Concepts and characteristics of the ‘COST Reference Microplasma Jet’

Abstract

Biomedical applications of non-equilibrium atmospheric pressure plasmas have attracted intense interest in the past few years. Many plasma sources of diverse design have been proposed for these applications, but the relationship between source characteristics and application performance is not well-understood, and indeed many sources are poorly characterized. This circumstance is an impediment to progress in application development. A reference source with well-understood and highly reproducible characteristics may be an important tool in this context. Researchers around the world should be able to compare the characteristics of their own sources and also their results with this device. In this paper, we describe such a reference source, developed from the simple and robust micro-scaled atmospheric pressure plasma jet (μ-APPJ) concept. This development occurred under the auspices of COST Action MP1101 'Biomedical Applications of Atmospheric Pressure Plasmas'. Gas contamination and power measurement are shown to be major causes of irreproducible results in earlier source designs. These problems are resolved in the reference source by refinement of the mechanical and electrical design and by specifying an operating protocol. These measures are shown to be absolutely necessary for reproducible operation. They include the integration of current and voltage probes into the jet. The usual combination of matching unit and power supply is replaced by an integrated LC power coupling circuit and a 5 W single frequency generator. The design specification and operating protocol for the reference source are being made freely available.

Full text

1 © 2018 IOP Publishing Ltd Printed in the UK
Journal of Physics D: Applied Physics
J Golda et al
Printed in the UK
029503
JPAPBE
© 2018 IOP Publishing Ltd
52
J. Phys. D: Appl. Phys.
JPD
10.1088/1361-6463/aae8c8
2
Journal of Physics D: Applied Physics
There is an incorrect representation of the expression for
resistances in parallel in equation(1) in section 4.1 ‘Voltage
probe calibration’ on page 6. The numerator and denominator
in the equationare reversed and should read:
I
=UC
R
m
+R
t
RmRt
.
Rm is the measuring resistor, Rt the terminating resistor at the
oscilloscope and UC is the voltage drop across Rm induced by
the current I.
None of the calculations and conclusions of the paper are
affected.
The authors apologise for any confusion that this transcrip-
tion error may have caused.
ORCID iDs
J Golda https://orcid.org/0000-0003-2344-2146
J Held https://orcid.org/0000-0003-1206-7504
N St J Braithwaite https://orcid.org/0000-0002-1586-3736
A Sobota https://orcid.org/0000-0003-1036-4513
S Reuter https://orcid.org/0000-0002-4858-1081
T Gans https://orcid.org/0000-0003-1362-8000
D O’Connell https://orcid.org/0000-0002-1457-9004
V Schulz-von der Gathen https://orcid.org/0000-0002-
7182-3253
Corrigendum: Concepts and characteristics
of the ‘COST Reference Microplasma Jet’
(2016 J. Phys. D: Appl. Phys. 49 084003)
JGolda1, JHeld1, BRedeker1, MKonkowski1, PBeijer2, ASobota2,
GKroesen2, NStJBraithwaite3, SReuter4, MMTurner5, TGans6,
DO’Connell6 and VSchulz-von der Gathen1
1 Experimental Physics II: Application Oriented Plasma Physics, Ruhr-Universität Bochum,
44801 Bochum, Germany
2 Eindhoven University of Technology, Eindhoven, Netherlands
3 School of Physical Sciences, The Open University, Milton Keynes, Buckinghamshire MK7 6AA, United
Kingdom
4 Leibniz Institute for Plasma Science and Technology, 17489 Greifswald, Germany
5 School of Physical Sciences and National Centre for Plasma Science and Technology, Dublin City
University, Dublin 9, Ireland
6 Department of Physics, York Plasma Institute, University of York, York YO10 5DD, United Kingdom
E-mail: svdg@ep2.rub.de
Received 8 October 2018
Accepted for publication 16 October 2018
Published 5 November 2018
Corrigendum
IOP
Original content from this work may be used under the terms
of the Creative Commons Attribution 3.0 licence. Any further
distribution of this work must maintain attribution to the author(s) and the title
of the work, journal citation and DOI.
2019
1361-6463
1361-6463/19/029503+1$33.00
https://doi.org/10.1088/1361-6463/aae8c8
J. Phys. D: Appl. Phys. 52 (2019) 029503 (1pp)

1 © 2016 IOP Publishing Ltd Printed in the UK
JGolda1, JHeld1, BRedeker1, MKonkowski1, PBeijer2, ASobota2,
GKroesen2, NStJBraithwaite3, SReuter4, MMTurner5, TGans6,
DO’Connell6 and VSchulz-von der Gathen1
1 Experimental Physics II: Application Oriented Plasma Physics, Ruhr-Universität Bochum,
44801 Bochum, Germany
2 Eindhoven University of Technology, 5612 Eindhoven, The Netherlands
3 Department of Physical Sciences, The Open University, Milton Keynes, Buckinghamshire MK7 6AA, UK
4 Leibniz Institute for Plasma Science and Technology, 17489 Greifswald, Germany
5 School of Physical Sciences and National Centre for Plasma Science and Technology, Dublin City
University, Dublin 9, Ireland
6 York Plasma Institute, Department of Physics, University of York, York YO10 5DD, UK
E-mail: svdg@ep2.rub.de
Received 15 October 2015, revised 18 November 2015
Accepted for publication 26 November 2015
Published 20 January 2016
Abstract
Biomedical applications of non-equilibrium atmospheric pressure plasmas have attracted intense
interest in the past few years. Many plasma sources of diverse design have been proposed
for these applications, but the relationship between source characteristics and application
performance is not well-understood, and indeed many sources are poorly characterized. This
circumstance is an impediment to progress in application development. A reference source
with well-understood and highly reproducible characteristics may be an important tool in this
context. Researchers around the world should be able to compare the characteristics of their own
sources and also their results with this device. In this paper, we describe such a reference source,
developed from the simple and robust micro-scaled atmospheric pressure plasma jet (μ-APPJ)
concept. This development occurred under the auspices of COST Action MP1101 ‘Biomedical
Applications of Atmospheric Pressure Plasmas’. Gas contamination and power measurement
are shown to be major causes of irreproducible results in earlier source designs. These problems
are resolved in the reference source by renement of the mechanical and electrical design and
by specifying an operating protocol. These measures are shown to be absolutely necessary for
reproducible operation. They include the integration of current and voltage probes into the jet.
The usual combination of matching unit and power supply is replaced by an integrated LC power
coupling circuit and a 5 W single frequency generator. The design specication and operating
protocol for the reference source are being made freely available.
Keywords: plasma medicine, COST reference microplasma jet, atmospheric pressure plasma
jet, biomedical applications of plasmas, power measurements, capacitively coupled radio
frequency discharge
(Some guresmay appear in colour only in the online journal)
Journal of Physics D: Applied Physics
Concepts and characteristics of the ‘COST
Reference Microplasma Jet’
J Golda et al
Concepts and characteristics of the ‘COST Reference Microplasma Jet’
Printed in the UK
084003
JPAPBE
© 2016 IOP Publishing Ltd
2016
49
J. Phys. D: Appl. Phys.
JPD
0022-3727
10.1088/0022-3727/49/8/084003
Paper
8
Journal of Physics D: Applied Physics
IOP
Original content from this work may be used under the terms
of the Creative Commons Attribution 3.0 licence. Any further
distribution of this work must maintain attribution to the author(s) and the title
of the work, journal citation and DOI.
0022-3727/16/084003+11$33.00
doi:10.1088/0022-3727/49/8/084003
J. Phys. D: Appl. Phys. 49 (2016) 084003 (11pp)

J Golda et al
2
1. Introduction
Microplasmas operated at atmospheric pressure have gained
very high attention. A series of review papers and topical
issues of leading scientic journals have discussed their tech-
nological and biomedical promise as well as their scientic
challenges [1–3]. A major part of the research on cold atmos-
pheric pressure microplasmas aims at the development and
optimization of plasmas for the production of reactive spe-
cies from a molecular precursor gas. Reactive microplasmas
have been developed for the deposition of functional organic
or inorganic coatings and thin lms. Other surface treated
materials range from polymers to living tissue in the emerging
and particularly promising eld of plasma medicine. These
applications include cancer treatment, sterilization and wound
healing [4–10]. So-called ‘jet’ devices are especially suited
for biomedical applications because they are small enough
to be easily handled and emit a cold gas stream. This gas
stream contains a multitude of reactive species which can be
directed towards a target surface. There, a localized interac-
tion can take place due to the small dimensions of the devices.
However, many groups around the world are investigating dif-
ferent, mostly self-made atmospheric pressure plasma jets for
biomedical applications [11–14]. Since each device behaves
differently, a comparison between the results is complicated
[15]. This leads to a huge delay in scientic progress. This pro-
gress could be improved by correlating the results from different
groups. Consequently, the basic understanding of atmospheric
pressure plasmas and their interaction with biological tissue is
often lagging behind. This impedes their scaling and prevents
the approval of processes by authorities [10].
To solve these problems, within the European COST
(Cooperation in Science and Technology) Action MP1011 on
‘Biomedical Applications of Atmospheric Pressure Plasma
Technology’ [16], a group was formed to discuss the possi-
bilities of dening a device that could be used as a reference
source for all groups doing research in the eld of plasma
medicine. Researchers around the world should be able to
compare the characteristics of their own sources and also their
results with this device.
A list of key requirements for a reference source was
dened. This list included:
• The design should be simple, robust, and inexpensive;
• The device should be usable in different laboratories
using standard electrical equipment and gas connectors;
• The device should be air-tight to minimize the inuence
of ambient air;
• The device components should be composed of inert or
medically certied (accepted) materials;
• Internal probes should be included to allow a continuous
monitoring of operation conditions;
• The device should be electrically safe and low in electro-
magnetic radiation;
• The design should be openly available to all interested
researchers.
Considering these requirements, the micro-scaled atmos-
pheric pressure plasma jet (μ-APPJ) developed by Schulz-von
der Gathen and co-workers [17] was selected as the basis for
the development of a reference source. Here, we present the
outcome of this development: a reference device for research
purposes—the COST Reference Microplasma Jet. We briey
introduce the technical details and then show a basic charac-
terization that can be used to demonstrate the reproducibility
of power, optical emission spectroscopy (OES), and gas
temper ature measurements.
2. State of the art radio-frequency excited
micro-scaled atmospheric pressure plasma jet
The micro-scaled atmospheric pressure plasma jet (μ-APPJ)
is based on the original APPJ as introduced by the group of
Hicks and Selwyn [18]. The μ-APPJ is a capacitively coupled
13.56 MHz RF-discharge with symmetric, co-planar, stainless
steel electrodes enclosed by two quartz panes and a discharge
volume of
××1mm1mm 30
mm, particularly designed for
optimized optical diagnostic access [19]. The standard opera-
tion condition is a homogeneous α-glow mode with a noble
gas ow (typically 1.4 slpm He) containing a small molecular
admixture of oxygen or nitrogen (typically 0.5%) [20, 21].
The μ-APPJ has been and is actively investigated at sev-
eral institutes. Up to today, about 40 articles on experimental
measurements as well as on models and simulations have been
published. These results include spatially resolved diagnostics
of reactive species such as radicals (e.g. atomic oxygen and
nitrogen, ozone, metastable oxygen molecules), gas temper-
atures, ow patterns, etc in the discharge region and in the
efuent [19, 22–26]. The emission was investigated down to
the vacuum ultraviolet as a potentially important contributor
to biomedical processes [27]. The investigations also com-
prise a variety of biomedical experiments [28–32].
Several modeling investigations have been carried out,
taking advantage of the simple geometry of the device.
One-dimensional models were used to describe the elec-
tron excitation within the electrode gap [33–36]. One- and
two-dimensional simulations have been used to investigate
the generation of reactive species [37–40] as well as power
Figure 1. Photograph of the ignited COST Reference Microplasma
Jet (COST-Jet). The homogeneous discharge is formed between
the plane-parallel electrodes of the electrode assembly that extends
from the housing.
J. Phys. D: Appl. Phys. 49 (2016) 084003

J Golda et al
3
modulation [41]. The μ-APPJ continues to be actively
investigated.
In preparing the specication for the COST Reference
Microplasma Jet, several drawbacks of the original design were
identied that needed an improvement to cover the goals of the
source.
3. Design of the COST Reference Microplasma Jet
The COST Reference Microplasma Jet (COST-Jet or
COST-RMJ) was developed based on the original μ-APPJ
concept. The complete device (see gure1) consists of two
main components: (i) the ‘head’ that comprises the electrode
stack with quartz panes, the gas connector and the gas tubing,
and (ii) the housing, that holds the head, provides the elec-
trical connections, and internal current and voltage probes.
Both components will be described in detail below.
3.1. Head
The head (see gure2) includes the electrode assembly com-
posed of a quartz pane / metal electrode / quartz pane stack.
This assembly forms the discharge channel. Using a two-comp-
onent glue suitable for high-vacuum applications (TorrSeal®),
the assembly is glued into a ceramic gas connector that attaches
the
1/4
inch stainless steel gas tubing to the head.
3.1.1. Electrode assembly. The two metal electrodes of the
head (see gure2) are symmetric and separated by a 1 mm
gap. The 52.5 mm long, 12 mm wide and 1 mm thick elec-
trodes are made of medical stainless steel (SS 316) and form
the gas channel which features three different zones: (i) A
region of about 11.5 mm length that is fully covered by quartz
panes. In this region, the electrode faces are widely spaced so
that no discharge is ever ignited. Here, gas from the supply
redistributes before entering the main discharge volume of the
head. This premix volume is 5 mm wide. (ii) The subsequent
part extends over 30 mm of length of closely spaced electrodes
each being 4.5 mm high, dening the 1 mm wide discharge
channel. At the exit region, the electrode width is reduced to
1 mm. (iii) The safety zone is formed by the quartz panes and
prevents contact with the electrodes.
These quartz panes (Corning 7980) are windows that
allow direct, broadband observation of the discharge down
to a wavelength of about 200 nm. They are 1.5 mm thick and
Figure 2. Detailed sketch of the head showing the two symmetric, stainless steel electrodes with connection wings and quartz panes
covering the symmetric plasma electrodes. The blue line at the circumference indicates seal made from TorrSeal® inside the groove formed
between the panes and the electrode.
Figure 3. Sketch of the complete assembly consisting of head, housing and adapter to external tubing. In the housing, the fastening and
guidance of the head, the electrical power connections with matching coil and capacitor and both internal probes are illustrated.
J. Phys. D: Appl. Phys. 49 (2016) 084003

J Golda et al
4
cover the electrodes over a length of 50 mm. They extend
slightly beyond the electrode conguration, forming 1 mm
deep grooves at its outer side. These grooves are used to
tightly glue together the entire stack with TorrSeal® while
preventing any contact between the feed gas and plasma with
the glue. During assembly, the stack is pressed together to
minimize any space between the electrode and pane and thus
preventing air intrusion during operation. The glue extends
from the wing of the electrode up to the at exit as indi-
cated by the blue boundary lines in gure2. The width of the
quartz panes decreases symmetrically to 5 mm over a length
of 3.5 mm, thus forming a at exit in combination with the
electrodes. This wedge shape conveniently enables the inser-
tion of the assembled tip into the cavities of a 24 well titer
plate.
The 1 mm safety gap formed by the extension of the quartz
panes beyond the electrode tip serves three purposes: (i) the
powered electrodes cannot be directly touched, (ii) electrical
contact with the target is prevented and (iii) even when coming
in contact with a surface, the gas ow is not shut off immedi-
ately, thus preventing irregular operation of the device.
3.1.2. Gas conguration. The electrode assembly is glued
into a slot of a cube of machinable ceramic that forms an elec-
trically isolated connection to the gas line (see gure2). A
1/4
inch stainless steel tube is inserted into a drilled hole on the
opposite side of this gas connector and tightly glued. The gas is
fed from the steel tube through a 1 mm diameter drilling in the
gas connector into the discharge channel. By selecting these
materials, we ensure that only stainless steel, quartz, and alu-
mina ceramic come in contact with the gas and the plasma.
Typically, the feed gas tube has a length of 60 mm, but can
also be shorter or longer than that, depending on the require-
ments. It can be connected to standard gas ttings using an
O-ring-to-standard tting adapter (right of gure3). Thus, the
complete head can be separated from the housing into which it
is inserted for operation.
3.2. Housing
The housing (see gure3) consists of a rigid metal casing (Fischer
Elektronik, AKG412450ME) of
××41mm24mm50
mm.
The casing is made of anodized aluminum and is therefore
electrically insulated, so that incidental electrical contact is
prevented. The complete head is inserted into the housing from
the front side with the
1/4
inch tubing. It is guided and mounted
by a metal clamp. This provides stable support and excellent
grounding to the device. The electrode wings are screwed with
M2 threads into a at copper conductor for connection to the
power supply. The complete COST-Jet device can be installed
on an optical post by using the M5 thread in the bottom of the
housing.
The front and back cover are tightly connected to the main
body of the housing. The front cover includes a slot for the head
and a thread to mount an electrode shielding for the grounded
electrode. All electrical connectors are combined into the back
cover of the housing. Here, the power connector (SMA), two
probe connectors (SMC), and the adjustable tuning capacitor
are located. To improve discernibility, different connectors for
power and probes have been selected.
The external connection to the power supply is provided by
a low loss coax cable (H-155 PE Low Loss) with a damping of
0.46 dB m−1at 2 GHz. The cable is tted with a female SMA
connector on the assembly side and a BNC connector on the
supply side.
Based on the work by Marinov and Braithwaite [42], an
internal resonance coupling is used (see gure4). Thus, there
is no need for an external tuning network (matchbox).
For this purpose, an LC circuit with a tunable capacitor
(Sprague Goodman
=C0.8
t
–8 pF) and an inductor (
=L9.6
μ H,
Amidon T68-2 core, 41 windings) was used to tune the circuit
into resonance at a frequency of 13.56 MHz. The capacitor is
installed in parallel to the electrode stack. This LC circuit has
a Q-factor of about 30. This means it provides a thirty-fold
increase of the applied voltage. Thus, a power supply capable
of delivering 7 V is enough to ignite the discharge.
3.2.1. Probes. For an easy control of experimental condi-
tions, two probes are integrated into the housing of the COST
Reference Microplasma Jet.
Voltage probe. The voltage probe is a pin of 5 mm length
positioned 4 mm below the powered at copper conductor
leading to the electrode. The voltage probe is capacitively
coupled to the electrode. Once calibrated using an external
voltage probe, the voltage between the electrodes is obtained
using an oscilloscope. The calibration procedure is described
in more detail in section4.1.
Figure 4. Electrical circuit scheme including LC circuit for matching, the capacitive voltage pick-up probe and
R
m for current
measurements.
J. Phys. D: Appl. Phys. 49 (2016) 084003

J Golda et al
5
Current probe. The current is measured via a resistive cur-
rent measurement. The precision lm resistor (
=R4.7
m
Ω
)
is positioned between the ground-side electrode and ground.
The voltage drop over this resistor is proportional to the dis-
charge current via Ohm’s law. To ensure that the current probe
only measures the current that crosses the discharge channel,
a grounded shielding is xed around the ground-side of the
electrode assembly (see gures1 and 3). Both probes are con-
nected to standard SMC sockets.
3.3. Power supply
The concept of the COST Reference Microplasma Jet pre-
sented here is accompanied by the development of a simple,
tailored, miniaturized power supply for 13.56 MHz (see
gure 5). It delivers the required voltage for the LC circuit
to safely operate the COST-Jet in helium with a 0.5% admix-
ture of oxygen. The maximum output power of the ampli-
er is 5 W. For reference purposes, the power can be xed
to a single power setting. This power supply can be replaced
by any power generator capable of delivering this voltage
range, when the required matching to the LC circuitry can be
achieved.
3.4. OES reference cover, reference ber spacer
A second possibility to control experimental conditions is
OES using an optical ber. To ensure reproducible OES
measurements, we introduced two simple accessories for the
COST-Jet: (i) The OES reference cover and (ii) the reference
ber spacer.
The OES reference cover is made of black cardboard and
encloses the electrode assembly completely. It has a rectan-
gular hole with a size of
×2mm 5
mm positioned at the center
of the discharge.
The ber-spacer is made of opaque plastic. The spacer
was manufactured for optical ber with SMA termination and
ensures a xed distance of 5 mm between the ber entrance
and glass pane of the COST-Jet. It has a core drilling of 3 mm
diameter that ts to the rectangular hole in the OES reference
cover. The ber spacer prevents any electric disturbance of the
COST-Jet operation by metallic ber jacketing. The combina-
tion of the OES reference cover and ber spacer ensures a
reproducible positioning of the observation hole in the center
of the discharge and prevents the collection of stray light by
the ber.
4. Basic characterization
To demonstrate the performance of the COST Reference
Microplasma Jet, a basic characterization was conducted at
the Ruhr-Universität Bochum. This characterization includes
electrical measurements, such as voltage, current and power
measurements, as well as OES and temperature measurements.
The built-in probes of the COST Reference Microplasma Jet
allow simple monitoring of electrode voltage and plasma cur-
rent of the device. Here, the results are presented.
For all measurements, the COST-Jet was operated at a
helium gas ow of 1.0 slpm of helium (purity 99.999%) and
an oxygen admixture of 0.5% (purity 99.998%). This gas ow
ensures a low gas temperature and yet keeps the ow low
enough to avoid or reduce evaporation with biological sub-
strates. The results of current and voltage measurements are
presented in rms values, unless denoted otherwise.
4.1. Voltage probe calibration
Prior to measurements, the internal voltage probe has to be
calibrated.
To calibrate the internal voltage probe, a commercial
voltage probe (see table1 for detailed description of equip-
ment) was connected to the electrodes of the COST-Jet. The
probe was properly matched to the oscilloscope (see table1)
to ensure high delity in the 13.56 MHz region. This was done
by connecting it to the calibration signal of the scope. This
Figure 5. Image of the home-made ‘on/off’ 13.56 MHz power supply tailored to the LC resonance circuit.
J. Phys. D: Appl. Phys. 49 (2016) 084003

J Golda et al
6
adjustment can make a signicant difference especially with
higher attenuation probes.
The internal voltage and current probes were connected to
two different oscilloscopes using a
50
Ω
termination to mini-
mize signal reection.
Comparing absolute amplitude values of the voltage meas-
ured with the commercial and the internal voltage probe, we
calculated a calibration factor for the internal probe. Internal
and commercial voltage probe measurements showed a linear
correlation (see gure 6). Therefore, a single calibration
factor could be calculated by a linear regression curve. The
calibration was done multiple times using two different oscil-
loscopes. The calibration factor from internal to commercial
probe voltage was
±2630 50
. The differences in calibration
factors can most likely be attributed to imprecise calibration
of the commercial voltage probe. This inaccuracy cannot be
entirely avoided.
It has to be noted that this calibration factor has to be meas-
ured and calculated for each COST-Jet. This recalibration is
necessary due to variations of probe positioning inside the
housing.
The discharge current was calculated from the voltage
measured by the internal current probe. Since the internal cur-
rent probe measures the voltage drop,
Uc
, over a resistor of
=R4.7
m
Ω
(see gure4) and is connected to the oscilloscope
using a termination of
=R50
t
Ω
, the current can be calculated
via Ohm’s law:
=
+
IU
RR
RR
cmt
mt
(1)
4.2. Power measurement
The power, that is dissipated in the plasma can be obtained
using current, voltage and the phase shift between them.
()=⋅⋅ΦPUIcos
(2)
The current and voltage signals are measured at different
locations and with different cables and therefore have a dif-
ferent phase than the signals at the electrode. Hence, it is nec-
essary to have a reference at which the value of the phase is
known. Without the discharge, the COST-Jet electrodes basi-
cally form a capacitor and the active power should be equal to
0 W. Therefore, the phase between current and voltage without
a discharge is 90°. The change in phase from this reference
can then be used to calculate the power, using equation(2).
For the power measurements, an oscilloscope with a minimum
sampling rate of 1 GS s−1 is recommended.
Voltage, current and power were measured using the
described method. At a voltage of approximately 150 V, break-
down occurred and the discharge was ignited (see gure7).
A stable operation of the device in a homogeneous mode
was possible in a voltage range of 155–335 V. This corre-
sponds to an active power of 0.2 W–1.7 W. This power range
agrees very well with model calculations by Waskoenig et al
Table 1. Equipment used for basic electrical and optical characterization.
Oscilloscope type Voltage probe Spectrometer
LeCroy WavePro 735Zi Tektronix P5100A Ocean optics HR4000
(40 GS s−1, 3.5 GHz) (100x)
Tektronix DPO 2024 Tektronix P5100A Ocean optics HR4000
(1 Gs s−1, 200 MHz) (100x)
Figure 6. Comparison of internal probe to commercial probe
(Tektronix P5100A). The linear regression curve provides a
calibration factor for the internal voltage probe. Each individual
COST-Jet has to be calibrated due to small manufacturing
differences. The legend indicates the used equipment. For the
measurement represented by the square symbols only one value out
of 20 is shown.
Figure 7. Active power from ignition through the transition to
constricted mode.
J. Phys. D: Appl. Phys. 49 (2016) 084003

J Golda et al
7
[43]. At higher voltages, a transition to a constricted mode
was observed. With the transition to this mode, the voltage
dropped and the power consumption strongly increased. In the
constricted mode, the plasma is located in a small area at the
very tip of the electrodes close to the exit. The gas temper-
ature considerably exceeds room temperature and might even
damage the COST-Jet if operated longer than a few seconds.
At approximately 240 V and 0.45 W (see gure7), a change
in the power-voltage-curve was observed. For voltages lower
than 240 V, the power rises linearly with voltage. For higher
voltages, an additional exponential trend becomes dominant.
This change corresponds to a transition of the discharge from
an α-like mode to a γ-like mode. This transition has been dis-
cussed previously [35, 44].
4.3. Optical characterization
OES allows for the estimation of stability and reproduc-
ibility of the operation as well as the inuence of impurities
in the different devices. To study the response of the plasma
chemistry to different plasma parameters, we investigated the
plasma emission using a broad band spectrometer (Ocean
Optics HR 4000, 200 nm–1100 nm, 3648 pixels) and ber
optics (Ocean Optics QP 600-2-UV-BX). To ensure a dened
distance between ber optics and discharge, we used the ber
distance spacer and the OES reference cover (see section3.4).
If not stated otherwise, all results were obtained at an active
power of 0.4 W (212 V) and a gas ow of 1 slpm helium and
admixture of 5 sccm oxygen.
Notably, atomic oxygen lines (777 nm, 844 nm) were the
most prominent emission lines in the emission spectrum (see
gure8). The helium atomic lines (706 nm, 587 nm, 668 nm)
were less pronounced. The weak molecular nitrogen emission
of the second positive band (336 nm, 357 nm) as well as the
hydroxide emission (308 nm) can be attributed to impurities
in the gas supply. Impurities from leakages or backow are
unlikely due to steel tubing and high gas ow. The low impu-
rity level in the spectrum was obtained using stainless steel
tubing and ushing with helium prior to ignition of the dis-
charge. This procedure has been shown to reduce humidity
in the gas supply due to water attached to the gas tubing [45].
Control of impurities is essential for reproducible results and
thus is a major issue in atmospheric pressure plasmas.
For ow variations (see gure9), the value at 1 slpm helium
ow was selected as the normalization point. Figure9 shows the
behavior of the strongest atomic lines (ratio of the 844 nm O line
and the 706 nm He line) for a ow variation from 0.25 slpm to
1.5 slpm. The error bars represent an upper limit of 5% devia-
tion of the absolute line intensity and are shown only for the rst
measurement point. The ratio of the two lines slightly decreases
by about 5% for increasing ow. This relative reduction of the
oxygen contribution can be attributed to the decreasing residence
time up to the measurement point at higher ows [19]. The ratio
of the oxygen line to the emission of the 357 nm molecular
nitrogen band of the second positive system gives an indication
of the impurities in the device. An increase in the He/N2 ratio
was observed with rising ow. This is explained by an apparent
decrease in nitrogen contribution either from the gas system or
back ow through the exit. The back ow argument is supported
by the steeper increase at the lowest ow of 0.25 slpm. For any
further interpretation, it has to be kept in mind that the emission
is inuenced by the number of species as well as the energy dis-
tribution of the exciting electrons.
5. Gas temperature
A crucial parameter for biomedical applications with living
tissue and to a lesser extent interactions with temperature sen-
sitive materials is the gas temperature of the efuent. Since
increased temperatures above 37 °C can damage living tissue,
it is crucial to control the heat impact. Therefore, the heating
of the efuent gas needs to be low.
Figure 8. Atomic oxygen lines (777 nm, 844 nm) are most prominent in the survey spectrum measured at 0.4 W (212 V), 1 slpm helium and
5 sccm O2.
J. Phys. D: Appl. Phys. 49 (2016) 084003

J Golda et al
8
The gas temperature of the efuent was measured using
an RF-insulated thermocouple in a distance of 3 mm of the
electrode tip (see gure10); this is also a typical distance used
for biomedical application interactions. Note that the 0 mm
position represents the COST-Jet exit, i.e. the tip of the elec-
trodes. Since investigations usually cover both the discharge
region and the efuent, it was chosen that positive ‘+’ coor-
dinates point into the efuent and negative ‘-’ values into the
discharge region, with zero dened at the electrode edge.
Temperature rises at an active power of 0.2 W within the
rst 5 min from ambient temperature of 19.9 °C (not dis-
played here) to 30.5 °C. After this increase, the temperature
reaches a steady state of approximately 32.5 °C within 30 min.
This corresponds to the typical heat-up time of the complete
system [46] and partially motivates our recommended warm-
up time for the system. The error bars for the temperature
represent a 0.5 °C uncertainty. In gure10, the development
of the emission lines of the most prominent atomic lines of
helium and oxygen is shown. The intensities for these meas-
urements were averaged over 4 min. The error bars represent
the standard deviation of the mean value. Here, the intensi-
ties of the observed lines increase by a few percent until they
reach a steady state after about 30 min. The increase corre-
sponds partially to the rise in temperature mentioned before.
Another reason for this behavior is the reduction of impurities,
mainly water and nitrogen, in the gas tubing. The duration of
Figure 9. Intensity ratios of oxygen 844 nm, helium 706 nm and molecular nitrogen (357 nm) emission for a ow variation measured in the
center at a power of 0.4 W (212 V).
Figure 10. Change in line intensity and efuent temperature in the rst 60 min after discharge ignition, measured at 0.2 W, 1 slpm helium,
5 sccm O2 at +3 mm distance.
J. Phys. D: Appl. Phys. 49 (2016) 084003

J Golda et al
9
the reduction process and the intensity change of the emission
lines strongly depends on the initial amount of impurities in
the gas tubing.
In gure11, we show the dependence of the steady state
temperature on a power variation from 0.1 W to 1 W meas-
ured under otherwise identical conditions at +3 mm distance.
Again, the error bars for the line intensities represent an upper
limit of 5% deviation and are exemplarily shown for the
strongest atomic oxygen line (777 nm). The error for temper-
ature measurement is 0.5 °C. The temperature of the efuent
showed a linear correlation to the active power in the dis-
charge rising from 30 °C to 75 °C. For powers below 0.25 W,
the temperature remains below 37 °C and hence is well-suited
for the treatment of heat-sensitive biological tissue. At 1.0 W,
the efuent temperature was above 70 °C. This has to be kept
in mind when conducting biomedical research.
The intensities of the prominent atomic oxygen lines
showed a linear correlation to the power dissipated in the dis-
charge (see gure11). This corresponds to the linear increase
of atomic oxygen with rising (generator) power that has been
observed by two-photon absorption laser-induced uores-
cence spectroscopy in the original μ-APPJ [20]. The inten-
sity of the most prominent helium line at 706 nm increased
exponentially with increased power since the helium emission
follows the electron excitation (see gure7).
6. Protocols
Based on various investigations of the COST-Jet, several addi-
tional decisions have been made on operation protocols:
• Gas tubing should consist of stainless steel tubing. Any
plastic tubing is prone to introduce impurities into the
feed gas by diffusion [45].
• Prior to any reproducible measurement, the COST-Jet
should be operated for at least 30 min continuously (see
gure10). This allows the temperature of the device to
stabilize [46] and removes a good part of the humidity
stored in the tubing.
• Valves closing the tubing should be installed as close as
possible to the COST-Jet to minimize humidity entering
the tubing.
• Humidity and temperature in the laboratory should be
recorded.
• When presenting results obtained with the COST-Jet, the
standard coordinate system should be used. In this coordi-
nate system, the origin represents the end of the discharge
channel, i.e. the tip of the electrodes. Positive coordinates
are in the space outside the discharge channel.
7. Further developments
Within the cooperation, a printed circuit board was designed
for the COST-Jet that replaces the voltage and current probes.
The circuitry yields a direct measure of the plasma power.
Details of the device are described in a publication by Beijer
et al [47]. Another publication presents a modied set of
COST-Jets to separate species and radiation components of
the plasma [48].
8. Summary and conclusions
Based on the μ-APPJ, a reference source was developed to
improve reproducibility and comparability of results obtained
by different research groups. The simple, inexpensive and
robust COST Reference Microplasma Jet was presented and
described in detail. Two probes integrated into the device allow
Figure 11. Efuent temperature measured at +3 mm distance and line intensity under variation of the power. Efuent temperature and the
atomic oxygen lines at 777 nm and 844 nm show a linear correlation to the active power.
J. Phys. D: Appl. Phys. 49 (2016) 084003

J Golda et al
10
a measurement of current and voltage. With high enough tem-
poral resolution, these measurements yield the input power into
the electrode system of the COST Reference Microplasma Jet.
This is a vast improvement compared to the usually available
generator readings for the power input. Those are measured at
the entrance and not at the exit of the matching network and
are hence inuenced by cables, matching elements and so on.
A set of basic power, temperature and OES measurements
was described that will be used for the comparison of different
devices.
The complete technical drawings of the COST-Jet and
information on acquisition or assembly will be available on
request online (www.cost-jet.eu). We would like to invite any
researcher interested in performing measurements using a
COST Reference Microplasma Jet to contact us.
To demonstrate the reproducibility of the performance
of the COST Reference Microplasma Jet, two sets of the
basic characterizations described above are presently being
performed with a total number of ve COST Reference
Microplasma Jets. One set describes the comparison of the
results at one institute, while the second set expands on the
comparison of measurements at the participating institutes.
The results of these investigations will be presented in a suc-
cessive publication. A further planned publication will give
details on the bio-medical protocols.
Acknowledgments
This project was initiated within the EU COST action MP1101
‘Biomedical Applications of Atmospheric Pressure Plasma
Technology’. The development and investigations presented
here were supported by the ‘German Research Foundation’
(DFG) in the frame of the ‘Package project PAK 816: Plasma
to Cell Interaction in Dermatology’ and the ‘Research Unit
FOR 1123: Physics of Microplasmas’ within the Research
Department ‘Plasmas with Complex Interaction’ of the
Ruhr-Universität Bochum. This work was also funded by the
UK Engineering and Physical Sciences Research Council
(EPSRC) grant numbers EP/ K018388 and EP/H003797.
We would like to thank V Scharf for proofreading the manu-
script and her continuous support, as well as S Burhenn and
B Biskup for experimental support and photography. Data
created during this research is available at the following
DOI:10.15124/8ee73b0c-4bbf-4d28-a860-287944a0bfb4 and
on the project’s website www.cost-jet.eu.
References
[1] BeckerK, SchoenbachK and EdenJG 2006 J. Phys. D: Appl.
Phys. 39R55–70
[2] LaroussiM and AkanT 2007 Plasma Process. Polym. 4777–88
[3] FoestR, SchmidtM and BeckerK 2006 Int. J. Mass Spectrom.
24887–102
[4] KongM, KroesenG, MorllG, NosenkoT, ShimizuT, van
DijkJ and ZimmermannJ 2009 New J. Phys. 11115012
[5] FridmanG, FriedmanG, GutsolA, ShekhterAB, VasiletsVN
and FridmanA 2008 Plasma Process. Polym. 5503–33
[6] GravesDB 2014 Phys. Plasmas 21080901
[7] LaroussiM 2009 IEEE Trans. Plasma Sci. 37714
[8] WeltmannKD, KindelE, von WoedtkeT, HähnelM, StieberM
and BrandenburgR 2010 Pure Appl. Chem. 821223–37
[9] WeltmannKD, PolakM, MasurK, von WoedtkeT, WinterJ
and ReuterS 2012 Contrib. Plasma Phys. 52644–54
[10] von WoedtkeT, MetelmannHR and WeltmannKD 2014
Contrib. Plasma Phys. 54104–17
[11] CoulombeS, LéveilléV, YonsonS and LeaskRL 2006
Pure Appl. Chem. 781147–56
[12] FoestR, KindelE, LangeH, OhlA, StieberM and
WeltmannKD 2007 Contrib. Plasma Phys. 47119–28
[13] VoracJ, DvorakP, ProchazkaV, EhlbeckJ and ReuterS 2013
Plasma Sources Sci. Technol. 22025016
[14] RobertE, BarbosaE, DoziasS, VandammeM,
CachoncinlleC, ViladrosaR and PouvesleJM 2009
Plasma Process. Polym. 6795
[15] EhlbeckJ, SchnabelU, PolakM, WinterJ, von WoedtkeT,
BrandenburgR, von dem HagenT and WeltmannKD 2011
J. Phys. D: Appl. Phys. 44013002
[16] MPNS COST Action MP1101: Biomedical Applications of
Atmospheric Pressure Plasma Technology http://bioplasma.
pointblank.ie/
[17] Schulz-von der GathenV, SchaperL, KnakeN, ReuterS,
NiemiK, GansT and WinterJ 2008 J. Phys. D: Appl. Phys.
41194004
[18] SchützeA, JeongJ, BabayanS, ParkJ, SelwynG and HicksR
1998 IEEE Trans. Plasma Sci. 261685
[19] KnakeN and Schulz-von der GathenV 2010 Eur. Phys. J. D
60 645
[20] KnakeN, NiemiK, ReuterS, Schulz-von der GathenV and
WinterJ 2008 Appl. Phys. Lett. 93131503
[21] WagenaarsE, GansT, O’ConnellD and NiemiK 2012 Plasma
Sources Sci. Technol. 2104002
[22] MaletićD, PuačN, LazovićS, MalovićG, GansT,
Schulz-von der GathenV and PetrovićZL 2012
Plasma Phys. Control. Fusion 54124046
[23] NiemiK, O’ConnellD, de OliveiraN, JoyeuxD, NahonL,
BoothJP and GansT 2013 Appl. Phys. Lett. 103034102
[24] NiermannB, HemkeT, BabaevaN, BökeM, KushnerM,
MussenbrockT and WinterJ 2011 J. Phys. D: Appl. Phys.
44485204
[25] SousaJS, NiemiK, CoxLJ, AlgwariQT, GansT and
O’ConnellD 2011 J. Appl. Phys. 109123302
[26] KnakeN, ReuterS, NiemiK, Schulz-von der GathenV and
WinterJ 2008 J. Phys. D: Appl. Phys. 41194006
[27] BahreH, Schulz-von der GathenV, LangeH and FoestR 2011
Acta Tech. 56T199–206
[28] O’ConnellD, CoxLJ, HylandWB, McMahonSJ, ReuterS,
GrahamWG, GansT and CurrellFJ 2011 Appl. Phys.
Lett. 98043701
[29] SchneiderS, LackmannJW, EllerwegD, DenisB,
NarberhausF, BandowJ and BenediktJ 2012 Plasma
Process. Polym. 9561
[30] LackmannJW, SchneiderS, NarberhausF, BenediktJ and
BandowJ 2012 Plasma for bio-decontamination, medicine
and food security Characterization of Damage to Bacteria
and Bio-Macromolecules Caused by (V)UV Radiation and
Particles Generated by a Microscale-APPJ (NATO Science
for Peace and Security Series: A) (Berlin: Springer) pp 17–19
[31] LackmannJW, SchneiderS, EdengeiserE, JarzinaF,
BrinckmannS, SteinbornE, HavenithM, BenediktJ and
BandowJE 2013 J. R. Soc. Interface 1020130591
[32] GibsonAR, McCarthyHO, AliAA, O’ConnellD and
GrahamWG 2014 Plasma Process. Polym. 111142–9
[33] O’NeillC, WaskoenigJ and GansT 2011 IEEE Trans. Plasma
Sci. 392588–9
[34] NiemiK, WaskoenigJ, SadeghiN, GansT and O’ConnellD
2011 Plasma Sources Sci. Technol. 20055005
[35] SchaperL, WaskoenigJ, KongMG, Schulz-von der GathenV
and GansT 2011 IEEE Trans. Plasma Sci. 392370–1
J. Phys. D: Appl. Phys. 49 (2016) 084003

J Golda et al
11
[36] KawamuraE, LiebermanMA, LichtenbergAJ, ChabertP
and LazzaroniC 2014 Plasma Sources Sci. Technol.
23035014
[37] LazzaroniC, ChabertP, LiebermanMA, LichtenbergAJ and
LeblancA 2012 Plasma Sources Sci. Technol. 21035013
[38] LazzaroniC, LiebermanMA, LichtenbergAJ and ChabertP
2012 J. Phys. D: Appl. Phys. 45495204
[39] HemkeT, WollnyA, GebhardtM, BrinkmannRP and
MussenbrockT 2011 J. Phys. D: Appl. Phys. 44285206
[40] KellyS and TurnerMM 2014 Plasma Sources Sci. Technol.
23065013
[41] KellyS and TurnerMM 2014 Plasma Sources Sci. Technol.
23065012
[42] MarinovD and BraithwaiteNStJ 2014 Plasma Sources Sci.
Technol. 23062005
[43] WaskoenigJ, NiemiK, KnakeN, GrahamL, ReuterS,
Schulz-von der GathenV and GansT 2010 Pure Appl.
Chem. 821209–22
[44] LiuDW, IzaF and KongMG 2008 Appl. Phys. Lett.
93261503
[45] WinterJ, WendeK, MasurK, IseniS, DünnbierM,
HammerMU, TrespH, WeltmannKD and ReuterS 2013
J. Phys. D: Appl. Phys. 46295401
[46] KellyS, GoldaJ, TurnerMM and Schulz-von der GathenV
2015 J. Phys. D: Appl. Phys. 48444002
[47] BeijerPAC, SobotaA, van VeldhuizenEM and
KroesenGMW 2016 J. Phys. D: Appl. Phys. submitted
[48] SchneiderS, JarzinaF, LackmannJW, GoldaJ, LayesV,
Schulz-von der GathenV, BandowJE and BenediktJ 2015
J. Phys. D: Appl. Phys. 48444001
J. Phys. D: Appl. Phys. 49 (2016) 084003