XVI International Conference on Gas Discharges and their Applications, Xi’an (China), September 11-15, 2006

EXPERIMENTAL OBSERVATIONS OF ATMOSPHERIC-PRESSURE, RADIO-FREQUENCY GAS DISCHARGES Wen-Ting Sun, Hua-Bo Wang, He-Ping Li, and Cheng-Yu Bao Department of Engineering Physics, Tsinghua University, Beijing 100084, China (Corresponding author. H.-P. Li: [email protected])

ABSTRACT

20° and never equal to zero. In this paper, a uniform, atmospheric-pressure, RF glow discharges operated

In this paper, a plasma torch with co-axial,

in different modes, e.g. α mode (sustained by

water-cooled, radio-frequency-powered bare copper

volumetric

electrodes is employed to study the discharge

(ionization by secondary electrons from the electrode

characteristics of non-thermal plasmas. Pure helium

surfaces is important) [3], using pure helium or

and mixture of helium/oxygen are used as the plasma

mixture of helium/oxygen as plasma-forming gas are

forming gas. With different operation parameters,

obtained

different discharge modes can be obtained. At certain

water-cooled copper electrodes. A specific discharge

operation parameters, a uniform glow discharge with

mode with the α-γ mode co-existing between

a α-γ co-existing mode between electrodes and with

electrodes and with nearly zero current-voltage phase

nearly zero current-voltage phase difference is

difference is observed at certain operation parameters.

observed in this study, which was not reported in

And a modified equivalent circuit model is proposed

previous papers. And correspondingly, a modified

for explaining this newly observed phenomenon

equivalent circuit model is proposed to explain this

qualitatively.

ionization

in

a

processes)

plasma

torch

and γ

with

mode

co-axial,

phenomenon qualitatively.

2. EXPERIMENTAL RESULTS 1. INTRODUCTION 2.1. Experimental setup The

atmospheric-pressure

non-thermal

plasmas

provide clear advantages over the traditional,

A schematic diagram of a plasma torch used for

low-pressure non-thermal plasmas due to the removal

generating glow discharge plasmas at atmospheric

of the vacuum system with reducing the capital cost

pressure is shown in Fig. 1. It consists of two 95 mm

of equipments and eliminating constraints on objects

long, coaxial, water-cooled copper electrodes. The

sizes imposed by vacuum compatibility in actual

inner diameter of the outer electrode is 19.2 mm,

applications,

deposition,

while the gap spacing is 1.6 mm. Plasma working gas

decontamination of chemical and biological warfare

(pure helium or helium/oxygen mixture is used in this

agents, etc. [1]. In previous papers, operation modes

study) passes through the annular space between the

of atmospheric-pressure, glow discharge non-thermal

electrodes, and an uniform glow discharge plasma is

plasmas driven by radio-frequency (RF) power

generated by applying RF power at 13.56 MHz to the

supply were discussed. In Ref. [2], it was indicated

center electrode.

that at different operation modes, such as abnormal

The rms values and waveforms of the discharge

glow mode, normal glow mode, recovery mode, the

voltage and current are measured using a high voltage

current-voltage phase difference was always above

probe (Tektronix P5100) and a current probe

such

as

etching,

(Tektronix TCP202), and recorded on a digital 413

XVI International Conference on Gas Discharges and their Applications, Xi’an (China), September 11-15, 2006

oscilloscope (Tektronix TDS3054B). The discharge image is taken by a digital camera (FUJIFILM S5500). Working Gas

Outer Electrode Nozzle Inner Electrode Plasma Jet

RF Power High Voltage Probe

Fig. 2. Image of a glow discharge in the α mode

Current Probe

using pure helium (QHe=10.0 slpm, Pin=96 W)

Water

without nozzle. Oscilloscope U

I

C RMS Voltage (V)

240

Fig. 1. Schematic diagram of the experimental setup. 2.2. Discharge with pure helium as working gas

α−γ mode D

B

E

160

F

120 80

Figure 2 shows a bright white/purple uniform glow

α mode

200

discharge with pure helium (99.99% pure) as plasma

α−γ mode

A 0.0 0.5 1.0 1.5 2.0 2.5 3.0 RMS Current (A)

forming gas in the α mode at a flow rate QHe=10.0 slpm and power input Pin=96 W. The corresponding

Fig. 3. V-I curve of the discharge with pure helium as

voltage-current characteristics (V-I curve) in different

plasma working gas (QHe=10.0 slpm).

operation regimes are shown in Fig. 3. It can be seen 100 Phase Difference (degree)

from Fig. 3 that (1) before breakdown (A-B), the V-I curve is nearly a beeline; (2) after breakdown, the plasma is in α mode (B-C), and the V-I curve is also nearly a beeline, but with smaller slope than that before breakdown; (3) with the increase of the current, a α-to-γ mode transition occurs at an upper limit about 1.5 A, where a α-γ coexisting mode appears

80 60

current to the upper limit of the power supply, this

α mode C

F

40

D α−γ mode

20 0

-20

between electrodes. With the further increase of the

B

A

α−γ mode

E

0.0 0.5 1.0 1.5 2.0 2.5 3.0 RMS Current (A)

α-γ coexisting discharge mode also sustains. And

Fig. 4. Variation of the current-voltage phase

then, with the decrease of the current, a transition

difference with the current (pure helium, QHe=10.0

from the α-γ coexisting mode to the pure α mode

slpm).

takes place at I=0.5 A. In this operation regime (E-D-

In the α-γ coexisting discharge mode, the discharge

F), the variations of the discharge voltage are small

region of α-mode can cover part or full region of the

with increasing or decreasing the current. The

space between electrodes, which depends on the RF

corresponding current-voltage phase difference (θ),

input power, the water cooling status, and the flow

as the function of the current, is shown in Fig. 4. In

rate of the working gas.

this study, the experimental measurements are

A remarkable phenomenon observed in this study is

repeated 3 times and the averaged values with error

that a discharge status with zero or even negative

bars are presented in Figs. 3 and 4. The maximum

current-voltage phase difference exists in the α-γ

standard deviations in Figs. 3 and 4 are 15 V and 5°,

coexisting mode, which has not been reported in

respectively.

previous papers on atmospheric-pressure RF glow 414

XVI International Conference on Gas Discharges and their Applications, Xi’an (China), September 11-15, 2006

discharges. The waveforms of current and voltage

current-voltage phase difference would always be

with zero current-voltage phase difference are

positive,

illustrated in Fig. 5. This new phenomenon may be

observation

helpful for understanding the unique features of

current-voltage phase difference as described in

atmospheric-pressure glow discharges compared to

Section 2.

those

of

conventional

low-pressure

which

cannot

with

and

the

even

present negative

discharge X 1 = 1 / ωC1 = d s1 / ωεA

plasmas. 4

RP

300

2

150

0

0 -150

-2

X 2 = 1 / ωC 2 = d s 2 / ωεA

Voltage (V)

U

I Current (A)

zero

explain

Fig. 6. A simplified equivalent circuit model of the discharge [2].

-300

-4 -50

0

50 100 Time (ns)

150

Based on the experimental results presented in

200

Section 2, there should exist an inductance Z in the

Fig. 5. Waveforms of current (dotted line) and

equivalent circuit model in order to explain the zero

voltage (solid line) in a α-γ coexisting mode with θ=0

and even negative current-voltage phase difference

(pure helium, QHe=10.0 slpm).

observed in this study. Therefore, a modified equivalent circuit model, as shown in Fig.7, is

2.3. Discharge with helium/oxygen mixture as

proposed in this section. After discharge, the plasmas

working gas

can be regarded as a combination of sheath capacitance (Cs), resistance (RP) and impedance (Z) connected in series.

With addition of 0.1% oxygen (in volume) into the base working gas (QHe=10.0 slpm in this study), a bright uniform glow discharge in the α mode can be

I rf

Cs

obtained. Decreasing He flow rate but keeping the O2 flow rate constant (0.1 slpm), a α- to γ-mode

RP

Vrf

transition occurs at QHe=2.0 slpm. And then, increasing the flow rate of helium back to 10.0 slpm,

Z

a α-γ coexisting discharge mode, similar to that obtained with pure helium as working gas in Section

Fig. 7. A modified equivalent circuit model.

2.2, is obtained and the glow discharge covers the full gap spacing. The measured current-voltage phase difference can be also nearly zero (θ=-0.1°).

After discharge, the permittivity of the plasmas εp can be expressed as [5]

3. DISCUSSIONS

ε p = ε 0 (1 −

ω P2 ) ω + ν m2

(1)

2

In previous papers, a simplified equivalent circuit

Where ε0 is the permittivity of vacuum, νm is the

model [2-4] was employed to analyze the discharge

collision frequency for the momentum transfer, ω is

characteristics, as shown in Fig. 6, where X1 and X2

the frequency of the RF power supply, and ωP is the

represent the reactance of the sheaths, ds1 and ds2 are

plasma frequency, respectively.

the sheath thicknesses, respectively, and RP represents the plasma resistance. From Fig. 6, it can be seen that

For atmospheric-pressure glow discharges, there always exists the inequality ν m2 >> ω 2 [5]. Thus, the

this

permittivity can be approximately expressed as

equivalent

circuit

is

capacitive

and

the 415

XVI International Conference on Gas Discharges and their Applications, Xi’an (China), September 11-15, 2006

ε p = ε 0 (1 −

ω2 ωP2 ) ≈ ε 0 (1 − 2P ) 2 νm ω +ν m

modified equivalent circuit model, which can be

(2)

2

used to explain this phenomenon qualitatively, is

In Fig. 7, if εp is negative, the corresponding

proposed in this study. More detailed investigations

impedance Z will be inductive, i.e. the plasma

on this modified equivalent circuit model need to be

behaves like an inductor [6]. From Eq. (2), it can be

done in future work.

seen that εp can be negative if ωP>νm. For helium glow discharges, we have the relations [5, 6] (3) ωP = 5.56 × 104 ne [cm −3 ] [s −1 ]

ACKNOWLEDGEMENT

ν m = 3.8 × 106 P [Pa ] Te [eV ] [s −1 ]

This work has been supported by the project

(4)

sponsored by SRF for ROCS, SEM. The authors

where ne and Te are the number density and

gratefully acknowledge Prof. Xi Chen, Tsinghua

temperature of electrons, P is the pressure. As

University, China, for his very helpful comments,

reported in Ref. [7], for an atmospheric-pressure, RF

and Prof. J. Laimer, Institut für Allgemeine physik,

discharge plasma operated with helium in the γ mode,

Vienna University of Technology, Austria, for his

P=105 Pa, Te=1.5 eV, thus, the critical value of ne is

helpful information.

7×1013 cm-3 for ωP=νm. In Ref. [7], the estimated value of ne is 1.8 × 1013 cm-3 at a power density ~2083 W/cm3. In this study, the power density is

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416

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