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

STABLE RADIO-FREQUENCY DISCHARGE WITH PURE NITROGEN AT ATMOSPHERIC-PRESSURE Hua-Bo Wang, Wen-Ting Sun, He-Ping Li, and Cheng-Yu Bao Department of Engineering Physics, Tsinghua University, Beijing 100084, China Xing Gao Beijing Center for Preventive Medical Research, Beijing 100013, China Corresponding author. H.-P. Li: [email protected] ABSTRACT

water-cooled

Pure nitrogen radio-frequency (RF) uniform glow discharge is obtained for the first time with two planar bare water-cooled copper electrodes at atmospheric pressure. The discharge characteristics of pure nitrogen, pure helium and nitrogen-helium mixture are studied in this paper. Experiments show that helium glow discharge can operate in two modes (α mode and γ mode), whereas, up to now, pure nitrogen glow discharge can only operate in the γ mode.

atmospheric-pressure is obtained for the first time.

bare

electrodes

at

Comparisons on the discharge characteristics of pure helium, pure nitrogen and mixture of He-N2 are also presented in this study.

2. EXPERIMENTAL SETUP 1. INTRODUCTION

A schematic diagram of the experimental setup is

As a new kind of non-thermal plasma source

shown in Fig. 1. The planar-type plasma generator is

developed in recent years, radio-frequency (RF),

composed of two 5×8 cm2 planar, bare, water-cooled

atmospheric-pressure glow discharge (RFAPGD)

metal electrodes, i.e. the RF (13.56 MHz) powered

plasmas, like other atmospheric-pressure glow

top electrode and the grounded bottom electrode. The

discharges (APGD), have notable advantages over

quartz spacers are used to seal the plasma generator

the traditional low-pressure glow discharges due to

on both sides and adjust the distance between the

the removal of the vacuum system, which reduces

electrodes from 0.4 to 1.6 mm with 0.1 mm step. The

the cost, and eliminates limitations on the

plasma forming gas (He with purity of 99.99% or

dimensions of the work-piece to be treated [1]. And

better, N2 with purity of 99.999% or better, or mixture

in contrast to atmospheric-pressure dielectric barrier

of He-N2 are used in this study) is admitted into the

discharge (DBD), RFAPGD does not need dielectric

plasma generator from the left side, ionized between

layers covered on the electrodes, which results in a

electrodes, and flows out of the generator from the

much

more

right side forming low-temperature plasma jet. The

homogeneous discharge [2]. Therefore, RFAPGD

rms values and waveforms of current and voltage, as

will have broad prospects in different fields, such as

well as the current-voltage phase difference, are

etching, deposition, decontamination of chemical

measured using a current probe (Tektronix TCP202)

and biological warfare agents, etc. [1]. Up to now,

and a high voltage probe (Tektronix P5100), and are

most of the researchers use helium (or with a small

recorded

fraction of O2, CF4, etc.) as the plasma forming gas

TDS3054B). The discharge images are taken by a

(e.g. [1], [3], [4]). But for actual applications, it is

digital camera (FUJIFILM s5500). The RF power input can be expressed as Pin = Vrms ⋅ I rms ⋅ cos θ ,

lower

breakdown

voltage

and

necessary to reduce the cost of the plasma forming gas. Wang et al [5] and Laimer et al [6] obtained

on

a

digital

oscilloscope

(Tektronix

where Vrms and Irms are the rms values of voltage and

RFAPGD plasmas with Ar-1.0%O2 and pure argon

current, respectively, θ is the current-voltage phase

as working gas, respectively. Yang et al [7] reported

difference. Thus, the average power density and

the discharge characteristics for the gas mixture of

current density of the discharge zone can be written as ρ in = Pin ( A ⋅ d ) and irms = I rms A , respectively,

He-0.4%N2. In this paper, a uniform, RF glow discharge for pure nitrogen with two planar

where A and d are the area of the discharge region and 377

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

the distance between electrodes.

Fig. 1. A schematic of the experimental setup. Similar to discharges at intermediate pressure, RFAPGD can exist in two distinctively different, but stable modes, i.e. α mode (sustained by volumetric

ionization

process)

and

Fig. 2. Images of RFAPGD at d=1.4 mm, f=13.56 MHz using copper electrodes. (a) α mode, Pin=108 W, θ=73°, χ=0, QHe=1.0 slpm; (b) α-γ mode, Pin=279 W, θ=22°, χ=0, QHe=1.0 slpm; (c) γ mode, Pin=156 W, θ=24°, χ=0, QHe=1.0 slpm; (d) γ mode, Pin=337 W, θ=42°, χ=0.5, QHe= QN2=1.0 slpm; (e) γ mode, Pin=214 W, θ=51°, χ=1, QN2=1.0 slpm.

γ mode

(ionization by secondary electrons from the electrode surfaces is important) [8, 9]. In this paper, RF glow discharge for pure nitrogen is obtained for the first time with the experimental setup shown in Fig. 1 as follows: the uniform glow discharge with He operated in α mode is obtained first; then, with increasing the input power, the discharge transfers

In this study, it is found that in a stable γ mode

into the γ mode; and then, with increasing the N2

discharge operated with He, N2 can be added into

flow rate and, at the same time, decreasing the flow

the plasma forming gas with sustaining a stable

rate of He, finally, a stable, uniform glow discharge

glow discharge, as shown in Fig. 2 (d), with

ρin=3786 W/cm3, and irms=2.3 A/cm2. For pure N2,

for pure N2 working in the γ mode is obtained when no addition of He in the plasma working gas any

the image of discharge in the γ mode is shown in

more.

Fig. 2 (e). Compared to Fig. 2 (c), a much clearer and brighter layer in the middle of the discharge region appears in the case with N2 addition.

3. RESULTS AND DISCUSSIONS 3.1 Visual description of the discharge

Because only a small fraction of the electrode area

Images of the RFAPGD operated with different gas

is covered with a discharge, the input RF power

mixing ratio χ (=QN2/(QN2+QHe)) at d=1.4 mm using

density and current density in Fig. 2 (e) becomes much larger (ρin=12170 W/cm3, irms=11.7 A/cm2).

copper electrodes are shown in Fig. 2. The volumetric homogeneous discharge (α mode) for

Figure 2 also shows that the current-voltage

pure helium at the flow rate QHe=1.0 slpm is shown

phase difference changes obviously for different

in Fig. 2 (a) with ρin=22 W/cm3 and irms=39 mA/cm2.

discharge modes and/or different working gases.

With increasing the RF input power, the coexistence

3.2 Voltage and current waveforms

of the α mode and γ mode (α-γ mode) can appear as shown in Fig. 2 (b). By disturbing the flow field artificially, a γ mode discharge can be obtained, shown in Fig. 2 (c), with higher values ρin=1420 W/cm3, irms=1.54 A/cm2, due to the significant reduction of the discharge region. 378

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

Ref. [9], in which the distortion of the current waveform was larger than that of the voltage waveform. 3.3 Voltage-current characteristics The features of the different operation modes of RFAPGD can be identified and characterized using the relationship between the amplitudes of the discharge voltage and the RF current, which is the so-called V-I curve. In Fig. 4 (a), a V-I curve is shown

for

pure

helium

discharge

using

water-cooled copper electrodes, which indicates three different operation regimes: (1) Prior to breakdown (A-B), the RF current increases with Fig. 3. Waveforms of voltage (solid line) and current (dashed line)

increasing the voltage linearly with a nearly 90°

for discharges operated in the γ mode at d=1.4 mm, f=13.56

current-voltage phase difference. (2) Once the

MHz using copper electrodes. (a) Pin=180 W, χ=0, QHe=1.0 slpm;

applied voltage reaches the breakdown voltage, the

(b) Pin=179 W, χ=0.5, QHe= QN2=1.0 slpm; (c) Pin=172 W, χ=1,

discharge is initiated in the α mode with a little bit

QN2=1.0 slpm; (d) Pin=300 W, χ=1, QN2=1.0 slpm.

lower discharge voltage than the breakdown voltage. Then, with increasing the RF input power, the RF

The current and voltage waveforms of RFAPGD

current and discharge voltage increase linearly

operated in the γ mode for different RF input

(B-C), but with a smaller slope than that before

powers at a gap spacing of 1.4 mm are shown in Fig.

breakdown. (3) When the input power (or the RF

3. It can be seen from Fig. 3 that: (1) the current and

current) increases to a certain value, a transition to

voltage waveforms of helium plasmas operated in

the γ mode occurs (C-D). In the γ mode regime,

the γ mode are both sinusoidal, as shown in Fig.

the discharge voltage varies very small with either

3(a); (2) with the addition of N2 to the original He

increasing (D-E) or decreasing (E-F) the RF current.

plasma working gas, the waveform of the voltage

When the RF input power or the RF current is lower

shows deviation from the sinusoidal form, as shown

than a certain value, the discharge transfers from the

in Fig. 3 (b). Fast Fourier transform (FFT) shows

γ mode to the α mode (F-G).

that the waveform of the voltage exhibits 6.8%

Up to now, direct pure nitrogen discharge in α

content of the third harmonic; (3) for pure N2

mode

discharge in the γ mode, the distortions of the

using

13.56

MHz

power

supply

at

atmospheric-pressure with bare metal electrodes

voltage waveform become more significant with the

cannot be obtained in our experiment, so, we can

increase of the power input. FFT shows that the

only provide the V-I curves of pure nitrogen

content of the third harmonic for the voltage

discharge operated in the γ mode. Figs. 4(b), (c) and

waveform increases from 9.6% in Fig. 3 (c) to

(d) show the V-I curves of the pure N2 discharge in

11.9% in Fig. 3 (d); (4) compared to the distortions

the γ mode using water-cooled copper, aluminum

of the voltage waveforms, the distortions of the

and stainless steel electrodes, respectively. From

current waveforms are smaller. FFT indicates that,

Figs. 4(a)~(d), it can be seen that: (1) for the case

in Fig. 3 (d), there exists no third harmonic in the

with copper electrodes, the V-I curves are almost

current waveform, and the content of the fifth

overlapped with increasing or decreasing the RF

harmonic is only 3.6%. This phenomenon differs

current; (2) the V-I curves for increasing or

from that of the helium glow discharge reported in

decreasing the RF current cannot be completely 379

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

overlapped for the cases with aluminum or stainless

driven by 13.56 MHz power supply. Experimental

steel electrodes; (3) for the same working gas flow

results show that helium glow discharge can operate

rate, the discharge voltage for sustaining the glow

in two stable operation modes (α mode and γ mode),

discharge of He in the γ mode is much smaller than

whereas, up to now, pure nitrogen glow discharge

that for N2 discharge.

can only work in the γ mode. And the different

As indicated in Section 1, ionization by

electrode materials have influence on the discharge

secondary electrons from electrode surfaces is very

characteristics of plasmas in the γ mode. The

important in the γ mode. So, the discharge

possibilities and methods for obtaining a uniform

characteristics are related with the electrode

glow discharge operated with N2 in the α mode need

materials due to the different values of the work

to be studied in future.

functions.

The

measured

averaged

discharge

voltages for sustaining N2 plasmas in the γ mode are

ACKNOWLEDGEMENT

284.7± 4.4 V, 269.6±4.7 V and 281.9±4.6 V for the

This work has been supported by the project

cases with copper, aluminum and stainless steel

sponsored by SRF for ROCS, SEM. The authors

electrodes, respectively.

gratefully acknowledge Prof. Xi Chen, Tsinghua University, China, for his very helpful comments, and Prof. J. Laimer, Institut für Allgemeine physik, Vienna University of Technology, Austria, for his helpful information.

REFERENCES [1]

[2]

[3]

[4]

[5]

[6] Fig. 4. V-I curves for different plasma forming gas at d=1.6 mm, f=13.56 MHz. (a) Copper electrodes, χ=0, QHe=1.0 slpm; (b) [7]

Copper electrodes, χ=1, QN2=1.0 slpm; (c) Aluminum electrodes,

χ=1, QN2=1.0 slpm; (d) Stainless steel electrodes, χ=1, QN2=1.0 slpm. [8]

4. CONCLUSIONS [9]

In this paper, a stable uniform RFAPGD operated with pure nitrogen is obtained for the first time using two bare planar water-cooled metal electrodes 380

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