Two-Color Operation of a Soft X-ray FEL with Alternation of Undulator Tunes
, Ivette J. Bermudez Macias 1, Martin Beye 1, Markus Braune 1, Marie Kristin Czwalinna 1, Stefan Düsterer 1, Bart Faatz 2, Rosen Ivanov 1, Ulf Fini Jastrow 1, Marion Kuhlmann 1, Juliane Rönsch-Schulenburg 1, Siegfried Schreiber 1, Andrey Sorokin 1,†, Kai Tiedtke 1, Mikhail Yurkov 1 and Johann Zemella 1Round 1
Reviewer 1 Report
Summary: The authors summarize performance characteristics of two-color FEL pulses generated at FLASH using alternatingly tuned undulator segments. This is enabled by the new variable-gap undulators in FLASH-2. This gain-modulated two-color scheme generates FEL pulses with higher efficiency and better spatial overlap. The authors presented the intensity, spectral and temporal properties of the two-color pulses, which could be useful for users in the future.
From a user perspective, the two-color range of 13.6 – 22.6 nm (40%) significantly exceeds that of a previous demonstration using alternately tuned undulator segments at LCLS where a ~1% difference in colors was achieved PRL 111, 134801 (2013). Although the scheme does not allow arbitrary pump/probe delay scans of the two colors as many users desire, the arrival time between the two colors could be monitored by THz streaking. For their particular FEL setup, a distribution of relative arrival times with 10 fs rms was observed, which is suitable to characterize some ultrafast processes.
I recommend publication after the authors address the following comments.
(1) The figures need attention; Figures 1-5 are exact replicas from a previous publication, Ref 17. ! This must be acknowledged in the associated figure captions.
(2) The authors did not show clearly the performance/improvement of the spatial overlap for the two-color pulses. They only place mentioned this is at section 4.2, line 176. Some data to quantify the good overlap and show the advantages of this two-color method would be helpful.
(3) For Figure 8, some description for the OPIS, such as what gas is used, which photoelectron peaks are used to indicating the FEL intensity would be helpful.
(4) The legend in Figure 10 should be corrected.
(5) A comparison between normal one-color gain and the two-color scheme would be interesting to include.
Author Response
Author Response File: 
Author Response.pdf
Reviewer 2 Report
The manuscript “Two-Color Operation of a Soft X-ray FEL with Alternation of Undulator Tunes” illustrates some interesting developments achieved at the FLASH Free Electron Laser. It reports the implementation and characterization of a “two-color” pulse generation for user experiments. The technique illustrated in this paper relies on a modulation of the gain of the undulators to amplify the SASE radiation at two distinct wavelengths. The authors describe a set of diagnostics to monitor the single-color intensities crucial for reliable user experiments and illustrate a set of more invasive diagnostics for a more complete characterization. The paper is well written, and I think it can be published with some minor adjustments, in particular:
- The precision of the presented diagnostics is not clearly reported. Are there any possible sources of systematic errors that one should keep in mind? Is it possible to have an estimation of the errors that can affect the presented diagnostic methods? (From a user’s perspective, what can be the most reliable combination for a quantitative measure of the relative energies?)
- Being produced by a SASE process, the energies of the pulses generated by FLASH are strongly fluctuating. I think that in plots such as the one in figure 5 it is helpful to display how much this fluctuation is relevant to the reported data (e.g. with error bars). How large are the expected uncertainties due to instrumental errors of the previous point in respect of the “natural” energy fluctuations?
- The authors report the presence of small time delays between the two pulses, does this mean that also small spatial fluctuations have to be expected?
- The style of the plots should be more homogeneous, (e.g. the font size of figure 6 is extremely small compared to the rest of the manuscript, some plots are in boxes and others are not, figure 3 has a black background, etc…)
- The axes labels from figure 3 are missing
- Maybe figure 8 can be clearer adding a second x-axis with the wavelength
Moreover, I have a couple of curiosities:
In figure 4 the gain curves are reported for the two colours. How was performed the measurement more in detail? Also, it is nicely visible how the two colours are amplified only by their respective sections of the undulator, maybe a bit more comments on that can be useful/interesting for the less specialized reader.
The behaviour of the two colours energies shown in figure 5 is very interesting. Do you know of some relation (even empirical) that can be used to estimate the ratio between the two energies knowing the wavelengths? (This piece of information could be very useful for potential users when planning the experiments)
Author Response
Please see the attachment
Author Response File: Author Response.pdf
