Design and Beam Dynamic Studies of an Injector for a Compact THz Coherent Radiation Source

Round 1

Reviewer 1 Report

The paper studies the possibilities of improving the quality of an electron bunch in a laser-driven photoinjector designed to power a tunable source of coherent undulator THz radiation. For this purpose, smoothed profiles of accelerating resonator cells and various spatial distributions of ultraviolet pulses, irradiating the photocathode, are considered. As a result of numerical simulations, rather attractive parameters of electron bunches were obtained.

The topic of the article is suitable for the journal "Particles". For its publication, I recommend that the authors consider the following remarks.

  I1) In the first sentence of the abstract, as a result, it is written about a relatively narrow-band source of THz radiation, which was calculated by the authors in an already published work but is almost not discussed in this paper.

22) In the both papers, the authors do not study the dynamics of dense bunches with a large charge of 1 nC, considering the effects of space charge in a long undulator with a strong field and the limitation of the radiation power and spectrum associated with these effects in the indicated very wide frequency range. It seems that it can be important at a high transverse and relatively low longitudinal velocity of particles.

33) The novelty of the authors' results is not sufficiently obvious from the text of the paper.

Author Response

Dear Reviewer,

We would like to thank the reviewer for taking your valuable time to read and give us your thoughtful comments and suggestions. We sincerely appreciate the time and effort that the reviewer has dedicated to reviewing our paper. The valuable feedback has helped us improve the overall quality of our work.

 

Comments and suggested points

The paper studies the possibilities of improving the quality of an electron bunch in a laser-driven photoinjector designed to power a tunable source of coherent undulator THz radiation. For this purpose, smoothed profiles of accelerating resonator cells and various spatial distributions of ultraviolet pulses, irradiating the photocathode, are considered. As a result of numerical simulations, rather attractive parameters of electron bunches were obtained.

The topic of the article is suitable for the journal "Particles". For its publication, I recommend that the authors consider the following remarks.

  I1) In the first sentence of the abstract, as a result, it is written about a relatively narrow-band source of THz radiation, which was calculated by the authors in an already published work but is almost not discussed in this paper.

Answer Thank you very much for your comment. We have carefully considered the comment. For a relatively narrow-band source of THz radiation mentioned in an abstract, it is the general information of THz radiation which can be produced from this source. Beyond this paper, the scope is about the design and simulation of beam dynamics of electron beam in an injector section. However, to provide a clearer explanation, we added the sentence of “Achieving higher power in generating THz radiation relies on the essential optimization of the electron beam.” In lines 24-25 of section 1. In addition, we have included the following sentence in lines 257-261 of Section 4 “However, the generation of THz undulator radiation could be affected by the presence of space charge, particularly in scenarios with high bunch charge beam under the condition of high-filed undulator. In the upcoming step, an in-depth investigation will be carried out to determine the influence of the undulator magnetic field on the electron beam.”

22) In the both papers, the authors do not study the dynamics of dense bunches with a large charge of 1 nC, considering the effects of space charge in a long undulator with a strong field and the limitation of the radiation power and spectrum associated with these effects in the indicated very wide frequency range. It seems that it can be important at a high transverse and relatively low longitudinal velocity of particles.

Answer Thank you very much for your comment. We agreed with you that a large charge of 1 nC with considering the effects of space charge can affect the generation of radiation in undulator with strong field and it would impact on the radiation power and spectrum. In this part of study, we plan to show and discuss in deep detail in the next study. However, we added the sentences of “However, the generation of THz undulator radiation could be affected by the presence of space charge, particularly in scenarios with high bunch charge beam under the condition of high-filed undulator. In the upcoming step, an in-depth investigation will be carried out to determine the influence of the undulator magnetic field on the electron beam.” In line 257-261 of Section 4.

33) The novelty of the authors' results is not sufficiently obvious from the text of the paper.

Answer Thank you very much for your comment. We have included the details in conclusion section.

For the study design of photocathode RF gun reported in this paper, it was found the mode separation of 44 MHz is higher than that of previous design from other studies as we mentioned this detail in the section2 line 115-123. From our finding (high mode separation), the researcher in this field can bring our design in order to develop and improve their source performance.

In section 3 of “Beam dynamics studies of injector”, the parameters varying for beam dynamics study, we got the results that impact for improvement of injector, and it would be applied for other accelerator sources and also the electron source for a synchrotron light source in the future.

Please find the attached document containing our response to your comments and suggestions.

Best regards,

Siriwan Jummunt

Author Response File: Author Response.docx

Reviewer 2 Report

The main subject of the paper  is design off the photocathode RF gun  for terahertz radiation source. In this design, authors mostly use some well known solutions and approaches. Nevertheless, I believe that publication of the paper would be useful, at least, for broad discussions with other experts in the field.

The technical quality of the manuscript should be improved  according to following remarks.

1. The Manuscript contains two Sections 2 (lines 43 and 69) with almost similar titles. Also, the description of the paper structure (lines 38-40) does not correspond to the actual one.

2. Line 58 states "The quality of the electron beam, such as its emittance, is crucial to the performance of the photocathode RF gun," which is illogical. Perhaps, the performance of the undulator radiation source is meant here.

3. In lines 98-99 the accelerating field amplitude is given, but no information on the RF source intended to power the accelerating structure is provided. Meanwhile, the accelerating amplitude is determined, among others, by the output power of that source. What RF power is needed to reach such an amplitude?

4. Line 121 contains a weird phrase "This design will shine light to increase the field gradient", which is, obviously, a bad translation. In all, the paper contains large amount of such examples of broken English, which should be carefully revisioned. Sometimes, the whole paragraphs are difficult to interpret.

5. The section 3 from line 128 gives an impression of experimental work, which is not the case. The Authors should correct such phrases as "Using a photocathode RF gun, we generated a high-brightness electron beam", "This beam was further accelerated", and like that.

6. The Authors count on the bunch charge of 1 nC with the transverse bunch size of less than 1 mm and the bunch duration of less or close to 10 ps. What type of the photocathode is intended to be used? How the Authors estimate the beam loading effect which may be prominent for such a big charge and can influence the accelerating field? Does the phrase "The electric field of the RF gun remains unchanged at 120 MV/m" in line 131 corresponds to this estimate?

7. The red gaussian-shaped pulse in Table 2 seems to be skewed.

8. The paragraph between lines 155 and 161 seems to be an excessive repetition of the previous text.

9. The sentence "While the ideal solenoid produces a magnetic field in the longitudinal direction only, the actual magnetic field distribution is shown in Fig. 5(left)" in lines 164-165 is somewhat between trivial and incorrect.

10. The description of the Fig. 5 in lines 165-167 does not correspond to  that Figure.

11. What is the nature of oscillations in the red line after 1.5 m in Fig. 6(left)? Is this the influence of the linac? The Manuscript does not specify the linac position. Why the x-axis limits are different in Fig. 6 (left) and (right)?

12. What is the transverse distribution of the electrons in the bunch in all described cases?

13. What are the laser pulse or the electron bunch durations for the Fig. 7?

14. Sentence in lines 193-194 almost repeats the sentence in lines 190-191.

15. Information in almost all figure captions is insufficient. In Fig. 9, what do different lines mean? What is the x axis? What is "Profile"? Why do the red and blue lines in left and right parts look similar?

The word "relation" in line 177 is inappropriate

Author Response

Dear Reviewer,

We would like to thank the reviewer for taking your valuable time to read and give us your thoughtful comments and suggestions. We sincerely appreciate the time and effort that the reviewer has dedicated to reviewing our paper. The valuable feedback has helped us improve the overall quality of our work.

Comments and suggested points

 

The main subject of the paper is design off the photocathode RF gun for terahertz radiation source. In this design, authors mostly use some well-known solutions and approaches. Nevertheless, I believe that publication of the paper would be useful, at least, for broad discussions with other experts in the field.

The technical quality of the manuscript should be improved according to following remarks.

1. The Manuscript contains two Sections 2 (lines 43 and 69) with almost similar titles. Also, the description of the paper structure (lines 38-40) does not correspond to the actual one.

Answer Thank you for your comment. We revised section 3 to “Electromagnetic Field Simulations of Photocathode RF Gun”. We revised and added the sentence of “This paper consists of four main sections. The general information of photocathode RF gun is described in Section 2. Section 3 presents the electromagnetic field simulation and the design of the photocathode RF gun, …”in the lines 36-38 of section 1.

 

2. Line 58 states "The quality of the electron beam, such as its emittance, is crucial to the performance of the photocathode RF gun," which is illogical. Perhaps, the performance of the undulator radiation source is meant here.

Answer Thank you for your comment. In order to clearly understand, we revised this sentence to “Regarding the quality of the electron beam in photocathode RF gun, the emittance…” in the line 65 of section 2.

 

3. In lines 98-99 the accelerating field amplitude is given, but no information on the RF source intended to power the accelerating structure is provided. Meanwhile, the accelerating amplitude is determined, among others, by the output power of that source. What RF power is needed to reach such an amplitude?

Answer Thank you for your question and comments. Regarding the RF power, in ASTRA simulation we used to study the beam dynamics, it requires only the peak accelerating field which relates to RF power. For clearly explanation, we revised the sentence to “For the RF gun parameters, the peak accelerating field (E0) of 120 MV/m was defined in ASTRA simulation. This field leads to the acceleration of the electron beam to the energy of 5.7 MeV.” in line 106 of section 3.

 

 

4. Line 121 contains a weird phrase "This design will shine light to increase the field gradient", which is, obviously, a bad translation. In all, the paper contains large amount of such examples of broken English, which should be carefully revisioned. Sometimes, the whole paragraphs are difficult to interpret.

Answer Thank you for your comment. To enhance comprehension, we revised this phase to “With using Model4 design, the electron beam would be obtained with low initial emittance and a linear longitudinal phase space.”

 

5. The section 3 from line 128 gives an impression of experimental work, which is not the case. The Authors should correct such phrases as "Using a photocathode RF gun, we generated a high-brightness electron beam", "This beam was further accelerated", and like that.

Answer Thank you for your comment. We decided to delete "Using a photocathode RF gun, we generated a high-brightness electron beam" because it mentioned before in the first paragraph of section3.

 

6. The Authors count on the bunch charge of 1 nC with the transverse bunch size of less than 1 mm and the bunch duration of less or close to 10 ps. What type of the photocathode is intended to be used? How the Authors estimate the beam loading effect which may be prominent for such a big charge and can influence the accelerating field? Does the phrase "The electric field of the RF gun remains unchanged at 120 MV/m" in line 131 corresponds to this estimate?

Answer Thank you for your comment. For the bunch charge estimation, in simulation we can define the initial bunch charge emitted from the cathode. However, if we would like to directly calculate the bunch charge, it can be done by defining the laser wavelength, laser energy and quantum efficiency of cathode material. In order to better explanation, we added the sentence to explain in lines 59-62 of section 3 “The amount of charge in the emitted electron bunch is determined by various factors including the properties of the laser, such as its energy and wavelength, as well as the quantum efficiency of the cathode material [3].”.

 

7. The red gaussian-shaped pulse in Table 2 seems to be skewed.

Answer Thank you for your comment. We revised the Gaussian shape.

 

8. The paragraph between lines 155 and 161 seems to be an excessive repetition of the previous text.

Answer Thank you for your comment. We rearranged this paragraph to “The solenoid field was applied to compensate for the transverse emittance, as it is used for beam focusing and transverse emittance conservation. The space-charge field produced by electrons emitted from the cathode can cause defocusing forces and lead to transverse beam expansion at the beginning of acceleration.”.

 

9. The sentence "While the ideal solenoid produces a magnetic field in the longitudinal direction only, the actual magnetic field distribution is shown in Fig. 5(left)" in lines 164-165 is somewhat between trivial and incorrect.

Answer Thank you for your comment. We revised the sentence to “A magnetic field generated by solenoid magnet is shown in Figure 5(left). It refers to the field distribution in longitudinal direction. The transverse beam emittance can be optimized by using the solenoid field. Figure 5(right) presents the transverse beam emittance at various solenoid magnetic field strength.”.

 

10. The description of the Fig. 5 in lines 165-167 does not correspond to that Figure.

Answer Thank you for your comment. We revised to “Figure 5(right) presents the transverse beam emittance at various solenoid magnetic field strength.”

 

11. What is the nature of oscillations in the red line after 1.5 m in Fig. 6(left)? Is this the influence of the linac? The Manuscript does not specify the linac position. Why the x-axis limits are different in Fig. 6 (left) and (right)?

Answer Thank you for your comment. The beam oscillation can occur in linac if laser spot size or laser pulse are not suitable. We added more detail, the phase was revised to “It was found that the laser spot size must be optimized not only for emittance compensation but also to match the beam properties before going to the linac, which is positioned at 1.35 m from the cathode.”

Why the x-axis limits are different in Fig. 6 (left) and (right)?

We edited the x-axis limits.

12. What is the transverse distribution of the electrons in the bunch in all described cases?

Answer Thank you for your comment. We added the detail of transverse distribution in all figure captions.

 

13. What are the laser pulse or the electron bunch durations for the Fig. 7?

Answer Thank you for your comment. We added the details and revised to “Figure 7. Transverse emittance at the bunch charge of 1 nC with using a pulse duration of 8 ps and rms beam size of 0.5 mm.”

 

14. Sentence in lines 193-194 almost repeats the sentence in lines 190-191.

Answer Thank you for your comment. We decided to delete the sentence in lines 190-191.

 

15. Information in almost all figure captions is insufficient. In Fig. 9, what do different lines mean? What is the x axis? What is "Profile"? Why do the red and blue lines in left and right parts look similar?

Answer Thank you for your comment. We revised Figure 9 for a clear explanation, the red and blue lines refer to magnet shape or magnet profile, so there are the same.

 

Comments on the Quality of English Language: The word "relation" in line 177 is inappropriate

Answer Thank you for your comment. We revised to “Figure 6 plots the dependences of the transverse emittance on the laser spot size and longitudinal laser pulse.”

Please find the attached document containing our response to your comments and suggestions.

Best regards,

Siriwan Jummunt

Author Response File: Author Response.docx