Space resolved density measurements of argon
and helium metastable atoms in radio-frequency generated
He-Ar micro-plasmas
Benedikt Niermann1 , Marc Böke1 , Nader Sadeghi2 , Volker Schulz - von der Gathen1 and Jörg Winter1
1
Ruhr-Universität Bochum, Institute for Experimental Physics II, Germany, [email protected]
2
Laboratoire de Spectrométrie Physique, Université Grenoble & CNRS, France
Abstract: Space resolved concentrations of helium He* (3 S1 ) and argon
Ar* (3 P2 ) metastable atoms in an atmospheric pressure radio frequency
micro-plasma jet were measured using tunable diode laser absorption spectroscopy. Even small absorptions down to 10−4 could be measured using
lock-in technique. The absolute density of metastable atoms densities at
different rf-power, flow rate and gas mixture was deduced from measured
absorption rates. Metastable concentrations range from 109 to 1011 cm−3 . The
spatial distribution of metastable atoms in the plasma volume was obtained
for various discharge conditions.
Keywords: TDLAS, Microplasma, Metastable atoms
1. Introduction
Atmospheric pressure micro-discharges recently attracted high attention due to their potential applications in
materials treatment and modification, biological and medical applications, sensors, micro-reactors, etc. [1], as well
as due to their independence of large and expensive vacuum systems. To optimize the performance of these devices, it is inevitable to develop reliable and robust diagnostics, to understand the processes of energy transfer
and the transport of species inside the discharge. Application of conventional probe diagnostics, is often impossible regarding the small plasma dimensions, high operating pressures, and high power densities in micro-discharges.
Optical diagnostic techniques, however, offer a noninvasive and effective way to characterize these plasma regimes
[2][3][4][5][6][7].
We have applied tunable diode laser absorption spectroscopy (TDLAS) to record the spectral profiles of two
argon and helium absorption lines, deducing He* (3 S1 )
and Ar* (3 P2 ) metastable densities for various discharge
conditions. Due to their long lifetime, atoms in metastable
states are a reservoir of energy in the discharge, and stepwise ionization through these states is known to be an
important ionization mechanism in rare gas plasmas, especially when the electron temperature is low. Since rare
gases like helium and argon are typically used as carrier
gases for micro-plasmas, transitions from the He* (3 S1 )
state and from the Ar* (3 P2 /1s5 ) level were analyzed,
both representing the lowest metastable states of these
atoms.
high pressures. The design concept of this discharge is
based on the plasma jet introduced by Selwyn et al. in
1998 [8] and advanced by Schulz-von der Gathen et al.
[9]. Feed gas flows between two closely spaced stainless
steel electrodes driven at 13.56 MHz radio-frequency in a
parallel plate configuration (Fig. 1). Electrodes, plasma
volume and effluent are enclosed by quartz windows, giving direct optical access to the plasma itself and the outflow volume behind the electrodes. The discharge uses a
feed gas consisting primarily of an inert carrier gas, such
as He, and small amounts of an additive to be activated,
in our case argon. One of the electrodes is grounded,
the other is powered with the radio frequency through
an impedance matching network. The electric field between the electrodes causes a breakdown in the gas and
produces a plasma with the electron temperature and
density of about 1 to 2 eV and 1010 cm−3 , respectively
[10][11]. Atoms and molecules in the feed gas become excited, dissociated or ionized by electron impacts. Since
the electrons are not in thermal equilibrium with the ions
and neutrals, the gas temperature is believed to remains
a few tens of K above the room temperature [12].
The here presented jet configuration features a dielectric extension of the gas channel to assure controlled gasflow in the effluent behind the plasma. Electrodes and
dielectric extensions are 4 cm and 5 cm in length, respectively. The cross-chapter of the gas channel in the y-zplane is 1 mm x 1.8 mm (depth x height). The relevant
jet parameters are summarized in table 1. Under these
conditions the discharge operates as a typical α-mode rf
glow discharge, as evidenced from our results.
2. The atmospheric pressure micro-plasma
All power values given in this paper refer to the generjet
ator output. The real power coupled into the plasma is
The atmospheric pressure micro-plasma jet is a capac- only a small fraction of it, due to losses in the impedance
itively coupled, non-thermal glow-discharge plasma at matching and cables. With a gas velocity between 10
Figure 1: Sketch of the micro-plasma jet discharge.
Shown is a 2-dimensional cross-chapter through
the discharge channel.
Table 1: Operational parameters of the micro-plasma jet.
rf-power
(generator output)
8 to 30 W
13.56 MHz
Gas flow
Carrier gas (He)
1 to 5 slm
Admixture (Ar)
up to 10 %
Gas velocity
(laminar flow)
ca. 20 ms−1
Plasma volume (L x W x H)
40 x 1 x 1.8 mm3
Figure 2: Experimental Setup for TDLAS measurements.
Pressure
1 bar
Shown is the configuration for pulsed plasma
and lock-in measurement. PD1 to PD3 are
Photodiodes to measure the transmitted beam
and 30 ms−1 , the flow can be estimated to be laminar,
intensity.
preventing the influence of turbulent gas motion on the
metastable distribution. All gases used in the presented
transmitted beam intensity was measured by photodiodes
measurements have 5.0 purity.
with on-chip transimpedance amplifier. The wavelength
was tuned to the 23 S1 → 23 P0J ; (J = 1, 2) transitions of
3. Diagnostic methods
He at around 1083 nm and to the 1s5 → 2p9 transition
The small dimensions of micro-discharges and their op- of Ar at around 811.5 nm, respectively. The laser beam
eration at atmospheric pressures are a challenge for op- was guided into the jet perpendicular to the quartz wintical diagnostics, since high sensitivity and high spatial dows that confined the gas channel. With a spot size of
resolution are required. For the TDLAS measurements, less then 300 µm at the focal point this provides a reatwo commercial diode laser (DL) systems were used. This sonable spatial resolution in the x-z-plane of the plasma
was an external cavity DL 100 in Littrow configuration volume. The absorption length is limited to 1 mm, the
for the 811.5 nm argon line and a DL DFB for the 1083 distance between the windows. Directing the laser beam
nm helium line. For both DLs, the linewidth of the laser through the longitudinal axis of the jet, i.e. through the
(<10 MHz) was much smaller than the width of the ab- complete gas channel, would provides an almost 40 times
sorption line at atmospheric pressure. Therefore the ab- longer absorption length and consequently a much strong
sorption profile could be recorded by scanning the laser absorption signal, but it wouldn’t be able to offer any
frequency across the absorption line without applying spatial resolution.
Since the absorption rate of the laser light by metastable
a deconvolution procedure. Figure 2 shows a sketch of
atoms
is very low, in the order of 10−3 after 1 mm abthe experimental setup. The laser beam from the DL
passed through two beam splitters. A part of the beam sorption length, lock-in technique was used to measure
was guided to a Fabry-Perot interferometer (1 GHz free the changes in signal intensity. Applying lock-in techspectral range) and an other part through a low pressure nique requires the pulsing of the signal to be measured.
reference cell to perform the calibration of the laser fre- This was realized by pulsing the rf-power coupled into
quency. The part of the beam transmitted through the the system, which consequently leads to a pulsing of the
first beam splitter was attenuated by neutral density fil- metastable density in the discharge. Details about the
ters with an optical density in the order of 3, and focused measurement process and the calculation of metastable
into the discharge with a beam power of less than 2 µW densities can be found in [17].
at 200 µm spot size, to avoid any saturation effects. After passing the discharge the beam was guided through a
set of apertures and filters to suppress the emission from
the plasma by reducing the collection angle and blocking
wavelengths different than the observed transitions. The
3 .0 x 1 0
-3
A b s o r p tio n
& s u p e r p o s e d V o ig t p r o file
H e
H ig h D e n s it y
1 .5
4 .0 E 1 0
3 .5 E 1 0
[m m ]
4 .0 x 1 0
-3
3
2 S
2 .0 x 1 0
1
3
- 2 P
2
-3
3
2 S
1 .0 x 1 0
V e r tic a l P o s itio n
A b s o r p tio n
-3
2 3S
1
5 .0 x 1 0
1
3
- 2 P
1
-3
2 .8 E 1 0
1 .0
2 .1 E 1 0
1 .4 E 1 0
6 .7 E 9
0 .5
-4 .8 E 8
L o w D e n s it y
0 .0
0 .0
0
F a b ry -P e ro t
-1 .0 x 1 0
5
1 0
1 5
2 0
H o r iz o n ta l P o s itio n
-3
2 5
3 0
3 5
4 0
[m m ]
R e fe re n c e
Figure 4: 2-dimensional map of the Ar* metastable density in the discharge volume for the Ar*
1s5 → 2p9 transition. Powered electrode at the
Figure 3: Pressure broadened absorption line profile of
top, grounded electrode at the bottom.
the He* 23 S1 → 23 P0J (J = 1, 2) transitions
(blue). The spectral profile was fitted with a
superposition of two Voigt profiles (red).
Spatial distribution of metastable atoms
0
2
4
6
8
1 0
1 2
1 4
1 6
1 8
R e la tiv e F r e q u e n c y
2 0
2 2
2 4
2 6
2 8
3 0
[G H z ]
4. Results and discussion
Broadening at atmospheric pressure
Figure 3 shows an example of the absorption profile
of He(1083 nm) lines. The helium profile results from
the superposition of two transitions lying close to each
other. They overlap due to their wide pressure broadened widths. All lines were fitted with a Voigt profile
which is clearly dominated by the Lorentzian component.
The calculated Doppler widths at room temperature are
1.72 GHz for the He* lines, and 0.73 GHz for the Ar* line,
respectively. Measured Lorentzian widths are 11.2 GHz
for He* and 12.7 GHz for Ar*. In helium, the profile
was expressed by the superposition of two Voigt profiles
with a separation of 2.3 GHz and with an intensity ratio
of 5:3, according to their statistical weights. The Gaussian part of the lines, resulting from Doppler broadening,
was calculated by assuming a gas temperature of around
300 K. The Lorentzian component was varied to fit the
measured line profile. The 11.2 GHz Lorentzian width of
the helium lines is about 40 % smaller than the 19.5 GHz,
recently published by Urabe et al, but measured in an
atmospheric pressure, room temperature DBD discharge
[?]. But only 20 % smaller than the 14.0 GHz, previously
published by the same group [3]. We do not have any
explanation for this scattered values and to exclude the
influences of the technique on the measurement, we have
recorded both profiles either by using the Lock-In amplifier technique or by direct method. Both methods provided similar line profiles. One possibility could be an
underestimation of the gas temperature, reported for the
RF micro-discharges used in this work [9][16].
Unless otherwise stated all measurements presented in
this paper have been taken directly in front of the powered electrode, where the metastable density is highest.
In the horizontal axis the point of measurement was between 5 and 10 mm from the edge of the electrode on the
gas-inlet side.
Figure 4 shows a 2D-map of the Ar* (3 P2 ) metastable
density in the discharge volume. Both, horizontal and
vertical axis show the exact area between the electrodes.
The map covers 2.000 reading points (40 vertical x 50
horizontal) of the absorption signal in the plasma volume.
Averaging the signal and subtracting the plasma emission
background intensity for every data point results in a
measuring time of about 20 hours for every map. Hence
the influence of temporal effects on the metastable profile
in the volume can not be excluded. We assume, however,
that the discharge reaches an equilibrated condition after
one hour of warming up before the measurement starts.
The measurement shown here was made in a He discharge with 3 % Ar admixture. The power was chosen
far away from the arcing threshold of the discharge to
ensure stable operation during the long measurement period. The low power, however, resulted in a shortening of
the plasma volume, and the area between the electrodes
was not completely filled with visible plasma. That impression is supported by the presented measurements,
showing a steady decrease in particle densities in the second half of the discharge volume. The shortening of the
plasma is caused by a small misalignment of the electrodes. This effects the electrode spacing at the end of
the jet to be marginally larger, thereby assumedly changing the breakdown condition and forcing the plasma to
retreat. Concerning the effluent region behind the electrodes, no metastables can be detected even at higher
power conditions, which is comprehensible regarding the
short lifetimes. With a feed gas flow of 2 slm in the discharge channel and a metastable lifetime less than 10 µs,
the metastable density is expected to vanish after about
300 µm (3 times their decay length) without further excitation.
The absorption measurements inside the jet volume reveal the highest metastable densities in a short distance
from the surface of the electrodes. This behavior is consistent with our understanding of the sheath structure in
an RF excited discharge. Directly in front of the electrodes, the metastable density is low since the electron
density is too low for an efficient excitation and ioniza-
1 0
5 .0 x 1 0
1 0
4 .0 x 1 0
1 0
3 .0 x 1 0
1 0
2 .0 x 1 0
1 0
1 .0 x 1 0
1 0
1 .4 x 1 0
1 1
1 .2 x 1 0
1 1
1 .0 x 1 0
1 1
8 .0 x 1 0
1 0
6 .0 x 1 0
1 0
4 .0 x 1 0
1 0
2 .0 x 1 0
1 0
behavior reveals the dominance of the α-mode regime in
this RF excited atmospheric microdischarge.
-3
[c m
M e ta s ta b le D e n s ity
1
0 .0
0 .0
0 .0
0 .2
0 .4
0 .6
D is ta n c e fr o m
0 .8
1 .0
1 .2
G r o u n d e d E le c tr o d e
1 .4
1 .6
3
H e 2 S
A r 1 s
5
M e ta s ta b le D e n s ity
[c m
-3
]
A r 1 s 5 M e ta s ta b le s
3
H e 2 S 1 M e ta s ta b le s
]
6 .0 x 1 0
1 .8
[m m ]
Acknowledgements
This project is supported by DFG (German Research
Foundation) within the framework of the Research Group
FOR1123, and by the Research Department ’Plasmas
with Complex Interactions’ at Ruhr-University Bochum.
References
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