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# The influence of gas humidity on the discharge properties of a microwave atmospheric-pressure coaxial plasma jet

Jie Yu, Wencong Zhang, Xiao Wu, Li Wu, Junwu Tao, Kama Huang

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Jie Yu, Wencong Zhang, Xiao Wu, Li Wu, Junwu Tao, et al.. The influence of gas humidity on the discharge properties of a microwave atmospheric-pressure coaxial plasma jet. AIP Advances, 2021, 11 (2), pp.025131. 10.1063/5.0033059. hal-03822066

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# The influence of gas humidity on the discharge properties of a microwave atmospheric-pressure coaxial plasma jet

Jie Yu ⚙️; Wencong Zhang ⚙️; Xiao Wu; Li Wu 📄; Junwu Tao ⚙️; Kama Huang

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AIP Advances 11, 025131 (2021)

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# 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

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Jie Yu,<fnref n="1" /> ID Wencong Zhang,<fnref n="2" /> ID Xiao Wu,<fnref n="1" /> Li Wu,<fnref n="1" />,ᵃ ID Junwu Tao,<fnref n="3" /> ID and Kama Huang<fnref n="1" />

## AFFILIATIONS

## 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

## I. INTRODUCTION

Low-temperature atmospheric-pressure plasma (APP) has been widely used in various biomedical applications,<fnref n="1" />⁻<fnref n="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), containing 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 density 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 typical 7/16 DIN connector (Deutsche Industrie Norm connector).

<footnote n="1">¹IAEM (Institute of Applied ElectroMagnetics), College of Electronics and Information Engineering, Sichuan University, Chengdu 610065, People's Republic of China</footnote>

<footnote n="2">²School of Electronic and Communication Engineering, Guiyang University, 550005 Guiyang, People's Republic of China</footnote>

<footnote n="3">³Laplace (Laboratoire Plasma et Conversion d'Energie), INPT - ENSEEIHT, Université de Toulouse, 31071, France</footnote>

<footnote>ªAuthor to whom correspondence should be addressed: wuli1307@scu.edu.cn</footnote>
<figure><img src="image_14.png" /><figcaption>FIG. 1. Structure of the plasma generator: the (a) vertical section and (b) bottom view.</figcaption></figure>

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 circuits. 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 structure 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 tangential 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

<figure><img src="image_15.png" /><figcaption>FIG. 2. Schematic of the experimental setup.</figcaption></figure>
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 experiment, for all the operating humidities, the total flow rate sustains at 8.1 L/min. The humidity of the working gas is controlled by adjusting the depth of the gas pipe into the water, and its value is measured by a humidity transmitter (LY60P-2X, ROTRONIC OEM, Switzerland). 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 discharge 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-P<sub>r</sub>/P<sub>in</sub>, where P<sub>r</sub>/P<sub>in</sub> 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%

<figure><img src="image_16.png" /><figcaption>FIG. 3. Energy efficiency varies with power under different humidities. Error bars represent the standard deviation of the measurements.</figcaption></figure>

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,

$$Z_0 = 60/\sqrt{\varepsilon_{r,eff}} \times \ln(D/d), \quad (1)$$

where Z₀ is the characteristic of a coaxial structure, D and d are the diameters of outer and inner conductors, respectively, and ε<sub>r,eff</sub> 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 relative permittivity can be obtained with the complex refractive index equation,<sup>22</sup>

$$(\varepsilon_{r,eff})^{1/2} = v_1(\varepsilon_1)^{1/2} + v_2(\varepsilon_2)^{1/2}, \quad (2)$$

where ε₁ and ε₂ represent the relative permittivity of water and argon gas, which is 1 – 0 × j and 79–8.77 × j at 20 °C, respectively. v₁ and v₂ are 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 humidities, 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,<sup>23</sup>

$$\nabla^2 \vec{E} + \omega^2 \mu_{r,eff} \varepsilon_{r,eff} \vec{E} = 0, \quad (3)$$

where ω is the angular frequency, μ<sub>r,eff</sub> is the magnetic permeability of mixed gas, which is 1 in our case, and ε<sub>r,eff</sub> is the relative permittivity of mixed gas. Its value can be calculated with Eq. (2). Ē is 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 permittivity is 1) and mixed gas with 10% humidity (relative effective permittivity is 2.95–0.154 × j) have been used to calculate the electric field intensity distributions at the nozzle. In order to compare the calculated consequences under these two gas humidity conditions at different 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 distributions in different cases are the same. The strongest electric field intensity is located around the inner conductor, and it becomes generally weaker from the inner conductor to the outer conductor. With the same gas humidity, higher input power leads to stronger electric field intensity at the nozzle. One can observe that the electric
<figure><img src="image_17.png" /><figcaption>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.</figcaption></figure>

field intensity with 10% humidity is much lower than that with pure argon at the same input power. These simulation results are consistent with the impedance mismatch theory mentioned above. Adding water content results in a reduction in the power, which could propagate to the nozzle and lower the electric field intensity. Higher electric 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 correlation 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 various humidities and input powers. These photos were captured by a camera (DSC-RX10M3, SONY, JAPAN) with 1/2500 exposure

<figure><img src="image_18.png" /><figcaption>FIG. 5. Photos of plasma plumes changing with power at different humidities.</figcaption></figure>

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. However, 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 humidity 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
<table><caption>TABLE I. Lengths of plasma plumes at different humidities and powers.</caption><thead><tr><th rowspan="2">Gas humidity (%)</th><th colspan="5">Plasma plume length (mm)</th></tr><tr><th>Power 30 W</th><th>Power 40 W</th><th>Power 50 W</th><th>Power 60 W</th><th>Power 70 W</th></tr></thead><tbody><tr><td>1</td><td>19.2</td><td>17.8</td><td>18.3</td><td>17.8</td><td>20.7</td></tr><tr><td>3</td><td>14.7</td><td>13.1</td><td>17.5</td><td>17.5</td><td>17.9</td></tr><tr><td>8</td><td>6.0</td><td>14.7</td><td>13.9</td><td>15.9</td><td>18.3</td></tr><tr><td>10</td><td>6.0</td><td>9.1</td><td>12.3</td><td>15.1</td><td>17.3</td></tr><tr><td>12</td><td></td><td>7.2</td><td>10.1</td><td>15.4</td><td>15.8</td></tr></tbody></table>

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 mentioned above, increasing the gas humidity leads to a more serious impedance mismatching problem, which causes more power reflection and less plasma density. It expresses why the plasma plume becomes thinner and shorter with increasing gas humidity. However, 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. Furthermore, the difference between the E-field intensities of inner and outer conductors at 70 W is the smallest among these input powers. 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(A²Σ⁺ - X²Π), Hα, and O(3s⁵S - 3p⁵P) increase. Meanwhile, other excited species mentioned in Sec. III, like N₂⁺ (B²Σ⁺u - X²Σ⁺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 intensities were also observed when adding other gas molecules into argon plasma.²⁰

<figure><img src="image_19.png" /><figcaption>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.</figcaption></figure>

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 humidity. 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 probability 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 N₂(A³Σ⁺) + H₂O → N₂ + H + OH³² makes the intensity of the spectra line of N₂⁺ (B²Σ⁺u - X²Σ⁺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
<figure><img src="image_20.png" /><figcaption>FIG. 7. Fitting result of OH radical emission at 12% humidity and 50 W.</figcaption></figure>

gas humidity on the rotational temperature $T_{\text{rot}}$ of plasmas at the nozzle under various input powers are investigated in this section.

In this paper, $T_{\text{rot}}$ is diagnosed by the OH($A^2\Sigma^+ - X^2\Pi$) 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, $T_{\text{rot}}$ of plasma at the nozzle is 1550 K.

Figure 8 shows the relationship between $T_{\text{rot}}$ and humidity at various input powers. Above all, $T_{\text{rot}}$ is increasing with increasing mixed gas humidity, which accords with the results in Ref. 35. However, $T_{\text{rot}}$ 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

molecular rotation and vibration modes, resulting in global heating of the plasma gas; thus, the gas temperature increases.³⁵

However, $T_{\text{rot}}$ 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 system 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 sustaining the plasma discharge. Moreover, a larger plasma size (Fig. 5), i.e., higher plasma density, will dilute the “average” energy of particles. 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 applications, 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 powers. 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 temperature 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 bacteria, such as *Escherichia coli*, *Bacillus subtilis endospores*, *Bacillus luteum*, and so on.⁸,³⁶ Inducing apoptosis of cancer cells is also reported to be induced by the concentration of reactive oxygen atoms.⁵ The 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 intensities of OH and O. However, the increment is not linear, which diminishes progressively generally as the input power grows further. 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 $H_2O$ is continuously mixed and increases slightly again when the humidity is at 10%. The same changing slopes repeat, but the corresponding turning points

<figure><img src="image_21.png" /><figcaption>FIG. 8. Rotational temperature varies with power under different humidities. Error bars represent the standard deviation of the measurements.</figcaption></figure>
<figure><img src="image_22.png" /><figcaption>**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.</figcaption></figure>

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

<figure><img src="image_23.png" /><figcaption>**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.</figcaption></figure>

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
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 temperature) 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 significant 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 Technology Planning Project of Sichuan Province under Grant No. 2018HH0107. It was also supported, in part, by the National Natural 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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