Generation of Large-Scale Plasma Jet with Excitation of Bipolar Nanosecond Pulse Voltage in Single-Spiral Electrode Configuration

Abstract

In this study, a single-outer-spiral electrode with inductance of 20 μH is employed to couple the energy input of a bipolar nanosecond pulse for the purpose of generating a large-scale atmospheric pressure plasma jet. When the spiral electrode is wrapped around a plasma jet tube with a length of 35 cm, the electrical field can be optimized, resulting in a stable laminar flow field, and a plasma jet with a length and diameter larger than 14 cm and 1.2 cm can be generated. A comparative study of the bipolar and unipolar pulse excitation voltages is also conducted, showing that the maximum lengths of the plasma jet excited by a bipolar pulse voltage, positive pulse voltage, and negative are 14 cm, 10 cm, and 7 cm, respectively. The temporal and spatially resolved spectra of the plasma jets excited by both bipolar and unipolar pulses are investigated, respectively, and the main physiochemical processes of the active species and the plasma dynamics’ evolution are discussed.

1. Introduction

Atmospheric pressure plasma jets (APPJs) have attracted a great deal of attention in the last several decades due to their various applications, such as biomedical tasks, surface modification, sterilization, etc. [1,2,3,4,5,6]. In various applications, a large-scale or major plasma jet is essential to achieve high application efficiency and sufficiently low costs [7,8]. However, limited by the shielding of the electrical intensity, gas flow turbulence, and discharge instability during long-term discharge, the longest APPJ that has been reported is about 10–12 cm in the surrounding air [9], and it is still unsatisfactory in terms of both the jet length and diameter for some applications.

The scale of an APPJ can be affected by the applied voltage, type of excitation voltage, frequency, gas flow rate, and electrode configuration [10,11,12,13,14,15,16]. Some studies have indicated that the length of an APPJ is strongly correlated with the applied voltage at lower levels; however, when the applied voltage exceeds a certain threshold, the APPJ’s length remains constant, despite further increases in the voltage [11,12,13]. In this process, the space charges play an important role in preventing further increases in the APPJ’s length, because the electric field built by the space charge can suppress the applied electric field at the trail of the plasma jet, when a long microsecond pulse or unipolar pulse voltage is employed [17]. The polarity of the excitation voltage pulse is another critical parameter in the generation of large-scale plasma jets [11,18]. It has been revealed that a plasma jet at positive polarity is typically composed of “bullet-like” plasma volumes, which can be considered as “guided ionization waves”, propagating with speeds of 10³–10⁶ m/s, and the length of the APPJ, which depends on the travel distance of the plasma bullet, is typically in the range of 3–8 cm [19]. Meanwhile, a plasma jet at negative polarity is only about 2–3 cm, with a shape similar to an arrow [20]. Moreover, theoretical and experimental studies in recent years have indicated that the flow patterns (laminar flow and turbulence flow) also have obvious effects on the scale of the plasma jet [11,21]. When the turbulence gas flow is formed by the electrode’s appearance or the gas flow rate, the fast diffusion of the working gas can limit the length expansion of the plasma jet significantly.

In this work, to achieve a large-scale atmospheric pressure plasma jet (LS-APPJ), we design a plasma jet structure featuring a single-spiral high-voltage electrode without the need for a ground electrode. The LS-APPJ is excited by a bipolar pulse voltage, positive unipolar pulse voltage, and negative unipolar pulse voltage in atmospheric-pressure helium. The waveforms of the voltage and current, discharge images, and temporally and spatially resolved spectra are investigated. Additionally, we discuss the effects of the pulse voltage polarity and the jet tube’s length on the length of the plasma jet.

2. Experimental Setup

The experimental setup of the LS-APPJ is illustrated in Figure 1a, and it is mainly composed of a high-voltage pulse power supply, a plasma jet device, an electrical measurement system, and an optical detection system. The plasma jet device consists of a jet tube and a copper high-voltage electrode, as shown in Figure 1b. The jet tube is composed of quartz, with an outer diameter of 1.2 cm and a length of 35 cm. A spiral copper wire is coiled around the outer surface of the jet tube for 120 turns as the high-voltage electrode. The inductance of the floating spiral electrode is about 20 μH, which has a good matching effect with the pulse power supply. Remarkably, there is no ground electrode inside the jet tube. The plasma jet device is fixed on a PTFE shelf with the nozzle placed about 20 cm above the ground. A positive unipolar pulse voltage, negative unipolar pulse voltage, and bipolar pulse voltage with a rising time of about 20 ns are employed to drive the plasma jet device to produce a positive pulse plasma jet (PPPJ), negative pulse plasma jet (NPPJ), and bipolar pulse plasma jet (BPPJ), respectively.

The waveforms of the voltage and current of the LS-APPJ are measured by a high-voltage probe (P6015A, 1000×, 75 MHz (Tektronix, Beaverton, OR, USA)) and a current probe (TCP312, 100 MHz (Tektronix, Beaverton, OR, USA)) and are recorded and displayed on an oscilloscope (TDS5054B, 500 MHz (Tektronix, Beaverton, OR, USA)). The plasma jet tube is long enough and contains no electrode to prevent the disturbance of the gas flow, so that a smooth gas flow can be obtained when the gas flow rate of helium (99.999%) is kept at 3.5 L/min. In order to measure the length of the plasma jet, discharge images are captured by a digital camera (5D IV (Canon, Tokyo, Japan)) with an exposure time of 100 ms. To obtain the spatially resolved spectra emitted from the plasma jet, the head of an optical fiber is placed close to the reactor, which is adjustable in the vertical and horizontal directions via a 3D displacement platform. Then, the optical emission spectra (OES) from the discharge region are collected by a grating monochromator (SR-750i (Andor, Oxford, UK)) (grating grooves are 2400 lines mm⁻¹ and 1200 lines mm⁻¹ to acquire the spectra in the range of 300–480 nm and 575–800 nm, respectively). After the diffraction of the grating, the output spectral light can be converted into an electrical signal by an ICCD camera (iStar DH334T (Andor, Oxford, UK)).

3. Results and Discussion

3.1. Electrical Features of LS-APPJ

The waveforms of the discharge voltage and current of the BPPJ, PPPJ, and NPPJ are shown in Figure 2. The waveforms of the three discharges are all measured under the conditions of a 24 kV pulse peak voltage, 150 Hz pulse repetition rate, and 3.5 L/min gas flow rate. It can be seen that both the bipolar and the unipolar pulses have a rising time of about 25 ns, and the current peak value of the BPPJ is higher than that of both the PPPJ and NPPJ. Specifically, the peak current of the positive pulse in the BPPJ is 1.7 times that of the PPPJ, while the peak current of the negative pulse in the BPPJ is 2.1 times that of the NPPJ.

The discharge energy of each pulse of the BPPJ and NPPJ can also be calculated from the waveforms of the voltage and current, as in the following equation:

$$E=\int P_{\mathrm{p},\mathrm{g}}\left(t\right)dt==\int U_{\mathrm{g}}\left(t\right)\times I_{\mathrm{p},\mathrm{g}}\left(t\right)dt,$$

(1)

where $P_{\mathrm{p},\mathrm{g}}\left(t\right)$, $U_{\mathrm{g}}\left(t\right)$, and $I_{\mathrm{p},\mathrm{g}}\left(t\right)$ are the instantaneous input power, the voltage, and the discharge current of the plasma jet, respectively. Then, the discharge power of the plasma jet can be determined by multiplying the energy per pulse by the pulse repetition rate. The average powers of the BPPJ, PPPJ, and NPPJ are 2.42 W, 1.59 W, and 1.93 W, respectively.

3.2. Morphological Features of LS-APPJ

Figure 3 shows the discharge images of the BPPJ, PPPJ, and NPPJ. The pulse peak voltage, pulse repetition rate, and gas flow rate are kept at 24 kV, 150 Hz, and 3.5 L/min, respectively. Since a large-diameter plasma jet tube, with a length of up to 35 cm, is employed and no electrode is placed inside, the gas flow remains very smooth, and the turbulence is effectively controlled. Therefore, the length of the plasma jet mainly depends on the electrical field density at the plasma jet head [17]. When the positive pulse voltage is employed, the PPPJ has a length of about 10 cm with a pulse peak voltage of 24 kV, which already reaches the maximum jet length at 7 L/min reported in ref. [22]. Compared with the PPPJ, the NPPJ has a length of about 7 cm because of the high applied voltage. When a bipolar pulse voltage is employed as an excitation voltage, the plasma jet with a diameter of about 1.2 cm can propagate over 14 cm (excluding the streamer filament in the trail of the plasma jet, as shown in Figure 3). It exhibits a higher discharge intensity and a longer jet length compared with the plasma jet excited by positive pulse voltage and negative pulse voltage.

The BPPJ presents a stable and uniform profile, and no turbulence is formed at the trail of the plasma jet, while the jet tube’s length significantly influences the length of the plasma jet. The corresponding comparison images are shown in Figure 4, where the BPPJs generated with the jet tube lengths of 35 cm and 10 cm are displayed on the left side and right side, respectively. When a shorter quartz tube (10 cm in length) is employed, the length of the BPPJ is only about 6.5 cm, since a disturbance still exists in the tube nozzle, and the position for the formation of turbulence is closer to the tube nozzle.

The length of the plasma jet reflects the propagation distance of the “plasma bullet”. The propagation is inhibited before the ignition of the secondary discharge, which is caused by the quenching process of the plasma by molecules of nitrogen or oxygen in the air diffused into the ionization channel. This limitation is called the transition point, which separates the plasma bullet from the inhibition zones [17]. Since the duration of the nanosecond pulse in our study is only about 50 ns, the plasma bullet can stop propagating immediately after the secondary discharge ignition [17]. Therefore, the propagation of secondary discharge ignition is directly influenced by the initial electric field intensity when the primary plasma bullet is formed. In the experiment, benefitting from the short voltage rising time and pulse matching with the spiral electrode, the applied pulse voltage could reach up to 24 kV without the formation of an arc or spark. In this case, the plasma bullet ignited by the secondary discharge, with a high local electric field intensity, can propagate over a long distance. This is the reason that the length of the plasma jet in this study is longer than that reported in previous studies.

The polarity of the pulse voltage is also a crucial factor influencing the plasma jet length [19,20]. When a unipolar positive pulse voltage (either positive or negative) is employed, the reversed built-in electric field formed by the space charges accumulated on the tube surface and in the plasma jet channel can limit the propagation of the plasma bullet. Therefore, even if a high pulse voltage is applied, the length of the plasma jet cannot propagate further when the pulse voltage is increased. In contrast, a bipolar pulse voltage, with a positive directional pulse following by a negative directional pulse alternately, is more beneficial for the excitation of an LS-APPJ [23]. Discharge in a negative pulse can remove the space charges in the discharge region effectively, and the charges accumulated on the tube surface at the previous pulse can participate in the subsequent pulse discharge. Therefore, compared with the PPPJ and NPPJ, the BPPJ exhibits a larger volume and higher discharge intensity.

Figure 5 illustrates the relationship between the plasma jet length and pulse peak voltage under three pulse polarities, the BPPJ, PPPJ, and NPPJ, with the same pulse repetition rate of 150 Hz. Under bipolar pulse excitation, the plasma jet length exhibits sustained growth with the rising pulse peak voltage. The maximum jet length reaches 14.5 cm with a pulse peak voltage of 26 kV, and then spark breakdown occurs when the voltage reaches up to 28 kV. In the case of the PPPJ, when the pulse peak voltage is below 22 kV, the plasma jet length increases in tandem with the pulse peak voltage, reaching a maximum length of 8.5–9 cm. However, when the pulse peak voltage exceeds 22 kV, the growth rate of the plasma jet length diminishes and it eventually stabilizes. For negative pulse excitation, the correlation between the plasma jet length and the pulse peak voltage is not clearly evident. Only a minor increase in jet length is observed at lower voltages. Once the pulse peak voltage surpasses 16 kV, the plasma jet length remains largely invariant with respect to further increases in the pulse peak voltage.

Existing research has demonstrated that the plasma jet length increases with the voltage and reaches saturation at elevated voltage amplitudes [10,11]. This saturation phenomenon can be attributed to two primary factors: the formation of a low-breakdown-voltage region due to the mixture of inert gas and the surrounding air and the propagation of bullet-like streamers. When the applied voltage is sufficiently high, streamers penetrate into regions with higher air content, where their propagation is inhibited. Consequently, to generate a plasma jet of adequate length, it is imperative to expand the mixing region of the inert gas and surrounding air, as previously mentioned. Furthermore, enhancing the electric field intensity is crucial to facilitate the elongation of the plasma jet.

3.3. Typical OES Emitted from LS-APPJ

OES and temporally–spatially resolved OES can be employed to record the behaviors of active species and plasma dynamics [24,25]. Figure 6 shows the typical OES emitted from the BPPJ in range of 300–480 nm and 575–800 nm to detect the active species produced in the plasma jet. The pulse peak voltage, pulse repetition rate, and gas flow rate are kept at 24 kV, 150 Hz, and 3.5 L/min, respectively. The emission spectra are mainly composed of the spectral lines of He (3p³P^° → 2s³S, 388.9 nm), He (3p¹P^° → 2s¹S, 501.6 nm), He (3d³D → 2p³P^°, 587.6 nm), and He (3d¹D → 2p¹P^°, 667.8 nm); the spectral lines of oxygen atoms O (3p⁵P → 3s⁵S^°, 777.4 nm); the bands of OH (A²Σ → X²Π); the second positive system of nitrogen N₂ (C³Π_u → B³Π_g); and the first negative system of nitrogen ions N₂⁺ (B²${\mathsf{\Sigma }}_{\mathrm{u}}^{+}$ → X²${\mathsf{\Sigma }}_{\mathrm{g}}^{+}$). The presence of these spectra indicates that excited species, such as O, OH, N₂ (C³Π_u), N₂⁺ (B²${\mathsf{\Sigma }}_{\mathrm{u}}^{+}$), and He, coexist in the plasma jet.

Table 1. The main physiochemical processes and their rate constants in a helium plasma jet.

No.	Reaction	Reaction Rate Constant *	Reference
1	e + N2 → N2 (C) + e	f(E/N)	[26]
2	e + N2 → N2+ + e + e	f(E/N)	[26]
3	e + He → He * + e	f(E/N)	[26]
4	He * + He * → e + He+ + He	8.7 × 10−10 cm3/s	[27]
5	He * + He *→ e + He2+	2.03 × 10−9 cm3/s	[27]
6	He * + N2 →N2+ + He + e	7 × 10−11 cm3/s	[27]
7	He * + He + H2O → 2He + H2O+ + e	2 × 10−29 cm6/s	[27]
8	He * + 2He → He2 (a) + He	3 × 10−34 cm6/s	[27]
9	He2 (a) + H2O → 2He + H2O+ + e	6 × 10−10 cm3/s	[27]
10	He2+ + H2O → OH + HeH+ + He	1.3 × 10−10 cm3/s	[27]
11	H2O+ + e → OH + H	3 × 10−7 cm3/s	[27]
12	He2 (a) +O2 → 2He + O2+ + e	1 × 10−10 cm3/s	[28]
13	He * +O2 → He + O2+ + e	2.54 × 10−10 cm3/s	[28]
14	He2++O2 → 2He + O2+	1 × 10−9 cm3/s	[28]
15	O2+ + e → 2O	1.2 × 10−8 Te−0.7 cm3/s	[29]

* The reaction rate constants are calculated when the gas temperature is 300 K.

lists the main physiochemical processes and their rate constants in the plasma jet for the production of the above active species.

The effect of the pulse peak voltage on the emission intensities of N₂ (C³Π_u → B³Π_g), N₂⁺ (B²${\mathsf{\Sigma }}_{\mathrm{u}}^{+}$ → X²${\mathsf{\Sigma }}_{\mathrm{g}}^{+}$) and He (3s³S → 2p³P, 706.5 nm) is investigated next, and the results are illustrated in Figure 7. It is clearly seen that all emission intensities rise with the increase in the pulse peak voltage, because the mean kinetic energy of electrons increases in a stronger electric field owing to the increase in voltage, and more excited particles are produced. Notably, the emission intensity of N₂⁺ (B²${\mathsf{\Sigma }}_{\mathrm{u}}^{+}$ → X²${\mathsf{\Sigma }}_{\mathrm{g}}^{+}$) presents the most significant growth trend. Furthermore, the intensities emitted from the BPPJ are obviously higher than those emitted from the PPPJ and NPPJ for all three particles. This indicates that a bipolar pulse voltage is more beneficial for the excitation of a plasma jet compared to a unipolar pulse voltage.

3.4. Spatially Resolved Spectra of LS-APPJ

To understand the dynamic behavior of the LS-APPJ and the spatial distribution of the excited active species, Figure 8 shows the effect of the distance from the plasma jet nozzle on the emission intensities of the N₂ (C³Π_u → B³Π_g), N₂⁺ (B²${\mathsf{\Sigma }}_{\mathrm{u}}^{+}$ → X²${\mathsf{\Sigma }}_{\mathrm{g}}^{+}$), He (3s³S → 2p³P, 706.5 nm), OH (A²Σ → X²Π) and O (3p⁵P → 3s⁵S^°, 777.4 nm) emitted from the BPPJ, PPPJ, and NPPJ. The pulse peak voltage, pulse repetition rate, and gas flow rate are kept at 24 kV, 150 Hz, and 3.5 L/min, respectively. The active species produced by the BPPJ have the highest emission intensity and the widest spatial distribution compared with the PPPJ and NPPJ. Regarding the spatial distribution of He (3s³S), it exhibits the maximum emission intensity at the position of the jet nozzle, and the emission intensity decreases with the increase in the distance from the nozzle. Regarding the emission intensities of N₂ (C³Π_u → B³Π_g) and N₂⁺ (B²${\mathsf{\Sigma }}_{\mathrm{u}}^{+}$ → X²${\mathsf{\Sigma }}_{\mathrm{g}}^{+}$), the maximum values occur at distances of 0 cm and 2 cm from the jet nozzle, respectively, and then decrease with the increase in the distance from the jet nozzle. However, there are some differences between the active species: at a distance of 1–6 cm from the nozzle, the dominant excited molecule is N₂⁺ (B²${\mathsf{\Sigma }}_{\mathrm{u}}^{+}$); when the distance is larger than 6 cm, N₂⁺ (B²${\mathsf{\Sigma }}_{\mathrm{u}}^{+}$) decreases sharply, while N₂ (C³Π_u) becomes the majority. The distributions of OH and O are similar to those of the active helium atoms He (3s³S), i.e., the highest intensities occur at the position of the jet nozzle, and then they decrease with the increasing distance.

The different spatial distribution characteristics of N₂ (C³Π_u → B³Π_g), N₂⁺ (B²${\mathsf{\Sigma }}_{\mathrm{u}}^{+}$ → X²${\mathsf{\Sigma }}_{\mathrm{g}}^{+}$), He (3s³S → 2p³P, 706.5 nm), OH (A²Σ → X²Π), and O (3p⁵P → 3s⁵S^°, 777.4 nm) are mainly caused by the change in the He atom concentration and electrical field intensity in the plasma jet. The transmission distance of the active helium atoms He (3s³S) is less than 7 cm, mainly due to the reduction in the He atom concentration caused by the diffusion of nitrogen and oxygen molecules in the air. Additionally, the excited He atoms can be rapidly quenched by nitrogen and oxygen molecules, as shown in reactions (6, 13). At the distance of 1–6 mm, due to the low concentrations of N₂ and O₂ in the core of the plasma jet, a large population of Penning ionization between metastable He atoms and other particles is an important process to produce OH (A), O, N₂⁺ (B), etc. Regarding N₂⁺ (B), at the distance of 1–6 mm, it is mainly produced by Penning ionization, as shown in reaction (6). However, when the plasma moves forward, with the reduction in the helium concentration, the role of the metastable He atoms becomes less important, and N₂⁺ (B) is produced by the high electric field at the head of the plasma bullet streamer.

3.5. Temporally Resolved Spectra of LS-APPJ

The temporally resolved OES of N₂ (C³Π_u → B³Π_g) is measured when an ICCD is employed as the spectrometry detector. Figure 9 shows the dynamic evolution of N₂ (C³Π_u → B³Π_g) at various distances from the jet nozzle for the discharges in both the positive pulse and negative pulse of the BPPJ. The gate width of the ICCD is set at 5 ns and the pulse peak voltage, pulse repetition rate, and gas flow rate are kept at 24 kV, 150 Hz, and 3.5 L/min, respectively. It is clearly shown that the N₂ (C³Π_u → B³Π_g) emission exhibits different behaviors in the discharges during the positive half-cycle and negative half-cycle. In the positive discharges, the temporally resolved OES of N₂ (C³Π_u → B³Π_g) can be observed over a much larger distance from the jet nozzle, which is about 10–15 cm. Near the plasma jet nozzle, the emission intensity increases sharply as the “plasma bullet” moves away from the nozzle, and it reaches the maximum at the distance of 3 cm, which is owing to the increase in the N₂ concentration. As the distance increases, the discharge is dominated by the secondary discharge ignition, where the discharge can be sustained after the pulse voltage extinguishes. Additionally, the relaxation time of the plasma can be influenced by the distance. The full width at half maximum (FWHM) of the N₂ (C³Π_u → B³Π_g) emission intensity is only about 100 ns at the distance of 3 cm; however, it is about 500 ns at the distance of 12 cm. However, in the negative pulse, the movement of the “plasma bullet” is significantly lower than that in the positive pulse discharge. The FWHM of the N₂ (C³Π_u → B³Π_g) emission intensity is larger than 1000 ns, and the signal can only be detected in the range of 0–6 cm, far from the jet nozzle.

4. Conclusions

In conclusion, an LS-APPJ with a length and diameter of up to 14 cm and 1.2 cm can be obtained through the excitation of a bipolar nanosecond pulse voltage. The OES shows that abundant active species with various spatial distributions are produced in the plasma jet. ROS such as O and OH are mainly produced in the region from 0 to 7 cm near the nozzle; at the far end of the plasma jet, the spectra of N₂ (C³Π_u → B³Π_g) and N₂⁺ (B²∑_u⁺ → X²∑_g⁺) continue to dominate. Moreover, a plasma bullet can be distinguished by the temporally and spatially resolved spectra of N₂ (C³Π_u → B³Π_g); under the current experimental conditions, the propagation velocity of the plasma bullet is about 3 × 10⁵ m/s, which is within the scale of the velocity of streamer breakdown.