Control Scheme of Phase-Shifter for Photon Energy Scan

Round 1

Reviewer 1 Report

 

 

The manuscript describes the output intensity stabilization at the Pohang Accelerator Laboratory X-ray Free-Electron Laser (PAL-XFEL) by appropriate tuning the gap of the phase shifters. The specific procedures at the Hard X-ray (HX) and a Soft X-ray (SX) FEL lines are described in chapters 3 and 4. High XFEL intensity can be maintained while scanning the photon energy: i) Applying the “scanning of individual phase shifter” method, the optimum gap position of the phase-shifters can be set for HX line. ii) A control scheme is proposed for the SX line, which uses the phase numbers n=0, 1, and 2, and a periodic sawtooth signal-like setting of phase-shifter gap distance. These novel procedures will surely help the users of the facility to carry out successful experiments in many scientific fields.

The manuscript can be accepted for publication if some minor, mostly technical revisions are made.

Figures could be shifted closer to their description. It is advisable to insert the figure as soon as possible after the explanatory text.

Contrast of the figures could be increased (especially Figs. 6, 7 and 12)

Line 9:  „Variable gap undulator widely used in X-ray free-electron laser enables…” please extend with the abbreviation XFEL: „Variable gap undulator widely used in X-ray free-electron laser (XFEL) enables…”

Lines 25-27: text modification is recommended. Text: “For the PAL-XFEL undulators, the European-XFEL undulator design (5-m long variable gap, out-vacuum undulator) was adopted and modified for PAL-XFEL [8-10].” Correction: “For the Pohang Accelerator Laboratory X-ray Free-Electron Laser (PAL-XFEL) undulators, the European-XFEL undulator design (5-m long variable gap, out-vacuum undulator) was adopted and modified [8-10].”

Lines 27-29: Left side of the page: the citation text (authors and title) should be given.

Lines 84-90: Figure caption of Figure 2. is shifted to page 3. It is recommended to keep figures and their captions on the same page.

Lines 93-93, 194, 273 275…: The text color varies between black and dark gray. It is recommended to select the whole the text of the document and set the character color to black.

Line 100: For the equation ??=?0cos(???) the external brackets are not required.

Line 110: This could be added: “λ_R is the wavelength of the generated x-ray radiation.”

Line 111: Please define the parameter T₀

Line 135: Please define the parameter T(K_u)

Lines 128 and 137: Please use non braking spaces to keep equations in one row.

Line 142: It is recommended to extend table1 with “drift space between undulator segments, m” (The value supposed to be 1.05 m for HX and SX lines)

Lines 145 Figure 3a: correct the description of y axis from “B filed” to “B field”

Lines 194-196 text: “The scanning starts from the beginning of the undulator line because the electron energy decreases along the downstream undulators due to the FEL process, and is difficult to know exactly at each undulator.” The end of the sentence is not clear. What is difficult to know? Please rephrase.

Lines 249-250 text: “Thus, setting the phase shifter phase integrals is only with the calculated values from Eq. (7). „ Correct to: „Thus, setting the phase shifter phase integrals is only possible with the calculated values from Eq. (7). „

Lines 251-252 “Figure 7 depicts the phase-shifter gap distance as a function of undulator K found  with n=1 and n=1 to 14 using Eq. (7).” It is recommended not to give the phase numbers in this sentence, since it is surprising having double definition for n. The n values for Figure 7a and 7b are given in the next sentences. Suggested correction: „Figure 7 depicts the phase-shifter gap distance as a function of undulator K using Eq. (7)."

Line 255 text: “Instead, a continuous line…” the extension of the text is suggested. „Instead, a continuous curved line…”

Lines 273-274 text: “The FEL intensity measured by the electron energy loss scan [24] was 0.44 mJ at 700 eV and 0.4 mJ at 900 eV,” The mJ units are not intensity units. The authors probably meant pulse energy. Please correct the text accordingly.

Line 306: Why is it necessary to define the gap distance with a new symbol as "gncal" and "gnmeas" when LD is already used?

Lines 314-315 text: “And the result at different Ks reveals an inconsistent value in Lc (Lc must be the same independent from the undulator K).” Please rephrase the sentece. Did you mean: “Furthermore, the result at different Ks reveals an inconsistent value in Lc (Lc must be the same, independently from the undulator K).”?

Lines 311-318: It is stated that “The minimum error point is not predominant in the figure because of low statistics samples.„ Is the inverted surface shown in Fig. 10 for the minimum search a result of fitting to measurement data? If yes, please mention in the text or in the figure caption.

Line 324, Figure 10: Please correct the undulator cell length (Lc) unit to m!

Lines 339-342: The rearrangement of the text is recommended in the following way:
“If an error in LC is only considered, the error of 26 T2·mm3 in the phase integral corresponds to 8.9 mm in LC. Supposing zero error in LC, the gap tolerances are 2.65 mm for n=0, 0.43 mm for n=1, and 0.25 mm for n=2. If the error in LC is assumed to be 3 mm, the gap tolerances become worse, particularly for the phase number n > 2.”

Line 363, Figure 12 (a) and (b): use the same scale for the y axis. It is recommended to use the same colors for phase numbers 1 and 2. E.g. use black color for datapoints “phase number n=1” and red color for “phase number n=2” also in Fig. 12 (a).

Lines 369-370: It is recommended to revise the text as follows:
Original text: „However, covering the wide range of photon energy along a single control line have the phase-shifter gap go down to below 20 mm at lower photon energy (higher undulator K), where the gap setting tolerance is very small.”
Recommended correction: “However, to cover a wide range of photon energy along a single control line, the phase-shifter gap must drop below 20 mm at lower photon energy (higher undulator K), where the gap adjustment tolerance is very small.”

Lines 367-377, Discussion: It is recommended to add a short discussion of the results of the HX line, too.

Lines 394-439, References: Please add all author names to references. The format for Journal Articles is:
1. Author 1, A.B.; Author 2, C.D. Title of the article. Abbreviated Journal Name Year, Volume, page range.

Line 408: Title of the article is missing.

Author Response

I appreciate the Reviewer for many thorough reviews and comments. Almost all comments are included in the revision except a few remarks like

1) Fig. 6 is difficult to improve the contrast of the figure because it is a sum of many different figures.

2) T0 and T(Ku) are the same definitions. So only T0 is used in the revision.

3) I tried to use non-breaking spaces to keep equations in one row.   But it did not work. 

4)  LD  is the drift distance between undulator segments. The gap distance with a new symbol as "gncal" and "gnmeas" are used to calculate the error.

I would like to thank again for finding many typos and suggesting improvements, which helps to improve my manuscript better.    

 

Author Response File: Author Response.pdf

Reviewer 2 Report

The paper presents a control scheme of phase-shifter for photon energy scan. The paper is interesting, however some matters need to be addressed before publication.

1. A better explanation of your contribution needs to be added in the Introduction Section. Please set differences of your work with respect to previous work. Please highligh the advantages, benefits and contributions of the new tecnique and results.

2. Provide more details of the methodology used.

3. There is a lot of paper dealing with this topic. Please cite references dealing with the topic of phase-shifter for photon energy scan.

4. It is needed a better description of the results. Please highligh the benefits, the advantages and contributions of your proposal.

Author Response

I appreciate the Reviewer for many thorough reviews and comments. 

Complying with the reviewer's comment, I describe more discussion and some benefits of my study in the discussion section.

I would like to thank again for suggesting improvements, which helps to improve my manuscript better.    

 

Author Response File: Author Response.pdf

Reviewer 3 Report

The control scheme of phase-shifter for photon energy scan at the PAL-XFEL has been introduced clearly and in detail. The slippage between the light and the electron can be compensated by a phase shifter in order to keep phase-matching with an integer multiple of 2 \Pi. The authors proposed a method to calculate the optimum gap of the phase-shifter by considering the uncertainties caused by the undulator parameter Ku and length L_D, however, failed to give exact values. Instead, the largest tolerance of the phase-shifter gap could be set by measuring FEL intensity vs the phase shifter gap, which can enable photon energy scan with high FEL intensities. It sounds and can be published with two minor modifications,

In Eq.(8), and line 129, the subscript ‘U’ of \lambda and K should be in lowercase letters.

In Figure 7, no labels ‘(a)’ and ‘(b)’ correspond to the citations in caption of Figure 7.

Author Response

I appreciate the Reviewer for many thorough reviews and comments. All comments are included in the revision

Complying with the reviewer's comment, I describe more discussion and some benefits of my study in the discussion section.

I would like to thank again for suggesting improvements, which helps to improve my manuscript better.    

Author Response File: Author Response.pdf