Effect of a Femtosecond-Scale Temporal Structure of a Laser Driver on Generation of Betatron Radiation by Wakefield Accelerated Electrons

Round 1

Reviewer 1 Report

This paper reports new results of interest for novel techniques of radiation generation. The investigated range of parameters can be reached in the near future, and the studied effects are new. I recommend this work for publication. However, the presentation of the results needs improvement:

Ref.12 is misspelled (wrong article number)

The palette used for density maps is counterintuitive (darker colors for lower densities) and therefore difficult to understand.

Some fonts in figures are too small and difficult to read. The size of the characters should be comparable to the size of the main text.

“A distinct electron channel” is not visible in Fig.4. Perhaps a couple of pixels is different from Fig. 1, but the effect is not clearly presented. There are fewer problems of understanding in Fig.5, because the prepulse is at least visible. The palettes and figure sizes need further optimization, perhaps by reducing the number of time slices.

The language could be improved, e.g., by comparing it to an automatic translator.

Author Response

We thank the reviewer for the comments. Below are our responses to them. We also marked up the significant changes made in the manuscript with red.

Ref.12 is misspelled (wrong article number)

The reference has been fixed.

The palette used for density maps is counterintuitive (darker colors for lower densities) and therefore difficult to understand.

We changed color palettes so that now the darker colors refer to higher densities.

Some fonts in figures are too small and difficult to read. The size of the characters should be comparable to the size of the main text.

We have redrawn some figures to increase font size.

“A distinct electron channel” is not visible in Fig.4. Perhaps a couple of pixels is different from Fig. 1, but the effect is not clearly presented. There are fewer problems of understanding in Fig.5, because the prepulse is at least visible. The palettes and figure sizes need further optimization, perhaps by reducing the number of time slices.

To improve visibility of the discussed electron channels we have added a new Fig. 6 and corresponding comments in the text. This figure shows lineouts of electron density in two cross-sections.

The language could be improved, e.g., by comparing it to an automatic translator.

We actually used the automatic translator from Google when preparing the text. Now we also checked it with Writefull, which, we hope, improved the quality of the text.

Reviewer 2 Report

INTRODUCTION

-Page 1, line 9: “… applications [7–9]. Among them …” I would recommend to replace by “… applications [7–9]. A compact source of directed radiation 13.5 keV based on an inductive discharge with the formation of an electron beam by a double electric layer was studied experimentally in [A1-A3]. Among them …”.

 

A1. Borgun, I.V.; Azarenkov, N.A.; Hassanein, A.; Tseluyko, A.F.; Maslov, V.I.; Ryabchikov, D.L. Double electric layer influence on dynamic of EUV radiation from plasma of high-current pulse diode in tin vapor. Physics Letters, Section A: General, Atomic and Solid State Physics 2013, 377(3-4), p. 307–309 . https://engineering.purdue.edu/CMUXE/Publications/AH/PLA13-IVB.pdf.

A2. Tseluyko, A.F.; Lazurik, V.T.; Ryabchikov, D.L.; Hassanein, A.; Maslov, V.I.; Borgun, I.V.; Sereda I.N. Influence of plasma nucleus form on radiation orientation in high-current pulse plasma diode Problems of Atomic Science and Technology, 2010, 6, 176–178. https://vant.kipt.kharkov.ua/TABFRAME.html.

A3. Tseluyko, A.F.; Lazurik, V.T.; Ryabchikov, D.L.; Maslov, V.I.; Azarenkov, N.A.; Yunakov, N.N.; Makienko A. A. Dynamics and directions of extreme ultraviolet radiation from plasma of the high-current pulse diode. Problems of Atomic Science and Technology, 2009, 1, 165–167. https://vant.kipt.kharkov.ua/TABFRAME.html.

 

-Page 2, line 10 from below: “…energy. Thus …” I would recommend to replace by “…energy. It was shown in [B1] that the complex temporal structure of a pulse consisting of several pulses, i.e. the pulse with the precursor led to the fact that a very small bunch is formed and accelerated under these conditions, which, of course, suppresses the betatron radiation. Thus …”.

 

B1. Maslov, V.I.; Bondar, D.S.; Grigorencko, V.; Levchuk, I.P.; Onishchenko, I.N. Control of characteristics of self-injected and accelerated electron bunch in plasma by laser pulse shaping on radius, intensity and shape. Problems of Atomic Science and Technology, 2019, 124(6), 39–42. https://vant.kipt.kharkov.ua/TABFRAME.html.

 

The results on this paper are very interesting and important, which are useful to publish after the removal of comments.

Author Response

We thank the reviewer for the comments. Below are our responses to them. We also marked up the significant changes made in the manuscript with red.

-Page 1, line 9: “… applications [7–9]. Among them …” I would recommend to replace by “… applications [7–9]. A compact source of directed radiation 13.5 keV based on an inductive discharge with the formation of an electron beam by a double electric layer was studied experimentally in [A1-A3]. Among them …”.
A1. Borgun, I.V.; Azarenkov, N.A.; Hassanein, A.; Tseluyko, A.F.; Maslov, V.I.; Ryabchikov, D.L. Double electric layer influence on dynamic of EUV radiation from plasma of high-current pulse diode in tin vapor. Physics Letters, Section A: General, Atomic and Solid State Physics 2013, 377(3-4), p. 307–309 . https://engineering.purdue.edu/CMUXE/Publications/AH/PLA13-IVB.pdf.
A2. Tseluyko, A.F.; Lazurik, V.T.; Ryabchikov, D.L.; Hassanein, A.; Maslov, V.I.; Borgun, I.V.; Sereda I.N. Influence of plasma nucleus form on radiation orientation in high-current pulse plasma diode Problems of Atomic Science and Technology, 2010, 6, 176–178. https://vant.kipt.kharkov.ua/TABFRAME.html.
A3. Tseluyko, A.F.; Lazurik, V.T.; Ryabchikov, D.L.; Maslov, V.I.; Azarenkov, N.A.; Yunakov, N.N.; Makienko A. A. Dynamics and directions of extreme ultraviolet radiation from plasma of the high-current pulse diode. Problems of Atomic Science and Technology, 2009, 1, 165–167. https://vant.kipt.kharkov.ua/TABFRAME.html.

We don’t think that these references are relevant to our research. First, they discuss one specific method of UV generation whereas there are plenty of them and there is nothing specifically remarkable about this one. Second, this method of generation is quite different from what we discuss and they cannot be compared. And third, it seems to aim to a specific application, namely, UV-lithograpy whereas the source investigated in our paper is not suitable for this particular application.

-Page 2, line 10 from below: “…energy. Thus …” I would recommend to replace by “…energy. It was shown in [B1] that the complex temporal structure of a pulse consisting of several pulses, i.e. the pulse with the precursor led to the fact that a very small bunch is formed and accelerated under these conditions, which, of course, suppresses the betatron radiation. Thus …”.
B1. Maslov, V.I.; Bondar, D.S.; Grigorencko, V.; Levchuk, I.P.; Onishchenko, I.N. Control of characteristics of self-injected and accelerated electron bunch in plasma by laser pulse shaping on radius, intensity and shape. Problems of Atomic Science and Technology, 2019, 124(6), 39–42. https://vant.kipt.kharkov.ua/TABFRAME.html.

This paper seems to be relevant to our research so we added reference to it.

Reviewer 3 Report

The paper ‘Effect of a femtosecond-scale temporal structure of a laser driver on generation of betatron radiation by wakefield-accelerated electrons’ addresses one crucial problem related to the use of short laser pulses produced by non-linear self-phase modulation in a thin plate. In particular, the complicated time structure, including pedestal, pre, and post-pulse, is investigated to find the best brightness for a betatron source.

The topic is timely, the paper is scientifically sound and well-written, and it deserves publication.

However, there is a minor problem that I would like to see addressed and better explained before I can recommend the paper for publication.

Some parts of the paper are very general. There are several scaling laws. I like this part. However, at a certain point, the focus is only on 1.5 PW and 11 fs, as the parameters in reference 23. It is a reasonable choice, and the results are very interesting, without a doubt. But the author must clearly state, maybe in the abstract, that their analysis is not general but focused on a particular case.

But the reader, like me, can have a simple request:  To have a comparison plot of the betatron radiation with and without laser compression. The authors investigated the effect of a0, the prepulse, the post-pulse, and even the pedestal only when there is compression. But if we do not compress the pulse, which is the spectrum of the betatron radiation, can we obtain the same laser parameters (except the duration that will be much longer)? In this case, we can skip pre and post-pulse and even the pedestal. It is a simple and natural question to understand the benefit of the compression technique and its limits.

 

I believe that having this additional information will benefit the readers. 

Author Response

We thank the reviewer for the comments. Below are our responses to them. We also marked up the significant changes made in the manuscript with red.

Some parts of the paper are very general. There are several scaling laws. I like this part. However, at a certain point, the focus is only on 1.5 PW and 11 fs, as the parameters in reference 23. It is a reasonable choice, and the results are very interesting, without a doubt. But the author must clearly state, maybe in the abstract, that their analysis is not general but focused on a particular case.

In fact, the parameters we used are similar to a different experiment [24]. However, we agree with this critic and added a clarification in the end of the introduction.

But the reader, like me, can have a simple request:  To have a comparison plot of the betatron radiation with and without laser compression. The authors investigated the effect of a0, the prepulse, the post-pulse, and even the pedestal only when there is compression. But if we do not compress the pulse, which is the spectrum of the betatron radiation, can we obtain the same laser parameters (except the duration that will be much longer)? In this case, we can skip pre and post-pulse and even the pedestal. It is a simple and natural question to understand the benefit of the compression technique and its limits.

We added results of simulations for non-compressed laser pulse. They are presented in a new Fig. 3 and described in the text.

Reviewer 4 Report

In this article were studied the changes in the betatron radiation spectrum caused by different shapes/configurations of the driving laser pulse. Specifically, the effects caused by the prepulses, postpulses, and pedestal were studied. In some way, it reminded me the resonant schemes for the plasma wake-field acceleration (although, for the beam driven cases), however I've never seen before such studies with regard to the betatron radiation. In my personal opinion, the practical use of that study is virtually non-existent. At best, it can be used to explain some effects observed during the experiment (if they will not be obscured by other effects), but I doubt it will ever be a viable method to control the spectrum of the betatron radiation. Actually, in the conclusion, authors them self wright about additional factors, that will appear on the practice (e.g. focusing). Nevertheless, as a theoretical study it can and should be published.

The article itself is written adequately, I had no problems reading it. Just a couple of minor recommendations:

1) Figures 1,4 and 5 contain a lot of data and it is not clear to me what information exactly the reader should take from them. Especially, since those images correspond to the three specific "prepulse" cases which, apparently, are compared eventually. A couple of words somewhere in the caption to one of the figures would solve this problem, I think. By the way, the font on the Fig.5 is much smaller than on the other images, a bit hard to read.

2) The article is too qualitative at times. For example: the core information/idea of this article is contained in one sentence on the page 3: "The stronger prepulse however can alter the spectrum significantly either enhancing or inhibiting it". But what does "significantly" means? Is it 5-10% or 2-3 times? The concluding chapter, "Discussion", is written in a similar manner. Yes, all the necessary information is embedded somewhere in the article body. However, if authors will add to the conclusion some "numerical information", I believe, the article only will benefit from it. 

Author Response

We thank the reviewer for the comments. Below are our responses to them. We also marked up the significant changes made in the manuscript with red.

1) Figures 1,4 and 5 contain a lot of data and it is not clear to me what information exactly the reader should take from them. Especially, since those images correspond to the three specific "prepulse" cases which, apparently, are compared eventually. A couple of words somewhere in the caption to one of the figures would solve this problem, I think. By the way, the font on the Fig.5 is much smaller than on the other images, a bit hard to read.

Those figures are shown to give a general impression on the acceleration dynamics. The details which are important for the discussion are underlined in the text. We also have added a new Fig. 6 in order to highlight the differences between three cases.

The font size of Fig. 5 has been fixed.

2) The article is too qualitative at times. For example: the core information/idea of this article is contained in one sentence on the page 3: "The stronger prepulse however can alter the spectrum significantly either enhancing or inhibiting it". But what does "significantly" means? Is it 5-10% or 2-3 times? The concluding chapter, "Discussion", is written in a similar manner. Yes, all the necessary information is embedded somewhere in the article body. However, if authors will add to the conclusion some "numerical information", I believe, the article only will benefit from it.

We tried to add the numerical information in the text in new revision.