Electromagnetic Simulations and Measurements of the K-800 Superconducting Cyclotron RF Cavity at INFN-LNS

Round 1

Reviewer 1 Report

This work is clearly presented and is worth publication after minor changes.

However, although the simulations tool and the parameters studied are well described and clearly presented, no evidence of how these are related to the main goal of improving the maximum beam power. Please add, and correlate when possible, the consequences that the improvements may have on the beam power.

The manuscript would improve after a general revision of the English writing.

Please also consider the following list of suggestions:

Page 1 – Introduction: please change “the maximum provided beam power” to “the maximum beam power provided”;

Page 1 – Introduction: please change “up to now” to the beginning of the sentence;

Page 1 – Introduction: please begin a new sentence after “[4-7]”;

Page 1 – Introduction: please change “which allows both to” to “which allows to simultaneously”;

Page 1 – Introduction: please be more explicit of which and/or how the planned RF cavities changes could result in, and how, an improvement of the beam power;

Page 1 – please change “The observed excellent agreement” to “the excellent agreement observed”;

Page 1 – Introduction: please change “of” in “will increase of two orders” to “by”;

Page 2 – section 2: please add a schematic or a photograph of the entire vacuum chamber, as in reference #6;

Page 2 – section 2: please rephrase “named “Dee” (because in the early cyclotron…”

Page 2 – section 2: please rephrase “to the upper and lower shorted inner”;

Page 2 – section 2: please change “to the facing surfaces distances” to “distance between facing surfaces”;

Page 3 – Figure 3: the legend should include the meaning of the 3 coloured arrows;

Page 4 –Figure 5: please specify the meaning of the colour scheme presented;

End of Page 4 – please rephrase “by our model: as an example….” To “by our model, exemplified with…”

Page 5 – Figure 7: why is there a blue line instead of a blue square for the measurement at around 39 MHz?

Figures 11, 12, 17 and 18: the vertical axis must be identified, somehow.

Page 8: “the upper one has been extracted of 55 mm while the lower one has been inserted of 50 mm,”: please specify how the authors chose these values.

Page 9 – Figure 14: please duplicate the graph from figure 9 for easier comparison

Page 10 – Figure 16: please duplicate the graph from figure 10 for easier comparison

Page 11 – please add figures 17 and 18 before the conclusion section.

Page 12: please normalize the references ass indicated by the journal:

• i) For instance, reference 1 only refers to “Calabretta, L.” while reference 2 indicates all the authors;
• ii) another example: the titles are sometimes shown in uppercases (e g. references 11 and 21) and sometimes not;
• iii) another example: the year of the references should be in bold all times

Author Response

We thank the reviewer for appreciating our work. We revised the grammar of the whole paper and applied all the minor changes. Please find attached also the “diff” file in which the changes with respect to the submitted version are highlighted.

Author Response File: Author Response.docx

Reviewer 2 Report

The new planned precise experiments in nuclear physics require high quality beams, applying new technology solutions. The proposed manuscript presents a necessary engineering step towards the experimental study of rare processes in nuclear physics using the K-800 Superconducting Cyclotron at INFN-LNS, in operation for more than 25 years, that requires the beam power to be increased by two orders of magnitude. Such a substantial beam upgrade needs intensive simulations for optimization of the electromagnetic cyclotron system. This manuscript reports on the simulations done on the existing structure as well as some improvements. In order to model the superconducting cyclotron RF cavity, the authors use two suitable commercial programs: CST Microwave Studio and COMSOL Multiphysics. Eigenmode simulations of the resonator with CST reproduce the TEM mode resonance frequency of the cavity and its tuning by two movable sliding shorts and a trimmer. The mesh of the model, as well the electric field vector-plot of the fundamental TEM mode are given. The obtained dependence of the sliding short distance from the median plane on TEM mode frequency is in a very good agreement with the measurements. Driven RF simulations are used to study |S₁₁|-parameter and analyze wave propagation and impedance. A detailed map of the electric field intensity is presented. The important coaxial-to-cavity impedance matching between the movable coupler tip and the cavity median plane is performed. Some relatively big discrepancy from 16 to 30% arises between the calculated and measured unloaded quality factor Q₀, and the authors explain this with many other factors, which is plausible. By means of 3D beam dynamics code under development in INFN-LNS, the authors study the vertical symmetry violation of the electric field inside the resonator and propose to neutralize it by introducing an asymmetry in the sliding shorts positions. Such a solution reduces the amplitude of the orthogonal electric field component and therefore the orthogonal size of the beam, improving its quality. The effect is shown in the figures with the calculated beam envelope.

Remarks:

1) Given the discrepancy, between the calculated and measured unloaded quality factor Q₀, it would be important if the influence of at least some of the mentioned factors was studied in depth.

2) COMSOL Multiphysics includes a particle tracking module, which could also be used for beam dynamics simulations and results obtained could be compared with these from the in-house developed 3D code. Is such a comparison planned?

3) There aren`t any references to the 3D beam dynamics code used by the authors. If such references still do not exist, a short description of the code would be useful.

4) Specify the COMSOL Multiphysics and CST Microwave Studio versions that you used to conduct the simulations.

5) I believe that including the equations that are solved will improve the quality of your paper. This applies to the boundary conditions that are set. For example, you mention metal boundary conditions (impedance boundary condition).

6) In Fig. 5 the color scale is missing. It makes it harder for the reader to interpret the results.

7) For better comparison of the envelopes, figures 10 and 16 could be as close as possible.

8) Proofread your paper again, as there are still some minor mistakes. For example, in the first sentence of subsection 3.1. Coaxial sliding shorts asymmetric position is written: "Even if the presented asymmetry and his effect on beam dynamics is compatible...".

Comments:

1. You mention that "the internal volume material was set as “vacuum” while a proper dielectric constant was used for the three alumina volumes". But did you use the standard materials with defined properties from COMSOL Multiphysics and CST microwave studio or you defined new materials?

2. In subsection 2.5., in the list with possible reasons for the discrepancy between the experimental and simulation results are included some factors which can cause changes in the values of constants such as conductivity and permeability. Could these changes be the cause of the obtained discrepancies? Or the answer is not that simple? 

Conclusion:

The present manuscript is devoted to an upgrade of K-800 Superconducting Cyclotron at INFN-LNS. Using commercial and own codes, the authors model the electromagnetic system of the cyclotron, calculating and optimizing some important parameters. The results obtained are depicted in 16 figures. The text is comprehensive and clear. This work is a valuable contribution to the upgrade of K-800 cyclotron and it could be publish in Applied Sciences taking into account the remarks above.

Author Response

We thank the reviewer for appreciating our work. We revised the whole paper and applied the minor changes. Please find attached the file in which the changes with respect to the submitted version are highlighted.

Author Response File: Author Response.docx