Jianjun Shi, Fangchuan Zhong, Jing Zhang and Jianjun Shi, Fangchuan Zhong, Jing Zhang and Michael KongMichael Kong

College of Science, Donghua University, Shanghai, 201620 ChinaDept of Electronic and Electrical Eng, Loughborough University, Loughborough, UK

Control of operation modes in radio Control of operation modes in radio frequency Atmospheric pressure frequency Atmospheric pressure

glow Discharges by excitation glow Discharges by excitation frequencyfrequency

Towards stable & cold atmospheric plasmas

OutlinesOutlines Introductions

Operation modes in RF APGDs• Numerical characteristics;• Experimental studies;

Excitation frequency on RF APGDs• Mode expansion studied by simulation• Experimental studies on excitation

frequency;

Conclusions

Chengdou- 2007: page-2

Towards stable & cold atmospheric plasmas

Introductions

Widely applied low pressure radio frequency plasmasWidely applied low pressure radio frequency plasmas

Atmospheric pressure radio frequency plasmasAtmospheric pressure radio frequency plasmas

Chengdou- 2007: page-3

Towards stable & cold atmospheric plasmas

Operation modes in RF APGDs

0 20 40 60 80 100 120200

250

300

350

400

RM

S A

pplie

d vo

ltage

(V

)

RMS current density (mA/cm2)

region

region

-

-

0.0 0.2 0.4 0.6 0.8 1.00.0

0.4

0.8

1.2

1.6

2.0

2.4

Time normalized to cycle period

Inte

rele

ctro

de

po

sitio

n (

mm

)

0.0 0.2 0.4 0.6 0.8 1.00.0

0.4

0.8

1.2

1.6

2.0

2.4

Time normalized to cycle period

Inte

rele

ctro

de

po

sitio

n (

mm

)

Mode: VolumetricMode: Volumetric

Differential conductivity: Differential conductivity: PositivePositive

Mode: dominated in sheath regionMode: dominated in sheath region

Differential conductivity: Differential conductivity: NegativeNegative

Modes in RF APGDs-NumericalModes in RF APGDs-Numerical

Ref: J. J.Shi and M. G. Kong, J. Appl. Phys., 97, 023306 (2005).

Chengdou- 2007: page-4

Towards stable & cold atmospheric plasmas

Modes characterization

0 20 40 60 80 100 120100

200

300

400

500

600

700

RMS current density (mA/cm2)

Sh

ea

th t

hic

kne

ss (

m)

50

100

150

200

250

300

350

Sh

ea

th vo

ltag

e (V

)

Modes in RF APGDs-NumericalModes in RF APGDs-Numerical

Ref: Jianjun Shi and Michael G. Kong, IEEE Transaction on Plasma Science, 33, 624 (2005)

100 200 300 400 500 6000.1

1

10

n e,m

ax (

101

1 c

m-3

)

RF voltage amplitude (V)

=0.3 =0.1 =0.03 Mode transition dependent on Mode transition dependent on

secondary emission coefficient. secondary emission coefficient.

Sheath thickness:Sheath thickness:

Change of slopeChange of slope

Sheath voltage:Sheath voltage:

Change of differential conductivityChange of differential conductivity

Chengdou- 2007: page-5

Towards stable & cold atmospheric plasmas

Operation modes in RF APGDsModes in RF APGDs-ExperimentalModes in RF APGDs-Experimental

0.0 0.2 0.4 0.6 0.8 1.00

100

200

300

400

500

600

(H)

(G)(F)

(E)

(D)

(C)

(B)

(A)

RM

S V

olta

ge (

V)

RMS Current (A)

Ref: J. J. Shi, X. T. Deng, R. Hall, J. D. Punnett and M. G. Kong, J. Appl. Phys., 94, 6303-6310 (2003)Ref: J.J. Shi, D. W. Liu and M. G. Kong, Applied Phys. Lett., 90, 031505 (2007)

Mode: Volumetric across the discharge gapMode: Volumetric across the discharge gap

Mode: Constricted single columnMode: Constricted single column

Chengdou- 2007: page-6

Towards stable & cold atmospheric plasmas

Evolution of RF APGDModes in RF APGDs-ExperimentalModes in RF APGDs-Experimental

Chengdou- 2007: page-7

Towards stable & cold atmospheric plasmas

Taken by nano-second ICCD.Taken by nano-second ICCD.

Sheath region formed above twoSheath region formed above two

electrodes alternatively.electrodes alternatively.

Excitation frequency on modes

1 10 100 1000250

300

350

400

450 f =6.78 MHz f =13.56 MHz f =27.12 MHz

RM

S A

pp

lied

vo

ltag

e (

V)

RMS current density (mA/cm2)

0 5 10 15 20 25 30 350

50

100

150

200

250

RM

S c

urre

nt d

ensi

ty (

mA

/cm

2 )

Frequency (MHz)

300

330

360

390

420

450

RM

S a

pplie

d vo

ltage

(V

)

Frequency on modes-NumericalFrequency on modes-Numerical

Ref: J.J. Shi and M. G. Kong, Applied Phys. Lett., 87, 201501 (2005)

FrequencyFrequency 6.78 MHz6.78 MHz 13.56 MHz13.56 MHz 27.12 MHz27.12 MHz

Voltage (V)Voltage (V) 420.2420.2 356.6356.6 339.4339.4

Current density (mA)Current density (mA) 17.6817.68 49.549.5 176.8176.8

Chengdou- 2007: page-8

Towards stable & cold atmospheric plasmas

Excitation frequency on modesFrequency on modes-NumericalFrequency on modes-Numerical

Ref: J.J. Shi and M. G. Kong, Applied Phys. Lett., 87, 201501 (2005)

Chengdou- 2007: page-9

5 10 15 20 25 30 356

8

10

12

14

Frequency (MHz)

Max

imum

ele

ctro

n en

ergy

(eV

)

0

4

8

12

16

20

Max

imum

ele

ctro

n de

nsity

(10

11cm

-3)

1 10 100 1000100

200

300

400

500

600

700

0

100

200

300

400

500

RMS current density (mA/cm2)

Sh

ea

th th

ickn

ess

(m

)

f =6.78 MHz f =13.56 MHz f =27.12 MHz

Sh

ea

th v

olta

ge

(V

)

Towards stable & cold atmospheric plasmas

Plasma reactivity:Plasma reactivity: Mean electron energy increases;Mean electron energy increases; Electron density increases.Electron density increases.Sheath structure:Sheath structure:

Sheath voltage: similar to applied voltage;Sheath voltage: similar to applied voltage; Sheath thickness: sustain in Sheath thickness: sustain in mode and collapse in mode and collapse in mode. mode.

Excitation frequency on modesFrequency on modes-ExperimentalFrequency on modes-Experimental

Excitation frequency: double & tripleExcitation frequency: double & triple Discharge current range expanded;Discharge current range expanded; Applied voltage reduced.Applied voltage reduced.

With increasing excitation frequency:With increasing excitation frequency: Stable operation Stable operation region expanded. region expanded.

Chengdou- 2007: page-10

0 50 100 150 200 250 3000

100

200

300

400

500

RM

S v

olta

ge

(V

)

RMS current (mA)

6.78 MHz 13.56 MHz 20.34 MHz

0 5 10 15 20 25 300

20

40

60

80

100

120

RM

S c

urr

en

t de

nsi

ty (

mA

/cm

2 )

Excitation frequency (MHz)

Before gas breakdown

region

region

Towards stable & cold atmospheric plasmas

Voltage regime in modesFrequency on modes-ExperimentalFrequency on modes-Experimental

Breakdown voltage:Breakdown voltage: Decreases with excitation frequency;Decreases with excitation frequency; maintain in higher frequencies.maintain in higher frequencies.

With increasing excitation frequency:With increasing excitation frequency: Voltage regime is constricted.Voltage regime is constricted.

Chengdou- 2007: page-11

0 5 10 15 20 25 30200

300

400

500

600

RM

S v

olta

ge (

V)

Excitation frequency (MHz)

Minimum sustain voltage - transition voltage

0 5 10 15 20 25 30200

300

400

500

600

RM

S b

rea

kdo

wn

volta

ge

(V)

Excitation frequency (MHz)

Towards stable & cold atmospheric plasmas

Excitation frequency on modesFrequency on modes-ExperimentalFrequency on modes-Experimental

With increasing excitation frequency:With increasing excitation frequency: More reactive species are obtained in RF APGDs.More reactive species are obtained in RF APGDs.

Chengdou- 2007: page-12

0 50 100 150 200 250 300

0

5

10

15

20

OE

I @ 7

06

nm

(a

.u.)

RMS current (mA)

6.78 MHz 13.56 MHz 20.34 MHz

Using optical emission intensity to indicate the energetic electron density.Using optical emission intensity to indicate the energetic electron density.

Towards stable & cold atmospheric plasmas

ConclusionsConclusions

• Different operation modes (- and -mode) are found in RF APGDs in numerical simulation and experiments;

• The mode in RF APGDs is suggested to be expanded by higher excitation frequencies in simulation results.

• The expansion of mode in RF APGDs by higher excitation frequency is also found in experimental studies.

Chengdou- 2007: page-13

Towards stable & cold atmospheric plasmas

Thank you!Thank you!

Chengdou- 2007: page-14

Towards stable & cold atmospheric plasmas