During electrical breakdown, two distinct operation modes, namely a diffuse mode and a concentrated mode, were observed with the rising applied voltage. Figure 57(a)–(d) present the images of the He/MMA DBD jet with variations on the discharge appearance as the applied voltage is changed. Immediately after the DBD jet is ignited a discharge with diffuse color (diffuse mode) is observed, as shown in Figure 57(a). Similar appearance of a DBD jet with uniform color distribution has been reported in several studies [98, 108]. As the applied voltage is raised, the central axis of the
discharge begins to be brighter than the outer parts of the discharge near the tube wall, as presented in Figure 57(b). The discharge at this point is in transition from the diffuse mode to the concentrated mode. The concentrated mode can be further divided into two types based on the discharge position; that is, centered concentrated mode and off- centered concentrated mode. Figure 57(c) displays the centered concentrated operation mode of the discharge by a sufficient increase of the applied voltage. The discharge appears to be a thin bright line located in the middle of the dielectric tube due to the contraction of the discharge in the radial direction. Further increase of the applied voltage results in the discharge movement from the tube centerline to the inner wall of the tube (off-centered concentrated mode), as can be seen in Figure 57(d). In this mode the direction in which the concentrated discharge moves is random and adjustment of the applied voltage can make the discharge shift to other positions on the tube inner wall. The reason for the variation on discharge position is yet unclear.
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Figure 57. Types of discharge appearance using a digital camera with 4-s exposure time (a) in diffuse mode, (b) during transition, (c) in centered concentrated mode, and (d) in off-centered concentrated mode.
Figure 58 shows the spatial light intensity profiles of the discharges for the various operation modes. To avoid the saturation of the CCD pixels in the camera, shorter exposure time (1/15 s) was employed to acquire the discharge images for
intensity estimation instead of the 4-s exposure time used for Figure 57(a)–(d). A region in the images extending from the dielectric tube exit to 0.5 mm downstream were used to calculate the intensity profiles as the reflection and refraction light from the dielectric tube influence the actual discharge intensity estimation inside of the tube. The intensity profile was obtained by taking an average of the grayscale values for each pixel and then calculating the mean value of each column of pixels. It can be seen from Figure 58 that the intensity profile is narrower and has a greater peak value when the operation mode changes from the diffuse mode to the concentrated mode. The off-centered concentrated discharge is observed to be located at –1.4 mm in this case.
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Figure 58. Intensity profiles of the discharge images at different operation modes.
Both the He DBD jet and He/MMA DBD jet have the mode change in their discharge appearance but visually with slightly different colors. The mode change in the discharge was found to significantly depend on several parameters, such as the precursor type, the precursor concentration, the position of the powered electrode, the nozzle-to- slide distance, the dimension and the material used in the dielectric tube, the parasitic capacitance in the circuit and the conductivity of the material placed on the holder. The occurrence of the off-centered concentrated mode is especially sensitive to these
parameters. It was found that the off-centered concentrated mode can occur directly after transition without occurrence of the centered concentrated mode when different gap distance, precursor concentration, or various experimental parameters are employed.
The He DBD jet and the He/MMA DBD jet show different relationships between the root-mean-square (RMS) applied voltage and the discharge power though similar
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operation modes were observed in both of them. It can be seen from Figure 59 that the He DBD jet requires lower applied voltage (RMS voltage of ~2 kV) for discharge breakdown, compared to the 2.5-kV RMS voltage required in the He/MMA DBD jet. This result indicates that precursors in a He DBD jet increase the required electric field for discharge breakdown, as might be expected. Additionally the different operation modes in both cases result in distinct slope changes in the data lines. As can be observed in the He DBD results, in the diffuse mode the power appears to increase exponentially with the RMS voltage up to around 1.3 W. It then exhibits nearly linear growth during transition and its slope increases to about 5 W/kV in the concentrated mode. Dashed trend lines along with their fit equations (obtained by a least squares method in Matlab) are shown on Figure 59 to assist in identifying the mode changes. These results imply that the electrical conductivity of the discharge in the He DBD jet is altered by the mode change. The concentrated mode has higher electrical conductivity than the diffuse mode. The trend (two distinct slope changes) in the He DBD jet is repeatable and similar to that observed by Walsh et al. although a metallic substrate was used in their study [108]. A slope change in the data curve is also observed in He/MMA DBD jet. Compared to the case in the He DBD jet, in the diffuse mode in the He/MMA DBD jet the data line increases linearly with a slope of around 0.6 W/kV, although in the transition and the concentrated modes, it has a slope comparable to 5 W/kV. Similar steep inclines of the data lines in the centered and off-centered concentrated modes are seen in both the He DBD jet and the He/MMA DBD jet. It may indicate that the electrical conductivity of the discharge shows no significant dependence on the position of the concentrated
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discharge. The horizontal error bars show the variation on the RMS voltages in multiple measurements when the discharge powers were set to those desired values. It should be reiterated that the mode change is very sensitive to the experimental parameters
mentioned previously. Therefore, the lines for differentiation between modes in Figure 59 may be located at different positions if some parameters are altered. The results shown in Figure 59 were measured immediately after the dielectric tube was cleaned (carefully controlled electrode surface properties) and the bubbler for the MMA monomer was refilled to a preset level (corrected the minor variations in precursor concentration).
Figure 59. Discharge powers in the He DBD jet and the He/MMA DBD jet as a function of the RMS applied voltages.
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Figure 60(a)–(d) show the waveforms of the He DBD jet at the powers of 0.7, 1.7, 3.0, and 6.0 W, respectively. At 0.7 W in the diffuse mode [Figure 60(a)], sporadic ignition is observed in each cycle, i.e., not every cycle has one current pulse in this mode. It should be noticed that the positive half cycle always has a more distinct current pulse than the negative half cycle likely due to the asymmetric electrode geometry in our DBD jet. As the applied voltage is increased, the probability of occurrence of the current pulses grows. When the discharge enters the transition to the concentrated mode, almost every positive and negative cycle has a current pulse but with different magnitude, as shown in the case at 1.7 W in Figure 60(b). Current pulses with similar magnitude are then observed when the discharge power keeps rising and the breakdown is seen to be periodic at the driving frequency. After the operation mode switches to the concentrated mode with the rising power, the duration of current pulses in the positive half cycles appears to be wider than those in the diffuse mode, as displayed in Figure 60(c) with a power of 3.0 W. Also, the current pulses in the negative cycles become comparable in magnitude. A power of 6.0 W in the He DBD jet leads to the off-centered concentrated mode, and its current waveform [Figure 60(d)] shows greater amplitude and longer duration in the current pulses, compared to those at a lower power.
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Figure 60. Voltage and current waveforms of He DBD jet at (a) 0.7 W, (b) 1.7 W, (c) 3.0 W, and (d) 6.0 W.
The voltage and current waveforms with identical power settings in the He/MMA DBD jet are given in Figure 61(a)–(d). Different current waveforms are observed in this case. At 0.7 W in the diffuse mode [Figure 61(a)], one current pulse is seen within each positive half cycle. However, one or two current pulses appear in each negative half cycle. Different from the case in the He DBD jet, these current pulses in the negative
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cycle are comparable to those in the positive cycle in the He/MMA DBD jet. When the discharge power is increased and the He/MMA DBD jet enters the transition condition, second pulses appear in the positive half cycles sporadically. Besides, the probability of occurrence of the second pulses in the negative half cycles grows with the rising voltage and finally two current pulses with different magnitude are observed in each negative half cycle with sufficient applied voltage. One example at a power of 1.7 W during the transition can be seen in Figure 61(b). As the power rises, similar to the pulses in the negative half cycles, the second pulses occur in the positive half cycles more frequently. Eventually two strong pulses in every positive half cycle are always observed when a relatively high power is applied and the discharge switches to the concentrated mode, as shown in Figure 61(c) with a power of 3.0 W. Also, in this case every negative cycle has three distinct current pulses. Further increase of discharge power can generate the third pulse in each positive half cycle. Figure 61(d) shows three strong current pulses in both the positive and negative cycle at 6.0 W. From these results, it can be concluded that in both cases (He DBD jet and He/MMA DBD jet) the higher applied voltage results in the longer duration of discharge breakdown in each half cycle. Additionally, in the
He/MMA case, it is seen that the number of pulses in the negative half cycle grows faster than that in the positive cycle. There is a stochastic nature to the breakdown in transitional modes nonetheless the average behavior and observable trends are repeatable.
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Figure 61. Voltage and current waveforms of He/MMA DBD jet at (a) 0.7 W, (b) 1.7 W, (c) 3.0 W, and (d) 6.0 W.
For a better understanding of the ‘structure’ of the discharge current pulses and the evolution of the current waveforms with respect to the power, the current waveforms over one complete cycle at various powers in Figure 60 and Figure 61 are extracted and re-plotted in Figure 62 for comparison.
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Figure 62. Evolutions of current pulses with rising power in (a) He DBD jet and (b) He/MMA DBD jet; the magnified current pulse which composed of small pulses (c) in He DBD jet at 1.7 W and (d) in He
DBD jet at 3.0 W in the concentrated mode with raised current edge.
In this figure, the zero-crossing point from negative voltage to positive voltage is located at the time of 0 µs, whereas the point from positive voltage to negative one is at the time of around 17.5 µs, as indicated by a dashed line. It can be clearly seen that the current pulses in the He DBD jets consist of several short-duration (1–2 µs) pulses in the
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positive half cycle though visually only one strong broad pulse (4–16 µs) appears on it, as shown in Figure 62(a). As the discharge power is increased from 0.7 W (diffuse mode) to 1.7 W (transition), the number of the relatively small (short-duration) pulses, which overlap to form the strong current pulse in the positive half cycle, increases. This further prolongs the pulse duration, Δt, as illustratively shown in Figure 62(a). Δt is determined by the time interval between the breakdown initiating time t0 [when the |dI/dt|>200 (A/s)] and the pulse disappearance time when the |dI/dt|<200 (A/s). A magnified waveform of the current pulse at 1.7 W is shown in Figure 62(c). As can be observed, after the greatest peak appearing at 6.8 µs, there are two relatively small peaks following it at 7.2 and 8.2 µs, respectively. It shows that a strong current pulse can be deconvolved into several short-duration pulses, and each pulse may correspond to a breakdown. To be clear, we refer to the strong current pulses as the “broadened current pulses”. It should be mentioned that at a fixed power not all current pulses at each half positive cycle contain the same numbers of short-duration pulses, i.e., some broadened pulses contain more peaks but some have fewer. The detailed breakdown phenomenon will be presented in the later subsections by using ICCD and PMT. As the discharge power rises, more small pulses following the most intensive peak occur, leading to a broadened pulse with longer duration. However, after the discharge in the He DBD jet switches to the concentrated mode, not only more small pulses in the positive half cycle are observed, but also the front edge of the broadened pulse is raised before the
occurrence of those short-duration pulses. A clear broadened current pulse in the concentrated mode can be seen in Figure 62(d), which is an enlargement of the case at
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3.0 W in Figure 62(a). It should be emphasized that the ‘raised front edge’, which is an initiating breakdown shown in the later subsections, is only observed in the concentrated mode in the He DBD jet, and not for the He/MMA case. Further increase of the
discharge power results in more subsequent short-duration pulses, such as the case at 6.0 W in Figure 62(a). Additionally, the initiating time (t0) of breakdown are seen to be shorter with the rising power. Note that the raised front edges begin in the negative half cycles of voltage (t0<0) when the power of 6.0 W is used in the He DBD jet. Similar to those DBD devices, the ‘early breakdown’ (t0<0) at higher power is due to the fact that more residual charges from the previous half cycle are present on the dielectric surface. Thus lower voltage potential difference is required to induce breakdown. The initiating time (t0) of breakdown and the broadened pulse duration (Δt) per each half cycle at various powers are summarized in Table 5.
Table 5. Initiating time (t0) of breakdown and the duration time (Δt) of the broadened current pulses during both positive and negative half cycles at various powers in He DBD jet
Power (W) Positive half cycle Negative half cycle
t0 (µs) Δt (µs) t0 (µs) Δt (µs)
0.7 7.0 4.0 18.8 8.2
1.7 5.2 4.8 18.0 11.0
3.0 1.6 11.9 16.7 15.3
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When the MMA is added to the He DBD jet, the current pulses in the He/MMA DBD jet evolve in a different way [Figure 62(b)], as compared with the He DBD jet. Each broadened current pulse in this case is similar to those in the He DBD jet during the diffuse mode and the transition as given in Figure 62(c). It is also composed of several small pulses (will be shown in the later results) though in this case with MMA no distinct subsequent short-duration current pulse can be observed. However, the current pulse in the He/MMA DBD jet is observed to have shorter duration (~1.9 µs at 0.7 W) than that of the pulse in the He DBD jet (~4.0 µs at 0.7 W). Also, a rising power leads to the generation of multiple separated broadened current pulses per half cycle instead of the increasing number of the subsequent short-duration pulses in every single broadened pulse. Thus it can be seen that every broadened pulse occurring in the He/MMA DBD jet has no significant difference between their pulse widths (about 2–4 µs) regardless of the power. Like the case in the He DBD jet, the initiating breakdowns (or the first broadened current pulses in this case) move toward the voltage zero with the rising discharge power, and the time between the broadened current pulses also decreases. When two broadened pulses in the positive half cycle are close to each other, the first current pulse is found to usually have weaker amplitude than the second one, as shown in the cases of 3.0 W and 6.0 W in Figure 62(b). However, the reason is yet unclear. In the negative half cycle, the number of the strong current pulses grows as the discharge power rises but the time between the pulses always remains short (~0 µs, i.e., successive negative current pulses). Similarly, summarized parameters of the broadened current pulses in the He/MMA DBD jet are given in Table 6.
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Table 6. Initiating time (t0) of breakdown and the duration time (Δt) of the broadened current pulses during both positive and negative half cycles at various powers in He/MMA DBD jet; the notations Δt1,
Δt2 and Δt3 represent the duration time of the first, second and third current pulse, respectively
Power (W) Positive half cycle Negative half cycle
t0 (µs) Δt1 (µs) Δt2 (µs) Δt3 (µs) t0 (µs) Δt1 (µs) Δt2 (µs) Δt3 (µs)
0.7 3.9 1.9 - - 20.9 2.0 - -
1.7 2.0 1.9 2.9 - 19.0 2.1 3.5 -
3.0 1.2 2.8 3.8 - 18.6 2.5 3.5 3.4
6.0 0 2.6 3.8 3.8 16.7 3.6 3.3 5.3