Ionizing Waves in Surface Dielectric Barrier Discharges Plasma Actuators

Round 1

Reviewer 1 Report

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Comments for author File: Comments.pdf

The writing must be formal and impersonal. This is not a report or opinion piece.

Author Response

First referee comments

R1) Extensive editing of English language required.

The writing must be formal and impersonal. This is not a report or opinion piece.

A1) We have rewritten the text, with the help of a English speaking collaborator, avoiding the use of personal and colloquial sentences.

The many modifications are highlighted in the revised text.

R2) The authors state that the term "ionic wind" is inappropriate and use it here. Inconsistent.

A2) We apologize. We added a sentence to comment on use of the term and avoided it in the following.

New line 44) “In particular, the role and the propagation of the ionizing wave, which switches the electrical discharge on, was studied. The wave promotes the formation of plasma filaments and the propagation of the electric currents which sustain the formation of the hydrodynamical flow.” 

New line 20) “This hydrodynamical phenomenon was sometimes referred to, although a bit improperly, as an ionic wind [4]. The term was introduced earlier along with observations made by the effects of the electrical discharges in air [5]. Also the nature of the particles involved in it or causing the phenomenon are not fully understood.”

New ref. 4) Moreau, E.; Sosa, R.; Artana, G. Electric wind produced by surface plasma actuators: a new dielectric barrier discharge based on a three-electrode geometry. Journal of Physics D 2008, 41, 115204.1-12.

New ref. 5) Robinson, M. History of the electric wind. American Journal of Physics 1962,  30, 366-372.

R3) To find out the details of the experiment, the reader has to read 5 different articles? I think this is unreasonable....

A3) We have modified the experimental description, avoiding too much references.

When needed for the experimental findings, we quote them.

Deleted line) “Also the diagnostics that have been developed to study the physical characteristics of the micro-discharges (MD) was presented elsewhere [13-15]”.

New line 60) “The early formation of a vortex flow in the discharge region can also by spotted by using optical measurements based on Background Oriented Schlieren technique [15].”

R4) According to the journal template, the figures should be inserted in the text. This would facilitate interpretation.

A4) We have moved the figures near their citation in the text.

R5) The manuscript should be written in a more formal language. In my opinion, it should not be written in the 1st person plural.

A5) We have rephrased our sentences not using the personal nouns.

R6) The discussion of the results is inadequate and needs to be improved. Otherwise, the document looks more like a report than a scientific article.

Results are mostly described rather than discussed and interpreted according to the literature.

A6) We have added some more sentences to discuss our results

New line 358) “It should be noted that the exact nature of the asymmetry between FD and BD strokes, as well as the their mutual or antagonist contribution to the hydrodynamical effects is yet to be demonstrated, and our findings are somewhat complementary to electron avalanche mechanism already pointed out in literature [2].”

R7) There is no discussion in this section... only conclusions.

A7) As stated above, we have added some more sentences to discuss our results

New line 358) “It should be noted that the exact nature of the asymmetry between FD and BD strokes, as well as the their mutual or antagonist contribution to the hydrodynamical effects is yet to be demonstrated, and our findings are somewhat complementary to electron avalanche mechanism already pointed out in literature [2].”

New line 369) “Mean electron energies or equivalently electric field strengths, during the ionizing wave, are a crucial input for radiative-collisional models [30] or for the modeling of the discharge and its effects, as a whole. Our results can already be used against some approach based on steady plasma conditions.”  

R8) This figure is not mentioned in the thext.

A8) Sorry for the typos, now all the figures are correctly referenced in the text.

Author Response File: Author Response.pdf

Reviewer 2 Report

Please find my comments in the attached file.

Comments for author File: Comments.pdf

Author Response

Second referee comments

R1) English language fine. No issues detected.

R2) The paper is interesting from a scientific point of view, and it is correctly referenced to a current literature. Its experimental part and data analysis is well designed and carried out but there are some confusions which must be explained and corrected or commented. The manuscript is also well written however, the following remarks must be addressed before the manuscript may be published.

R3) Line 20: why do you argue that the “directed airflow in the gas…above the surface” which is induced by the plasma should be not named as “ionic wind”? Please pay attention that you actually use term “ionic wind” in line 43 and then in line 104. Please explain.

A3) As stated above (answer A2 to first referee comments), we added a sentence to comment on use of the term and avoided it in the following.

New line 44) “In particular, the role and the propagation of the ionizing wave, which switches the electrical discharge on, was studied. The wave promotes the formation of plasma fila-ments and the propagation of the electric currents which sustain the formation of the hydrodynamical flow.”

New line 20) “This hydrodynamical phenomenon was sometimes referred to, although a bit improperly, as an ionic wind [3]. The term was introduced earlier along with observations made by the effects of the electrical discharges in air [4]. Also the nature of the particles involved in it or causing the phenomenon are not fully understood.”

R4) Line 40: “relevant for the understanding” – “the” seems to be not required here.

A4) We have corrected the sentence

New line 42) “that are relevant for understanding the link between the discharge characteristics and the induced hydrodynamical phenomena in the surrounding gas.”

R5) Line 51: Rogowski coil (not “Rogoskii”).

A5) Sorry for the typo. We have corrected it.

R6) Lines 69-72: please precisely specify the thickness of the dielectric layers as now the “combined” dimensions are little bit confusing. Probably the best idea would be to include the vital geometrical dimensions in Figure 1.

A6) We have added the relevant dimensions on Figure 1.

New Figure 1)

New line 91) “or slightly shifted, 10 mm apart, as shown in Figure 1, were relevant quotes are reported.”  

R7) Lines 81-98: it is not clear how many optical fibers were actually used to direct light emitted by the MD to the matrix PMT (16 or 4 or less) and how the fibers were actually located over the analyzed region. Were they organized in a single line or as 4x4 matrix? What was their spacing? What was the gap between the dielectric specimen surface and the front of the optical fiber? Please explain and include a photo of the measurement setup detailing the location of all the optical fibers.

A7) We have modified the description to include the relevant details.

New line 114) “As it is shown in Figure 1, the discharge region could be observed through a series of optical fibers, whose viewlines were vertical and could be arranged to look small spot areas on the dielectric surface, hopefully intersecting the pattern of a single discharge filament.”  

R8) Lines 95-98: please provide an exemplary snapshot of PMT output signals recorded by the Infinium MSO-8104 scope. It is not clear how many PMT output signals were actually recorded (therefore it is not clear in how many places the MD filaments were actually analyzed. As the scope has 4 input channels only 4 locations may be analyzed at the same time – how it is related to the total number of 16 anodes in PMT, which is strongly accented in line 98?

A8) We have inserted a new figure with a typical recorded time series. We have also corrected the text, to explain the actual setup that was used in the reported experiments.

New figure 3)

New line 132) “A scope trace is shown in Figure 3 to demonstrate the quality of the measured signals and the absence of cross-talk. Using such instrument only up to four simultaneous viewlines could be recorded.” 

R9) Lines 99-103: please explain how the light collected by the optical fibers (plural suggests that there was more than one optical fiber used) from the discharge was directed to a single-input spectrophotometer (Avantes, AvaSpec-ULS4096CL-EVO)? Was any optical fiber summator used? What was the integration time used to record emission spectrum of a short-lived MD?

A9) We have added a sentence to address this point.

New line 149) “Acquisition times however are in the order of the seconds, so only averaged properties of emission spectra were measured.”

New line 254) “Here the comparison were made using  an optical fiber viewline with a spot area of about 5 mm in diameter, centered at the edge of the exposed electrode.”

R10) Lines 140-142: please clearly indicate where the optical fibers were located on the photo in Figure 3 to show which part of the MD and its filament was actually analyzed. How can you ensure that the “second” optical fiber was actually collecting light from the MD filament originating from the “discharge spot“ (located at the GND electrode rim) observed by the first optical fiber? If you look at Figure 3 it is obvious that two MDs were ignited and they produced a “combined” filament. By the way, what was the exposure time of the photo shown in Figure 3? Such information would help to judge whether the images of the MD filaments are captured as more-or-less stationary or whether they are blurred because they actually wobbled side-to-side during long exposure time.

A10) We have added a sentence to address this point. We have also modified the Figure to show the scale, the position of the electrode edge and the corresponding spots of the viewlines.

New line 149) “Acquisition times however are in the order of the seconds, so only averaged properties of emission spectra were measured.”

New Figure 4) “Figure 4. A snapshot of the discharge region, imaged from above, showing the fil-ament patterns elongating above the dielectric and with a bright spot at the edge of the exposed electrode (indicated by the black line). Also the approximate location of the optical fiber viewline spots is shown.”

R11) Lines 155: “…despite the electrons moves…” – this statement seems to be truncated.

A11) Sorry for the typo.

New line 221) “This demonstrates that the discharge filament evolves as a cathode directed streamer [20], despite the electrons moves towards the anode.”

R12) Figure 4 – it would be probably better to include a graphical legend or to use arrows to help the reader properly distinguishing which set of points illustrates the ionizing wave velocity (and which is related to the measured delay). Providing such information just in the caption may be misleading.

A12) we have modified the Figure

New Figure-5)

R13) Lines 155-156: I do not agree with the statement “The propagation velocity is somewhat controlled and increases with the applied voltage amplitude”. As the ionizing wave velocity is almost doubled thus the increase is significant and moreover, it seems to be linearly proportional to the voltage. Was such linear characteristic observed for the first time or can you provide a literature reference for such phenomenon?

A13) We have modified the statement since it appears not clear.

New line 227) “The propagation velocity appears to increase, roughly linearly, with the applied voltage amplitude. The order of magnitude of the speed agrees with estimates based on cross-correlation spectroscopy and theoretical expectations for such kind of streamers [25,26].”

R14) The manuscript does not clearly state in which geometrical location (in relation to the electrode rim) was the propagation velocity actually measured. Thus, it is interesting to know whether the ionizing wave velocity may be actually dependent on the distance from the discharge origin. Please comment, whether the wave velocity may depend on the location in which it is measured (saying location I mean the distance from the discharge spot producing the filament).

A14) This is true, we have added a sentence to clarify the point.

New line 226) “Actually this velocity corresponds to the average propagation speed over the first 2.4 mm besides the exposed electrode edge.”

R15) What do you man by “the instantaneous voltage” in case of AC voltage used to supply the actuator? “Instantaneous” means “momentary”. So, what is the sense of giving “monetary” value in case of AC variable voltage as it changes in evert time point? Did you mean the amplitude of the AC supply voltage, its mean or RMS value? By the way, was the AC supply voltage sinusoidal?

A15) We have modified the sentence and the Figure since it appears not clear.

New Figure 6)

New line 215) “Figure 6 shows the measured delays and the ionizing wave propagation velocity, for different values of potential difference across the electrodes. This was achieved by recording the PMT signals only during a limited phase portion of the voltage sinusoidal oscillation, using the segmented memory acquisition option of the scope.”

R16) Line 160: Figure 6 should be referenced to, not Figure 5. Moreover, it would be valuable to mark at least the most distinct emission lines of the “N2 second positive system (emissions from the vibrational states of the C3Πu level)” as well as those for “atomic oxygen, from OH radicals and also from nitrogen ions N2+” which are then used for calculations (mentioned in line 174). Another question also arises regarding Figure 6: what is the purpose of those two superimposed emission spectra characteristic for “Teflon” and “Plexiglas”? Please make the spectra separated in order to show if there is any difference between them. And please explain what is the purpose of “Plexiglas” spectrum if plexiglas base material was not in contact with the discharge filament.

A16) ) Sorry for the typos. We have corrected the reference to the Figure.

We modified the figure to include the most relevant emission lines and removed the Plexiglas spectrum, since we choose to not discuss comparison of different dielectric material.

New figure 6) 

R17) Line 163: Figure 7 should be referenced to, not Figure 6.

A17) Sorry for the typos. As stated above, now all the figures are correctly referenced in the text.

R18) Line 169: what OES stands for?

A18) We added a sentence in the Materials and Methods section about it.

New line 64) “Optical Emission Spectroscopy (OES) have been performed too, using optical fibers and a low resolution, low speed UV-Vis spectrometer [11].”

R19) Lines 168-171: Your explanation suggest that the discharge channels turns filamentary only at (or around) 22 kHz AC supply voltage repetition rate. Why just such frequency should be preferred in your plasma actuation system? Is it related to some other system parameters (like its geometry, air pressure etc.)?

A19) We have modified the sentence to avoid confusion.

New line 263) “. This could happens since sometimes the discharge displays a pattern of strong fila-ments and such spatial pattern appears to be somewhat wandering in the direction transverse to the electrodes [28].”   

R20) Line 179: Figure 8 should be referenced to, not Figure 7.

A20) Sorry for the typos. As stated above, now all the figures are correctly referenced in the text.

R21) Line 181: Figure 9 should be referenced to, not Figure 8.

A21) Sorry for the typos. As stated above, now all the figures are correctly referenced in the text.

R22) Line 196: Figure 10 should be referenced to, not Figure 9.

A22) Sorry for the typos. As stated above, now all the figures are correctly referenced in the text.

R23) Lines 199-206: Statements presented in this paragraph, related to the “velocity field pattern” must be supported by at least one figure or other measurement data presentation. Without such data it is not possible to judge whether the presented statements are true and correct (as well as it is made impossible to reproduce such experimental results which is mandatory in case of peer-reviewed scientific paper).

A23) We added a new figure showing the velocity profiles measured at three different downwards position to help understanding of the surface jet flow.

New Figure 12)

New line 157) “Some more defined vertical profile were measured using a capillary of 0.1 mm diameter.”

New line 318) “The jet was tracked downstream up to 25 mm away from the exposed electrode edge and its vertical profile and extension was measured and found to be quite independent of the downstream position, as it is shown in Figure 12.”

Author Response File: Author Response.pdf

Reviewer 3 Report

The work is devoted to experimental investigation of ionization wave propagation in surface dielectric barrier discharge (SDBD). The authors have measured its propagation velocity and electric field. The work is continuation of the author’s research topic and contribute to fundamental description of SDBDs. Unfortunately some parts of the manuscript were unclear for me and I recommend to revise the manuscript for resolving number of confusion described below.

Major comments

1) Figure 1 is very similar to figure 1 from [15], I would suggest to say in the figure caption that it was adopted from [15]. However, in [15] the electrodes are co-aligned and in present manuscript a gap between electrodes is shown. I did not found a description of the gap and the used geometry is unclear.  

2) In [15] the reported velocity of the ionizing wave is 220km/s. In the current manuscript reported 100-200 km/s. The difference between the experiments or their similarity should be formulated and both results should be discussed.  

3) The meaning of the figure 3 is unclear. It looks that it does not account the charge accumulated on dielectric surface and presents the potential and electric field before discharge ignition. Thus, this figure do not have any relation to the other results of the manuscript. During a steady-state SDBD operation this characteristics should look differently.

4) The reported reduced electric field should be compared with discharge breakdown electric field. 

5) The reduced electric field was estimated from peak intensities of the molecular bands. The determination of the peak intensities is ambiguous and should be demonstrated in a figure, which explains: a) background subtraction, b) which pixels were used for the intensity measurements (is a fixed pixel used or maximum was determined separately for each experiment?), c) influence of intensity overlap with other molecular bands.    

6) Numeration of the figures is completely wrong that makes reading difficult. 

7) Figure 9 (energy level diagram) is confusing, ground state of a molecule should be X, also two levels B is drown. In addition, this diagram do not help to explain that time-average measurements are not adequate. This explanation in the manuscript is confusing, however I agree with the conclusion. The manuscript do not develop the collisional radiative model, but use results of other works. It is better to show a plot of final curve, which connects intensity ratio and electric field, and make a proper reference.  

Minor comments. 

1) First sentence of introduction should also refer original work of Roth, probably:Roth J R, Sherman D M and Wilkinson S P 2000 AIAA J.38 1166

2) Line 163, figure 10 ? (there are more mismatched figures)

3) Lines 165-167, why not to show intensities normalized on discharge frequency?

4) Lines 191-192, confusing, please mention some other application for SDBD

5) Figure 2. Electrode positions should be better shown

6) Figure 4. Time pulses??? -> emission/light pulses recorded over time

7) Figure 7. Some emission lines??? Please state wavelength. It is very unfortunately to detect lines with low resolution spectrometer used in the work. Intensity at xxx nm should be stated instead, and can be added that it belongs to a certain molecular band.  

8) Concerning determination of electric field a following article should be mention: Mrkvičková et al Plasma Sources Sci. Technol. 32 (2023) 065009 (21pp) https://0-doi-org.brum.beds.ac.uk/10.1088/1361-6595/acd6de

Author Response

Third referee comments

The work is continuation of the author’s research topic and contribute to fundamental description of SDBDs. Unfortunately some parts of the manuscript were unclear for me and I recommend to revise the manuscript for resolving number of confusion described below.

R1)  Figure 1 is very similar to figure 1 from [15], I would suggest to say in the figure caption that it was adopted from [15]. However, in [15] the electrodes are co-aligned and in present manuscript a gap between electrodes is shown. I did not found a description of the gap and the used geometry is unclear. 

A1) As stated above, we have modified Figure 1 adding dimensions. We also modified the text to explain better the lay-out.

New Figure 1)

New line 90) “The two electrodes thus lie parallel to each other where the edges just corresponding in order to avoid any overlap or slightly shifted, 10 mm apart, as shown in Figure 1, were relevant quotes are reported.”

New line 190) “The buried electrode edge corresponds exactly to this line in these experiments.”

R2) In [15] the reported velocity of the ionizing wave is 220km/s. In the current manuscript reported 100-200 km/s. The difference between the experiments or their similarity should be formulated and both results should be discussed. 

A2) We have added a sentence about it. However in our paper [15] we used a single PMT tube and the signal of two viewlines was combined together. Also the experimental conditions are similar but not exactly the same.

New line 230) “It also agrees with our previous results obtained with a single PMT and combining light of different optical fiber viewlines [14].”

R3) The meaning of the figure 3 is unclear. It looks that it does not account the charge accumulated on dielectric surface and presents the potential and electric field before discharge ignition. Thus, this figure do not have any relation to the other results of the manuscript. During a steady-state SDBD operation this characteristics should look differently.

A3) We agree. We have added a sentence to comment on this point.

New Line 99) “However, it is also noteworthy that the charge transported by the discharges and ac-cumulated on the dielectric surface will distort this pattern and the actual size and shape of current filaments cannot be predicted. Once discharge is ignited, as it will be discussed below, microdischarges  happens in two phase portions of the sinusoidal cycle, during which the intensity of the external applied voltage changes even strongly.”

R4) The reported reduced electric field should be compared with discharge breakdown electric field.

A4) We have added a sentence to comment on this point.

New Line 293) “To put it in perspective, this is about five times the dielectric strength of air at stp.”

R5) The reduced electric field was estimated from peak intensities of the molecular bands. The determination of the peak intensities is ambiguous and should be demonstrated in a figure, which explains: a) background subtraction, b) which pixels were used for the intensity measurements (is a fixed pixel used or maximum was determined separately for each experiment?), c) influence of intensity overlap with other molecular bands.   

A5) ] We have added two sentences, to clarify the point. As we used a formula published by other researcher, we believe that a separate figure (now there are12)  is unneeded.

New line 291 ”, where a formula was reported connecting the intensity ratio to the field strength.”

New line 274) “Line intensity ratio was calculated from both the peak maximum and area (after back-ground subtraction), which agrees, as in our low resolution instrument the line shape depends on the spectrometer rather than on the specific emission line.”    

R6) Numeration of the figures is completely wrong that makes reading difficult.

A6) Sorry for the typos. As stated above, now all the figures are correctly referenced in the text.

R7) Figure 9 (energy level diagram) is confusing, ground state of a molecule should be X, also two levels B is drown. In addition, this diagram do not help to explain that time-average measurements are not adequate. This explanation in the manuscript is confusing, however I agree with the conclusion. The manuscript do not develop the collisional radiative model, but use results of other works. It is better to show a plot of final curve, which connects intensity ratio and electric field, and make a proper reference. 

A7) We separated in the diagram the excitation from neutral nitrogen molecules and that from (nitrogen molecular) ions, which however can be created only after ionization, which involve even larger energy transfer.

New Figure 10)  

R8) First sentence of introduction should also refer original work of Roth, probably:Roth J R, Sherman D M and Wilkinson S P 2000 AIAA J.38 1166

A8) We have added this reference, although this journal is more difficult to access.

New ref.2)

R9) Line 163, figure 10 ? (there are more mismatched figures)

A9) Sorry for the typos. As stated above, now all the figures are correctly referenced in the text.

R10) Lines 165-167, why not to show intensities normalized on discharge frequency?

A10) We tried by most on the lines at 20 and 30 KHz now are superimposed. So we left the vertical scale unchanged. The fictional line 1.5x multiplied should help to judge the effect of frequency normalization.

R11) Lines 191-192, confusing, please mention some other application for SDBD

A11) We have slightly enlarged the scope of our sentence.

New line 306) “As a final remark, it could be also pointed out that our device can function as a plasma actuator, among other applications which benefit from the hydrodynamical effects associated with such plasmas  [32].”

R12) Figure 2. Electrode positions should be better shown.

A12) We have modified Figure 2, highlighting the electrodes.

New Fig 2)

R13) Figure 4. Time pulses??? -> emission/light pulses recorded over time

A13) We have modified the caption. With the new Figure 3, it should be clear how pulses appear.

New Figure 5 Caption) “between the time arrival of pulse signals recorded by two channels of the PMT”

R14) Figure 7. Some emission lines??? Please state wavelength. It is very unfortunately to detect lines with low resolution spectrometer used in the work. Intensity at xxx nm should be stated instead, and can be added that it belongs to a certain molecular band.

A14) We have added the emission line wavelengths to the caption

New Figure 8 Caption) “Figure 8. Intensity of some emission lines of the second positive system of nitro-gen (black, λ=315.9 nm (v-vf)=(1-0) transition; blue, λ=375.5 nm (1-3); red, λ=399.8 nm (1-4)) as a function of the discharge amplitude and repetition frequency” 

R15) Concerning determination of electric field a following article should be mention: Mrkvičková et al Plasma Sources Sci. Technol. 32 (2023) 065009 (21pp) https://0-doi-org.brum.beds.ac.uk/10.1088/1361-6595/acd6de

A15) We have added this new reference and a sentence.

New line 292) “A recent paper discussing measurements of electric field strength can also be quoted [31].”

New ref. 31) “Mrkvičková, M.; Kuthanová, L.; Bílek, P.; Obrusník, A.; Navrátil, Z.; Dvořák, P.; Adamovich, I.; Šimek, M.;  Hoder, T. Electric field in APTD in nitrogen determined by EFISH, FNS/SPS ratio, α-fitting and electrical equivalent circuit model, Plasma Sources Sci. Technol. 2023, 32, 065009.”

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Revision of actuators-2795839_Round#2

First of all, I would like to thank the authors for accepting the recommendations and making an effort to really improve the quality of the manuscript.

However, I think you can and should make one change to make the work even better.

Please consider removing the discussion from the Conclusions section and moving it to the Results section. I think this makes more sense, is more explicit, and is more in line with the Actuators journal standard. Keep the Conclusions section only for the specific observations of this work.

In this sense, I recommend a Minor Revision.

Comments for author File: Comments.pdf

In general, it is ok.

Author Response

First referee comments

R1) First of all, I would like to thank the authors for accepting the recommendations and making an effort to really improve the quality of the manuscript. However, I think you can and should make one change to make the work even better. Please consider removing the discussion from the Conclusions section and moving it to the Results section. I think this makes more sense, is more explicit, and is more in line with the Actuators journal standard. Keep the Conclusions section only for the specific observations of this work. In this sense, I recommend a Minor Revision.

A1) We have moved the discussion of the results in the section 3 and changed the section titles.

Reference numbers have been rearranged to reflect the new order of the text.

New line 241) “It should be noted that the exact nature of the asymmetry between FD and BD strokes, as well as the their mutual or antagonist contribution to the hydrodynamical effects is yet to be demonstrated, and our findings are somewhat complementary to electron avalanche mechanism already pointed out in literature [2].”

New line 297) “These values can be used also as input to develop suitable discharge models aimed at obtaining an energy transfer to the neutral molecules and to the development of hy-drodynamical effects. Mean electron energies or equivalently electric field strengths, during the ionizing wave, are a crucial input for radiative-collisional models [30] or for the modeling of the discharge and its effects, as a whole. Our results can already be used against some approach based on steady plasma conditions [32].”

New line 354) “It could be underlined that the configuration was kept as simple as possible in or-der to focus more on the properties of the discharges rather than on the optimization of the hydrodynamics efficiency. However, the produced flow will affects the bounda-ry layer above the free surface downstream of the device, where the hydrodynamical effects are more useful, as needed by a sensible plasma actuator. We also recall that our supply system, based on driving the resonant circuit built of the electrodes system and the secondary of a HV transformer, allows to change frequency and voltage, but cannot control the microdischarges properties or the hydrodynamical efficiency, that are be-yond the scope of our experiments [34].”

New line 160) Section heading: “3. Presentation and discussion of results”

New line 364) Section heading: “4. Conclusions”

Author Response File: Author Response.pdf