2. Numerical Simulation of Atmospheric Pressure Discharges Controlled By Dielectric Barrier Wang Dezhen, Wang Yanhui, Zhang Yuantao State Key Laboratory for Materials Modification by Laser, Ion and Electron Beams, Department of Physics, Dalian University of Technology, Dalian 116024, P.R. China

3. Outline Model and equations Multipeak and mode of the homogeneous discharges in atmospheric pressure helium Radial evolution of the glow discharge in atmospheric pressure helium Inhomogeneity of the discharge in atmospheric pressure helium

4. Model and equations  n e /  t+  ·j e =S e  n i /  t+  ·j i =S i j e = – D e  n e –  e E n e j i = – D i  n i +  i E n i  ( x )  E /  t+j c (x,t)=j T (t)  G /  t = - e [ j i ( x 1,t) – j e ( x 1,t) ]  P /  t = e [ j i ( x 2,t) – j e ( x 2,t) A C Power Dielectric

5. Multipeak and mode of the homogeneous discharges Progress S.J.Okazaki, et al, “Stable glow plasma at atmospheric pressure”, J. Phys. D:Appl. Phys., 21, 836(1988) J.R. Roth, et al, “Surface Modification of Fabrics Using a One- Atmosphere Glow Discharge Plasma to Improve Fabric Wettability, Textile Res. J. 67(5), 359-369(1997) F. Massines, et al, “Experimental and Theoretical Study of A Glow Discharge at Atmospheric Pressure Controlled by Dielectric Barrier”, J. Appl. Phys. 83(6), 2950(1998) Yu B. Golubovskii, et al, “Modelling of the homogeneous barrier discharge in helium at atmospheric pressure”, J. Phys. D: Appl. Phys. 36, 39(2003)

6. Multipeak and mode of the homogeneous discharges Electrical characteristics with one current peak Fig.1 Electrical characteristics of the discharge with only one current pulse per half-cycle of the applied voltage. Simulation parameters: the discharge gap width is 0.7 cm, the amplitude and frequency of applied sinusoidal voltage are 3kV and 5kHz, respectively, the thickness and permittivity of dielectric barriers are respectively 0.1 cm and 7.5.

7. Multipeak and mode of the homogeneous discharges Multipeak characteristics Fig. 2 Current characteristic for discharge gap width equal to (a)0.5 cm, (b)0.4 cm, (c)0.3 cm, (d)0.2 cm. to (a)0.5 cm, (b)0.4 cm, (c)0.3 cm, (d)0.2 cm. (other conditions as fig.1.) (other conditions as fig.1.)

8. Multipeak and mode of the homogeneous discharges Electric field and electron and ion densities for glow mode Fig. 3 Spatial variation of the electrical field and the ion and electron densities over a half cycle for the gap width of 0.5cms. 1 corresponds to the first current peak, 2 corresponds to the second current peak. Cathode is at x=0.5cm. (other conditions as Fig.1)

9. Multipeak and mode of the homogeneous discharges Electron and ion densities for glow mode Fig. 4 Spatio-temporal developments of (a) electron density and (b) ion density over a half cycle for the gap width and (b) ion density over a half cycle for the gap width of 0.5cm. The instantaneous cathode is at x=0.5cm. of 0.5cm. The instantaneous cathode is at x=0.5cm. (other conditions as Fig.1) (other conditions as Fig.1) ( a )( b )

10. Multipeak and mode of the homogeneous discharges Electric field distributions for two modes (a) Glow mode(b) Townsend mode Fig. 5 Spatio-temporal distributions of the electric field over a half cycle for the gap width of (a) 0.5cms over a half cycle for the gap width of (a) 0.5cms and (b) 0.3cm.(other conditions as Fig.1) and (b) 0.3cm.(other conditions as Fig.1)

11. Multipeak and mode of the homogeneous discharges Electric field and electron and ion densities for Townsend mode Fig. 6 Spatial structure of the discharge at 0.3cm gap width. (a) electric field. (b) densities of electrons and ions. The numbers 1,2,3,4 corresponds four current peaks. The cathode is at 0.3cm. (other conditions as Fig.1)

12. Multipeak and mode of the homogeneous discharges Electron and ion densities for Townsend mode Fig. 7 Spatio-temporal developments of (a) electron density and (b) ion density over a half cycle for the gap width and (b) ion density over a half cycle for the gap width of 0.3 cm. The instantaneous cathode is at x=0.3cm. of 0.3 cm. The instantaneous cathode is at x=0.3cm. (other conditions as Fig.1) (other conditions as Fig.1)

13. Multipeak and mode of the homogeneous discharges Electric field and electron and ion densities for two modes Fig. 8 Glow mode of homogenous barrier discharge with one current pulse per half cycle. Simulating conditions: f=5kHz, U 0 =2400V, d=0.1cm, d g =0.7cm, ε=7.5. Fig. 9 Townsend mode of homogenous barrier discharge with one current pulse per half cycle. The barrier thickness d=0.2cm. Other conditions are the same as Fig 8.

14. Multipeak and mode of the homogeneous discharges Multipeak discharge operating in two modes Fig. 10 Multipeak discharge operating in two modes. Driving frequency f=10kHz, U 0 =3000V, d=0.1cm, d g =0.3cm, ε=7.5.

15. Multipeak and mode of the homogeneous discharges Current characteristic for different driving frequency Fig.11 Current characteristic under driving frequency equal to (a) 40kHz, (b) 10kHz. The gap width is equal to 0.3cm and other conditions are as Fig.1

16. Multipeak and mode of the homogeneous discharges Influence of the amplitude of applied voltage on the the discharge current characteristic Fig. 12 Influence of the amplitude of applied voltage on the behavior of the discharge current. The applied voltage is respectively equal to (a) 4000kV, (b) 3000kV, (c) 1800kV. The gap width is equal to 0.3cm and other conditions are as Fig.1

17. Multipeak and mode of the homogeneous discharges Conclusions 1.A homogenous atmospheric pressure DBD in helium, whether single peak discharge or multipeak discharge, even the same breakdown series of multipeak discharge, can operate in two different modes, i.e. Townsend and glow modes. 2. Discharge modes are governed by external parameters. The glow discharge usually requires thin dielectric layer, wide gas gap, big dielectric constant, high driving frequency or big peak voltage. Otherwise, Townsend discharge occurs. Moreover, in case of multipeak discharge, the Townsend discharge is more easily developed.

18. Radial evolution of the glow discharge in atmospheric pressure helium Progress L. Mangolini et al, “Radial structure of a low-frequency atmospheric- pressure glow discharge in helium”, Appl. Phys. Lett.,80(10), 1723(2002)

19. Radial evolution of the glow discharge in atmospheric pressure helium Radial evolution of the discharge current ExperimentSimulation The gap length is 6mm,driving frequency is 5khz,votage amplitude is 2.4KV L.Mangolini,et al, Appl. Phys.Lett. 80, 1722(2002) The gap length is 3mm,driving frequency is 10KHz,Votage amplitude is 2.0KV. The thickness and permittivity of dielectric barriers are respectively 0.1 cm and 7.5.

20. Radial evolution of the glow discharge in atmospheric pressure helium Radial evolution of the discharge current The gap length is 6mm,the driving frequency is 5KHz,and voltage is2.9Kv L.Mangolini,et al, Appl. Phys.Lett. 80, 1722(2002) The gap length is 3mm,the driving frequency is 10KHz,and voltage is 2.3Kv ExperimentSimulation

21. Radial evolution of the glow discharge in atmospheric pressure helium Radial evolution of ion and electron densities (One peak) Ion densities at time (a) 358.8μs, (b)359.1μs Electron densities at time (c) 358.8μs, (d) 359.1μs

22. Radial evolution of the glow discharge in atmospheric pressure helium Radial evolution of ion and electron densities (two peaks) Ion densities at time (a) 364.5μs, (b)364.8μs Electron densities at time (c) 364.5μs, (d) 364.8μs

23. Radial evolution of the glow discharge in atmospheric pressure helium Conclusions 1.In the case of single current peak, the breakdown firstly begins in the central region, and then expand to the edge. 2.In the case of two current pulses, the radial evolution of first peak is similar to the case of single current peak, but the breakdown for second peak firstly ignite at periphery and propagate toward to the center region. 3.Ion and electron densities are almost uniform along the radial direction in wide central region.