Atmospheric pressure helium plasma jets are widely used in biomedical applications. Researchers normally introduce small amounts of nitrogen and oxygen (0.2–1.0%) into helium to enhance the electron density and electron energy, thus increasing the concentration of active species in plasma. To further explore why the combination of impurity gases N2/O2 leads to an increase in the electron density from the discharge mechanism, we used a microwave Rayleigh scattering method with excellent time-varying characteristics to monitor the temporal electron density changes when different concentrations of N2/O2 were mixed. The research revealed that even trace amounts of N2/O2 (0.2%) can increase the peak electron density, with this effect being more pronounced when N2 is added, increasing from 3.3 × 1019 to 4.6 × 1019 m−3 in pure helium. As the concentration increases, the introduction of O2 leads to a rapid decrease in the electron density. When 1.0% oxygen is mixed, the electron density decreases from 3.3 × 1019 to 2.4 × 1019 m−3. However, the situation is different when N2 is added, at 0.5% proportion of nitrogen, the electron density increases to its maximum at 6.5 × 1019 m−3. These effects are due to the electronegativity of the oxygen-containing particles or the Penning ionization related to excited nitrogen species.

Atmospheric pressure plasma jets (APPJs) have recently attracted much attention due to their diverse applications.1–7 Especially in biomedical applications, the APPJs have been successfully applied in sterilization, wound healing, and transdermal delivery.8–12 Typically, APPJs are driven by AC or pulsed high voltage, employing noble gases, such as helium or argon, as a working gas. These noble gases are usually mixed with small amounts of O2 or N2, which can significantly improve the density of reactive oxygen and nitrogen species (RONS).13–18 

However, even after much research being done on plasma medicine, the mechanism of discharge after the introduction of nitrogen and oxygen remains unclear, particularly in the diagnosis of electron density. The electron density serves as a crucial parameter to describe the dynamics of plasma, but it is also difficult to diagnose APPJs under various conditions, especially the temporal electron density with high temporal resolution. Traditional diagnostic methods for plasma density, such as Langmuir probe and optical emission spectroscopy methods, have their notable limitations, especially in temporal resolution. Hence, there is an urgent need for a diagnostic method with high temporal resolution to address these challenges.

Shneider et al. proposed a microwave Rayleigh scatter method of measuring the electron density.19 An advantage of this approach is that it has excellent performance in time resolution and continuity, and the temporal resolution can reach a nanosecond scale. It is a noninvasive measurement that has no impact on the plasma and can directly interact with electron. Furthermore, the experimental setup is relatively simple compared to laser diagnosis. More importantly, the applicability of the Rayleigh scattering method is very extensive. It can be applied to various gas environments and is not constrained by pressure, making it highly suitable for a wide range of applications under atmospheric pressure conditions.

In this paper, the temporal electron density in a pulsed helium plasma jet at atmospheric pressure is obtained by the microwave Rayleigh scattering system. The effect of mixing with different percentage of O2 or N2 and the pulse width of applied high voltage on the electron density have been investigated. Additionally, simulations are conducted using COMSOL software under identical conditions. We specifically compared the electron density amplitude and decay time with the experimental results and analyzed the impact of impurity gases on the electron density from the perspective of particle reactions in Sec. IV.

The microwave Rayleigh scattering system comprises a radiation antenna, a receiving antenna, an amplifier, an I/Q mixer, an absorbing material, and an oscilloscope. The rectangular radiation antenna is used to emit 12 GHz microwave signal, while a similar receiving antenna is used to capture the microwave Rayleigh signal scattered by the plasma. The antenna's radiating and receiving aperture is a rectangular shape with dimensions of 7 × 10 cm2. Each component needs to be accurately positioned; two antennas must be situated within the same horizontal plane and ensure their vertical alignment. Additionally, both the radiation antenna and the receiving antenna should be positioned at a fixed distance of 20 cm from the scatterer. The scatter is placed at the center of the intersection area of two antennas, with the axis of the plasma or dielectric material parallel to the direction of the microwave electric field E, as shown in Fig. 1(b). The diagram of the system used to measure the time-resolved electron density of atmospheric plasma jet is shown in Fig. 1.

A microwave absorbing material is employed to build a small microwave anechoic chamber, effectively minimizing the stray microwave reflection and the interference of the surrounding environment. Note that all metallic elements, including the discharge electrode and cable lines, which have a strong reflection on microwaves, must be carefully excluded from the scattering region. When the plasma is generated in the scattering region, the Rayleigh signal is collected synchronously by the receiving antenna. Subsequently, the receiving antenna transmits the scattered microwave signal to an amplifier and processed by an I/Q mixer. More details about the system parameters and microwave simulations are available in our previously published work.20 

Finally, the mixer exports two independent in-phase (UI) and quadrature-phase (UQ) outputs to oscilloscope. The total voltage amplitude of microwave Rayleigh scattering signal can be expressed as
U out = U I 2 + U Q 2 .
(1)
According to the theory of microwave Rayleigh scattering, the output voltage U out is directly proportional to the total current excited within the scatter I s .19 For conductive scatterers, the current can be expressed as I s = σ E i S (where σ is the conductivity, E i is the amplitude of incident microwave field, and S is the cross-sectional area). For dielectric scatterers, the current can be written as I s = ε 0 ε 1 ω E i S ( where ε 0 is the vacuum permittivity, ε is the relative permittivity of the dielectric material, and ω is the angular frequency of microwave). The calibration parameters associated with the scattering system are denoted as A, and after simplification, the U out can be expressed as follows for different materials:20 
U out = A · e 2 m v · N e for plasma , A · V · ε 0 · ε 1 · ω for dielectric material ,
(2)
where A is the calibration coefficient, which is related to experimental setup. e is the electron charge (1.6 × 10−19 C), m is the electron mass (9.109 × 10−31 kg), v is the electron–gas collision frequency that is estimated to be 1.46 × 1012 s−1 in this case, V is the volume of the dielectric material, ε 0 is the vacuum permittivity (8.85 × 10−12 F/m), ε is the relative permittivity of the dielectric material (3.8 in this work), and ω is the frequency of the microwaves (12 GHz). N e is the total number of electrons in the scattering area.
In order to obtain the absolute value of electron density from the amplitude of Uout, it is necessary to calibrate the whole system. The calibration parameter A is related to the antenna geometry structure, microwave power, and other inherent properties. Once the system is fixed, it can be used as a constant without calibrate repeatedly. We record the I/Q channel signal with or without calibration material, and the difference between them is used as the Rayleigh scattering signal of calibration given as
U cal = ( U I U I ) 2 + ( U Q U Q ) 2 .
(3)

Five measurements repeats are needed to reduce the uncertainties in calibration results.

Referring to formula (1) for dielectric material, the output signal is proportionate to the volume of the dielectric material. In this experiment, calibration of the entire system is conducted using quartz rods of varying lengths, each of them having a same constant dielectric coefficient of 3.8. These rods are made into cylinders with a diameter of 3 mm with the length varying from 5 to 50 mm. Sequentially, each rod is placed on a ceramic platform at the center of the scattering region, and the corresponding Rayleigh scattering values are recorded. Note that the dielectric coefficient of the platform (relative dielectric constant of 9.8) must have a noticeable difference from the calibration material to avoid unnecessary influences. Moreover, due to the existence of the platform and the disturbance of the surrounding environment, there is still a stable Rayleigh scattering signal even if no calibration material is placed. The calibration result by dielectric material is shown in Fig. 2.

It can be seen from Fig. 2 that the measured Rayleigh signal exhibits proportionality to the length of dielectric rods, but the fitting line does not pass through the origin point due to the rods being not exactly symmetrical in the middle of the incident microwave beam. To address this, the fitting line is artificially adjusted to pass through the origin point, and taking the signal of 20.04 mV at the length of 25 mm as the calibration point. Then, we can obtain the calibration parameter A = 0.802.

In this work, plasma is produced by pulsed high voltage and the working gas is helium or helium mixed with a small amount of oxygen or nitrogen. The tungsten needle electrode is placed in the glass tube (dielectric coefficient of 6.0) with an inner diameter of 3 mm, and the tip of the needle is situated 2 cm away from the nozzle. The plasma is sprayed from the tube to the outside and there is no additional ground, as illustrated in Fig. 3(b). The pulsed high voltage is generated by a high-voltage DC power supply (Spellman SL300) and a pulse chopper (DEI PVX-4110). The working gases, helium, oxygen, or nitrogen (99.999%) are all controlled by a gas flow controller. The voltage applied to the needle electrode is measured by a high-voltage probe (Tektronix P6015), and the current flowing through the high-voltage electrode is measured by a current probe (Tektronix TCPA300). All measured signals are displayed on an oscilloscope (Tektronix DPO3034).

As the glass tube is also placed within the Rayleigh scattering area, there is still a stable scatter signal when no plasma is generated, and we record this scatter signal as the background value. Note that the needle electrode is made of metal, which has a strong reflection of Rayleigh signal; the scattering region must be a considerable distance away from the electrodes. To establish a reference point, the tip of the needle is defined as the position where z = 0. Importantly, the measured Rayleigh signal only provides information about the total number of electrons Ne, obtaining that the absolute electron density requires fixing the volume of the plasma in the microwave Rayleigh scattering region. In this work, the plasma is confined to a glass tube, so the volume inside the thin tube can be used to approximate the plasma volume.

After calibration, the absorbing material is placed in front of the radiating antenna to shape the emitted microwaves into a 1 × 10 cm2 rectangle, achieving a higher spatial resolution. Plasma jet is positioned at the center of the Rayleigh region, the electron density in the area from z = 5 to 15 mm is measured, with the other area being blocked by the absorbing material. When pure helium is used as the working gas with a flow rate of 2 l/min, the applied voltage of 7 kV, pulse width of 200 ns, frequency of 5 kHz, the temporal electron density along with the corresponding voltage and current is shown in Fig. 4.

It can be found that there are two peaks in the discharge current, the current reaches 25 mA in the rising edge discharge and the negative peak value of the current is about −34 mA. The measured temporal electron density can be divided into three stages. In the range from 0 to 100 ns, the electron density experiences a gradual increase, attributed to the fact that the plasma has not fully developed to the core scattering region. In the range of 100–160 ns, there is a rapid surge in the electron density, reaching 1 × 1019 m−3 at 160 ns due to the rising edge discharge. Subsequently, a falling edge discharge occurs after 200 ns, with the electron density reaching 3.1 × 1019 m−3 at about 400 ns. Interestingly, the electron density is maintained at about 5 × 1018 m−3 at 1200 ns.

It is worthy to note that there is a delay of 100 ns between the peak of the electron density and peak of the discharge current. This is mainly caused by the delay of plasma developed in microwave Rayleigh scattering region. As many researchers have studied the velocity of plasma generation and propulsion, a typical velocity for plasma jet propulsion is 105 m/s, aligning with the measured delay of 100 ns.21,22

Furthermore, we focus on adding a small amount of oxygen or nitrogen to helium, which is the widely used method to enhance the density of RONS and plays a key role in biomedical applications. Initially, we maintain the discharge conditions unchanged, while mixing three different proportions of oxygen (0.2%, 0.5%, and 1.0%) with pure helium (keeping the total gas flow rate of 2 l/min). When the oxygen content surpasses over 1.0%, the discharge is obviously weakened, so we do not care about the situation of introducing more oxygen. The temporal electron density by helium mixed with different proportions of oxygen obtained by microwave Rayleigh scattering is presented in Fig. 5.

When 0.2% O2 is mixed with helium, the peak electron density reaches 3.7 × 1019 m−3 at 360 ns, which is slightly higher than pure helium of 3.3 × 1019 m−3. Notably, mixing with oxygen accelerates the decay of electron density, which makes it to decay to detection limit at about 1400 ns, but in pure helium, the electron density is still maintained at about 4 × 1018 m−3. As the oxygen content increases, the peak electron density decreases to 3.2 × 1019 m−3 of 0.5% O2 mixed and 2.4 × 1019 m−3 of 1.0% O2 mixed. The decay of electron density also becomes progressively faster with higher oxygen content, leading to an earlier reaching of the detection limit.

Next, while maintaining the discharge conditions, three different proportions of nitrogen (0.2%, 0.5%, and 1.0%) are introduced into pure helium (the total gas flow rate of 2 l/min). The temporal electron density of mixing with nitrogen is presented in Fig. 6.

The situation of mixing nitrogen is obviously different from oxygen. When 0.2% N2 is mixed, the peak electron density increases to 4.6 × 1019 m−3, which has a substantial rise compared to 3.3 × 1019 m−3 in pure helium. In addition, while the peak electron density clearly increased with the addition of a small amount of nitrogen, the decay rate of high electron density also accelerates, causing the electron density to almost overlap at 1 μs. With the addition of 0.5% N2, the peak electron density further surges to 6.5 × 1019 m−3, which is nearly twice as high as the electron density in pure helium. Along with more nitrogen added, the peak electron density begins to decrease, but is still much higher than pure helium when 1.0% N2 is mixed. Compared to adding oxygen, mixing with a small amount of nitrogen significantly increases the peak value of electron density with less influence on the decay rate of electron density.

Due to limit of detection, the tendency of electron density under low magnitude is difficult to be determined precisely. A zero-dimension simulation is conducted using COMSOL, and main reactions along with the corresponding rate coefficient between electron and N2/O2 are listed in Table I. The helium with varying proportions of oxygen or nitrogen is employed as the raw gas inlet to simulate the electron reaction process of the plasma.

In this simulation, we utilized the electron temperature under similar conditions, as measured by Thomson scattering in our previous studies.23 The observed trend of the electron temperature was found to be relatively stable, with a mean electron temperature to be 3 eV. Additionally, based on our earlier measurements, the generation of APPJs did not lead to a significant increase in temperature.24 Therefore, the gas temperature is set to the ambient temperature during the experiment, which is 290 K. For the solution of the electron energy distribution function (EEDF), we employed the Boltzmann solver BOLSIG+. Subsequently, the electron impact reaction rate coefficients were calculated based on the mean electron energy and gas composition. In simulation, the applied voltage is 7 kV with a frequency of 5 kHz and pulse width of 200 ns; the simulation result is shown in Fig. 7.

Consistent with experimental findings, a small amount of oxygen and nitrogen proves beneficial to the increase in the electron density. When oxygen is added to helium, the peak value of electron density reaches its maximum of 5.1 × 1019 m−3 at 0.2% O2 mixed. However, owing to the electron attachment by oxygen, both the peak value and decay time of the electron density decrease significantly. The decay of electron is particularly rapid at the initial stage after voltage disappearance (from 200to 400 ns). Moreover, when mixing with nitrogen, the electrons were produced more efficiently than oxygen. Especially after 0.5% N2 is added, the electron density reached 8 × 1019 m−3 at 200 ns. Since nitrogen has no electronegativity, the decay of electron density shows no obvious difference when a small amount of nitrogen is added.

On comparing the decay of electron density when nitrogen and oxygen are mixed, the decay rate of electron density is greatly affected by the proportion of mixed oxygen. This effect is primarily attributed to oxygen atoms and excited oxygen particles, whose decay time ranges from several hundreds of nanoseconds to a few microseconds. To further investigate this influence, we explored the effect of discharge frequency, and by increasing the discharge period, oxygen particles can achieve more complete reduction.

The discharge frequency is adjusted to 1 kHz while keeping other conditions unchanged. Figure 8 illustrates that a decrease in frequency has a minimal impact on the decay of electron density when a small amount of oxygen (0.2%) is present, as compared to pure helium. Because the interval between two adjacent discharge is relatively long in the case of low frequency, the long-lived particles decay to a relatively low level, and these particles play a key role in the electron attachment, which has a great influence on the decay rate of electron density. However, as the oxygen content continues to increase, even with longer intervals, the initial oxygen species is enough to attach electron, leading to the faster decay of electron density.

In contrast to the sensitivity of long-lived particle accumulation to frequency, the electron decay rate is extremely rapid compared to the discharge period, and altering the frequency does not produce a noticeable effect. Therefore, we investigate a discharge with long pulse width closing to period, which has an extremely short interval between the falling edge and the rising edge of the next discharge. Figure 9 reveals that at a frequency of 1 kHz and pulse width of 140 ns, the discharge channel is bright and successive. However, with a pulse width of 999.86 μs, the brightness of jet inside the tube is obviously diminished, creating a dark area in the tube, and the outside jet becomes dim with a shortened length.

From Fig. 10, it is found that the falling edge discharge is remarkably strong, reaching its peak value of 3 × 1019 m−3 around 200 ns. As the electron density decreases to 2.5 × 1019 m−3, the next rising edge discharge occurs, which has little effect on the amplitude of electron density, but slightly prolongs the decay time of the electron density. Additionally, Fig. 11 shows that compared to pulse width of 999.86 μs, the electron density is significantly lower with a pulse width of 140 ns, reaching its peak value of 1.4 × 1019 m−3.

This study investigates changes in the electron density when a small amount of nitrogen or oxygen is mixed with a pure helium gas jet within a conduit. It is observed that the introduction of oxygen leads to a significantly accelerated decay rate of electron. This phenomenon is attributed to the strong electronegativity of oxygen gas, where even a small number of oxygen atoms or excited oxygen particles can exert a strong adsorption effect on electrons. Consequently, as the oxygen content in the mixture increases, the electron density experiences a rapid decay. When oxygen content exceeds a threshold of 2%, the plasma becomes weak and may even fail to generate. The primary electron consumption reactions are as follows:
e + O 2 O + O , e + O 2 + M O 2 + M , e + O 2 + O + O O * .
In contrast to oxygen, nitrogen exhibits a different behavior. More high-energy electrons in the plasma will ionize and excite nitrogen molecules, especially when we have introduced only a small amount of nitrogen into helium.28 Penning ionization of N 2 C 3 Π u plays a significant role in this context,
N 2 C 3 Π u + H e m N 2 + B 2 Σ u + + H e + e .

However, it should be noted that the intensity of Penning ionization does not exhibit a continuous increase with the rise in nitrogen concentration. As the nitrogen concentration reaches a relatively high proportion, the electron density experiences a rapid decrease as well. Additionally, compared to pure helium gas, especially in the case of a stable composition of plasma gas inside the tube, the introduction of impurity gases, such as oxygen or nitrogen, significantly enhances the photoionization in helium core. This enhancement results in the generation of seed electrons over a larger range, thereby increasing the charge density and the speed of discharge generation and propulsion.29,30

Moreover, it is interesting to note the changes in the characteristic decay time (decays to 1/e) of the electron density following the introduction of impurity gases. In the case of pure helium, the characteristic decay time of the electron density is 500 ns. However, with a small trace of oxygen (0.2%) mixed in, the electron density peak slightly increases, but the decay time significantly decreases to only 320 ns. However, when a small amount of nitrogen (0.2%) is introduced, the electron density rises by approximately 40%, but the decay time only decreases to 420 ns. However, with a higher concentration of nitrogen (0.5%), the decay time rapidly decreases further to 280 ns. Similar patterns can also be inferred from our simulation results in Fig. 7.

Based on the above-mentioned analysis, changes in the electron density are influenced by the presence of N or O atoms and excited particles introduced by the admixed gas. These particles often exhibit lifetimes in the order of microseconds. Therefore, we increased the discharge period from 200 μs (5 kHz) to 1 ms (1 kHz) to explore the decay of long-lived particles further. Consequently, when a small amount of oxygen (0.2%) is mixed, the change in helium gas attenuation is relatively small compared to the situation at 5 kHz.

Furthermore, unlike long-lived particles, electrons decay very rapidly, decreasing to a very low level within a few microseconds, making it challenging to control the electron density by adjusting the frequency. To address this issue, we adjusted the pulse width to 999.86 μs at 1 kHz, ensuring a 140 ns interval between the falling edge of the previous discharge and the rising edge of the next discharge. This change resulted in a significant transformation of the plasma discharge dynamic, with distinct dark regions appearing within the tube, and there was a substantial increase in the electron density. This increase in the electron density can be attributed to the decrease in electric field and electron temperature, leading to a reduction in excitation, and consequently, a decrease in brightness.

In this study, we employed the method of microwave Rayleigh scattering to diagnose the time-varying electron density in helium jet mixed with a small amount of nitrogen or oxygen. This method offers excellent temporal continuity and is relatively more convenient to set up compared to techniques like Thomson scattering. More importantly, it does not require discharges to have strict stable repeatability, making it possible to capture transient instantaneous changes of the electron density. While this method is relatively straightforward, certain limitations should be acknowledged. The plasma volume needs to be determined, and ensuring the absence of metallic electrodes in the scattering region is essential. Moreover, further research is needed to explore related electron temperatures and the high-spatial resolution distribution of the electron density.

The accuracy of Rayleigh scattering method is mainly influenced by factors from two aspects: the calibration process and the determination of the volume of the scatterer. In Shneider's previous research, calibration was carried out by using a dielectric bullet, and the deviation from this process did not exceed 10%–15%.19 In our study, the calibration results shown in Fig. 2 reveal a robust linear relationship in the fitting of calibration parameters, with a total deviation within 10%. For the determination of the volume of scatterers, we confine the plasma inside a thin glass tube to minimize errors in volume estimation. However, due to the hollow distribution of the plasma jet, the actual electron density may be slightly higher.23 In previous studies, we also utilized Thomson scattering method to measure the electron density of helium APPJs. The deviation of Thomson scattering method comes from the fitting error of Raman scattering and the deviation of laser energy, which is estimated as 10%, where the two methods have an accuracy of a similar magnitude.23 

The microwave Rayleigh scattering method is employed to measure the temporal electron density when helium is mixed with impurity gases (oxygen or nitrogen). The results revealed that an excessive amount of impurity gas mixing inhibited the generation and propulsion of the plasma. However, when trace amounts (less than 1%) of impurity gas were introduced, it significantly increased the electron density of the plasma. Notably, the addition of trace amounts of nitrogen significantly enhanced the electron density due to the photoionization and Penning ionization. Additionally, the study demonstrated that the variations in discharge frequency and pulse width exerted a significant impact on the electron density.

This work was supported by the National Natural Science Foundation of China (Grant Nos. 52130701, 51977096, and 52277150) and the National Key Research and Development Program of China (Grant No. 2021YFE0114700).

The authors have no conflicts to disclose.

Xu Li: Methodology (equal); Resources (equal); Writing – original draft (equal). Lanping Wang: Formal analysis (equal); Writing – review & editing (equal). Lanlan Nie: Funding acquisition (equal); Project administration (equal); Writing – review & editing (equal). Xinpei Lu: Writing – review & editing (equal).

The data that support the findings of this study are available within the article.

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