Effect of Molybdenum Coatings on the Accelerating Cavity Quality Factor

Round 1

Reviewer 1 Report

The paper presents theoretical estimates for the ohmic quality factor degradation in a copper cylindrical resonator covered by a thin layer of molybdenum. This paper leaves an overall feeling of a senior undergraduate level research project and does not have any scientifically significant results worthy of being published in a refereed journal. Therefore I suggest that the paper shall be rejected. Some particular comments are the following:

1. There is no much evidence that dark current in accelerating structures increases probability of RF breakdown. Therefore reducing dark current does not necessarily improve high gradient performance of the structure.

2. Molybdenum coating of a cylindrical resonator is of little interest to practical applications because real accelerating cavities have more complicated shapes and coating the whole cavity is not always the best solution.

3. Please explain how you define "delta f sensitivity" and "Q0 sensitivity" and what are the sensitivity goals that you are trying to achieve.

4. Although theoretical results may look good in principle, creating a high quality coating is not a trivial matter. Do you have any plans of validating the results of your computation in practice and see if there is any agreement.

English language is understandable but the write up may still benefit from proofreading to correct some mistakes.

Author Response

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Reviewer 2 Report

Pushing the accelerating gradient of the normal conducting RF cavities to a higher limit is a very interesting topic not only for large-scale particle accelerators but also for commercial applications like medical accelerators. Using MoO3 coating to improve the work function, thus improve the limitation on peak E field, is an interesting idea. Some of the authors did pioneer work on this (refs 15,16), and continuous studies might bring a break through thus should be encouraged.

In section 2.1 electromagnetic analysis, the authors gave a comprehensive study that lead to the surface impedance of the coated material. However, the outcome is very similar to, if not the same as, that in N. Klein's paper (Klein N, Chaloupka H, MOHer G, Orbach S, Piel H, Roas B, Schultz L, Klein U and Peiniger M 1990 The effective microwave surface impedance of high Tc thin films Journal of Applied Physics 67 6940), which is widely used in both normal conducting and superconducting (dirty limit only) RF applications. This make the numerical study more like a home work or tech note (it is still very valuable though). Also in Figure 2 (and in Figure 3) I would expect that rho_MoO3/rho_Cu=1 would overlap with bulk Cu curve, and curve for 20 should be closer to the one for bulk Cu, curve for 2000 should be further away from the one for bulk Cu, there is something not right in this figure.

I would strongly recommend the authors to have experimental results (even one will be great) to compare with the numerical study to make this paper complete.

Author Response

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Reviewer 3 Report

This manuscript presents a new approach towards high gradient in accelerating cavities by performing molybdenum thin-film coating on copper. The authors performed numerical analysis on the effect of the molybdenum coating to key figures of merit of the cavity performance. Overall, I find the analysis solid and the writing clear. Therefore, I recommend the publication of the manuscript.

I do have a few minor comments/suggestions:

1. This work reminded me of the previous results by CLIC and SLAC on developing the clamped, molybdenum-iris structures for the CLIC structure prototypes. I see some correlations even though the previous work did not involve thin films. I leave it to the authors to decide whether it’s worth referring to the molybdenum-iris studies.

2. Line 97: “For the "ideal" cavity case, Rs = Xs = 0, thus Q0 and Δf / f0 may be written as follows”

 

I understand what the authors were trying to say, while this sentence reads a little awkward with the word “thus”, and it seems to indicate that Eq. (1) were for the ideal cavity case. Maybe rephrase it to something like: in general, with lossy materials the quantities are written as Eq. (1).

3. I recommend explaining the meaning of \rho ratios in the figure captions (Fig. 2 and 3) so that the figures are more readable independently.

4. I recommend trying to make the curves in Fig. 4 more readable by experimenting with different line types, for example, but it may be hard.

Author Response

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Round 2

Reviewer 1 Report

The paper reports studies of possible effects of molybdenum coatings on the quality factor and performance of the accelerating cavity. The title of the paper still states that the study is numerical, however, the paper now includes a section on experimental results, which suggests that the title must be modified.

The paper is much improved compared to the previous version. The introduction is expanded. Appropriate references are added. A section that reports the results of measurements of the surface resistivity of copper coupons coated with a thin layer of molybdenum is added and provides some verification of computational results. The measurements of the coated coupons also add novelty to the research described in the paper.

I believe, however, that the experimental measurements must be expanded further to provide more validation to the proposed coating concept. The authors should coat a thicker layer of molybdenum onto their copper coupon to demonstrate that degradation of the quality factor will indeed be observed for a sufficiently thick layer. This will also demonstrate that their measurement system operates correctly. Finally, a thicker coating will allow measurement of resistivity of the coated molybdenum. Very wide range of possible resistivities is used in simulations, and it would be important to get at least some estimates for what realistic resistivities of the molybdenum layer would be.

Finally, please add a paragraph on the future plans to your Conclusion section. Is there a plan to coat and test an actual accelerating cavity?

The paper still requires serious proofreading to correct multiple grammatical mistakes.

Author Response

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Reviewer 2 Report

I am happy to see experimental results, even they did not "fully prove" the calculations in the paper.

N. Klein's paper actually included results with metal substrate (with dielectric between film and substrate though, Fig1b in that paper), it is not difficult to modify it to remove the dielectric and the result should be the same as eq (3).

I agree with the authors that results in Fig 2&3 are counter-intuitive but correct, I also agree with authors explanation that it is due to the skin depth much larger than its thickness. I used my own code, which is not based on eq (3), it is based on normal/anomalous field distribution inside the metal, and I got quite similar results shown in Fig 2 at 300nm. It will be great if the authors can add one plot in Fig2 about RRR (in Log scale) vs Q0 with fixed thickness (at 300nm would be fine), it should be a v shape curve with lowest point at around RRR 2.5.

Author Response

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Round 3

Reviewer 1 Report

The authors stated they are unable to easily perform more experiments with different thicknesses of Mo layer. Without additional experimental validation the novelty of the paper remains low. I do not see enough merit for the paper to be published.

English is improved.

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Reviewer 2 Report

The added figure of Rs & Xs vs resistivity ratio for certain layer thicknesses helps clarify the counter-intuitive results, thanks to the authors for this effort. I still do not agree though, that the authors claimed that there is a substantial difference between Klein’s work and this work. In Klein's work he studied dielectric-metal film case and metal substrate-dielectric-metal film, and in the second case if you use 0 as the dielectric thickness, it gives the same results as in this paper, also in the studies https://arxiv.org/abs/2101.11678 & https://arxiv.org/abs/2004.11462 metal substrate-metal film, metal substrate-dielectric-metal film for both normal conductors and superconductors, and for both normal and anomalous cases, were calculated. Using the same method, the results in this paper can also be reproduced. This paper is worth publishing, my suggestion is that the authors mention Klein's work without emphasizing the difference.

Author Response

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