High Harmonic Generation Seeding Echo-Enabled Harmonic Generation toward a Storage Ring-Based Tender and Hard X-ray-Free Electron Laser

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

I think the paper is suitable for publication in this journal. The idea of seeding EEHG with HHG is explored in sufficient analytical detail to be of interest to readers in the field. I find no major errors. Below are a few points the authors may wish to consider:

• The first sentence in the abstract should be changed: I think the authors can motivate this work better than simply ‘following a trend’.

• HHG fluctuations are known to detrimentally affect direct seeding. I would appreciate further discussion of how these would affect the proposed scheme of the authors.

• I would appreciate some further discussion of how the authors propose to generate the HHG parameters they claim.

• Some figures, e.g. Fig. 2 are unwieldy large and looks like they’ve been photocopied. The aspect ratio of some of the other figures look like this too. This should be addressed.

• Could the authors provide an illustrated schematic of the Two-frequency Crab-Cavity Scheme? This would aid understanding of this section, which is interesting.
  

Comments on the Quality of English Language

Overall good quality

Author Response

Dear Reviewer, 

We appreciated you spending your valuable time reviewing our paper and made those important suggestions.

We made all those changes, which greatly improved the quality of our manuscript.

See the attached file for line-by-line response. 

Thanks, -xi

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

The Authors discuss a novel approach to use a conventional 256 nm laser and an HHG source for a SR based EEHG seeding, as an alternative to cascaded EEHG with only conventional 256 nm seeds. The goal is to further shorten the output wavelength, reduce the costs, and the used space in the SR. This idea is innovative and I agree that this should be published in this Journal. Since I am not an accelerator physicist I will mostly give comments on the laser and HHG part, which I think are useful for the project and will improve the manuscript.

Line 48:
Please add a citation for 1.28 nm, I guess a paper from FERMI.

Figure 1(a) and (b):
Please increase the text size a bit and use big letters for EEHG instead of eehg.
This combination between laser labs and SR needs long optical beamlines. Both beamlines, HHG and 256 nm (at least recommended for 256 nm) need ultra-high vacuum chambers. Additionally, a laser stabilization system is usually used, in order to overcome any drifts and keep the spacial overlap in the modulators. Is there anything planned at this direction?
Figure 1(b):
HHG need diagnostics in the laser lab (for optimization), but I guess this goes above the scope of this publication. But, are there any diagnostics after the radiator, like a spectrometer? How do you want to make the temporal overlap between HHG pulse and electron bunch? With a streak camera? Of course you need imager screens for the spacial overlap. For HHG, a laser pulse is focused into a gas target, but the HHG radiation is a bit divergent! Did you make any simulations of the HHG diameter at the modulator 2 and compared it to the electron bunch diameter? I expect that refocusing (imaging) of the HHG source into the modulator is needed. This in turn needs multilayer mirrors for HHG under almost normal incidence and even more mirrors (and losses) for guiding the beam. Please add some discussions about that in the text around line 130-137.
Did you estimate how many HHG photons/pulse are needed at modulator 2 for successful EEHG?

-> I recommend the Authors to read these two papers from the FEL FLASH, where an HHG source was used for direct seeding:
PRL 111 114801 (2013)
Appl. Phys. B 115 45 (2014)
The Authors report of a HHG transmission of 5% into the accelerator tunnel and not ~ 100%.
Please note that the FLASH facility decided to go this year to HGHG seeding (like FERMI), because HHG seeding worked out in an R&D phase but it was technically too difficult. Note, your proposal is very ambitious, technically difficult, and will need a lot of scientists and engineers.

Line 87:
This figure has no number or caption and I also think it is not really needed at all.

Figure 2(a) caption, line 93 and 94:
(a) shows only spectrum, not number of photons per shot, repetition rate, and pulse duration. You also write this again in line 195.

Figure 2(b):
X- and Y-axis do not have ticks and I also think this figure is not needed. Increasing repetition rate increases the photon flux, but this is self evident. More important is, can the optical drive laser keep its output (and therewith the HHG performance) up to 5 kHz? Usually, with increasing repetition rate the laser single-shot pulse energy goes downwards.
Another point is the stability of the drive laser and HHG source: Is E9 photons/pulse for the HHG a "record" happened "once in a while" in the laser lab, or is this 24/7 reliable available for EEHG? What is the uncertainty of E9 photons/pulse?

Line 118:
The abbreviation SASE is introduced much later in line 312.
In the SR the electron bunch duration is 20 ps, for FELs this is ~ 1 ps. Is this still SASE background (with micro-bunching) or is it more something like synchrotron background?

After the electron bunch goes from modulator 1 (256 nm), modulator 2 (HHG), and radiator it will circulate around the synchrotron and come back to modulator 1. Will any electron bunch modulation remain, or is this always a pristine electron bunch before modulator 1?

Line 144:
Please cite an experimental HHG paper where eta_SBW ~ 1 is shown.

I would save space and put figures 4, 5, 6, and 7 together in one figure and name it (a), (b), (c), and (d), respectively.

Line 254:
To fully explore the time resolution one could split the drive laser pulse into two branches, one for HHG production and one could be transmitted to the experimental hutch for pump-probe experiments. This way, radiator light and laser would be perfectly synchronized. Are there any plans for that?

Chapter 2.3:
Is self-seeding an option for your SR based FEL?

Line 302:
Please cite a paper for XFELO.

Line 336:
Abbreviation CSR is not introduced.

Author Response

Dear Reviewer, 

We appreciated the reviewer spending valuable time in reviewing our paper and giving extremely important comments and suggestions on our manuscript. We made all those changes, which greatly improved the quality and readability of our manuscript.

See the attached file for line-by-line response.

Thanks, -xi

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

I am happy with the changes made by the authors. I think the manuscript is suitable for publication in this journal.

Comments on the Quality of English Language

The quality of written English is overall high.

Reviewer 2 Report

Comments and Suggestions for Authors

Dear Authors,

the paper became much better, all fine from my side.

I wish you good luck with this demanding project!