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Acoustic emission within an atmospheric helium
discharge jet
V. J. Law1, C. E. Nwankire2, D. P. Dowling2 and S. Daniels1
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Dublin City University, National Center of Plasma Science and Technology, Collins
Avenue, Glasnevin, Dublin 9, Dublin, Ireland
School of Electrical, Electronic and Mechanical Engineering, University College
Dublin, Belfield, Dublin 4, Ireland
Abstract: This paper describes the thermal gas effluent and ion acoustic pressure
wave interaction between the fundamental drive frequency and its harmonics within
an atmospheric helium discharge. Deconvolution of the acoustic signal and the
electrical signals reveal that the plasma jet undergoes a change in operational mode
from chaotic (where the plasma is spatially and temporally inhomogeneous at the
electrode surface) to stable (periodic in nature) when the plasma expands away from
the electrodes and into the reactor cylinder. This effect is strongly influence by the
helium flow and input power. In addition the generated acoustic signals is found to
have a frequency response to that of a closed-end cylinder column which supports
antinodes of n = 1 and 3. Decoding of the acoustic signal allows the helium thermal
gas temperature to be obtained: Tgas ~ 290 K. The signal allows the axial gap distance
between the jet nozzle and work surface to be estimated which has technology
importance in terms of plasma metrology and in the basic understanding of
atmospheric pressure plasma jet physics.
Keywords: corona discharge, phase-space diagrams, and acoustic emission.
1. Introduction
Atmospheric pressure plasmas are used in many engineering [1, 2], biological
applications [3], and medicine [4]. In recent years multivariate analyses tools
have been deployed to explore their plasma-surface interactions [5]. Acoustic
metrology is one such promising plasma metrology tool that has been
deployed on plasma welding processes [6] and reel-to-reel plasma process
systems [7]. This paper is focused on the examination of a low temperature
atmospheric pressure Corona discharge jet system called PlasmaStream™,
which was developed by Dow Corning Plasma Solutions. Amongst the
applications of this PlasmaStream™ source are: enhancing silicone adhesion
to steel [8, 9], control of cell adhesion [10] antimicrobial applications [11]
and the deposition of plasma polymerised coatings [12, 13, 14]. For this
plasma device the initial gas breakdown produces a corona discharge which is
spatially and temporally inhomogeneous, and then proceeds to stable and
homogenous volume discharge at high flow rates and powers. Understanding
this mode change in terms of electro-acoustic emission and gas flow
dynamics shall help in the control of low temperature treatment of heat
Chaos Theory: Modeling, Simulation and Applications
C. H. Skiadas, I. Dimotikalis and C. Skiadas (Eds)
© 2011 World Scientific Publishing Co. (pp. 255 - 264)
V. J. Law et al.
sensitive biological materials. The electro-acoustic being generated from the
abundance of fast flow thermal molecules and rarer abundance of activated
species (metastable and ions) that are rapidly quenched in the plume and at
the surface. This paper reports on a systematic study of the non-invasive
acoustic emission measurement of the PlasmaStream™ system as function of
plasma nozzle to work-surface gap distance and helium gas flow. The
analysis of which enables the physical geometry around the plasma-surface
interaction to be understood in terms of gas flow dynamics, and surface
response outcomes to discharge plume exposure. This paper is organised into
the following sections. Section 2 provides the measurement methodology.
Section 3 details the electrical measurements, and section 4 provides the
acoustic measurements and deconvolution. Finally, section 5 forms the
discussion of the data.
2. Measurement methodology
This work extends plasma acoustic metrology to an atmospheric helium
discharge jet (see figure 1). The aim here is to correlate the plasma acoustic
emission and electrical signals under varying spatial and temporal conditions
by varying the applied power, gas flow rate and axial gap distance (d)
between the jet nozzle and the work-surface.
HV 20 kV
Circuit current
Plasma Technics Inc,
ET series frequency agile
power supply
Closed end constrains
Node to minimum vibration
HV Probe
Discharge current
Volume discharge
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Quartz tube
(72 mm x 16mm)
(maximum vibration)
Figure 1: Schematic of Corona discharge atmospheric pressure jet, and model of closed
cylinder column.
The approach employed here is to model the discharge plasma jet as an
acoustic source that comprises a closed cylinder helium column of length (L
= 72 mm + end correction), csound is the sound velocity of the gas and
where the acoustic molecular / ion oscillation frequency (xn) is modelled in
Acoustic emission within an atmospheric helium discharge jet
equation 1. For this equation to hold true an end correction factor of 0.6 x the
cylinder radius is used [15]. For this reactor the end correction is 0.6 x 16
mm = 4.8 mm.
xn =
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For a closed cylinder column, the aperture defines the maximum pressure
vibration antinode, n, which is always an odd number. The initial value of csound
is determined from equation 2.
csound =
In equation 2, γ is adiabatic constant for helium (1.6), R is the gas constant
(8.123), Tgas is the helium gas temperature, and M is the molecular mass of
helium (0.004 kg mol-1).
For the Corona discharge cylinder jet geometries (see figure 1) reported here,
equation 1 yields an acoustic frequencies of X1 ~ 3.3 kHz, X3 ~ 10 kHz and X5
~ 16.5 kHz when the helium column is not baffled. Here we note that when
the jet nozzle is at an extended distance (~10 to 7 mm) from the work-surface
the helium jet aperture (201 mm2) is unrestricted. However as the jet nozzle
is moved progressively closer to the work-surface the diminishing annular
aperture ratio ((π x nozzle diameter) x d) between the jet nozzle and worksurface acts as a second aperture, resulting in an increased back-pressure in
the processing zone. To investigate the effect of second aperture on the
plasma acoustic emission, acoustic measurements of the plasma jet close to
the equality of nozzle-surface aperture area / nozzle aperture area = 1. Here
the values of d = 2.5 mm, 5 mm and 7.5 mm are used which correspond to
aperture ratio values of 1.8, 1.25, and 0.62.
2.1. Electrical measurements
The current voltage phase-space diagram, for 1, 4 and 10 standard litre per
minute (slm) of helium flow rates are shown in figure 2, where the current is
plotted on the x-axis and the voltage on the y-axis. The discharge impedance
is given along with the power; the data is plotted over 3 periods. For the 1
slm helium flow rate case (figure 2a), there is a chaotic, or strange, attractor,
where one positive current spike at maximum negative voltage is generated
along with a series of negative current spikes as the voltage becomes
positive In the positive voltage region, the negative current spikes have a
chaotic behaviour (that is for the three the periods the negative current spikes
do not overlay each other). With increasing helium flow the phase space
attractor become periodic, or cyclic, where a single negative current spike is
produced and grows in magnitude and width. The discharge impedance and
the drive frequency also falls. The associated digital image of each discharge
is shown to the right of each phase space figure. These images illustrates that
V. J. Law et al.
the discharge is constricted around the electrodes at 1 slm, and then
progressively expands in the chamber as the helium flow is increased. This
change in current waveform morphology is accompanied by an expansion of
the discharge into the Quartz cylinder.
1 slm
1.5 W
23 M Ω
4 slm
3.6 W
11 M Ω
10 slm
5.2 W
5 MΩ
Applied voltage (kV)
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Discharge current (mA)
Figure 2: Current-voltage phase space diagram as a function of molecular helium flow
rate for a fixed gap height of 2.5 mm. To the right of each phase-space portrait is the time
average discharge image.
3. Acoustic measurements and deconvolution
An omni directional condenser microphone placed next to the work piece and
within 20 mm of the plasma plume is used to capture the plasma sound
pressure energy. The information is processed with the soundcard of a
computer and analysed in the frequency-domain using LabVIEW 8.3
software [5 and 7]. For the experiments reported here the sound level of the
discharge was measured to be between 86 to 88 dB(A). Figure 3 depicts the
acoustic base-line of the system with no helium plasma and the power set to
zero (black line) and with the plasma present (blue line) for a discharge
Acoustic emission within an atmospheric helium discharge jet
He plasma
No plasma
17.87 kHz
35.72 kHz
Gain (dB)
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power = 5.2 W and helium flow rate = 5 slm. In this figure the base-line
acoustic power spectral density is confined to 0 to 5 kHz with discrete
emission peaks at the drive frequency of the power supply 17.87 kHz and its
harmonics. The helium plasma however exhibits the predicated acoustic
emissions at n = 3.5 kHz and n = 9.5 kHz with -3db bandwidths of ~2 kHz.
The fifth node may be present but is difficult to identify due to masking by
the power supply frequency. Further features of note are: acoustic signal of
the drive frequency is highly attenuated with harmonic number (-10dB per
harmonic integer) and each consecutive bandwidth increases from 0.2 kHz at
-10 dB at the fundamental to 1 kHz at the -10 dB level for the 4th harmonic.
3.5 kHz
53.67 kHz
10 kHz
71.5 kHz
Frequency (kHz)
Figure 3: Base-line acoustic measurements identifying predicted helium acoustic nodes
plus power supply drive frequency.
The following three figures (Figures 4, 5 and 6) survey the acoustic emission
range from 0 to 70 kHz as a function of axial gap distance (2.5, 5 and 7.5
mm) for fixed helium flow rates of 2, 4, 6 and 8 slm. In this survey the
discharge power is held constant at 5.2 W.
The analysis approach taken here is to deconvolve the complex acoustic
emission into peaks which have specific origins. For this to be successful the
plasma is required to have a degree of spatial and temporal stability, where
the phase relation between signals of different origins is preserved. This
intermodulation process is most likely to be present in the plasma volume
expansion mode and to a lesser degree in the Corona discharge mode. This
intermodulation between periodic time-vary signals is mathematically
expressed in equation 3.
f n ± x → f n− x , f n + x
V. J. Law et al.
Where fn is the fundamental drive frequency or one of its harmonics, and X is the
identified acoustic frequency due to molecule / ion oscillations.
X1 (3.5 kHz) mixes with f1 (18 kHz) to produce sidebands at 15 kHz and
21 kHz
X2 (9.5 kHz) mixes with f1 (18 kHz) to produce sidebands at 8.5 kHz and
27.5 kHz
The result of both of these intermodulation processes generates the
appearance of a double peak in the 8 to 10 kHz frequency region. Further to
this, the process also occurs on each of the drive frequency harmonics, but
this more difficult to observe due to the noise floor of the measurement.
8 slm
Gain (dB)
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The acoustic measurement (0 to 70 kHz) for d = 2.5 mm and an aperture ratio
0.62 is shown in Figure 4. The experimental measurement data reveals that
the acoustic signals originating from the molecules / ion oscillations (3.5 and
10 kHz) increase in amplitude to +10 dB above the noise floor as the helium
gas flow is increased from 2 to 8 slm. There is also a degree of
intermodulation with the drive frequency and its harmonics. For example in
the of the 8 slm case the flowing relationships are observed:
6 slm
4 slm
2 slm
20 24
28 32
36 40
48 52
60 64
Frequency (kHz)
Figure 4: 2.5 mm gap distance at a discharge power of 5.2 W
Figure 5 shows the results for d = 5 mm and aperture ratio of 1.25. The first
feature here to note is that drive frequency has been pulled to 17.6 kHz,
indicting an increase in plasma capacitance to the work-surface [16].
Intermodulation process is also occurring. However the intermodulation of f1 Xn=3 product now overlays the original Xn=3.acoustic signal. This superposition
generates strong sidebands around fo and f1 and f2 at low helium flow rates.
Acoustic emission within an atmospheric helium discharge jet
Figure 6 provides further measurements for d = 7.5 mm, and aperture ratio of
1.8 at a discharge power of 5.2 W. Under these conditions the acoustic
emission is found to be greater with respect to the small gap distance of 5 and
2.5 mm. The acoustic emissions are obtained some 15 dB above the noise
floor at high helium flow rate (8 slm). Intermodulation between the molecular
/ ion acoustic signals and the drive and its harmonics are observed to be
present at middle helium flow rates (6 and 4 slm). At low helium flow rates
only X1 interacts with the fundamental drive frequency.
Gain (dB)
6 slm
4 slm
2 slm
12 16 20 24 28
32 36 40
44 48
52 56 60 64 68
Frequency (kHz)
Figure 5: 5 mm gap distance at a discharge power of 5.2 W.
8 slm
Gain (dB)
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8 slm
6 slm
4 slm
2 slm
12 16
20 24
28 32
40 44
48 52
56 60
Frequency (kHz)
Figure 6: 7.5mm gap distance at 5.2 W discharge power.
64 68
V. J. Law et al.
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4. Discussion
This work has examined the electro-acoustic emission from a low temperature
discharge jet. In this configuration the jet exhibits a clarinet-like acoustic, or
closed-end cylinder behaviour. The jet is found to have two operational
modes: The electrical voltage-current phase-space diagrams map these two
modes. The first mode is a Corona / Filamentary discharge [17] that has
chaotic behaviour at low helium flow rates of 1-2 slm. Above this level of
helium flow the voltage-current phase-space attractor becomes periodic
(cyclic) with one major plasma ignition per cycle. Under these conditions, the
discharge fills the cylinder with a visible plume issuing from the nozzle to the
It is also found that there is a correlation of these modes with the electroacoustic emission. When the discharge is in the Corona / Filamentary mode
[17], the acoustic fundamental is present and the acoustic 3rd harmonic
(overtone) is weakly supported. When the discharge fills the cylinder
generating only one current event per cycle both the acoustic fundamental
and it 3rd harmonic (overtone) under go an intermodulation with the power
supply drive frequency.
The strongest acoustic measurement of the sound velocity is found at Xn=3
frequency (~10 kHz) at high flow rates when the plasma jet is at extended
distance from the work-surface with an aperture ratio of 1.8. Under these
conditions, the influence of the work-surface can be considered to be
As the axial gap distance is decreased to d = 5 mm and aperture ratio of 1.25,
the first acoustic node (Xn=1 = 3.5 kHz) signal increase in strength and
interacts with the drive frequency. Under these conditions, the second
aperture can be considered to be influencing the gas flow properties within
the plume-surface interaction zone.
Finally as the axial gap distance is reduced further (d = 2.5 mm) where the
aperture ratio is 0.62, the Xn=3 frequency signal increase with helium gas
Based on the electro-acoustic measurements made, it can be concluded that
the reactor geometries provide a unique electro-acoustic emission signature.
Within the acoustic signature an intermodulation process is identified with
the power source drive frequency and its harmonics. A simple acoustic model
(equations: 1 and 2) based on chamber geometry is used to describe the
thermal molecular acoustic emission. The model predicts a series of odd
number acoustic nodes (n = 1, 3, 5 ...). The value of n = 1 and 3 are
Acoustic emission within an atmospheric helium discharge jet
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confirmed by experimental measurements. The thermal gas temperature of
the discharge effluent is found by using equation (2). Consider first the
acoustic emission model (equation 2) for this reactor where L = 0.0768 m (72
mm + end correction of 4.8 mm) and a resonant fundamental frequency value
of 3.2 kHz. From these values a gas velocity of 983 m s-1 is obtained which
equates to a gas temperature of ~290 K (~170C). This value agrees well with the
thermal gas temperature (18oC) reported in reference [14] using an alcohol
thermometer. As the even 2nd harmonic (overtone) is not present (as predicted
by the closed-end cylinder model (equation 1) the 3rd harmonic (overtone) may
be used and does produce a similar gas velocity and helium gas temperature.
For the intermodulation process, electromagnetic interaction between activated
low temperature (400 ±50 K) nitrogen ions and radicals [17] and drive
frequency could be a possible mechanism. This relationship is mathematically in
equation (3) as a RF mixing mechanism. Undoubtedly the intermodulation
model must be more complex as the two electromagnetic sources have different
origins: the electrodes for the drive frequency signal and the nozzle aperture for
the acoustic. The coherence of these two signals will therefore be function of
axial distance and position of the microphone. Nevertheless the electro-acoustic
emission measurement proves a means of process monitoring by linking the
acoustic signature to gas temperature and gap distance and mode of operation.
The above experimental observations lead to a number of non-invasive acoustic
plasma metrology innovations to reference [7] and patent [18] in atmospheric
pressure plasma processing.
5. Acknowledgements
This work is partly supported by Enterprise Ireland grant CDT/7/IT/304 and the
Science foundation Ireland under grant 08/SRC/1141.
6. References
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liquid interface in atmospheric-pressure micro plasma with solution. Thin Solid Films
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[2] B. Twomey, D. P. Dowling, G. Byrne, L. O'Neill and L O'Hare. Properties of
siloxane coatings deposited in a reel-to-reel atmospheric pressure plasma system.
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V. J. Law et al.
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[18] Patent: GB 2459858 A (published: 11 November 2009), and Patent: WO
209/135919 A1 (published: 12 November 2009).
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