Simulation on the transition of electrostatic instabilities in EAST steady-state scenario

1.   Introduction

Non-thermal plasma is an indispensable technology that has attracted increasing interest in a variety of environmental and energy applications involving plasma catalysis, such as removal of gaseous pollutants [1–3], nitrogen fixation [4, 5], greenhouse gas conversion [6, 7], and ozone generation [8, 9], etc. Packed bed dielectric barrier discharge (DBD) reactors are one of the most popular non-thermal plasma catalytic reactors due to their straightforward structure, scalability and gentle operating conditions. In this type of reactor, dielectric material (such as beads or pellets) is usually fully or partially filled in the electrode gap. Due to the polarization of the dielectric material, the electric field near the dielectric material is enhanced, which results in electrons being accelerated to high energies. Therefore, the collision reaction of high-energy electrons is more intense and more chemically active species are produced, which can effectively improve plasma chemistry and reaction performance [9–12]. However, the structure of packed bed reactors needs to be continuously improved to optimize the plasma characteristics and meet various industrial requirements. The discharge structure has a significant impact on the plasma characteristics and plays a crucial role in applications; hence, it is imperative to study the influence of the discharge structure within the packed bed reactor on the plasma characteristics.

During the last few years, various experiments and simulations have been conducted to investigate the plasma characteristics in different packed bed reactors. Wang et al [13] investigated plasma behavior with various packing dielectric materials in a single bead plate–plate DBD reactor by fluid modeling and optical imaging experiments. They observed polar discharge and surface streamer discharge within the reactor. Polar discharge occurs at a low discharge voltage, while surface streamer discharge requires a higher voltage to initiate. Additionally, it was found that as the dielectric constant increases the transition voltage to the streamer regime also increases. Mujahid and Hala [14] used a time-resolved intensified charge coupled device (ICCD) camera to study plasma dynamics within a simple planar packed bed DBD reactor. They found that the plasma is sustained in three modalities: filamentary discharges in the volume, micro-discharges at the contact points, and surface ionization waves at the dielectric surface. Notably, they revealed that discharge occurs primarily at the contact points and emphasized their critical role in plasma dynamics. Zhu et al [15] employed rod-shaped packing materials to regulate the streamer discharge behavior in a visible cylindrical packed bed DBD reactor. They discovered that adjusting the parameters, number and position of the dielectric rods can significantly change the intensity, width and length of the streamer discharge. Ren et al [16] utilized a two-dimensional fluid model to investigate the effect of residual surface charge on coupled energy in a coaxial packed bed DBD reactor. They observed that in a positive polarity pulse, the accumulated residual charge from the previous discharge limited the coupled energy of the subsequent discharge. However, in a negative polarity pulse and bipolar pulse, the accumulated residual charge from the previous discharge promotes the coupled energy of the subsequent discharge. These findings underscore the pivotal role of the packed bed reactor structure and residual surface charge in regulating discharge dynamics.

In fact, for a pulsed packed bed DBD, two main discharges are usually contained within a single voltage pulse period: the positive discharge (PD) ignited by the external enhanced electric field and the negative discharge (ND) ignited by the accumulated residual charge on the surface of the dielectric material [17]. Therefore, residual surface charges will not only affect the discharge dynamics of the next voltage pulse period but also the generation and propagation of ND within a single voltage pulse period. The distinct plasma generation mechanisms of PD and ND lead to notable differences in the plasma behavior and the generation of active species. For examples, Lu et al [18] observed that in PD the luminous region of the plasma jet is mainly focused at the plasma bullet head, while in ND the entire channel of the plasma jet emits light. Sun et al [19] studied the propagation of surface ionization waves on curved gas–solid interfaces through experiments and simulations. They discovered that in PD the distribution of surface ionized waves is the same as that of the heavy particles ${\text{N}}_{\text{2}}\text{(}{\text{A}}^{\text{3}}{{ \Sigma }}_{\text{u}}^{\text{+}}\text{)}$ and O₂(a¹∆_g), which diffuse from the edge of the ground mesh to the center of the space electric field. However, in ND, electrons and ${\text{N}}_{\text{2}}\text{(}{\text{A}}^{\text{3}}{{ \Sigma }}_{\text{u}}^{\text{+}}\text{)}$ dissipate, O₂(a¹∆_g) accumulates steadily and no surface ionization waves are generated in this case. Therefore, it is crucial to focus on the characteristics of PD and ND in a pulsed packed bed DBD. This will help clarify the behavior of plasma and the generation mechanism of active species, and ultimately promote the application of packed bed DBD in plasma catalysis.

In this study, considering that various industrial applications urgently need to generate a large-volume stable discharge at atmospheric pressure, self-designed multi-hollow needle electrodes are used as a high-voltage electrode in the packed bed dielectric barrier discharge reactor to facilitate fast gas flow through the active discharge area and achieve a large-volume stable discharge. The plasma behavior and the generation mechanism of active species are need to be clarified in both PD and ND for optimizing discharge. An ICCD camera is employed to study the generation and propagation process of plasma on the dielectric beads, and the effect of the discharge gap between the dielectric beads on the plasma behavior is the main focus. Additionally, the generation mechanisms of three key excited state species are investigated by using time-resolved optical emission spectra (OES). Furthermore, the vibrational temperature (T_vib) and rotational temperature (T_rot), which are crucial for exploring the energy transfer pathways inside the plasma, are both calculated and discussed.

2.   Experimental setup

The experimental setup, as depicted in figure 1(a), comprises a discharge reactor, a high-voltage (HV) nanosecond pulse power supply (HVP-20P, Xi’an Smart Maple Electronic Technology Co. Ltd), an electrical measurement system, an optical detection system, and a gas intake system. The discharge reactor is mainly composed of a quartz reaction vessel, an HV needle array electrode, a dielectric bead layer, a dielectric plate, and a ground electrode, etc. The HV needle array electrode consists of seven stainless-steel hollow needles with an inner diameter of 1.6 mm and an outer diameter of 2.1 mm. This design allows the gas to flow between and perpendicular to the electrodes, and can effectively promote fast gas throughput through the active discharge zone. The stainless-steel ground electrode is positioned 3 mm below the hollow needles and has a diameter of 30 mm. The dielectric plate, made of alumina ceramics with a diameter of 50 mm and a thickness of 0.5 mm, is placed above the ground electrode. A layer of Al₂O₃ dielectric beads with a diameter of 2.5 mm and a relative permittivity (ε) of 10 is filled within the electrode gaps. The waveforms of pulse voltage and discharge current are measured using a 1:1000 HV probe (Tektronix P6015A, 1000×, 3.0 pF, 100 MΩ) and a current probe (Tektronix TCP312, bandwidth 100 MHz), respectively. These measurements are displayed on an oscilloscope (Tektronix, TDS5054B, 500 MHz). In the optical detection system, the OES in the discharge region are collected by optical fiber and recorded by a grating monochromator (Andor SR-750i, grating groove 2400 lines mm⁻¹, blazed wavelength 200 nm). The macro discharge image is captured by a Canon EOS 70D camera with an exposure time of 100 ms, as shown in figure 1(b). The dynamic behavior of the discharge is captured by an ICCD camera (Andor, New iStar DH334T) and displayed on a computer. The experiment is carried out at atmospheric pressure with pure nitrogen and air (80% N₂/20% O₂, N₂ 99.999% purity and O₂ 99.999% purity), and the total gas flow is controlled by two mass flow controllers (S48-300, HoribaStec). In the experiment, the pulse parameters are set as follows: pulse peak voltage 15 kV, pulse repetition rate 1 kHz, pulse width 50 ns, and pulse rising and falling times 50 ns each. The total gas flow is maintained at 200 ml min⁻¹.

Figure  1.  (a) Experimental setup and (b) discharge images under nitrogen and air.

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3.   Results and discussion

3.1   Electrical and visualization characteristics of a multi-hollow needle plate packed bed DBD

Figure 2 shows the waveforms of pulse voltage, discharge current and the corresponding calculated coupled energy. The current waveform exhibits two distinct discharges, occurring at the rising and falling edges of the pulse voltage, respectively, i.e. PD and ND. PD is ignited by an externally enhanced electric field, corresponding to positive current values. ND is ignited by the accumulated surface residual charge on the dielectric plate and Al₂O₃ dielectric beads in PD, corresponding to negative current values. In ND, the accumulated surface residual charge is regarded as a ‘high-voltage point’ [17].

Figure  2.  (a) Waveforms of pulse voltage and discharge current and (b) coupled energy under nitrogen and air.

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As shown in figure 2(a), the peak current value in PD significantly exceeds that in ND. This disparity arises from the insufficient accumulation of surface residual charge in PD, which fails to generate a substantial reverse electric field in ND. Therefore, the maximum potential formed by the accumulated surface residual charge is lower than the pulse voltage applied in PD. In addition, because the accumulated surface residual charge has a long survival time, the current duration in ND is longer than that in PD [20]. Figure 2(b) shows that the discharge coupled energy in PD is greater than that in ND, which will lead to the discharge intensity in PD being greater than that in ND. The gas composition also has obvious effects on the discharge current. The peak value and duration of the discharge current in nitrogen are both greater than those in air. This is because the oxygen molecules in an air discharge can accelerate the consumption of electrons through the electron attachment reaction [21, 22], which results in a decrease in electron density. Figure 2(b) also shows that the discharge energy input of a single voltage pulse in nitrogen (28.1 mJ) is over twice that in air (11.2 mJ), which means that the discharge in nitrogen has a higher discharge energy input than that in air and causes a stronger discharge intensity in nitrogen than in air.

Figure 3 shows ICCD images of a multi-hollow needle plate packed bed DBD under nitrogen and air. For PD, the start and exposure times of the ICCD camera are set to 0 ns and 150 ns, respectively, which correspond exactly to the current of the PD shown in figure 2(a). Similarly, for ND, the start and exposure times of the ICCD camera are both set to 150 ns, which correspond exactly to the current of the ND shown in figure 2(a). It is evident from the images that each hollow needle electrode contains multiple discharge channels. This is because the bottom of the hollow needle electrode resembles a ring structure and can be regarded as the tips of multiple needle electrodes, which contributes to forming multiple main discharge channels. In addition, the discharge current and image under the multi-needle electrode and the multi-hollow needle electrode are also compared, as shown in figures S1 and S2 (see details in the supporting information), respectively. Figure S1 shows that the values of the discharge current of multi-hollow needle electrodes are significantly larger than those of needle electrodes, which means that the discharge intensity is enhanced after utilizing a multi-hollow needle electrode to replace a multi-needle electrode. It can be clearly observed from figure S2 that more discharge channels are distributed under multi-hollow needle electrodes than under multi-needle electrodes, and the larger discharge volume under multi-hollow needle electrodes can also be directly observed.

Figure  3.  ICCD images of a multi-hollow needle plate packed bed DBD under nitrogen and air.

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Comparing the discharge under nitrogen and air in figure 3, it can be found that the luminous intensity under nitrogen is stronger than that under air in PD, while there is little difference between the luminous intensity under nitrogen and air in ND. This is mainly due to the fact that PD is ignited by an external enhanced electric field at the rising edge of the pulse voltage. In PD, electrons increase sharply and gain more energy from the continuously increasing applied electric field [19]. Most electrons concentrate on the discharge gap between the multi-hollow needle electrodes and the dielectric beads and the surface of the dielectric beads, thus giving these parts stronger luminous intensity in the images taken by the ICCD. Compared with the discharge under nitrogen, oxygen molecules under air discharge can significantly reduce the discharge current by adsorbing electrons, resulting in a reduction in the discharge intensity and luminous intensity. However, ND is initiated by the accumulated surface residual charges, and electrons do not accumulate but dissipate rapidly [17, 19]. The light emission can be attributed to continuous electron collisions and residual heavy particles in space. Combined with figure 2(a), it can be found that the discharge current of ND in nitrogen is slightly larger than that in air, which leads to little difference in discharge intensity and luminous intensity.

3.2   Spatial-temporal evolution of plasma

To clearly observe the evolution of plasma in the hollow needle plate packed bed reactor we focus on the discharge under a single hollow needle electrode. Under a single hollow needle electrode, three dielectric beads labeled (1), (2) and (3) are placed next to each other. There is a gap of about 250 μm between dielectric bead (1) and dielectric bead (2) and no gap between dielectric bead (2) and dielectric bead (3). Figure 4 shows the plasma evolutions in both PD and ND under nitrogen. The exposure time and step time of the ICCD camera are both set to 5 ns.

Figure  4.  Plasma evolutions in both positive and negative discharges under nitrogen. The time labels inserted in images are the delay with respect to the start time of the voltage pulse.

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It can be seen from figure 4(a) that the surface streamer forms and develops on the surface of dielectric bead (2) in PD, so the discharge is mainly in a streamer mode. This phenomenon in PD stems from the beneficial effects of rapidly rising voltage which generate greater instantaneous power and excite a higher space electric field. As a result, electrons can gain more energy from the continuously increasing applied electric field [19], which promotes the generation and propagation of streamers. While in ND, as shown in figure 4(b), the discharge is initiated by the accumulated surface residual charges and there is no secondary electron emission from the dielectric surface [23]. The accumulated residual charges in PD tend to suppress the development of surface streamers in ND [24], which leads to a diffusion-like mode. Moreover, the electric field corresponding to ND is notably lower than that corresponding to PD, and the decrease of the electric field results in a corresponding decrease in the energy gained by electrons, which is not conducive to the generation of surface streamer [25].

The effect of the gap between dielectric beads on the plasma evolution in PD under nitrogen can also be observed in figure 4(a). When there is no gap between the dielectric beads, partial discharge occurs preferentially at the contact point between dielectric bead (2) and dielectric bead (3) at 40 ns. This is because dielectric beads can lead to strong polarization and a high electric field at the contact point. Therefore, there is sufficient electron energy and electron temperature in this region to initiate a discharge [26]. However, the discharge can only propagate above the contact point and only produce a small polarized discharge area below the contact point. In this case, the surface streamer cannot propagate to the bottom of the dielectric beads.

When there is a gap between dielectric beads, the surface streamer does not propagate directly to the bottom of dielectric bead (2), but first propagates from dielectric bead (2) to the surface of the adjacent dielectric bead (1), and a standing filamentary micro-discharge is formed in the discharge gap. This is because when the surface streamer propagates on the surface of dielectric bead (2), the electrons charge the bead surface and the positive ions hit the bead surface to produce secondary electron emission [23, 27], which results in a difference between the electric field at the upper surface of dielectric bead (2) and the lower surface of dielectric bead (1). The electric field formed between dielectric bead (2) and dielectric bead (1) can promote the surface streamer to approach the adjacent dielectric bead (1) and lead to the generation of filamentary micro-discharge in the gap between dielectric bead (1) and dielectric bead (2) [23, 28]. After about 50 ns, the discharge rapidly propagates to the bottom of dielectric beads (1) and (2). This is because the local electric field caused by the filamentary micro-discharge promotes the surface of dielectric bead (2) to have a higher positive charge density, which leads to enhancement of the electric field of the streamer head, thus promoting the rapid propagation of surface streamers [29, 30]. In a word, the gap between the dielectric beads plays a crucial role in the packed bed DBD reactor. It allows the plasma to leave the local area and is conducive to the generation and propagation of surface streamer discharge and filamentary micro-discharge, which is beneficial to the generation of active species [14]. Figure 4(b) shows the effect of the gap between the dielectric beads on plasma evolution in ND under nitrogen. The results reveal that the discharge only exhibits a diffusion-like mode between the gap of dielectric beads, regardless of whether there is a discharge gap.

Figure 5 shows the plasma evolutions in both PD and ND under nitrogen. The exposure time and step time of the ICCD camera are both set to 5 ns. Comparing figures 4 and 5, it can be seen that there are similar discharge phenomena under nitrogen and air, such as the streamer discharge mode in PD, the diffusion-like discharge mode in ND, the filamentary micro-discharge and the partial discharge. However, the difference is that the surface streamer under nitrogen has a narrower channel diameter than that under air in PD. This is because the photoionization range under nitrogen is shorter than that under air, thus a narrower streamer channel diameter is required to provide sufficient field enhancement for streamer propagation [31]. Additionally, the discharge under air starts at 15 ns, approximately 5 ns earlier than that under nitrogen. This is also due to the fact that the photoionization range under air is longer than that under nitrogen, resulting in a lower breakdown voltage under air.

Figure  5.  Plasma evolutions in both positive and negative discharges under air. The time labels inserted in images are the delay with respect to the start time of the voltage pulse.

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Moreover, in order to depict the variation of the brightness of the discharge channel, MATLAB is used to process the discharge image in gray values, as shown in figure 6. It is evident that the mean gray value under nitrogen is notably higher than that under air in both PD and ND, and the mean gray value in PD is greater than that in ND. The mean gray values of the discharge image can reflect the PMT signal of the discharge to some extent, which in turn reflects the variation of the discharge intensity [32]. Therefore, these results indicate that the discharge intensity under nitrogen is stronger than that under air, and the discharge intensity in PD is obviously stronger than that in ND.

Figure  6.  The mean gray value as a function of the time under nitrogen and air.

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3.3   The OES of a multi-hollow needle plate packed bed DBD

Figure 7 shows the OES of the multi-hollow needle plate packed bed DBD in the wavelength range 300–450 nm under nitrogen and air. The exposure time of the ICCD camera is set to 0.2 s. It mainly contains the bands of $\text{OH(}{\text{A}}^{\text{2}}{{ \Sigma }}^{\text{+}}\text{→}{\text{X}}^{\text{2}}\text{Π}\text{)}$, the second positive system of molecular nitrogen ${\text{N}}_{\text{2}}\text{(}{\text{C}}^{\text{3}}{\text{Π}}_{\text{u}}\text{→}{\text{B}}^{\text{3}}{\text{Π}}_{\text{g}}\text{)}$ and the first negative system of the nitrogen molecular ion ${\text{N}}_{\text{2}}^{\text{+}}\text{(}{\text{B}}^{\text{2}}{{ \Sigma }}_{\text{u}}^{\text{+}}\text{→}{\text{X}}^{\text{2}}{{ \Sigma }}_{\text{g}}^{\text{+}}\text{)}$. The enlarged spectra of $\text{OH(}{\text{A}}^{\text{2}}{{ \Sigma }}^{\text{+}}\text{→}{\text{X}}^{\text{2}}\text{Π}\text{)}$ and ${\text{N}}_{\text{2}}^{\text{+}}\text{(}{\text{B}}^{\text{2}}{{ \Sigma }}_{\text{u}}^{\text{+}}\text{→}{\text{X}}^{\text{2}}{{ \Sigma }}_{\text{g}}^{\text{+}}\text{)}$ are also presented in detail in figure 7.

Figure  7.  The OES of a multi-hollow needle plate packed bed DBD in the wavelength range 300–450 nm under nitrogen and air.

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At atmospheric discharge plasma, the OES of $\text{OH(}{\text{A}}^{\text{2}}{{ \Sigma }}^{\text{+}}\text{→}{\text{X}}^{\text{2}}\text{Π}\text{)}$, ${\text{N}}_{\text{2}}\text{(}{\text{C}}^{\text{3}}{\text{Π}}_{\text{u}}\text{→}{\text{B}}^{\text{3}}{\text{Π}}_{\text{g}}\text{)}$, and ${\text{N}}_{\text{2}}^{\text{+}}\text{(}{\text{B}}^{\text{2}}{{ \Sigma }}_{\text{u}}^{\text{+}}\text{→}{\text{X}}^{\text{2}}{{ \Sigma }}_{\text{g}}^{\text{+}}\text{)}$ are derived from the spontaneous emission of the radicals and active species, such as ${\text{N}}_{\text{2}}\text{(}{\text{C}}^{\text{3}}{\text{Π}}_{\text{u}}\text{)}$, ${\text{N}}_{\text{2}}^{\text{+}}({\text{B}}^{\text{2}}{{ \Sigma }}_{\text{u}}^{\text{+}})$, and $\text{OH(}{\text{A}}^{\text{2}}{{ \Sigma }}^{\text{+}})$, (R1–R3 in table 1). In nitrogen, the excited state ${\text{N}}_{\text{2}}\text{(}{\text{C}}^{\text{3}}{\text{Π}}_{\text{u}}\text{)}$ is mainly derived from collisions between high-energy electrons (11.03 eV) and ${\text{N}}_{\text{2}}({\text{X}}^{\text{1}}{{ \Sigma }}_{\text{g}}^{\text{+}})$, electron impact of metastable ${\text{N}}_{\text{2}}\text{(}{\text{A}}^{\text{3}}{{ \Sigma }}_{\text{u}}^{\text{+}}\text{)}$ and the pooling reaction between the metastable ${\text{N}}_{\text{2}}\text{(}{\text{A}}^{\text{3}}{{ \Sigma }}_{\text{u}}^{\text{+}}\text{)}$ (R4–R6). The excited state ${\text{N}}_{\text{2}}^{\text{+}}({\text{B}}^{\text{2}}{{ \Sigma }}_{\text{u}}^{\text{+}})$ is mainly generated by collisions between high-energy electrons (18.75 eV) and ${\text{N}}_{\text{2}}({\text{X}}^{\text{1}}{{ \Sigma }}_{\text{g}}^{\text{+}})$ and by electron impact of metastable ${\text{N}}_{\text{2}}\text{(}{\text{A}}^{\text{3}}{{ \Sigma }}_{\text{u}}^{\text{+}}\text{)}$ (R7, R8). The radical $\text{OH(}{\text{A}}^{\text{2}}{{ \Sigma }}^{\text{+}})$ is mainly generated by direct electron collisions with H₂O molecules, and the resonant energy transfer process between metastable ${\text{N}}_{\text{2}}\text{(}{\text{A}}^{\text{3}}{{ \Sigma }}_{\text{u}}^{\text{+}}\text{)}$ and $\text{OH(}{\text{X}}^{\text{2}}\text{Π)}$ (R9–R12). In this experiment, the H₂O molecules mainly originate from the small amount of residual H₂O in the reactor. However, as a kind of electronegative gas, oxygen molecules in an air discharge can weaken the discharge by capturing free electrons and exhaust ${\text{N}}_{\text{2}}\text{(}{\text{A}}^{\text{3}}{{ \Sigma }}_{\text{u}}^{\text{+}}\text{)}$ within 1.25 × 10⁻¹⁵ ns (R13–R17), thus $\text{OH(}{\text{A}}^{\text{2}}{{ \Sigma }}^{\text{+}})$, ${\text{N}}_{\text{2}}\text{(}{\text{C}}^{\text{3}}{\text{Π}}_{\text{u}}\text{)}$ and ${\text{N}}_{\text{2}}^{\text{+}}({\text{B}}^{\text{2}}{{ \Sigma }}_{\text{u}}^{\text{+}})$ can be sharply decreased. Therefore, the emission intensities of $\text{OH(}{\text{A}}^{\text{2}}{{ \Sigma }}^{\text{+}}\text{→}{\text{X}}^{\text{2}}\text{Π}\text{)}$, ${\text{N}}_{\text{2}}\text{(}{\text{C}}^{\text{3}}{\text{Π}}_{\text{u}}\text{→}{\text{B}}^{\text{3}}{\text{Π}}_{\text{g}}\text{)}$ and ${\text{N}}_{\text{2}}^{\text{+}}\text{(}{\text{B}}^{\text{2}}{{ \Sigma }}_{\text{u}}^{\text{+}}\text{→}{\text{X}}^{\text{2}}{{ \Sigma }}_{\text{g}}^{\text{+}}\text{)}$ under nitrogen are greater than their respective values under air.

Table  1.  The principal reactions of active species.

Reaction processes	Reaction constant	No.	Ref.
${\text{N} }_{\text{2} }({\text{C} }^{\text{3} }{\text{Π} }_{\text{u} })\text{ → }{\text{N} }_{\text{2} }({\text{B} }^{\text{3} }{\text{Π} }_{\text{g} })\text{ + }{h\nu}$	3.0 × 107 s−1	R1	[33]
${\mathrm{N}}_2^+\text{(B}^\text{2}{ \Sigma }^\text{+}_{\mathrm{u}}\text{) →} {\mathrm{N}}_2^+({\mathrm{X}}^2{ \Sigma }^\text{+}_{\mathrm{g}} )+{h\nu}$	1.5 × 107 s−1	R2	[34]
$\text{OH(A}^\text{2}{ \Sigma }^\text{+}\text{) → OH(X}^\text{2}\text{Π) + }{h\nu}$	1.4 × 106 s−1	R3	[35]
${\text{e}}\text{ + }{\text{N} }_{\text{2} }({\text{X} }^{\text{1} }{ { \Sigma } }_{\text{g} }^{\text{+} })\text{→ }{\text{e}}\text{ + }{\text{N} }_{\text{2} }({\text{C} }^{\text{3} }{\text{Π} }_{\text{u} })$	4.0 × 10−14 cm3 s−1	R4	[33]
${\text{e}}\text{ + }{\text{N} }_{\text{2} }\text{(}{\text{A} }^{\text{3} }{ { \Sigma } }_{\text{u} }^{\text{+} }\text{) → }{\text{N} }_{\text{2} }({\text{C} }^{\text{3} }{\text{Π} }_{\text{u} })\text{ + }{\text{e}}$	1.1 × 10−10 cm3 s−1	R5	[33]
${\text{N}}_{\text{2}}\text{(}{\text{A}}^{\text{3}}{{ \Sigma }}_{\text{u}}^{\text{+}}\text{) +}{\text{N}}_{\text{2}}\text{(}{\text{A}}^{\text{3}}{{ \Sigma }}_{\text{u}}^{\text{+}}\text{) →}{\text{N}}_{\text{2}}\text{(}{\text{C}}^{\text{3}}{\text{Π}}_{\text{u}}\text{) +}{\text{N}}_{\text{2}}\text{(}{\text{X}}^{\text{1}}{{ \Sigma }}_{\text{g}}^{\text{+}}\text{)}$	2.0 × 10−12 cm3 s−1	R6	[33]
${\text{e}}\text{ + }{\text{N} }_{\text{2} }({\text{X} }^{\text{1} }{ { \Sigma } }_{\text{g} }^{\text{+} })\text{ → } {2{\mathrm{e}}}\text{ + }{\text{N} }_{\text{2} }^{\text{+} }({\text{B} }^{\text{2} }{ { \Sigma } }_{\text{u} }^{\text{+} })$	2.4 × 10−12 cm3 s−1	R7	[36]
${\text{e}}\text{ + }{\text{N} }_{\text{2} }\text{(}{\text{A} }^{\text{3} }{ { \Sigma } }_{\text{u} }^{\text{+} }\text{) → } {2{\mathrm{e}}}\text{ +}{\text{ N} }_{\text{2} }^{\text{+} }({\text{B} }^{\text{2} }{ { \Sigma } }_{\text{u} }^{\text{+} })$	>2.4 × 10−12 cm3 s−1	R8	[37]
${\text{e}} \text{ + H}_\text{2}\text{O }{\to {{\text{e}} } + \text{H } + }\text{ OH(A}^\text{2}{ \Sigma }^\text{+}\text{)}$	2.6 × 10−12 cm3 s−1	R9	[36]
${\text{e}}\text{ + H}_\text{2}\text{O → H}^{-} \text{ + }\text{OH(A}^\text{2}{ \Sigma }^\text{+}\text{)}$\$	2.6 × 10−12 cm3 s−1	R10	[36]
${\text{N} }_{\text{2} }\text{(}{\text{A} }^{\text{3} }{ { \Sigma } }_{\text{u} }^{\text{+ } }\text{) + }{\text{H} }_{\text{2} }\text{O → }{\text{N} }_{\text{2} }\text{ + H + OH(X}^\text{2}\text{Π)}$	4.2 × 10−11 cm3 s−1	R11	[36]
${\text{N} }_{\text{2} }\text{(}{\text{A} }^{\text{3} }{ { \Sigma } }_{\text{u} }^{\text{+} }\text{) + OH(X}^\text{2}\text{Π) → } {\text{ N} }_{\text{2} }({\text{X} }^{\text{1} }{ { \Sigma } }_{\text{g} }^{\text{+} })\text{ + OH(A}^\text{2}{ \Sigma }^\text{+}\text{)}$	1.1 × 10−10 cm3 s−1	R12	[38]
${\text{e}}\text{ + }{\text{N} }_{\text{2} }({\text{X} }^{\text{1} }{ { \Sigma } }_{\text{g} }^{\text{+} })\text{ → }{\text{N} }_{\text{2} }\text{(}{\text{A} }^{\text{3} }{ { \Sigma } }_{\text{u} }^{\text{+} }{)} \text{ + } {\text{e}}$	1.1 × 10−10 cm3 s−1	R13	[36]
${\text{N}}_{\text{2}}\text{(}{\text{A}}^{\text{3}}{{ \Sigma }}_{\text{u}}^{\text{+}}\text{) +}{\text{O}}_{\text{2}}\text{→}{\text{N}}_{\text{2}}({\text{X}}^{\text{1}}{{ \Sigma }}_{\text{g}}^{\text{+}})\text{+ O + O}$	2.54 × 10−12 cm3 s−1	R14	[33]
${\text{N}}_{\text{2}}\text{(}{\text{A}}^{\text{3}}{{ \Sigma }}_{\text{u}}^{\text{+}}\text{) + O →}{\text{N}}_{\text{2}}({\text{X}}^{\text{1}}{{ \Sigma }}_{\text{g}}^{\text{+}})\text{+}{\text{O(}}^{\text{1}}\text{S)}$	2.1 × 10−11 cm3 s−1	R15	[33]
${\text{N} }_{\text{2} }\text{(}{\text{A} }^{\text{3} }{ { \Sigma } }_{\text{u} }^{\text{+ } }\text{) + }{\text{O} }_{\text{2} }^{ {-} }\text{ → }{\text{N} }_{\text{2} }\text{ + }{\text{O} }_{\text{2} }\text{ + } {\text{e}}$	2.1 × 10−9 cm3 s−1	R16	[39]
${\text{N} }_{\text{2} }\text{(}{\text{A} }^{\text{3} }{ { \Sigma } }_{\text{u} }^{\text{+} }\text{) + }\text{O}^ {-} \text{→}{\text{N} }_{\text{2} }\text{ + O + }{\text{e}}$\$	2.2 × 10−9 cm3 s−1	R17	[39]

 | Show Table

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Figure 7 also shows that the emission intensity of $\text{OH(}{\text{A}}^{\text{2}}{{ \Sigma }}^{\text{+}}\text{→}{\text{X}}^{\text{2}}\text{Π})$ under air is nearly zero. This is because $\text{OH(}{\text{A}}^{\text{2}}{{ \Sigma }}^{\text{+}}\text{)}$ can be rapidly transformed into $\text{OH(X}^\text{2}\text{Π)}$ within a very short time through collision and radiation in atmospheric air. For this short lifetime, it is difficult to measure using OES alone. Moreover, among the three emission intensities shown in figure 7, that of ${\text{N}}_{\text{2}}\text{(}{\text{C}}^{\text{3}}{\text{Π}}_{\text{u}}\text{→}{\text{B}}^{\text{3}}{\text{Π}}_{\text{g}}\text{)}$ is the highest. This indicates that a large amount of ${\text{N}}_{\text{2}}\text{(}{\text{C}}^{\text{3}}{\text{Π}}_{\text{u}}\text{)}$ is generated during the discharge process, which is attributed to its high reaction constant and short radiation lifetime of 36.6 ns [40].

3.4   The generation process of active species

In order to elucidate the generation and propagation mechanisms of multi-hollow needle plate packed bed DBD plasmas, the time-resolved OES of ${\text{N}}_{\text{2}}\text{(}{\text{C}}^{\text{3}}{\text{Π}}_{\text{u}}\text{→}{\text{B}}^{\text{3}}{\text{Π}}_{\text{g}}\text{, 337 nm)}$, ${\text{N}}_{\text{2}}^{\text{+}}\text{(}{\text{B}}^{\text{2}}{{ \Sigma }}_{\text{u}}^{\text{+}}\text{→}{\text{X}}^{\text{2}}{{ \Sigma }}_{\text{g}}^{\text{+}}\text{, 391.4 nm)}$ and $\text{OH(}{\text{A}}^{\text{2}}{{ \Sigma }}^{\text{+}}\text{→}{\text{X}}^{\text{2}}\text{Π, 309 nm)}$ under nitrogen and air were measured and are shown in figures 8 and 9. The data in figure 8 were obtained by collecting 30 times in succession when the gate width and gate delay step of the ICCD camera were both set to 10 ns. The data in figure 9 were obtained by collecting 28 times in succession when the gate width and gate delay step of the ICCD camera were both set as 40 ns. In order to obtain a better signal-to-noise ratio, all the data are accumulated 50 times.

Figure  8.  Time-resolved emission intensities of $\text{N}_{\text{2}}\text{(}\text{C}^{\text{3}}\text{Π}_{\text{u}}\text{→}\text{B}^{\text{3}}\text{Π}_{\text{g}},$$337\mathrm{\ nm}\text{)}$ and ${\text{N}}_{\text{2}}^{\text{+}}\text{(}{\text{B}}^{\text{2}}{{ \Sigma }}_{\text{u}}^{\text{+}}\text{→}{\text{X}}^{\text{2}}{{ \Sigma }}_{\text{g}}^{\text{+}}\text{, 391.4 nm)}$ under nitrogen and air.

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Figure  9.  The emission intensity of $\text{OH(}{\text{A}}^{\text{2}}{{ \Sigma }}^{\text{+}}\text{→}{\text{X}}^{\text{2}}\text{Π, 309 nm)}$ and the voltage and current vary with time during one pulse voltage period under nitrogen and air.

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As shown in figure 8, the OES of $\text{N}_{\text{2}}^{\text{+}}\text{(}\text{B}^{\text{2}}\Sigma_{\text{u}}^{\text{+}}\text{→}\text{X}^{\text{2}}\Sigma_{\text{g}}^{\text{+}}, 391.4\ \mathrm{nm})$ and ${\text{N}}_{\text{2}}\text{(}{\text{C}}^{\text{3}}{\text{Π}}_{\text{u}}\text{→}{\text{B}}^{\text{3}}{\text{Π}}_{\text{g}}\text{, 337 nm)}$ are only detected in both PD and ND, and the emission intensities in PD are higher than those in ND. This is because ${\text{N}}_{\text{2}}^{\text{+}}({\text{B}}^{\text{2}}{{ \Sigma }}_{\text{u}}^{\text{+}})$ and ${\text{N}}_{\text{2}}\text{(}{\text{C}}^{\text{3}}{\text{Π}}_{\text{u}}\text{)}$ are mainly generated by collisions between high-energy electrons and ${\text{N}}_{\text{2}}({\text{X}}^{\text{1}}{{ \Sigma }}_{\text{g}}^{\text{+}})$. The higher value of the discharge current peak, number of streamer channels and plasma luminous intensity in PD can result in higher generation of ${\text{N}}_{\text{2}}^{\text{+}}({\text{B}}^{\text{2}}{{ \Sigma }}_{\text{u}}^{\text{+}})$ and ${\text{N}}_{\text{2}}\text{(}{\text{C}}^{\text{3}}{\text{Π}}_{\text{u}}\text{)}$. In addition, according to the studies of Obrusník et al, Bílek et al and Paris et al [41–44], the intensity ratios of ${\text{N}}_{\text{2}}^{\text{+}}\text{(}{\text{B}}^{\text{2}}{{ \Sigma }}_{\text{u}}^{\text{+}}\text{→}{\text{X}}^{\text{2}}{{ \Sigma }}_{\text{g}}^{\text{+}}\text{, 391.4 nm)}$ and ${\text{N}}_{\text{2}}\text{(}{\text{C}}^{\text{3}}{\text{Π}}_{\text{u}}\text{→}{\text{B}}^{\text{3}}{\text{Π}}_{\text{g}}\text{, 394.2 nm)}$ (R_391/394) can be considered as an indicator of the reduced electric field (E/N). Therefore, the time-resolved R_391/394 ratio is calculated to reflect the difference between PD and ND, as shown in figure 10. It can be also concluded that the higher E/N in PD is the key factor leading to the higher generation of ${\text{N}}_{\text{2}}^{\text{+}}({\text{B}}^{\text{2}}{{ \Sigma }}_{\text{u}}^{\text{+}})$ and ${\text{N}}_{\text{2}}\text{(}{\text{C}}^{\text{3}}{\text{Π}}_{\text{u}}\text{)}$ [45]. Figure 8 also shows that the OES of ${\text{N}}_{\text{2}}^{\text{+}}\text{(}{\text{B}}^{\text{2}}{{ \Sigma }}_{\text{u}}^{\text{+}}\text{→}{\text{X}}^{\text{2}}{{ \Sigma }}_{\text{g}}^{\text{+}}\text{, 391.4 nm)}$ can be only detected in PD. The reason is that ${\text{N}}_{\text{2}}^{\text{+}}({\text{B}}^{\text{2}}{{ \Sigma }}_{\text{u}}^{\text{+}})$ is mostly generated by direct collisions between ${\text{N}}_{\text{2}}({\text{X}}^{\text{1}}{{ \Sigma }}_{\text{g}}^{\text{+}})$ and high-energy electrons (18.75 eV) which are usually generated at the streamer head. As discussed above, the discharge propagates as a streamer mode in PD and a diffusion-like mode in ND. In addition, E/N in ND is significantly lower than that in PD. Therefore, for ND, there are insufficient high-energy electrons to generate ${\text{N}}_{\text{2}}^{\text{+}}({\text{B}}^{\text{2}}{{ \Sigma }}_{\text{u}}^{\text{+}})$ [46].

Figure  10.  Time-resolved intensity of R_391/394 in both positive and negative discharges under nitrogen and air.

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It can also be seen from figure 8 that the peak emission intensity of ${\text{N}}_{\text{2}}^{\text{+}}\text{(}{\text{B}}^{\text{2}}{{ \Sigma }}_{\text{u}}^{\text{+}}\text{→}{\text{X}}^{\text{2}}{{ \Sigma }}_{\text{g}}^{\text{+}}\text{, 391.4 nm)}$ comes approximately 10 ns earlier than that of ${\text{N}}_{\text{2}}\text{(}{\text{C}}^{\text{3}}{\text{Π}}_{\text{u}}\text{→}{\text{B}}^{\text{3}}{\text{Π}}_{\text{g}}\text{, 337 nm)}$. This discrepancy arises from the fact that both the electric field strength and neutral gas density are diminished behind the streamer head, which leads to a more rapid decrease in the creation frequency for ${\text{N}}_{\text{2}}^{\text{+}}\text{(}{\text{B}}^{\text{2}}{{ \Sigma }}_{\text{u}}^{\text{+}}\text{→}{\text{X}}^{\text{2}}{{ \Sigma }}_{\text{g}}^{\text{+}}\text{, 391.4 nm)}$ compared with that for ${\text{N}}_{\text{2}}\text{(}{\text{C}}^{\text{3}}{\text{Π}}_{\text{u}}\text{→}{\text{B}}^{\text{3}}{\text{Π}}_{\text{g}}\text{, 337 nm)}$. Consequently, the electric field strength becomes insufficient to accelerate electrons beyond the excitation threshold of ${\text{N}}_{\text{2}}^{\text{+}}\text{(}{\text{B}}^{\text{2}}{{ \Sigma }}_{\text{u}}^{\text{+}}\text{→}{\text{X}}^{\text{2}}{{ \Sigma }}_{\text{g}}^{\text{+}}\text{, 391.4 nm)}$ [47].

In figure 9, it is noteworthy that although the OES of $\text{OH(}{\text{A}}^{\text{2}}{{ \Sigma }}^{\text{+}}\text{→}{\text{X}}^{\text{2}}\text{Π, 309 nm)}$ can be detected in both PD and ND under nitrogen. The emission intensity in ND is obviously higher than that in PD, and the peak intensity occurs at the end of the ND, which is completely opposite to the intensity distributions of ${\text{N}}_{\text{2}}\text{(}{\text{C}}^{\text{3}}{\text{Π}}_{\text{u}}\text{→}{\text{B}}^{\text{3}}{\text{Π}}_{\text{g}}\text{, 337 nm)}$ and ${\text{N}}_{\text{2}}^{\text{+}}\text{(}{\text{B}}^{\text{2}}{{ \Sigma }}_{\text{u}}^{\text{+}}\text{→} {\text{X}}^{\text{2}}{{ \Sigma }}_{\text{g}}^{\text{+}}\text{, 391.4 nm)}$. As discussed before, there are two main pathways for generating the radical $\text{OH(}{\text{A}}^{\text{2}}{{ \Sigma }}^{\text{+}})$ in nitrogen: through collision of high-energy electrons with H₂O molecules or the resonant energy transfer process between metastable ${\text{N}}_{\text{2}}\text{(}{\text{A}}^{\text{3}}{{ \Sigma }}_{\text{u}}^{\text{+}}\text{)}$ and $\text{OH(}{\text{X}}^{\text{2}}\text{Π)}$ [38, 48]. Considering that the higher discharge current in PD does not lead to higher emission intensity of $\text{OH(}{\text{A}}^{\text{2}}{{ \Sigma }}^{\text{+}}\text{→}{\text{X}}^{\text{2}}\text{Π, 309 nm)}$, the radical $\text{OH(}{\text{A}}^{\text{2}}{{ \Sigma }}^{\text{+}})$ is mainly generated by resonant energy transfer between metastable ${\text{N}}_{\text{2}}\text{(}{\text{A}}^{\text{3}}{{ \Sigma }}_{\text{u}}^{\text{+}}\text{)}$ and $\text{OH(}{\text{X}}^{\text{2}}\text{Π)}$. This is because metastable ${\text{N}}_{\text{2}}\text{(}{\text{A}}^{\text{3}}{{ \Sigma }}_{\text{u}}^{\text{+}}\text{)}$ can accumulate in the preceding PD period due to electron-impact excitation, the indirect radiative/quenching cascade and the vibrational relaxation processes [49]. Direct electron collision reactions have less effect on the generation of the radical $\text{OH(}{\text{A}}^{\text{2}}{{ \Sigma }}^{\text{+}})$ under this condition. Furthermore, figure 9 reveals that the quenching process of the radical $\text{OH(}{\text{A}}^{\text{2}}{{ \Sigma }}^{\text{+}})$ in the afterglow lasts for more than 800 ns. This long existence time of $\text{OH(}{\text{A}}^{\text{2}}{{ \Sigma }}^{\text{+}})$ is maintained by the long radiative lifetime and high energy of ${\text{N}}_{\text{2}}\text{(}{\text{A}}^{\text{3}}{{ \Sigma }}_{\text{u}}^{\text{+}}\text{)}$ [50, 51].

3.5   The plasma temperature

T_vib and T_rot, which can be obtained by fitting experimental OES of ${\text{N}}_{\text{2}}\text{(}{\text{C}}^{\text{3}}{\text{Π}}_{\text{u}}\text{→}{\text{B}}^{\text{3}}{\text{П}}_{\text{g}}\text{, 0–2, 1–3)}$ with SPECAIR software, play important roles in exploring the energy transfer pathways in plasmas, and well-fitted OES are shown in figure 11 [52–54]. It is shown that T_vib and T_rot are approximately 2150±50 K and 610±10 K, respectively, under nitrogen, and approximately 3300±50 K and 725±10 K, respectively, under air. T_vib and T_rot under air are higher than those under nitrogen, which is probably due to the effect of oxygen. With the presence of oxygen molecules under air, more effective energy is transferred from electrons to oxygen molecules, driven by the excitation of the rotational and vibrational levels within a large number of negatively charged oxygen molecules. This causes both T_vib and T_rot to increase through an effective energy relaxation process [55]. Additionally, the higher T_rot observed under air is also attributed to the presence of oxygen molecules, which increases the gas-phase electrical resistance and causes Joule heating [56]. In this experiment, T_rot can be used to represent the gas kinetic temperature (T_gas) of the plasma because of the narrow energy gap between rotational levels [57].

Figure  11.  Simulated and experimental OES of a multi-hollow needle plate packed bed DBD under (a) nitrogen and (b) air in the range of ${\text{N}}_{\text{2}}\text{(}{\text{C}}^{\text{3}}{\text{Π}}_{\text{u}}\text{→}{\text{B}}^{\text{3}}{\text{Π}}_{\text{g}}\text{, 0–2, 1–3)}$.

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Time-resolved T_vib and T_rot under nitrogen and air are both depicted in figure 12. In figure 12(a), T_vib decreases first and then increases in PD (during the time 40–150 ns), and increases obviously in ND (180–240 ns) under nitrogen and air. This indicates that there are obvious energy relaxation processes of electron-to-vibration and vibration-to-vibration in both PD and ND. Figure 12(b) shows that T_rot remains constant in both PD (40–150 ns) and ND (180–240 ns) under nitrogen and air. This reflects that the energy relaxation process of vibration-to-rotation is not obvious in either PD or ND [58, 59]. Therefore, the heating of nanosecond pulse discharge is not obvious during the breakdown of the gas gap.

Figure  12.  Time-resolved (a) vibrational temperature and (b) rotational temperature under nitrogen and air.

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4.   Conclusions

In this study, fast-exposure ICCD images and OES are used to investigate the characteristics of plasma generated in a multi-hollow needle plate packed bed DBD under nitrogen and air. The discharge behavior, the generation mechanism of active species and the energy transfer mechanism are investigated in both PD and ND. Compared with a multi-needle electrode, the discharge intensity, the number of discharge channels and the discharge volume are significantly increased when a multi-hollow needle electrode is utilized. During a single voltage pulse period, PD mainly develops in a streamer mode, the corresponding discharge current, luminous intensity and E/N are stronger than ND, which presents a diffusion-like mode. Besides, in PD, as the gap between dielectric beads changes from 0 to 250 μm the discharge between the dielectric bead gap changes from partial discharge to standing filamentary micro-discharge, which allows the plasma to move from the local area and is conducive to the propagation of surface streamers. In ND, the discharge between the dielectric bead gaps only exhibits a diffusion-like mode, regardless of whether there is a discharge gap. Moreover, the excited states ${\text{N}}_{\text{2}}^{\text{+}}({\text{B}}^{\text{2}}{{ \Sigma }}_{\text{u}}^{\text{+}})$ and ${\text{N}}_{\text{2}}\text{(}{\text{C}}^{\text{3}}{\text{Π}}_{\text{u}}\text{)}$ are only generated in the discharge stage, and mainly in PD. This is attributed to ${\text{N}}_{\text{2}}^{\text{+}}({\text{B}}^{\text{2}}{{ \Sigma }}_{\text{u}}^{\text{+}})$ and ${\text{N}}_{\text{2}}\text{(}{\text{C}}^{\text{3}}{\text{Π}}_{\text{u}}\text{)}$ being mainly generated by electron-impact excitation, and the E/N in PD is higher than that in ND. However, ND is conducive to generating the radical $\text{OH(}{\text{A}}^{\text{2}}{{ \Sigma }}^{\text{+}})$, which reflects that $\text{OH(}{\text{A}}^{\text{2}}{{ \Sigma }}^{\text{+}})$ is not directly dominated by E/N but by the resonant energy transfer process between metastable ${\text{N}}_{\text{2}}\text{(}{\text{A}}^{\text{3}}{{ \Sigma }}_{\text{u}}^{\text{+}}\text{)}$ and $\text{OH(}{\text{X}}^{\text{2}}\text{Π)}$. Furthermore, both PD and ND demonstrate obvious electron-to-vibration and vibration-to-vibration energy relaxation processes, and no vibration-to-rotation energy relaxation process is observed. This study will help to clarify the plasma behavior, the mechanism of active species generation, and the energy transfer mechanisms in both PD and ND, and provide some insights for optimizing the application of multi-hollow needle plate packed bed DBDs in plasma catalysis.

Appendix A. Supplementary data

Supplementary data to this article can be found in the supporting information.