WDS'14 Proceedings of Contributed Papers — Physics, 271–274, 2014.
ISBN 978-80-7378-276-4 © MATFYZPRESS
∗
Department of Experimental Physics, Comenius University, Mlynská dolina F-2, 842 48 Bratislava, Slovakia.
Abstract. This work presents an experimental study of the high pressure alternative current (AC) microdischarges over wide range of frequencies in Argon. The microdischarges were generated between two plan-parallel molybdenum electrodes in a high vacuum chamber filled with pure Ar gas. The breakdown voltages from the right side of the Paschen curves and the corresponding Volt–Amper (VA) characteristics of the microdischarges were measured as a function of the electrode separation (from 2.5 um to 100 um), pressure (from 150 mbar up to atmospheric pressure) and the frequency range of 100 Hz up to 13.56 MHz. For higher frequency lower breakdown is obtained because the ions are trapped in the gap and only the electrons are still lost to the walls.
In many applications, the non-equilibrium plasma source operating in the glow mode, are highly desired [1, 2]. However, stable diffuse glow discharge at high pressure is a challenging problem due to their transition to an arc [3, 4] One of the ways how to stabilize such a high-pressure plasma is to spatially confine it to the dimensions below 1 mm, leading to the so -called microplasmas [3]. The microdischarges have a relatively short history and many mechanisms and phenomena occurring in microdischarges are not clearly understood especially in case of AC discharges. In the recent years, the microdischarges have been applied in different fields of technology, such as material processing, chemical synthesis, plasma medicine etc. [5–7]. Therefore the knowledge of the physical phenomena occurring in the microdischarges is very important [8–12]. The electric breakdown of the discharge is described by the Paschen’s law, where the breakdown voltage depends on the product of pressure and electrode distance — pd . In microdischarges different processes responsible for electric breakdown in gases are present in comparison to standard macroscopic discharges [10–12]. These processes are related to the high electric fields and thus deviations from Paschen law were observed. The main goal of this paper is to study the breakdown phenomena of high pressure microdischarges over wide range of frequencies in Argon and their comparison to the breakdown phenomena in macroscopic discharges.
The schematic view of the experimental apparatus is shown in Figure 1. It consists of vacuum chamber with a glass cross vacuum chamber and two xyz and tilting manipulators of electrodes. The electric discharge was generated in argon (99.996 % Linde Gas) between two plan-parallel molybdenum electrodes. The diameter of the electrodes was 8 and 10 mm respectively while the upper electrode had Bruce profile. The distance between the electrodes was set by micrometer linear feedthrough from 2.5 μm to 100 μm. The Volt –Amper characteristics were measured using fast oscilloscope (Tektronix TDS 3032), connected to high-voltage probe (Tektronix P6015A) and to current probe (Pearson 2877) for frequency range from 100 Hz up to 13.56 MHz. For such a wide range of frequencies three different power supplies were used. For the frequency range from 100 Hz to
10 kHz the power supply consists of wave generator connected to an acoustic amplifier and then through the high voltage transformer and 100 kΩ resistor to the discharge tube. For the frequencies between 2.4 MHz and 6 MHz homemade high frequency generator was used. The third source was a high frequency generator (400 W) connected with π – matching unit. Moreover, direct current (DC) measurements were carried out for comparison to AC microdischarges.
In this experiment the breakdown voltages of the microdischarges were studied according to three parameters (distance between electrodes, pressure and frequency). All measured data are presented as
∗ ladislav.moravsky@fmph.uniba.sk
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MORAVSKÝ ET AL.: STUDY OF RF MICRODISCHARGES IN ARGON root mean square Voltages (V rms
). In Figure 2, the breakdown voltage has been measured as function of pressure and frequency at constant electrode distance of 100 μm (Fig ure 2). As it is seen from the figure, the highest values of the breakdown voltages were measured for the DC electric field if V rms was taken for the AC discharges. If we consider the AC voltage amplitude, the breakdown voltage for
DC and AC below 13.56 MHz are more or less identical, weakly dependent on the frequency. The lowest measured values of the breakdown voltage were determined for 13.56 MHz, which are three times lower than DC breakdown voltage. This frequency is high enough so that the electrons with high drift velocities are lost on the walls but the ions are trapped in the gap since their drift velocity is significantly lower than of the electrons [13]. In Fig ure 3 is shown the dependence of the breakdown voltage as function of the distance between electrodes at constant gas pressure of 930 mbar. The measured data have similar character as in Figure 2. The highest values were achieved for DC field and the lowest for 13.56 MHz. The wave form indicates a capacitive discharge.
Figure 1. Vacuum chamber for microdischarges with different type of sources.
350
300
250
200
150
V r
100
50
DC - Argon BP 2014
AC - LF 0.1 KHz
AC - LF 1 KHz
AC - LF 10 KHz
AC - RF 2.4MHz
AC - RF 13.56 MHz
0 1
Figure 2. Breakdown voltages V rms
2 3 4 5 6 7 8 pd ( Torr.cm
)
for different pressures at the electrodes distance of 100 um.
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MORAVSKÝ ET AL.: STUDY OF RF MICRODISCHARGES IN ARGON
350
300
250
200
V r
150
100
50
DC Argon BP 2014
AC - LF 10 kHz ( 100-2.5
m m )
AC - RF 2.4 MHz ( 100-2.5
m m
AC - RF 13.56 MHz ( 100 -5 m m
)
)
1 10 100
Distance ( m m )
Figure 3. Breakdown voltages V rms
(930 mbar).
for different frequency and electrodes distances at ATM pressure
Figure 4. VA characteristics for f = 2.4 MHz, distance = 100 μm, [A] 150 mbar, pd = 1.1 Torr·cm.
Figure 5. VA characteristics for f = 2.4 MHz, distance = 100 μm, [B] 400 mbar, pd = 3 Torr·cm.
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MORAVSKÝ ET AL.: STUDY OF RF MICRODISCHARGES IN ARGON
Figure 6. VA characteristics for f = 2.4 MHz, distance = 100 μm, [C] 930 mbar, pd = 6.7 Torr·cm.
In Figures 4–6 the VA characteristics of the microdischarge at constant electrode separation of
100 μm and frequency of 2.4
MHz and at pressure of 150mbar, 400mbar and 930mbar are shown in sequence. The dotted waveform represents the current and voltage when discharge is off, and the strong lines represent waveforms when discharge is on. At low pressure both waveforms are smooth and nearly sinusoidal. At higher pressure current waveforms exhibit minor distortion due to higher ion and electron densities.
The experimental results have shown the differences in electrical properties of high pressure AC microdischarges in Argon. At low frequencies (from 100 Hz up to 3 MHz) no significant effects on breakdown voltages were measured however as the frequencies turns to 3 MHz rapid increase of breakdown voltage were observed. We assumed the frequency above 3 MHz is high enough so that the ions are trapped in the gap and the electrons with higher drift velocities are lost to the wall.
Acknowledgments. This work was supported by the Slovak Research and Development Agency, project
Nr. APVV-0733-11.
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