When Ar gas flow of 1.0 slm and microwave pulses are applied on the hairpin resonator, an arched plasma plume can be generated at the opened end of the hairpin resonator. Figure 2 shows a group of discharge images captured by the ICCD camera. The pulsed frequency of 10 kHz, duty ratio of 0.2, and pulsed power of 5 W are maintained. Hence each pulse has a time of with microwave power and without microwave power. As shown in Fig. 2, during the first from 0 to , the arched lightened pattern seems dim, suggesting that it is the afterglow of the previous discharge. In the time of , a light point appears at the tip of the main electrode. At about the time of , the ionization develops to the slave electrode and thereafter the more lightened arched pattern is formed. From to , the thickness of plasma plume grows and the discharge domain stretches backward to both powered electrodes. In the time of , the discharge domains around the metal wires decay, but the forward arched plume becomes stronger in light intensity. After till the next pulse, the discharge comes into the afterglow period and the light intensity damps more and more weakly. In order to check how long the afterglow of microwave Ar plasma could persist, we reset the pulsed frequency of 2 kHz, the duty ratio of 0.1, and the pulsed power of 8 W. Then for each pulse microwave power on time changes to and microwave power off time reaches . Figure 3 shows this group of discharge images. In comparison to Fig. 2, the discharge process and the distinctive formation are similar. It should be mentioned that the afterglow of microwave Ar plasma plume can persist longer than , which is the longest time for the afterglow of Ar APPJ as far as we know.

Fig. 2.

	Figure Option ViewDownloadNew Window
	Fig. 2. (color online) Sequence of discharge images obtained using the ICCD camera with different exposure times. The time spans are indicated in each image. A pulse frequency of 10 kHz, duty cycle of 0.2, peak input power of 5 W, and an argon gas flow of 1.0 slm are used.

Fig. 3.

	Figure Option ViewDownloadNew Window
	Fig. 3. (color online) A group of discharge images obtained by ICCD camera with exposure times of . A pulse frequency of 2 kHz, duty cycle of 0.1, peak input power of 8 W, and an argon gas flow of 1.0 slm are used.

Moreover, in combination with Figs. 2 and 3, the ionization process of pulsed microwave Ar APPJ could be divided into three develop phases, called reigniting, growing up, and finally shrinkage. At the reigniting phase, the microwave power rises higher than 2 W, then the electrons located at the tip of the powered electrode will be accelerated by the local enhanced electric field, and hot electrons collided frequently with Ar atoms will induce discharge. Due to the main electrode having stronger voltage, the ionization occurs at its tip firstly. Then the ionization process enters into the second phase of growing up. The ionization front develops along the previous discharge channel because there is an amount of Ar excitation state atoms and lower plasma density left in the afterglow. Once the arched discharge channel reignites in whole, the ionization domain will further occupy around the powered electrode tips, and meanwhile the forward arched plume grows thicker. In the final phase of shrinkage, the pump power disappears, and the electrons around the metal wire will be cold and be absorbed by electrodes quickly. The forward arched plume continues to be overdense about because it has been heated too long and is with higher energy. In after the microwave power is off, the forward arched Ar APPJ plume damps to very a weak state and waits for the next microwave pulse.

Next, we will continue to experiment on He discharge. Besides using He gas instead of Ar gas, the input parameters (pulsed frequency, duty ratio, microwave power, and gas flow) are set to the same as Ar discharges in order to compare each other. Figures 4 and 5 show the He discharge images, in which their input parameters are the same as those in Figs. 2 and 3, respectively. As shown in Fig. 4, the ionization starts at the inner side between both electrodes but near the main electrode at about . After that time, the ionization front develops to the slave electrode and the discharge channel forms in between both electrodes. From to , the discharge channel becomes thicker. When the microwave pulse passes away, the He lightened discharge channel damps more quickly than Ar discharges besides that its middle domain of the discharge channel disappears at last. As for Fig. 5, the total situation is similar to Fig. 4 except that the reigniting stage is started somewhat slowly because there are little He atoms in the excitation state and lower electron density (shrinking too long).

Fig. 4.

	Figure Option ViewDownloadNew Window
	Fig. 4. (color online) A group of discharge images obtained by ICCD camera with the different exposure times. The discharge conditions are the same as those for Fig. 2 except using He gas instead of Ar gas.

Fig. 5.

	Figure Option ViewDownloadNew Window
	Fig. 5. (color online) Sequence of discharge images obtained by ICCD camera with the different exposure times. The discharge conditions are the same as those for Fig. 3 except using He gas instead of Ar gas.

In comparison with He discharge and Ar discharge, the main difference is that He plumes generate only a contracted plasmas in between both tips of the U-shaped powered electrode, whereas Ar plasmas form an arched plasma plume at the opened end of the hairpin resonator, as shown in Figs. 2–5. Despite this difference, their discharge processes have three similar characteristics: (i) the ionization occurs at the main electrode firstly and then develops to the slave electrode, (ii) during the shrinking stage the middle domains of the discharge channels disappear at last, and (iii) even at zero power input (in between pulses) a weak light region always exists at the discharge channels. The difference in discharge formation might be induced by the interaction between plasma plume and electric field or from the different ionization attributes of Ar/He gasous.^[4,22,31]

For an electromagnetic wave distributed in a transmission line, the input microwave power of 1 W will excite the maximum voltage amplitude of 10 V located at the opened end of the electrode. Without plasma load, the input voltage wave will be reflected from an opened end and should form a standing wave with a maximum voltage amplitude of 20 V.^[22] The total length is three quarters of the wavelength and the discharge device resonates for the proposed hairpin resonator. The maximum voltage located at the opened end of the powered electrodes may get to 200 V.^[27,28] The local enhanced electric field can easily ionize gas discharge. However, once the plasma load appears, the distribution of standing wave changes and the maximum voltage at the opened end loses probably. In order to study how the plasma load influences the distribution of standing wave, simple electromagnetic simulation models are developed by using a high-frequency structure simulator (HFSS, Ansoft). The detailed settings of our models can be referred in previous reports.^{[14,21,34,35]}

Here, we will introduce the equivalent values of plasma parameters, which are input to the simulation model. In simulation, an equivalent dielectric is used to stand for plasma domain. HFSS simulator uses the electrical conductivity σ, relative permittivity , and dielectric loss tangent tan δ. Thence, we should deduce these three parameters from plasma parameters as^[36–38] 1 2 3 where is the imaginary unit, is the electron mass, e is the charge of an electron, ω is the angular frequency of microwave, is the effective electron–neutral collision frequency, is the electron temperature in the unit of eV, is the electron density, and is the angular frequency of plasma electrostatic oscillation. So if we know the values of and , by using Eqs. (1)–(3) , σ, and can be counted, respectively. Our pervious simulation work had calculated step by step the distribution of and in the ionization process.^[31] We select three typical data groups in terms of ( , , like (0.05, 10¹⁷), (0.1, 10²¹), and (0.1, 10¹⁹), representing the different discharge processes of ‘pulse before’, ‘pulse on’, and ‘pulse after’, respectively. Inputting them into Eqs. (1)–(3), another three digit groups in forms of ( , σ, tan δ) can be approximated correspondingly as (1, 0.008, 0.064), (−13.6, 60, 34), and (0.85, 0.6, 5.4). For Ar plasma plume, an arched plasma domain with radius of 0.25 mm is set at the opened end of powered electrodes. As for He plasma, a cylindrical plasma domain with radius of 0.35 mm is arranged in between both electrode tips.

Figure 6 shows the simulated electric field distributions of Ar plasma model. Figure 6(a) gives the discharge process of ‘pulse before’ with the input power of 0.1 W and ( , σ, tan δ) of (1, 0.008, 0.064). The standing wave is distributed along the powered electrodes and the electric fields are enhanced at the metal tips of both electrodes. The electric field located at the main electrode is stronger than that of the slave electrode. Figure 6(b) and 6(c) present the discharge processes of ‘pulse on’, where ( , σ, tan δ) is selected as (−13.6, 60, 34) but the input powers are applied in 3 W and 5 W, respectively. At the pulse rise time about (corresponding to 3 W), the discharge process is in the phase of reigniting, and thence a half of the arched plasma domain is set at the tip of the main electrode. Comparing Fig. 6(b) with Fig. 6(c), the strongest electric field is placed at the up end of the half arched plasma domain as shown in Fig. 6(b), while the end of the main electrode again obtains the maximum value of electric field as shown in Fig. 6(c). Figure 6(d) displays the discharge process of ‘pulse after’ with the input power of 0.5 W and only a forward arched plasma domain with parameter ( , σ, tan δ) of (0.85, 0.6, 5.4). As shown in Fig. 6(d), the standing wave pattern forms due to the plasma density being too weak.

Fig. 6.

	Figure Option ViewDownloadNew Window
	Fig. 6. (color online) Simulated electric field distributions for Ar plasmas with different plasma domains or different plasma parameters.

As for an electromagnetic simulation of the He gas discharge process, we change the location of the plasma domain. Figure 7 shows four kinds of simulated electric field distributions corresponding to the different discharge processes of ‘pulse before’, ‘pulse on’, and ‘pulse after’, respectively. Also the parameters used in Fig. 7 correspond to those used in Fig. 6. Despite their differences in the details, the standing waves distribute along the electrode and the strongest electric fields take place at the opened end of the hairpin resonator.

Fig. 7.

	Figure Option ViewDownloadNew Window
	Fig. 7. (color online) Simulated electric field distributions for He plasmas with different plasma domains or different plasma parameters.

We also sample the simulated electric field distributions in one microwave period. Figure 8 shows the simulated results varied along with the different microwave phases. The plasma parameters used in Fig. 8(a) and Fig. 8(b) are the same as those in Fig. 6(c) and Fig. 7(c) representing Ar discharge and He discharge, respectively. It is interesting to note that a hollow shape of the electric field distributed in the plasma domains is always exhibited in disregard of the microwave phases and gas discharges, as shown in Fig. 8. This character suggests that conduction within the discharge channel is sufficiently strong that the microwave energy can be able to conduct through the plasma jet plume between the powered electrodes. As shown in Fig. 8, another interesting point is at phase 0.25π the enhanced electric field emerging around the plasma domain, which indicates that the standing wave mode located at the plasma domain is the SPPs.^[38] At the transient state of phase 0.25π, the standing wave mode is pure SPPs. We can conceive that at other phase states the standing wave mode would be the hybrid wave of an electromagnetic wave plus the SPPs. The electromagnetic standing wave is driven by the input microwave and locates along the metal electrode of the hairpin resonator, while the standing wave of SPPs merely takes place at the dielectric with negative refractive index like plasma domain as the frequency of plasma electrostatic oscillation is larger than the frequency of pumping wave. Although both waves are with the different attributes, their wave modes belong to transverse magnetic (TM) modes and they can be transformed to each other by wave mode transforming.^[37,38]

Fig. 8.

	Figure Option ViewDownloadNew Window
	Fig. 8. (color online) Simulated electric field distributions with the different microwave phases for (a) Ar plasmas and (b) He plasmas, respectively.

Combining the discharge images with the electromagnetic simulations, the ionization process and the characteristic morphologies of atmospheric Ar and He plasmas should be analyzed. Before discharge, once the input microwave is applied on the hairpin resonator, the standing electromagnetic wave would be formed and the maximum electric field should be located at the opened end of electrodes. Due to the larger gap of 2 mm, in experiment we often need to apply a microwave power higher than 6 W to ignite the discharge. After the igniting started, the applied microwave power could be decreased to a lower value but higher than 2 W, which suggests that the previous discharge can active the latter ionization process. We assume that the influence factor is the afterglow effect. Because of the main electrode with much stronger electric field, the reigniting would easily happen there, which has been verified by experiment as shown in Figs. 2–5 (at about ) and by simulations as indicated in Figs. 6(a) and 7(a). At atmospheric gas pressure, the ionization is governed by streamer theory and the developed electron avalanche performs as an ionization wave (plasma bullet), which has an enhanced static electric field at its head part, called the ionization front.^[1–3] In addition, the proposed hairpin resonator is with three quarters wavelength and both tips of electrodes have maximum voltages with opposite polarities. Thence, the inner domain between both electrodes has much stronger electrodes, which would favor the ionization direction developing from the main electrode to the slave electrode. At about , the discharge channel forms at the opened ends of both electrodes. After that stage, on the one hand the discharge channel becomes thicker; on the other hand the discharge develops backwards to electrodes till to the microwave pulse damping. The discharge development is still induced by the changeable stronger electric field distributions as shown in Figs. 6(c) and 7(c). Once the pumping microwave pulse disappears, the discharge domain around the metal electrodes will decrease quickly because of the thinner sheath region at atmospheric discharges and sheath region damping quickly. The middle domain of the discharge channel shrinks at last also due to this effect in part, and another reason might be the plasma maintained by SPPs for a longer time. There exists wave mode coupling among standing electromagnetic wave, SPPs, and static plasma electron wave.^[37,38] When the input microwave pulse is applied, the order of wave mode transform is from the standing electromagnetic wave to SPPs and thereafter from SPPs to the static plasma electron wave. The electrons accelerated by the electric field of the static plasma electron wave become hot and thereby advance the ionization development. When the input microwave pulse is passed, the transform order might be from the static plasma electron wave to SPPs, then from SPPs to the standing electromagnetic wave and finally wave modes transformation conversely. From the view of energy conservation, the microwave power is mainly consumed by the plasma plume. That is to say, the wave mode coupling can delay the ionization development. This effect can be used to interpret the middle domain of the discharge channels disappearing at last, as shown in Figs. 2–6.

The above discharge processes are the common characteristics of the hairpin resonator despite Ar discharge or He discharge. But as for why they perform different plasma plume patterns, the different ionization and excitation energies and the gas flow effects for Ar and He atoms might be two main affected factors.^[39–41] As is well known, the molecular weight of Ar atom is far more than that of He atom and the effect of Ar gas flow is far greater than that of He gas. In our previous experiment, we have observed that the radian of the arched Ar plasma plume is mainly affected by the gas flow rate.^[31] The develop direction of the ionization front is from the main electrode to the slave electrode, while the gas flow direction is along the axis of the quartz tube. So the arched Ar plasma plume pattern forms. For He plasma, however, it is mainly affected by the direction of strong electric field and the He discharge channel is merely in existence between the inner part of both electrodes. Moreover, the different ionization and excitation energies for Ar and He atoms also affect the formation of discharge morphologies. Actually, the electron density of the hairpin resonator is in the amplitude of 10²¹ m⁻³. Even at its shrinking stage, the electron density is still approximated in the order of 10¹⁹ m⁻³.^[22,31] Under these intense conditions for atmospheric Ar plasma, the Ar* (argon metastable) population would be strongly depleted from the center of the discharge channel due to two-step ionization.^[27] The depletion of Ar* from the center of the discharge channel would induce the reducing of visible emission.^[4,28] In other words, the energy damping used for light emission would be reduced and the lightened arched domain could exist with the weak light region in the discharge channel even at zero power input (in between pulses). For Ar microwave APPJ, Ar* populations located around the discharge channel are more intense,^[40] the Ar* lifetime is longer than , and its afterglow is too intense to shrink immediately in view of space and time, which in all induce the formation of the arched plasma plume morphology. As for He microwave discharge, on the one hand, the plasma shrinking quicker than Ar plasma is due to its ionization energy being much higher than that of Ar atom.^[41] At the shrinking stage, the local enhanced electric field weakens much more quickly to lower than the threshed value that could ionize the He atoms. On the other hand, the He APPJ might not suffer from metastable depletion and the energy damping used for light emission would be more quickly.^[27,42,43] The energetic electrons that can ionize He atoms are only generated in between both tips of the metal electrodes, and thence only a contracted discharge occurs, as shown in Figs. 4–5.