucr,3

Ucrpa,1

Ucrpa,2

Ucrpa,3

Ucrpa,1 = Ucr,1

A

B

B B

C

0 1 2 3 4 5 6 7 8 9

10

0 3 6 9 12 15 18 21 Discharge gap d, mm

Igni

tion

and

burn

ing

volta

ge U

br, k

V

A

B

C

Barrier: b = 3 mm (alkaline glass)

B

B

Ubr,1

Ubr,2

Ubr,3

Real

pow

er su

rface

den

sity

p S, W

/m2

100 300 500 700 900

1100 1300 1500 1700

0 Voltage URMS, kV

5 10 15 20 25 30

9 12

15

Parameter: Discharge gap d, mm

0

Third operation mode (С)

0

200 400

600 800

1000

1200 1400 1600

0 5 10 15 20 25 30Voltage URMS, kV

Real

pow

er su

rface

den

sity

p S, W

/m2

6 3

12

21

21

9 15

Parameter: Discharge gap d, mm

Second operation mode (В)

0 1 2 3 4 5 6 7 8 0

5

10

Voltage URMS, kV

25

15

20

(9.883; 98)

Real

pow

er su

rface

den

sity

p S, W

/m2

3

Parameter: Discharge gap d, mm

9

12

15 6

21

First operation mode (А)

65

second case – in the stoichiometric mixture (air) of three gases (total 99.79 weight percent) – two of them are molecular (oxy-gen: 23 weight percent, and nitrogen: 75.5 weight percent) gases, and the third - atomic (argon: 1.29 weight percent) gas.

Fig. 9. Change of real power surface density pS depending on

the discharge gap size d at voltages URMS up to 6 kV (experiment) for helium barrier discharge at atmospheric pressure p0 and room temperature T0.

Fig. 10. Change of real power surface density pS depending

on the discharge gap size d at voltages URMS from 6 to 10 kV (experiment) for helium barrier discharge at atmospheric pres-sure p0 and room temperature T0.

A glance on the attached pictures of the discharge, fig. 2, is sufficient to note that there is a homogeneous barrier discharge, but only within the volume of the discharge space. In the border area, along the edge of the grounded electrode, there are distinct separate microdischarges, and they are located at the same place, regardless of the voltage value. This edge effect is due to the

electrode's geometry (strictly planar), which increases the electric field intensity along the length of its "sharp" edge. On the other hand the microdischarges along this edge identify specific points of local electric field unevenness with increased intensity.

Fig. 11. Change of real power surface density pS depending on the discharge gap size d at voltages URMS from 10 to 21 kV (ex-periment) for helium barrier discharge at atmospheric pressure p0 and room temperature T0.

The introduction of a certain radius of curvature along the edge of the grounded electrode GE (Rogovski’s electrodes) can reduce the number of observed microdischarges to their complete elimination, fig. 1.

Fig. 12. Change of real power surface density pS depending

on the discharge gap size d at voltages URMS from 22 to 30 kV (extrapolation) for helium barrier discharge at atmospheric pressure p0 and room temperature T0.

The different aspects, concerning the realisation of homoge-neous (diffuse, glow) atmospheric pressure barrier discharge, are being elaborated for a long time by the research groups of S. Okazaki (1987), F. Massines (1992), R. Roth (1995), N. Gheradi (1998).

1800

0 3 6 9 12 15 18 21 Working gap d, mm

27 28 29 30

24 25 26

23 22

Parameter: Voltage URMS, kV

Barrier: b = 3 mm (lkaline glass)

C B A

600

1000

1400

1600

Activ

e po

wer s

urfa

ce d

ensit

y p

S, W

/m2

1200

800

3 6 9 12 15 18 21 Working gap d, mm

0

100

200

300

400

500

600

Activ

e po

wer s

urfa

ce d

ensit

y p

S, W

/m2

700

800

900

1000

10 11 12

15 14 13

16

19 18 17

21 20

Parameter: Voltage URMS, kV

A B

Barrier: b = 3 mm (alkaline glass)

C

10

9

8

7

6

A

C

Barrier: b = 3 mm (alkaline glass)

Parameter: Voltage URMS, kV

B

0 3 6 9 12 15 18 21 Working gap d, mm

0

50

100

150

200

250

300

Activ

e po

wer s

urfa

ce d

ensit

y p S

, W/m

2

5 4 3 2

6 Parameter: Voltage URMS, kV

Barrier: b = 3 mm (alkaline glass)

C

B

A

0 3 6 9 12 15 Discharge gap d, mm

18 21 0

5

10

15

20

25

30

35

40

45

Real

pow

er su

rface

den

sity

p S, W

/m2

66

Fig. 13. Change of real power density pS depending on the

co-planar diode system's discharge gap size d at different voltage URMS = const, revealing three characteristic areas with ex-pressed maximum A, B and C, corresponding to different barrier discharge burning mechanism in air at atmospheric pressure.

New approaches to the elementary processes are searched for. The new engineering approach demonstrated for the study of DBD in helium, allows not only to set apart its operation modes, but also to get an integral picture of the elementary processes development, determining the three stages in the discharge de-velopment. Due to the limited space we won't make this analysis, but this can be easily done by comparing the typical peaks and their characteristics, shown on fig. 9, 10, 11 and 12 (for helium) and on fig. 13 (for air) for the same co-planar diode system.

Conclusion A full analysis has been made of the existing operation

modes of atmosphere pressure dielectric barrier discharge, burn-ing in half-opened co-planar diode plasmatron. The technological characteristics are shown allowing getting an idea of the real power surface density size, respectively of the discharge real power, burning in helium (99.5 weight percent).

The difference observed in the behaviour of the atmospheric pressure dielectric barrier discharge, burning in helium and in air has been analysed. This analysis has been presented, based on an original approach, using the “real power surface density – dis-charge gap size” characteristic. The different elementary proc-esses, characterising the different stages through which pass the atmospheric pressure dielectric barrier discharge, are presented integrally by the characteristic peaks observed.

Attention has been drawn to the fact, that with the exception of the first operation mode, the current and power critical pa-rameters of the other operation modes do not coincide. This difference depends on the two burning voltages of the adjacent operation modes and of the current critical values.

At the same general conditions (burning voltage and dis-charge gap size), the helium technological plasmatron, provides a plasma volume with much greater real power, which in turn means greater chemically active particles (CAPs) concentration and greater productivity in processing different types of surfaces.

A forthcoming improvement of the presented plasmatron should be aimed at reducing the edge effect observed. It is the main reason for the observing microdischarges on the grounded electrode's edge. This can provide conditions for homogeneous helium atmospheric barrier discharge in half-opened plasmatron.

Acknowledgements The present work was carried out with the exclusive financial

support of the National scientific research fund with the Minis-try of education, youth and science through research project HS-TSc 2005/2006.

References [1] Kogelschatz, U. Filamentary Patterned and Diffuse Bar-

rier Discharges. IEEE Transaction on Plasma Science, 2002, Vol.: 30, Issue: 4, pp. 14001408.

[2] Golubovskii, Y., V. Maiorov, J. Behnke1, J. F. Behnke1. Influence of Elementary Processes Over an Homogeneous Bar-rier Discharge in Helium. Journal of Physics D: Applied Physics, 2003, Vol.: 36, Issue: 1, pp. 39 44.

[3] Tepper J., M. Lindmayer. Investigations on two Different Kinds of Homogeneous Barrier Discharges at Atmospheric Pres-sure. HAKONE VII International Symposium on High Pressure, Low Temperature Plasma Chemistry, Greifswald, Germany, September 1013, 2000, pp. 16.

[4] Wagnera, H.-E, R. Brandenburga, K. Kozlovb, A. Son-nenfeldc, P. Michela, J. F. Behnke. The barrier discharge: basic properties and applications to surface treatment. Vacuum, 2003, Vol.: 71, pp. 417436.

[5] Dineff, P. Industrial plasma surface processes at atmos-pheric pressure. Х. International scientific conference on cutting-edge materials and processings „АМО‘10”, June 2729, 2010, Varna, Bulgaria. Abstracts collection, 2010, pp. 104105. Pro-ceedings, Sofia, Publishing house „St. Ivan Rilski”, 2010, pp. 182190 (in Bulgarian).

[5] Dineff, P., D. Gospodinova, М. Shopov. Plasma tech-nologies. – Laboratory exercise manual. Sofia, Avangarde-Prima, 2008 (in Bulgarian).

working gap d, mm

10

15

20

25

30

35

40

45

0 3 6 9 12 15

Activ

e po

wer s

urfa

ce d

ensit

y p S

, W/m

2

22

21

20 19

18 17 16

15

14

13 12 11 10 9

8

Al-Al

A B

C

Parameter: Voltage URMS, kV

Barrier: b = 3 mm (alkaline glass)

67