![Simulated electric field intensity distributions at the nozzle in (a) pure argon and (b) 10% gas humidity at different powers: [(a1) and (b1)] 30 W, [(a2) and (b2)] 40 W, [(a3) and (b3)] 50 W, [(a4) and (b4)] 60 W, and [(a5) and (b5)] 70 W.](https://www.researchgate.net/publication/349292267/figure/fig4/AS:11431281219475073@1706022810814/Simulated-electric-field-intensity-distributions-at-the-nozzle-in-a-pure-argon-and-b_Q320.jpg)
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AIP Advances ARTICLE scitation.org/journal/adv
The influence of gas humidity on the discharge
properties of a microwave atmospheric-pressure
coaxial plasma jet
Cite as: AIP Advances 11, 025131 (2021); doi: 10.1063/5.0033059
Submitted: 21 December 2020 •Accepted: 26 January 2021 •
Published Online: 12 February 2021
Jie Yu,1Wencong Zhang,2Xiao Wu,1Li Wu,1,a) Junwu Tao,3and Kama Huang1
AFFILIATIONS
1IAEM (Institute of Applied ElectroMagnetics), College of Electronics and Information Engineering, Sichuan University,
Chengdu 610065, People’s Republic of China
2School of Electronic and Communication Engineering, Guiyang University, 550005 Guiyang, People’s Republic of China
3Laplace (Laboratoire Plasma et Conversion d ’Energie), INPT - ENSEEIHT, Université de Toulouse, 31071, France
a)Author to whom correspondence should be addressed: wuli1307@scu.edu.cn
ABSTRACT
This paper investigated the influence of gas humidity (1%, 3%, 8%, 10%, and 12%) on the characteristics of a microwave-induced atmospheric
plasma jet. The plasma discharge was generated by a microwave solid-state source with a H2O–Ar mixture gas flow of 8.1 L/min. The variation
in energy efficiency, O and OH concentrations, rotational temperature of heavy species, shapes of plasma plumes with different humidities,
and microwave input powers were recorded and analyzed. The results showed that the concentrations of O and OH increase monotonously
with gas humidity at higher input powers while they fluctuate with gas humidity at lower input powers. With an increase in the H2O/Ar ratio
from 1% to 12%, the energy efficiency of the plasma generator decreases, and the plasma plumes become shorter and thinner. The rotational
temperature of plasma at the nozzle also showed positive correlation with increasing humidity. Adding more input power would make all the
values of these parameters increase. This paper is supposed to be helpful for the research of the interaction mechanism of mix gas plasma and
microwave power and for improving the effect of plasma treating biomedical materials.
©2021 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license
(http://creativecommons.org/licenses/by/4.0/). https://doi.org/10.1063/5.0033059
., s
I. INTRODUCTION
Low-temperature atmospheric-pressure plasma (APP) has
been widely used in various biomedical applications,1–3 for instance,
in killing cancer cells,4–6 sterilization,7–11 inducing cell apoptosis,12,13
whitening tooth,14 cell adhesion,15,16 regenerating skin,17,18 and so
on. To date, most investigations concerning low-temperature APP
have upheld the fact that chemically active species such as reactive
oxygen species (ROS) and reactive nitrogen species (RNS), contain-
ing oxygen atoms (O), oxygen negative ions (O2−), ozone (O3),
hydroxyl radicals (OH), nitric oxide radicals (NO), and hydrogen
peroxide (H2O2), play an essential role in biomedical areas as well
as other relevant research domains.15,19–21 To obtain a higher den-
sity of the wanted active species at low temperature, researchers
tend to use mixed gas as the working gas to initiate the plasma.
However, reports show that a slight change in the composition of
working gas may lead to significant variation in plasma properties,
especially active species and their doses, which will affect the plasma
treating results. Therefore, it is necessary to investigate how the gas
composition influences the plasma properties, especially the active
species.
In this paper, the influence of argon humidity variation on the
plasma properties especially on the doses of O and OH is studied
with emission spectra based on a self-designed microwave-induced
coaxial plasma generator. The change law is studied and discussed
by adjusting the gas humidity and microwave input power.
II. EXPERIMENTAL SETUP
The microwave coaxial plasma generator structure is provided
in Fig. 1. The size of its bottom is designed according to a typ-
ical 7/16 DIN connector (Deutsche Industrie Norm connector).
AIP Advances 11, 025131 (2021); doi: 10.1063/5.0033059 11, 025131-1
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FIG. 1. Structure of the plasma genera-
tor: the (a) vertical section and (b) bottom
view.
The diameters of its inner and outer conductors are 7 mm and
16 mm, respectively. This enables the generator to connect to the
microwave source directly without extra impedance-matching cir-
cuits. The top of the device, i.e., the plasma initiating point, is open-
circuited. To further focus the microwave power at the discharge
point and maintain impedance matching, a gradually tapered struc-
ture from the coaxial structure to the open end is employed. Four
6 mm holes are bored on the side of the coaxial body to form tangen-
tial gas inlets. A short metal annulus is welded to the outer conductor
of the plasma generator to help assembling of gas inlet pipes and the
plasma generator.
Figure 2 shows the schematics of the experimental setup. A
solid-state source (WSPS-2450-200M, Wattsine, Chengdu, China)
working at 2.45 GHz is employed to provide power to the plasma
generator. A circulator is connected to the power source for
protecting it from the reflected power. A dual-direction coupler
(L00PE22DC40A10N, Euler Microwave Element Ltd., Chengdu,
China) is linked to port 2 of the circulator while the matching load is
FIG. 2. Schematic of the experimental setup.
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connected to port 3 to absorb the reflected power. Two power meters
(AV2433, the 41st Institute of China Electronic Technology Group
Corporation, Qingdao, China) are used to respectively measure the
input and reflected power of this system. Finally, in order to couple
the microwave power to the coaxial structure, a waveguide-to-coax
converter is assembled at the end of this system.
To mix with H2O, pure argon gas flows through deionized
water before entering the plasma generator. A flow controller
(LF420-S, LAIFENG TECHNOLOGY CO. LTD., Chengdu, China)
is used to control and gauge the inflow of the argon gas. In our exper-
iment, for all the operating humidities, the total flow rate sustains at
8.1 L/min. The humidity of the working gas is controlled by adjust-
ing the depth of the gas pipe into the water, and its value is measured
by a humidity transmitter (LY60P-2X, ROTRONIC OEM, Switzer-
land). Before plasma ignition, the argon gas was flowing through
the ventilation system for 15 min to exhaust the air in the pipes
and the water jar. The emission spectrum of plasma at the nozzle is
measured by a spectrometer (AVSRACKMOUNT-USB2, Avantes,
Apeldoorn, The Netherlands) from 290 nm to 820 nm. The optic
probe is placed horizontally at a distance of 8 cm from the nozzle.
To catch the plasma spectrum clearly, a focusing lens and a laser pen
are employed. By adjusting the lens, the light from the plasma dis-
charge at the nozzle can be focused and captured by the optical fiber
probe.
III. EXPERIMENTAL RESULTS AND DISCUSSION
A. The energy efficiency at different humidities
Figure 3 shows that the relationship between energy efficiency
(1-Pr/Pin, where Pr/Pin indicates the ratio of reflected power to input
power) and the humidity of mixed gases at various input powers.
It is obvious, from Fig. 3, that the energy efficiency of this device
decreases with increasing gas humidity under the same input power.
An input power of 30 W cannot ignite the argon gas with 12%
FIG. 3. Energy efficiency varies with power under different humidities. Error bars
represent the standard deviation of the measurements.
humidity to generate plasma. By increasing the input power, the
energy efficiency enlarges at the same gas humidity.
These phenomena are easy to address. The dimensions of this
coaxial plasma generator are designed for pure argon gas, according
to the formula of coaxial impedance,
Z0=60√εr,eff ×ln(Dd), (1)
where Z0is the characteristic of a coaxial structure, Dand dare
the diameters of outer and inner conductors, respectively, and εr,eff
is the effective relative permittivity of the material (working gas in
this paper) between inner and outer conductors of the coax. Since
the working gas is a mixture (argon gas mixed with H2O), its rela-
tive permittivity can be obtained with the complex refractive index
equation,22
(εr,eff )1/2=ν1(ε1)1/2+ν2(ε2)1/2, (2)
where ε1and ε2represent the relative permittivity of water and argon
gas, which is 1 −0×j and 79–8.77 ×j at 20 ○C, respectively. v1
and v2are their homologous volume fractions, which satisfy the
relationship of v1+v2= 1.
With Eq. (2), it is obvious that mixing with water will change
the effective permittivity of the working gas, thus altering the
impedance of the coaxial plasma generator. Impedance mismatch
will lead to higher reflected power in the system, which makes the
energy efficiency lower and plasma generation harder. The more
water the argon gas mixes with, the more serious the impedance
mismatch is and the lower the energy efficiency is. That is why the
energy efficiency of this system decreases with higher humidity when
the microwave input power is the same.
To explore more about the reasons causing energy efficiency
variations at different microwave input powers and gas humidi-
ties, the electric field intensity distributions at the nozzle at various
microwave input powers and two humidities are calculated with the
finite element method in COMSOL Multiphysics and the Helmholtz
equation,23
∇2Ð→
E+ω2μr,eff εr,eff Ð→
E=0, (3)
where ωis the angular frequency, μr,eff is the magnetic permeability
of mixed gas, which is 1 in our case, and εr,eff is the relative permit-
tivity of mixed gas. Its value can be calculated with Eq. (2).
Eis the
electric field intensity. In the simulations, the bottom of the plasma
generator is set as a coaxial microwave excitation port. The four gas
inlet ports and the plasma nozzle are the scattering boundaries. The
other edges are set as perfect conductor boundaries.
In this paper, the parameters of pure argon (relative permittiv-
ity is 1) and mixed gas with 10% humidity (relative effective permit-
tivity is 2.95–0.154 ×j) have been used to calculate the electric field
intensity distributions at the nozzle. In order to compare the calcu-
lated consequences under these two gas humidity conditions at dif-
ferent input powers better, the same color range has been employed.
Results are shown in Fig. 4.
It is obvious, from Fig. 4, that the electric field intensity distri-
butions in different cases are the same. The strongest electric field
intensity is located around the inner conductor, and it becomes gen-
erally weaker from the inner conductor to the outer conductor. With
the same gas humidity, higher input power leads to stronger elec-
tric field intensity at the nozzle. One can observe that the electric
AIP Advances 11, 025131 (2021); doi: 10.1063/5.0033059 11, 025131-3
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FIG. 4. Simulated electric field intensity distributions at the nozzle in (a) pure argon
and (b) 10% gas humidity at different powers: [(a1) and (b1)] 30 W, [(a2) and (b2)]
40 W, [(a3) and (b3)] 50 W, [(a4) and (b4)] 60 W, and [(a5) and (b5)] 70 W.
field intensity with 10% humidity is much lower than that with pure
argon at the same input power. These simulation results are consis-
tent with the impedance mismatch theory mentioned above. Adding
water content results in a reduction in the power, which could prop-
agate to the nozzle and lower the electric field intensity. Higher elec-
tric field intensity could ionize more particles and generate higher
density plasma, which enables the plasma discharge absorb more
power. That is why the energy efficiency shows a positive corre-
lation with input power. However, higher reflected power due to
impedance mismatch and lower density plasma at higher humidity
make the energy efficiency lower.
B. The change in plasma plume shape and length
at different humidities
Figure 5 shows the photos of plasma plumes generated at var-
ious humidities and input powers. These photos were captured by
a camera (DSC-RX10M3, SONY, JAPAN) with 1/2500 exposure
FIG. 5. Photos of plasma plumes changing with power at different humidities.
time. Their lengths are listed in Table I. The gas flow rate is stable
at 8.1 L/min. Clearly, the plasma plumes have dominant streamers
and many small streamers when the humidity is low. It is worth
noticing that the branch streamers change quickly while the main
stream seems fixed over time. With increasing input power, the
dominant streamers get thicker, and more small streamers are born
at the ionization front. The higher the humidity, the less the small
streamers and the shorter the dominant streamers. The dominant
streamers will also become thinner with increasing humidity. How-
ever, the changes in plasma plumes, especially the lengths, do not
monotonously obey the above-mentioned variation in humidity and
input power. One could observe from Table I that the plume length
increases with slight fluctuation with input power when the humid-
ity is 1%, 3%, and 8%. The lower the gas humidity is, the larger
the change in the magnitude of the plume length is. The longest
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TABLE I. Lengths of plasma plumes at different humidities and powers.
Plasma plume length (mm)
Power Power Power Power Power
Gas humidity (%) 30 W 40 W 50 W 60 W 70 W
1 19.2 17.8 18.3 17.8 20.7
3 14.7 13.1 17.5 17.5 17.9
8 6.0 14.7 13.9 15.9 18.3
10 6.0 9.1 12.3 15.1 17.3
12 7.2 10.1 15.4 15.8
flame reaches 20.7 mm under a humidity of 1% and an input power
of 70 W.
These experimental phenomena can be interpreted with the
assistance of streamer theory and impedance mismatch. As men-
tioned above, increasing the gas humidity leads to a more serious
impedance mismatching problem, which causes more power reflec-
tion and less plasma density. It expresses why the plasma plume
becomes thinner and shorter with increasing gas humidity. How-
ever, when the gas humidity is fixed and the microwave input power
is increasing, the higher electric field intensity ionizes more gas.
More frequent electron collision ionization accelerates the avalanche
of electrons in the form of ionization waves, which will raise the
local electric field at the head of the plasma ionization front.24 When
the local E-field intensity is stronger than the critical value, the
plasma plume could generate many ionization fronts.25–27 As have
been indicated in Refs. 28 and 29, the ionization front with the local
enhanced electric field prolongs the dominant discharge, while many
small streamers abandoned by the ionization front grow up in the
axial direction around the main discharge. Moreover, those small
streamers can produce many much smaller streamers but with less
strength. As a result, plasma plumes become longer and thicker. Fur-
thermore, the difference between the E-field intensities of inner and
outer conductors at 70 W is the smallest among these input pow-
ers. The potential plasma initiation area is thus the largest, which
may also generate more plasma discharges and get a longer plasma
plume.
C. The emission spectra under different humidities
In order to check the influence of humidity on the plasma
property, the emission spectra of plasma at the generator’s nozzle
from 290 nm to 820 nm under different humidities of argon were
observed. To show and compare the spectra better, they are divided
into segments and demonstrated in Fig. 6 from 290 nm to 600 nm
and from 600 nm to 820 nm.
Figure 6 clearly indicates that, with increasing humidity, the
intensities of species related to H2O such as OH(A2∑+−X2∏),
Hα, and O (3s5S−3p5P) increase. Meanwhile, other excited species
mentioned in Sec. III, like N2+(B2∑+
u−X2∑+
g, shown in Fig. 6(a)),
present a great growth. However, the spectral intensities of Ar
I(3s23p54s−3s23p54p, shown in Fig. 6(b), speedily drop when the
humidity increases. Analogical changes of different Ar I line intensi-
ties were also observed when adding other gas molecules into argon
plasma.30
FIG. 6. Spectrums of plasma plumes under an input power of 70 W (a) from 290 nm
to 600 nm and (b) from 600 nm to 820 nm.
Since the intensity of the emission spectrum is approximately
proportional to the number of atoms spontaneously transitioned,31
Fig. 6 reveals that the densities of OH, Hα, O, and N2+increase while
that of Ar I (3s23p54s−3s23p54p) decreases with increasing humid-
ity. As mentioned above, the total gas flow speed is 8.1 L/min, and
the density of argon in the working gas becomes smaller when the
gas humidity is higher. Less argon leads to less collision probabil-
ity of argon atoms with other particles, which makes the densities of
Ar I (3s23p54s−3s23p54p) drop. On the contrary, higher humidity
enhances the collision chances of H2O and other relevant particles,
like N2. The reaction path N2A3∑++H2O→N2+ H+OH32 makes
the intensity of the spectra line of N2+(B2∑+
u−X2∑+
g)higher.
D. Changes in rotational temperature
under different humidities
Gas temperature of plasma is another significant factor one
should pay attention to if it is used for medical treatment. It is
reported that the gas temperature of plasma used to be considered
as the temperature of heavy particles, which is also close to the
molecular rotational temperature Trot.33 Therefore, the impacts of
AIP Advances 11, 025131 (2021); doi: 10.1063/5.0033059 11, 025131-5
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FIG. 7. Fitting result of OH radical emission at 12% humidity and 50 W.
gas humidity on the rotational temperature Trot of plasmas at the
nozzle under various input powers are investigated in this section.
In this paper, Trot is diagnosed by the OH(A2Σ+−X2∏) radical
and fitted by the LIFBASE software.34 Figure 7 shows a fitting result.
For the case with 12% gas humidity and 50 W input power, Trot of
plasma at the nozzle is 1550 K.
Figure 8 shows the relationship between Trot and humidity at
various input powers. Above all, Trot is increasing with increasing
mixed gas humidity, which accords with the results in Ref. 35. How-
ever, Trot of plasma does not increase with increasing input power
but decreases slightly when the gas humidity is 1% and 3%.
This phenomenon is relative to the fact that water has a larger
heat capacity than single atom Ar. Even when less water is added
to the plasma gas, significantly more thermal energy is stored in the
FIG. 8. Rotational temperature varies with power under different humidities. Error
bars represent the standard deviation of the measurements.
molecular rotation and vibration modes, resulting in global heating
of the plasma gas; thus, the gas temperature increases.35
However, Trot of plasma does not increase with increasing input
power but decreases slightly when the gas humidity is 1% and 3%.
This is explicable. Even though the energy efficiencies of the sys-
tem at these two humidities increase with input power, the collisions
among different particles are more frequent, which may make the
energies of heavy particles transmit to other particles for sustain-
ing the plasma discharge. Moreover, a larger plasma size (Fig. 5),
i.e., higher plasma density, will dilute the “average” energy of par-
ticles. These two make the energy of the heavy particle to decrease,
leading to lower rotational temperature. When the humidity is lower
(1% and 3%), these two factors add greater weight to the rotational
temperature than water heat capacity. Therefore, their rotational
temperatures decrease with increasing power.
It is worth noticing that the rotational temperatures obtained
as shown in Fig. 8 are those at the nozzle. However, in real appli-
cations, the treated sample is usually put around the plasma plume
head, rather than at the nozzle. To check if this plasma jet can be
employed for material surface treatment, especially for biomedical
treatment, a quartz plate is placed 26 mm above the nozzle and
treated by the plasma jet at different gas humidities and input pow-
ers. A thermal imager (VarioCAM hr, Infretec, Germany) is used
to record the maximum temperature of the quartz plate after being
treated for 2 min, as shown in Fig. 9(a). The measured results are
demonstrated in Fig. 9(b). It is obvious that the quartz plate is heated
up, which indicates that the energy of electrons and ions in plasma
could treat the target surface indirectly. The maximum temperatures
of the plate are low enough for biomedical treatment. Furthermore,
by adjusting the microwave input power and gas humidity, the tem-
perature above the plasma plume head would be much lower and
more suitable to treat human skin or cells.
E. Influence of humidity on spectral intensity
of 309 nm OH and 777 nm O
The excited oxygen atoms can significantly deactivate various
bacterials, such as Escherichia coli,Bacillus subtilis endospores,Bacil-
lus luteum, and so on.8,36 Inducing apoptosis of cancer cells is also
reported to be induced by the concentration of reactive oxygen
atoms.5The hydroxyl radical has strong oxidation and can be used
for tooth whitening to remove pollutants efficiently.14 Since the dose
of these active species plays an essential role on the treating effect,
the influences of humidity and input power on their intensities are
investigated in this section. Results under cases with five different
gas humidities and input powers are shown in Fig. 10.
A glance at Fig. 10 tells that the spectrum intensity of 309 nm
OH and 777 nm O is related to argon humidity and microwave input
power. Increasing the microwave input power increases the inten-
sities of OH and O. However, the increment is not linear, which
diminishes progressively generally as the input power grows fur-
ther. For the impact of humidity, one can observe that the intensities
of OH and O do not always monotonically increase with the gas
humidity. Instead, both OH and O show a wavy change tendency at
lower input powers. The intensity of OH increases from a humidity
of 1%–3% with 30 W. It starts to decline if more H2O is continuously
mixed and increases slightly again when the humidity is at 10%. The
same changing slopes repeat, but the corresponding turning points
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FIG. 9. Temperatures above the head of plasma plumes under various cases: (a)
a thermal imager being placed H= 26 mm above the nozzle to catch the glass
temperature and (b) the maximum temperature on the glass plate with different
input powers and different humidities.
emerge at higher humidity for 40 W and 50 W. Variations in O spec-
tra line intensity with gas humidity are a little bit different when the
input powers are 30 W and 40 W. The intensity curves show only
one turning point.
The hydroxyl radical (OH) is generated mainly through the
collisions between molecules and other particles like electrons and
excited-state argon atoms37,38 (e + H2O+→H + OH, Ar∗+ H2O
→H + OH + Ar). It could also be produced via thermal dissocia-
tion of H2O39 when the gas temperature of plasma is higher than
1500 K (factor 1). The active oxygen atoms are obtained by ioniz-
ing excited state H2O molecules (H2O∗→O + H237). Combining
with the expressions in Secs. III A and III E, it is easy to understand
that the line intensity of OH and O increases with increasing gas
humidity and microwave input power because of more frequent col-
lisions and higher electric field intensity (factor 2). Meanwhile, as
the H2O molecule has strong electronegativity and a large electron
attachment surface, the electrons and H2O molecules will combine
FIG. 10. Absolute intensity of emission spectra varies with humidities at different
powers: (a) the OH 309 nm line (integrated time of 200 ms) and (b) the O 777 nm
line (integrated time of 30 ms). Error bars represent the standard deviation of the
measurements.
and form negative ions. This leads to fewer collisions between H2O
and electrons and less ionization of excited-state H2O (factor 3). The
densities of OH and O thus decrease. When the gas humidity and
microwave input power are lower, factor 2 dominates the reactions,
and the line intensities of O and OH are proportional to these two
parameters. On increasing the gas humidity further, the influence
of factor 3 appears. When the gas humidity is larger than 8%, fac-
tor 1 begins to work. The detected intensities of OH and O spectra
lines are results of these three factors. That is why the intensities of
these two show a wavy tendency with increasing gas humidity and
microwave input power.
IV. CONCLUSION
Water was added to the feeding gas of a microwave-induced
atmospheric argon plasma jet to experimentally investigate its
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influence on the plasma characteristics. Spectral diagnosis, microwave
theory, and simulation were employed to analyze the interaction
between microwave power and plasma discharge. Results showed
the following:
(1) Increasing the water fraction in the working gas reduces the
energy efficiency of the plasma generator and the plasma
jet length due to impedance mismatching. The more water
the argon gas mixes with, the more serious the impedance
mismatch is and the lower the energy efficiency is.
(2) Increasing microwave input power could obtain more OH
and O. However, adding more water does not lead to higher
densities of OH and O because the electronegativity and the
large electron attachment surface of H2O molecules impeded
the occurrence of collisions and formation of OH and O.
(3) Water addition makes the rotational temperature (gas tem-
perature) higher because the collisional relaxation of H2O
molecules dominates the impact factors. However, when the
gas humidity is lower, the gas temperature remains almost the
same at different input powers.
In short, gas humidity and microwave input power play sig-
nificant roles in plasma properties, especially the intensities of ROS
(OH and O). One could obtain proper ROS densities and gas
temperatures for biomedical applications by controlling these two
parameters.
ACKNOWLEDGMENTS
This work was supported, in part, by the Science and Tech-
nology Planning Project of Sichuan Province under Grant No.
2018HH0107. It was also supported, in part, by the National Nat-
ural Science Foundation of China under Grant Nos. 61801313 and
61731013.
DATA AVAILABILITY
The data that support the findings of this study are available
from the corresponding author upon reasonable request.
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AIP Advances 11, 025131 (2021); doi: 10.1063/5.0033059 11, 025131-8
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