Improving the Performance of an Ultrashort Soft X-ray Free-Electron Laser via Attosecond Afterburners

Round 1

Reviewer 1 Report

The manuscript presents a novel method to shorten the pulse duration of Free-Electron Laser pulses useful for configurations that present a strong dominant spike and where slippage effects are relevant.

The method requires a strongly micro-bunched electron bunch part, much shorter than the X-ray pulse duration, and subsequently uses microbunching rotation and short afterburners to produce a shorter and cleaner pulse.

The simulations are based on the eSASE scheme and include the modulator section, the radiator section, the microbunching rotation with offset quadrupoles and the afterburners. Optimization of the radiator and afterburner lengths is adequately presented and the microbunching rotation with the offseted quadrupoles is adequately described. The stability section shows a clear improvement of the pulse duration that is shorter, and with a smaller absolute jitter. For the peak power stability there is also an improvement, at least when considering the 13.65 m radiator, 0.72m afterburner working point.

A final chapter is devoted to the longitudinal space charge effect (LSC). LSC has a severe impact on the energy chirp profile, quoting the manuscript lines 207-209 "... a strong energy chirp with a peak to peak energy variation of about 24 MeV is produced by LCS effect. Such a large energy spread is much larger then the FEL bandwidth, and can degrade FEL interaction". I understand that this relevant effect has been neglected in the simulation result presented. If this is true, a simulated result including this effect should be presented.

The research is interesting, original and important, but I cannot recommend publication unless it is clarified that LSC was included in simulations. If it was not included, a simulated result with it included should be presented.

Point to point list:

1) If LSC was included in the presented simulated results it should be adequately stated. If not, a simulated result with it included should be presented.

Author Response

Please see the attachment

Author Response File: Author Response.docx

Reviewer 2 Report

The authors presented a clear study on the impact and improvements of an afterburner undulator for a Free-Electron Laser operated in SASE mode. For the use of a few cycle laser, that laser needs to be coupled in and out (or blocked) that leads to the questions a) what does an excess in R_56 do to the beam and b) are there other effects to consider.

Also, while the plots and figures are clear in their meaning, the quality is not optimal. When zooming in one can see compression artefacts. Also, for figures including subfigures, the subfigure indices are doubled (once in the image, once as a seperate one below the images). If the plots are redone, this is easy to be removed.

Some formatting needs to be redone. e.g. in the experimental setup (Fig. 1), the units should be seperated from the numerical value, and also all units should not be italic. This needs to be checked throughout the manuscript. (in this Figure it is "GeV").

Apart from these minor issued I see no reason for not to publish the research once those have been resolved.

Author Response

Please see the attachment.

Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

The Authors have responded to my question satisfactorily, therefore I recommend publication.