Study of Coherent Smith–Purcell Radiation in the Terahertz Region Using Ultra-Short Electron Bunches

Round 1

Reviewer 1 Report

There are several expressions like in the incipit of the abstract that are not grammarly correct in English in my opinion, please revise the full manuscript.

Please provide more details about the R parameter, or an implicit expression.

I think that the second harmonic in Fig. 6 is placed at 3.1 THz instead of 3.4 THz.

Authors should discuss the expectations on polarization from 3D simulations, comparing them to measurements. They only discussed the disagreement with a simplified 1D model.

Overall the paper is well written and the results are clear. After the improvements I suggested I may accept the paper for publication, especially for the interest that it may have for beam diagnostics of ultra-short particle bunches.

Please revise the full text avoiding expressions as in the incipit of the abstract, that are grammarly not corrected.

Author Response

Refree 1

Comments and Suggestions for Authors

We thank the reviewers for their suggestions to improve the manuscript. We have revised the manuscript in accordance with the reviewers' comments. Below is a point-by-point explanation of our responses to each reviewer's comment.

There are several expressions like in the incipit of the abstract that are not grammarly correct in English in my opinion, please revise the full manuscript.

Response 1 : The entire manuscript was revised to correct inaccuracies and clarify issues.

Please provide more details about the R parameter, or an implicit expression.

Response 2 : We provided more details about the grating efficiency R^2.

I think that the second harmonic in Fig. 6 is placed at 3.1 THz instead of 3.4 THz.

Response 3 : Thank you for pointing out the typo. We have corrected the values.

Authors should discuss the expectations on polarization from 3D simulations, comparing them to measurements. They only discussed the disagreement with a simplified 1D model.

Response 4 : A description was added stating that the polarization expected by the 3D calculation, including the shading effects, was 99.9%. This result arises from the fact that the contribution from the second facet is very small owing to shading effects, which is qualitatively consistent with the measured results.

Overall the paper is well written and the results are clear. After the improvements I suggested I may accept the paper for publication, especially for the interest that it may have for beam diagnostics of ultra-short particle bunches.

Thank you very much for reviewing our manuscript and recognizing the quality of our manuscript.

Comments on the Quality of English Language

Please revise the full text avoiding expressions as in the incipit of the abstract, that are grammarly not corrected.

Reviewer 2 Report

My main questions and notes are following:

1.There should be described a grating "tooth" shape in Fig.1 (angle alfa, period, groove depth).

2.There are no explanations concerning so-called "surface current vector"(see Fig 3b). I don't understand the value of such a vector  for X<-1000 micrometers if the maximal groove depth is equal Xdepth= 600*Sin[6 deg] micrometers.

3.The statement "..change in bunch length from 80 to 150 fs" (see p.5) is absolutely incorrect. The bunch length is fixed (100 fs) and doesn't depend on an observation angle.

4.Authors have to explain  strong suppression of the second orders SPR lines in Fig.6b.

5.Authors should estimate  spectral resolution of the Michelson interferometer and compare it with measured line widths.

Author Response

Refree 2                                                        

Comments and Suggestions for Authors

We thank the reviewers for their suggestions to improve the manuscript. We have revised the manuscript in accordance with the reviewers' comments. Below is a point-by-point explanation of our responses to each reviewer's comment.

My main questions and notes are following:

1. There should be described a grating "tooth" shape in Fig.1 (angle alfa, period, groove depth).

Response 1 : Figure 1 was replaced to include the blaze angle alfa.

2. There are no explanations concerning so-called "surface current vector"(see Fig 3b). I don't understand the value of such a vector  for X<-1000 micrometers if the maximal groove depth is equal Xdepth= 600*Sin[6 deg] micrometers.

Response 2 : Thank you for pointing out the incorrect expression. We have corrected the typo in the horizontal title in Fig. 3.
         incorrect : x-position
         correct : y-position
A brief explanation of the current density vector is also provided.

3. The statement "..change in bunch length from 80 to 150 fs" (see p.5) is absolutely incorrect. The bunch length is fixed (100 fs) and doesn't depend on an observation angle.

Response 3 : We have corrected the misleading explanations.

4. Authors have to explain  strong suppression of the second orders SPR lines in Fig.6b.

Response 4 : A strong suppression for the second harmonics measured at 45°(3.2 THz) is explained by the small form factor (< ~0.1) for the 100 fs bunch.  On the other hand, it is expected that the second harmonics observed at 60° (1.9 THz) is suppressed to ~30% owing to the form factor. Considering the wavelength dependence of the Z-cut quartz window, it is further reduced by approximately 20%, which results in the expected intensity being a few times larger than the measured intensity. Although the further study is needed to investigate in more detail, it is considered that there was no significant discrepancy with the measured results because of the poor signal intensity in the preliminary measurements.

5. Authors should estimate  spectral resolution of the Michelson interferometer and compare it with measured line widths.

Response 5 : We have revised the measured values and included the errors. The source of the measurement error is the maximum optical path length in the interferogram and the finite angular acceptance to be transported to the detector.  Spectral resolution of the inferferometer is estimated to be 0.037 THz, while the error due to finite acceptance is 0.023 and 0.052 THz for the observation angle 60 and 45 deg., respectively. The expected line width is estimated as the error by the quadratic sum of these values and natural line width (~ 1/mN, m:harmonics, N:No. of grating periods) of the radiation.

Round 2

Reviewer 2 Report

The manuscript may be published as it stands now.