Multi-Beam-Energy Control Unit Based on Triple-Bend Achromats

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

This manuscript discusses a novel method of a triple bend achromat as an electron beam delay system in order to send high energy electron beams to different undulator lines in superconducting x-ray free-electron laser facilities. This method offers a promising solution to significantly vary the electron beam energy without distorting the phase space and beam qualities. The manuscript is well written and the results are clearly presented. There are a few points that I would like to see addressed by the authors before I can recommend publication on Photonics.

 

I noticed that in Fig. 6 lower row, with increasing delay the electron phase space starts to show some microbunching instability, especially for panel (h). Will this degrade the FEL lasing in the undulators? The authors have done a great job quantifying and presenting the twiss parameters and emittance change through the TBA, and it would be useful to quantify the strength of microbunching instability introduced by TBA, as it might be harmful for FEL lasing. 

 

By introducing the quadrupoles does it significantly increase the difficulty of optimization during operation?

 

Fig. 6 needs lager fonts and labels.

 

What does VHF stand for in line 76?

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

This article discusses a proposal for controlling the electron-beam energy in an FEL in order to tune the photon energy. In a previous work, two of the authors proposed a delay system based on double-bend achromats; in this work, the authors propose a delay system based on triple-bend achromats, which achieves better performance. Start-to-end simulations of this system using parameters of the SHINE FEL facility in China have been run; the key results are presented and discussed in this work.

The work is relevant to this journal and presents an important scheme that could improve FEL performance and tunability. There are a few areas that could be improved, particularly concerning the discussion of the scheme and results. Detailed comments are provided below.

 

General comments

1. This work builds upon previous work based on a scheme with DBA cells, and simulation results presented in this paper include a comparison of both schemes. Although the previous work is mentioned in the introduction, I would suggest including a more extensive comparison of the two schemes in this section to help the reader understand the differences.

2. Section 4 includes a detailed comparison between the DBA and TBA schemes: the differences in beam parameters for the two schemes are presented, and it is shown that the TBA scheme generally performs better. However, there is a lack of discussion as to why the TBA scheme outperforms the DBA scheme. Expanding this discussion would improve this paper.

3. There is a lack of connection between properties of the electron beam and properties of the photon beam. Given the journal to which this paper has been submitted, it is essential to explain how the electron-beam properties discussed in Section 4 influence the photon-beam properties.

 

Specific comments

1. Line 76: VHF is undefined.

2. Page 3: The frequencies of the cavities is not clear. Perhaps they could be added to Fig. 1.

3. Line 94: The undulator description and period for FEL-II is missing.

4. Line 100: What is the length of the “break section”?

5. Fig. 2: This schematic is confusing. According to the text, the kickers deflect vertically but the dipoles horizontally. Assuming this schematic is in the X-Z plane, there should therefore not be deflection visible after the kickers. Also, are the TBA cells bending in opposite directions? This should be made clear.

6. Line 110-111: The terms “achromatic”, “isochronous”, and “R_56” ought to be defined for the general reader.

7. Line 128: Are all quadrupoles allowed to vary independently?

8. It would be helpful to include a labeled diagram with the terms appearing in Eq. (2) and (3). Do all dipoles in the TBA have angle theta? This should be explicitly stated. Why do the first two terms in Eq. (2) not add (they are both positive for small theta)?

9. Eq. (4): What about the delay due to the kickers?

10. Lines 138-139: If the delay distance can be effectively controlled by adjusting the dipole angles, how are L1 and L2 chosen?

11. Table 1: It would be clearer if DBA and TBA were labeled in the emittance column rather than at the far left of this table. What is the emittance at the start of the delay section?

12. Fig. 4: What bin length is used, and how many macroparticles are in each slice? It looks like there is a lot of noise. It would be helpful to plot the longitudinal charge profile as well. What does this slice emittance look like before the delay section? The plot shows the slice emittance for two bend angles. Do the authors expect to see a noticeable difference?

13. Fig. 5: It would be clearer to add the energy label to each subplot.

14. Line 170: Why does the TBA system give lower vertical beta values?

15. Table 2: The emittance values from lines 193-194 should be included.

16. Why did the TBA lattices give lower emittances for all cases for the double-horn profiles but not for the flat-top?

17. Fig. 6: Should label the subplots with the delays. It should be mentioned in the caption that this is for the flat-top profile.

18. Line 226: Is it just the center dipole angle that is adjusted? If so, this is not clear from the rest of the paper.

Comments on the Quality of English Language

There are a number of errors that need correcting. Some of the ones I noticed are:

• “In addition” (lines 35 and 53) is used to indicate the second reason or method. It would be clearer to use “The second…”.

• Unnecessary “the”, e.g. in line 54, 62 etc.

• Line 55: “would be” instead of “is”, assuming this system has not yet been installed.

• Unnecessary “of” in line 66.

• Lines 78-79: Repetition of “accelerator section” confusing.

• Line 117: Should be “separates”, not “separating”.

• Line 158: typo.

• Line 164: I think “as” is meant instead of “and”.

• Line 169: “While” instead of “with”.

• Line 209: Should replace comma by “and”.

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

In this article, the authors report a novel delay system consisting of four three bend achromatic filters (TBAs). Based on the parameters of Shanghai high repetition rate XFEL and extreme light facility, simulations from start to finish show that the delay system based on TBA achieves better electron beam quality while providing a wide beam energy tuning range.

This job is quite specific, but in order to publish, the following issues still need to be addressed,

1. The abstract in this article is suggested to be rewritten by the authors. The background explanation needs to be reduced, as the introduction section has already provided a detailed introduction. The abstract needs more information about one's work, what they have done, how they did it, and what applications they have.
2. Figure 1 is drawn very simply and needs to be redrawn.
3. Regarding Figure 3, it is necessary to label a and b. Moreover, in Figure A, the curves overlap together, and it is recommended that the author redraw them.
4. Figure 5, the author's main text lacks relevant explanations and requires clarification.
5. The introduction of references in the summary section is not allowed.
6. The English expression of the whole article needs to be further improved.

 

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

The authors have addressed my questions, and I think the manuscript can be published.

Reviewer 3 Report

Comments and Suggestions for Authors

Accept in present form