An Ultra-Compact X-ray Regenerative Amplifier Free-Electron Laser

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The authors report an XRAFEL design based on the proposed UC-XFEL configuration and present simulation results to demonstrate the feasibility and performance of the method. The paper is well-written, clear, and concise. The topic is timely and relevant for the field of X-ray FELs. The results show that the proposed scheme can produce fully coherent and high-flux hard X-rays in a compact footprint, which could have a wide range of applications in both academy and industry. I recommend the paper for publication, subject to some minor revisions as follows:

In Fig. 3a, the curves for pass 4, 5, and 6 are hard to distinguish in the plot. I suggest using different line styles to make them more visible.

In Fig. 3c, the peak power along the bunch is less than 80 GW, but the caption says it is 85 GW. Please correct this inconsistency.

Author Response

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Author Response File: Author Response.docx

Reviewer 2 Report

Comments and Suggestions for Authors

The authors reported the results of numerical simulation studies on the regenerative amplifier configuration applied to a compact X-ray free-electron laser (XFEL) scheme. There are increasing demands on XFEL and　a compact solution discussed in this paper could be a significant scientific advance. I have some questions that should be answered by the authors. My questions are listed below.

1. Since the slippage in the undulator is about 100 nm as mentioned in the main text, the FEL pulse generated from each current spike of the electron beam has no phase correlation if the seed pulse duration generated from each current spike is less than 1 micro-m. I suppose the micro-pulse duration of the seed is longer than 1 micro-m and this gives the phase correlation of at least neighboring spikes. The authors are advised to add information on the temporal profile of the seed pulse at the entrance of the undulator. In addition, in the case of the spikes with the 3 micro-m intervals, having a phase correlation between spikes gets more difficult. Even with the 3 micro-m intervals, can the comb feature be preserved?

2. Are the properties of the comb shown in Fig. 5 (b) always the same in different runs of the simulation having different seed numbers for random distribution generators? In other words, how is the stability of the comb properties? The stability of the comb properties is important information and determines the applicable applications.

3. The authors mentioned that the UC-XRAFEL reaches a steady state within 7 electron shots in the case of 1-micro-m current. As depicted in Fig. 5 (a), the FEL peak power of passes 8 and 7 are almost the same. That’s why the authors mentioned, “reach steady state within 7 electron shots.” But the power evolution of pass 8 in the undulator is largely different from that of pass 7. This means that the FEL process is not in the steady state. The authors are advised to change the description. I expect the properties of the FEL pulse in passes 7 and 8 are not the same. This comment is also applicable to the results of 3-micro-m current.

4. In the cavity-based FEL system, the required accuracy of the cavity length is an important parameter. To judge the feasibility of UC-XRAFEL discussed in this manuscript, the authors should provide information on the required accuracy of the cavity length in this configuration. 

5. I suppose that the detuning amount of the cavity length has a strong impact on the temporal and spectral properties of the FEL. Regarding the synchronization condition between the cavity roundtrip time and the electron bunch spacing, only a very rough number (cavity roundtrip length = 12 m and bunch spacing =40 ns) is given in the manuscript. Is this calculation performed under perfectly synchronized conditions? More detailed information about the synchronization condition should be provided.

6. The tapering condition of SASE calculation was given in the caption of Fig. 4 and 8 but not given in the main text and Fig. 6. The authors are recommended to add the information on the tapering condition in the case of SASE in the main text and the caption of Fig. 4.

Author Response

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Author Response File: Author Response.docx

Reviewer 3 Report

Comments and Suggestions for Authors

The paper provides ultra-compact regenerative amplifier free electron laser using multiple pulse trains with higher peak current by electron beam – laser modulation. This proposed scheme is promising to improve the quality of x-ray such as higher peak brightness and lower intensity jitter compared to SASE scheme. So I believe the result will arouse interest in the x-ray free electron laser community. This is a high quality paper and I suggest it being published in the present form.

Author Response

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Author Response File: Author Response.docx

Reviewer 4 Report

Comments and Suggestions for Authors

This paper reports on an X-ray Regenerative Amplifier FEL (XRAFEL) design study using an ultra-compact X-ray FEL configuration. The undulator is very short, only 4 m long, and the footprint is also very short, 5.5 m long excluding the accelerator part. The design incorporates the ESASE scheme to get a high peak current of 4 kA. The proposed concept is not new and uses the ideas published before. The authors carried out the simulation study of two cases of energy modulation wavelengths, 1 um and 3 um. The 3um case shows a better result due to the slippage effect. The most dominating factor determining the simulation result is the transverse emittance. The emittance used in the simulation is relatively small, 0.075 um, and difficult to achieve. It is recommended to describe the accelerator part more, focussing on the emittance part.

 

 

Author Response

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Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

The authors have addressed the previous comments and suggestions.  I recommend the paper for publication in its current form.

Author Response

Dear Reviewer 1,

 

We are pleased to hear we have adequately addressed your comments and extend our gratitude for reviewing the revised manuscript. We have submitted our most up-to-date manuscript.

Reviewer 2 Report

Comments and Suggestions for Authors

I read the revised manuscript and the author's response. The authors answered all of my comments in the previous review round. The authors mentioned that there is an appendix related to the study on the tolerance of the cavity length in the reply to comments 4 and 5. However, I couldn't find any description related to this topic in the appendix. Could the authors explain this point?

In addition, there is some description of the synchronization condition, but there could be mistakes in units (ns and fs). Please check the attached PDF file.

The authors should explicitly describe the synchronization condition used in their simulation study shown in the manuscript. For example, the head of the electron beam was synchronized with the middle of the seed laser. In the current manuscript, information on the temporal synchronization condition is missing.

I found several typos in the manuscript and put comments/corrections on the attached PDF file. Please check it.

Comments for author File: Comments.pdf

Author Response

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Author Response File: Author Response.pdf

Reviewer 4 Report

Comments and Suggestions for Authors

I checked the reply from the authors and the revised manuscript. They have revised the manuscript according to the comments. 

Author Response

Dear Reviewer 4,

 

We would like to extend out thanks for reviewing our revised manuscript. We are pleased to hear you are satisfied with the revisions. Our updated manuscript is submitted.

Round 3

Reviewer 2 Report

Comments and Suggestions for Authors

The authors answered all of my comments in the previous review round. The authors misunderstood one point. I asked authors to explicitly describe the synchronization condition used in their simulation study shown in the manuscript. However, no additional information was provided. I understand the torelance of the cavity length but the synchronization condition used in the author's simulation is not given in the manuscript. Please add the information of the synchronization condition to the manuscript. If the e-bunch repetition rate is perfectly matched with the roundtrip frequency of the cavity, please write this information to the manuscript.

Author Response

Dear Reviewer 2,

Thank you for re-reviewing our submitted manuscript. Your concern has been addressed in the new version: we explain that the roundtrip cavity length of 12 m  is matched to the electron bunch repetition rate (40 ns). We have also included the explicit calculation of this synchronization condition. We would like to extend our gratitude for your thorough reviews, as we feel the paper has greatly benefitted from your comments. 

Best,

 

Madison Singleton