Experimental Investigation on Atmospheric Pressure Plasma Jet under Locally Divergent Magnet Field

Abstract

Regulating the parameters of the atmospheric pressure plasma jet (APPJ) is meaningful for industrial applications. Since plasma is a typical functional fluid in the magnetic field, it is possible to control the discharge characteristics via the Lorentz force. In this study, the effects of a locally divergent magnetic field on the generation and propagation of APPJ were examined experimentally. The experiments used a coplanar dielectric barrier discharge (CDBD) device driven by a 30 kHz AC high-voltage source to generate a helium APPJ. A locally divergent magnetic field of 250 mT (maximum) was applied coupled with the electric field, and noticeable enhancement was observed. The results showed that the magnetic field changed the motion state of electrons and promoted collision ionization, leading to a 40% improvement in the APPJ length (0.6 cm) and a 23% increase in the intensity of line O (777.2 nm). In addition, the spatiotemporal evolution and flow field of APPJ were diagnosed by ICCD and schlieren technique. The combination of electric and magnetic fields may effectively optimize the APPJ in practical applications.

1. Introduction

The atmospheric pressure plasma jet (APPJ) was widely adopted as a cold plasma source in biomedicine [1,2], environmental protection [3], material surface modification [4,5], and other interdisciplinary fields [6,7] because it has advantages of relatively high electron temperature, low gas temperature, rich content of active particles, and good controllability [6,8,9]. The simplicity of the APPJ device and its ability to operate in an open environment without a vacuum system make it an attractive option for research and industry. Plasma processing and catalysis offer new ways to produce fuels, chemicals, and other products while reducing carbon emissions [10,11]. To achieve this, it is important to improve the stability, controllability, and efficiency of APPJ. Further research on the plasma characteristics and optimization of discharge parameters is needed to advance the technology and its applications.

Although the spatial scale of APPJ based on dielectric barrier discharge (DBD) has reached millimeters, it still needs to meet the demand of industrial applications. Expanding the plasma scale and achieving a large-volume (length or diameter) cold plasma jet has excellent academic value and application prospects [12]. At present, a large-volume cold plasma jet is mainly produced in three ways under atmospheric pressure: expand the scale of a single cold plasma jet source, run multiple jets in parallel, and design a large-area device with a combined structure. As for the first way, the nozzles are mostly narrow and long slits, and the generated plasma is lamellar [13,14]. This way is more suitable for flat processing materials. The second way is many of the same cold plasma jet sources being arranged in parallel. The question is whether coupling between discharge units will affect the uniformity and stability [15,16,17,18]. Third, an annular coplanar dielectric barrier discharge (CDBD) device can obtain a large-area jet [12]. However, discharge uniformity dramatically depends on the gas spacing and electrode parameters. Therefore, we need to explore new ways to change the size of the jet.

The magnetic field has a more profound influence on the sparse low-pressure or vacuum discharge plasmas. Due to the magnetic field’s apparent confined effect on electrons, plasma characteristics often have a noticeable change. For a low-pressure environment, the mean free path of the electron is longer and the collision frequency is lower. Thus, applying a magnetic field will significantly affect the discharge [19,20,21,22,23]. Ono et al. [22] studied the enhancement of a magnetic field on the plasma jet in a vacuum environment. The results showed the jet was constricted radially and the center had high brightness. Chen et al. [24] designed a source with ring magnets, which produced large-area helicon plasma. Kaganovich et al. [25] reported plasma propulsion and similar technologies with E × B discharge.

Previous studies have also investigated the effect of magnetic fields on high-pressure discharge [26,27,28,29,30,31,32]. Jiang et al. [26] studied the performance of a DBD slit jet with a transverse magnetic field at atmospheric pressure and found that the jet could be lengthened by about 2 mm. Zhu et al. [27] researched CDBD with an external transverse magnetic field, and the magnetic field bent the jet. Some other scholars paid attention to improving the chemical and biomedical effects via applying a magnetic field. Wang et al. [28] investigated the surface wettability of PET films treated by DBD with magnets, which was better than that without magnets. Pekárek [29] studied nitrogen oxide and ozone generation by corona-like DBD with a NdFeB toroidal magnet. Liu et al. [30] compared the bactericidal function of APPJ with and without an axial magnetic field, indicating the application of the magnetic field increased the bactericidal rate by ~1.23 times. Jin et al. [31] increased the sterilization efficiency of the jet threefold by using a magnetic ring.

While these studies have shown that a magnetic field can affect discharge in a high-pressure environment and improve the application effect of the jet, there has been little attention paid to the physical levels of APPJ formation and propagation under applied magnetic field conditions. Therefore, it is necessary to use a variety of diagnostic methods to analyze the macroscopic and microscopic characteristics of plasma jet under the magnetic field. This can lead to a better understanding of the fundamental physics of plasma jet development and optimization, which can ultimately benefit industrial and biomedical applications.

In this paper, we present a device that combines a magnetic ring with a CDBD structure to enhance jet propagation. This phenomenon was reported for the first time at a conference, but in the follow-up study, we found that there may be a new physical mechanism for the interaction between magnetic field and plasma [32]. Therefore, we used wider diagnostic methods for research. The performance of the jet was evaluated using a combination of electrical and optical diagnostics, including discharge imaging and emission spectroscopy. Schlieren imaging was also employed to visualize the flow field. Furthermore, we conducted a detailed investigation into the influence of the magnetic ring on the APPJ and discussed the underlying mechanisms that contribute to jet growth.

2. Basic Principles and Experimental Methods

2.1. Basic Principles

The interactions between magnetic fields and plasmas have long been investigated, and the single-particle orbit theory is well covered in textbooks. When studying the influence of magnetic fields on a single charged particle in discharges, there are two cases according to the different directions of electric field and magnetic field. One is that the magnetic field is perpendicular to the direction of the electric field (E ⊥ B), and the other one is that the magnetic field is parallel to the direction of the electric field (E // B).

When E ⊥ B, the charged particle will experience Lorentz force when it has an initial velocity perpendicular to the direction of the magnetic field. The motion equation of a charged particle in an orthogonal electromagnetic field is expressed by

$$m\frac{d{\mathit{v}}_{\perp }}{dt}=q\left(\mathit{E}+{\mathit{v}}_{\perp }\times \mathit{B}\right)$$

(1)

where m is the mass of the charged particle, v_⊥ is the velocity component perpendicular to the magnetic field, and q is the amount of charge of the charged particle. The right side of the equation represents Lorentz force and electric field force, respectively. The charged particles circulate around the magnetic line of induction under the action of Lorentz force, with the cyclotron radius R_c = mv_⊥/qB and the angular frequency ω_c = qB/m. At the same time, the charged particles are subjected to the electric field force, and electric drift occurs in the direction of E × B. The motion diagram is shown in Figure 1a.

When it comes to the case of E // B, since the direction of the thermal velocity of the charged particle is the most random, when the random thermal velocity has a component perpendicular to the magnetic field, the energy of the charged particle is acted on by the Lorentz force applied by the parallel magnetic field. It will spiral with increased pitch, as illustrated by Figure 1b.

Specially, when the magnetic field is not uniform, an extra force (F_∇B = −μ∇B) pointing to the weaker field strength region is produced, which brings the effect of gradient drift as well as the kinetic energy conversion between the two components, parallel and perpendicular to the magnetic line [33]. Generally, for either the E × B or E // B case, the precondition is that the charged particle is well confined (or magnetized) by the magnetic field.

2.2. Experimental Methods

Figure 2 depicts the experimental setup used in this study. A CDBD system with helium gas produced APPJ. The CDBD reactor consisted of a quartz (relative permittivity ε_r of 3.7) tube with an inner diameter of 3 mm and an outer diameter of 5 mm. Two aluminum electrodes with a width of 3 mm were tightly wrapped around the quartz tube with a distance of 10 mm between them. An AC high-voltage power supply with a frequency of 30 kHz was applied on the left electrode, while the right electrode was grounded through a 2 kΩ sampling resistor (R_s). High-purity helium flow (Beijing AP BAIF Gases Industry Co., Ltd., 99.999%) was controlled by a gas flow meter (Seven Star CS200) and injected into the quartz tube at a constant flow rate of one standard liter per minute (slm) from the left side of the tube. The discharge voltage (V_s) and the discharge current were measured using a high-voltage probe (Tektronix P6015A) and a low-voltage probe (Tektronix TPP1000), respectively. A current probe (PEARSON current monitor, model 4100) was used to measure the total current waveform of the discharge circuit. The discharge behavior was observed using a digital oscilloscope (Tektronix DPO4104B). Time-integrated and time-resolved images of the plasma jet were recorded by a CCD camera (Nikon D5100) and an ICCD camera (Andor iStar DH334), respectively. The emission spectrum information was recorded by a spectrometer (AvaSpec-3648 for 250–900 nm). Figure 2 shows the real discharge structures. To protect the electrode, a high-temperature-resistant insulating polyimide tape was used. In the experiment involving an external magnetic field, a magnetic ring was placed coaxially at the high-voltage electrode.

A local divergent magnetic field was generated by a NdFeB toroidal magnet of outer/inner diameter D₂ = 10 mm/D₁ = 6 mm. The height of the magnet was l = 5 mm. Figure 3 shows the magnetic field intensity of the magnetic ring. Figure 3a illustrates the maximum axial magnetic field intensity of the magnetic ring, which was measured using a Gaussian meter. The maximum field strength on the central axis was approximately 0.25 T. Figure 3b illustrates the intensity distribution of the magnetic ring obtained through theoretical calculations. In the image, the color represents magnetic flux density and the arrow represents the direction of the magnetic field. The theoretical calculation results were found to be in close agreement with the measured maximum value. The experiments were carried out with and without the magnetic ring.

The helium flow field was also visualized with a schlieren optical bench in Z-type configuration, as shown in Figure 4. The parabolic mirrors were 15 cm in diameter and had a focal distance of 1.5 m. The light source was halogen. A CCD camera (Nikon 610) was used to visualize the helium flow expansion in the ambient air. The knife edge was set at the focal point of the parabolic mirror so that the knife blocked all nonrefracting beams. The refracted beam was not focused on the knife, forming a filtered image with higher contrast.

3. Results

3.1. General Performance of APPJ

The CCD camera captured the discharge images with different applied voltages, as shown in Figure 5. The jet morphology did not change, even with a magnetic ring for the same type of discharge. When the APPJ propagated in the dielectric tube, the brightest part of the jet was concentrated on the central axis. However, when the jet reached the nozzle of the medium, the jet expanded radially, clung to the pipe wall, and shrank to the central axis. The jet’s variation trend with driving voltage was same for the two cases with and without the magnetic ring. Namely, as the driving voltage rose, the electric field enhanced, the electron energy increased, collision ionization increased, and the jet became longer. The jet length remained unchanged when the voltage exceeded 12 kV.

Further comparing the discharge images under the same driving voltage in Figure 5, it could be found that the jet length increased with the magnetic ring. The jet growth was significantly observed at 9–11 kV, as shown in Figure 6. Here, the jet length was measured outside the instantaneous cathode (the jet inside the tube was 1 cm). Generally, 0.25 cm or more than a 10% increment was achieved after the magnetic field was applied. The maximum increase in jet length was 0.6 cm at 9.5 kV. Nonmagnetic metal rings of the same size as the magnetic ring were used to verify the influence of the magnetic field, as shown in Figure 7. Ring 1 and ring 2 were made of ferrite and steel, respectively. It also could be seen that the magnetic ring’s influence was the most obvious. It was indeed the magnetic field that affected the discharge. In Figure 8, the jet was almost unchanged when the magnetic ring was placed on the ground electrode. If the magnetic ring was placed between the DBD or downstream of the ground electrode, the experimental results showed no positive effect on the jet.

Figure 9 presents the schlieren photographs of the helium flow in the ambient obtained with and without the magnetic ring. The right column of Figure 9a,b is the processed schlieren image to observe the flow path more clearly. In the picture, the path of the flow field is always curved. Because the helium mass was lighter than air, the helium flow channel shifted upwards when it developed into the air. When the driving voltage was applied, the flow field deflected, and with the increase in voltage, the trajectory of the flow field tended to be flat. In order to analyze the propagation trajectory of the flow field, we interpreted the overall bending degree by observing the height of the tail of the flow field. The position of the arrow pointing downward was the position of the jet head, marked according to Figure 6.

Comparing Figure 9a,b, we can see the effect of the external magnetic ring on the flow field was not noticeable. The impact of the magnetic ring on the discharge did not further affect the flow field.

3.2. Propagation of APPJ

We studied the discharge characteristics under the maximum jet growth value (when V_s = 9.5 kV) to better understand the influence of the local divergent magnetic field. ICCD recorded time-resolved images of APPJ with or without the magnetic ring in the experiment. Figure 10 shows the typical waveforms of the applied voltage and DBD current. The waveforms of the DBD current were similar for the two cases, with a single peak in each half-cycle. In the positive half-cycle of discharge, the peak value of the DBD current was the same. In the negative half-cycle of discharge, the DBD current with the magnetic ring was more significant than without the magnetic ring. When there was a magnetic ring, the surface charges accumulated on the dielectric layer grew at the end of the positive half-cycle discharge. The electric field generated by the surface charge was stronger, thus making the negative half-cycle DBD current greater.

Figure 11 shows time-resolved images of the jet in the positive half-cycle. Each image frame corresponds to the moment marked by the five-pointed star in the current waveform. The discharge first occurred at the instantaneous anode, then developed to the instantaneous cathode, forming a jet outside the instantaneous cathode and propagating along the central axis (in a straight line). However, when the magnetic ring was added, the plasma jet developed further for 3.8 μs or more after t = 37 μs, which was the terminal of the jet propagating without the magnetic ring. The jet was about 0.6 cm longer than that without the magnetic ring.

The propagation velocity of the plasma jets at a given time was estimated by the axial position of the light emission front in two continuous images. The propagating velocity of the plasma jet is shown in Figure 12. In the quartz tube, the jet velocity rose rapidly from the instantaneous cathode and remained unchanged at the position close to the tube nozzle. Then, the velocity gradually decreased to a minimum value, but the jet velocity increased again in the air outside the tube and came to a stop. This result was similar to previous research [34,35,36]. It is worth noting that when the magnetic ring existed, the velocity of the jet inside the tube became lower, with a maximum reduction of 32%, but it was still on the same order of 10³ m/s.

3.3. OES of Discharge

The emission spectrum of the jet can reflect the particle concentration indirectly. Figure 13 shows the emission spectra at and 3 mm away from the tube nozzle under the condition of V_s = 9.5 kV. Since the applied voltage frequency was 30 kHz, the single shot contained thousands of discharge pulses, and the obtained spectra were time-integrated. Here, the integral time was 20 s and three times on average to obtain relatively stable data. The emission spectra of the plasma jet were in the range of 250–900 nm, and the lines mainly distributed in the range of 300–500 nm and 600–800 nm. Because helium diffused outside the tube and the air seeped into the jet, electrons in the discharge channel collided readily with nitrogen and oxygen in the air, which had lower excitation and ionization energy, with nitrogen and oxygen-related spectral lines in the emission spectrum.

Figure 13a shows the emission spectra near the nozzle. The strongest emission was 706.5 nm, corresponding to the line of He (3³S→2³P). Figure 13b shows the emission spectra at 3 mm from the nozzle. The intensity of atomic and ion lines decreased due to a decrease in electron energy downstream of the discharge channel. However, the spectral line intensities of He, N₂⁺ (FNS: B²Σ_u⁺→X²Σ_g⁺), and O (777.2 nm: 3⁵P→3⁵S) were all slightly enhanced with the magnetic ring in the two positions.

4. Discussion

As shown above, the magnetic ring produced a divergent field at the high-voltage electrode. This magnetic field affected the propagation of the jet. The jet length increased by 40%, and the intensity of the spectral lines in the jet also increased.

Previous research explored the mechanism of DBD jet propagation without the magnetic ring [37]. After the formation of DBD between electrodes, the high-density positive ions near the surface of the cathode generated a strong axial electric field. Under the electric field, the seed electrons in space moved to the high-density space charge of positive ions, resulting in an electron avalanche. The positive ions produced by ionization stayed behind the electron avalanche. They attracted electrons to form the second electron avalanche in the path, leaving more positive ions in space as the new jet head and producing an axial electric field such that the jet continued to develop. Positive ions played an important role in jet propagation.

As for the jet propagation process, there was no apparent difference with or without the magnetic ring. The magnetic ring did not change the propagation form of the jet. However, the jet length increased in this experiment. It could be deduced that there were more positive ions outside the instantaneous cathode to maintain jet development. To further observe the spatial distribution of positive ions in the discharge channel, we took the time-resolved images of emission around 395.3 nm. The ICCD camera recorded images with the narrow bandpass filter placed on the lens. Each image frame corresponds to the moment marked by the five-pointed star in the current waveform of Figure 10. As demonstrated in Figure 14, the concentration of positive ions outside the instantaneous cathode was higher with the magnetic ring. At the same time, the emission spectrum also reflected the change in ion concentration. There was an enhancement of the N₂⁺ line at 391.4 nm with the magnetic ring. In both cases, we found an increase in the concentration of positive ions, which might explain the jet growth. At the instantaneous anode, the magnetic field’s existence made the electron subject to Lorentz force, which increased the impact ionization. When the discharge developed to the instantaneous cathode, more positive ions were left near the cathode to maintain the jet development.

The jet velocity with the magnetic ring was lower than without the magnetic ring in the quartz tube. When the magnetic ring existed, the number of positive ions outside the instantaneous cathode increased. Due to the greater mass of the ion, they stayed behind the avalanche. They formed a trail, which affected the energy distribution in the discharge channel and the jet velocity. The development of the jet lagged when the magnetic ring existed. We can also see this result indirectly in Figure 11 when t = 35.15 μs and 36 μs.

The generation of plasma would disturb the flow field. Since the external jet of the ground electrode developed in the form of a positive corona, the positive corona caused the ionic wind effect, interacted with the flow field, and promoted the propagation of the flow field. When the voltage increased, the positive corona ionic wind effect and the effect on the flow field increased. From the schlieren image, the flow field’s trajectory tended to be flat. The magnetic ring promoted the development of the jet. Still, the ionic wind effect generated by the magnetic ring was not enough to disturb the flow field, and the schlieren image obtained had no noticeable change. Therefore, it was not the flow but the magnetic field that affected the jet.

The intensities of the OES were characteristics of the excited species. By measuring the emission spectra at different positions of the jet, it was found that the intensity of the spectral lines increased with the presence of the magnetic ring. Previous researchers have also proposed that a magnetic field enhances the spectral intensity of plasma [26,38]. In this experiment, more electrons were attracted to collision excitation and ionization due to the increase in positive ion concentration in the discharge channel. Furthermore, the luminous intensity and the content of active particles increased.

From the above results, the magnetic field’s existence changed the jet’s length, development speed, and spectral line strength. The main reason for these changes was the increase in positive ion concentration outside the instantaneous cathode. These results showed the effect of the magnetic field on the plasma jet. It also could be inferred that the stronger the magnetic field intensity, the more pronounced the influence on the plasma jet.

5. Conclusions

In this study, we investigated the development of a plasma jet in a coplanar dielectric barrier discharge with and without a magnetic ring. When the magnetic ring was placed at high-voltage electrode, the jet length was increased by up to 0.6 cm at 9.5 kV. The ICCD recorded the jet propagation process and revealed an increase in spatial distribution density of positive ions outside the instantaneous cathode. The intensity of the lines in the jet was also enhanced, and the concentration of the active particles increased. The experimental results showed that introducing a permanent magnet at the high-voltage electrode effectively optimized the plasma jet. The combination of electric and magnetic fields increased the collision frequency between electrons and gas molecules, promoting ionization to produce more ions in some areas. Although the magnetic field did not disturb the flow field, it led to the generation of a longer plasma jet at a lower driving voltage and increased the concentration of active groups in the jet. In summary, this study provides valuable insights into the effects of a magnetic field on the development of a plasma jet in coplanar dielectric barrier discharge. The findings can be applied to optimize plasma-based industrial processes, such as surface treatment and sterilization, and can lead to the development of more efficient plasma sources.