Magnetic Noise Mitigation Strategies for the Einstein Telescope Infrastructure
Round 1
Reviewer 1 Report
Comments and Suggestions for AuthorsIn this manuscript, the authors discuss the magnetic noise mitigation strategies under development, based on their experiences from Virgo. Two passive strategies applied to the test mass tower are studied, i.e. introducing a layer of high magnetic permeability material to redirect magnetic field lines, or leveraging eddy currents by enclosing the interferometer arms and the tower at the intersection regions with hollow cylinders made of conductive material. Additionally, a new facility is planed to enable the characterization of devices' magnetic emissions, facilitating the development of effective mitigation strategies. The study will help the Einstein Telescope improve low-frequency sensitivity significantly, which is thus of crucial importance for the development of the next generation gravitational wave detectors. The manuscript is well written and well organized.
Here I have some minor suggestions. I believe this manuscript should be accepted for publication after these suggestions are implemented in a revised version.
(1) P1, Line 27, in the expression of $pT / \sqrt(Hz)$, the symbols of $p$ and $T$ are not defined.
(2) P2, Line 65, a full stop should be placed after Equation (1), but not before the equation.
(3) P3, Line 72, in this Section 2, I only see one subsection, which is "2.1 MANET facility". I did not find the Subsection 2.2 . In this case, I think it is not necessary to divide Section 2 into subsections. Please re-arrange this section.
(4) P4, the authors present their results in Section 3 in the form of a number of tables, i.e. Tables 1 -- 8. These results are obtained from numerical simulations. However, the authors did not describe how the simulations were performed. It is suggested that the authors should describe their methods in some detail. For example, did they use any particular softwares (tools) ? They could also consider to give some necessary equations used in the calculations.
(5) P8, Line 176, Kagra ---> KAGRA
Author Response
Thank you very much for taking the time to review this manuscript. Please find the detailed responses below.
Comments 1: P1, Line 27, in the expression of $pT / \sqrt(Hz)$, the symbols of $p$ and $T$ are not defined.
Response 1: pT is a unit of measurement of magnetic field (pico Tesla).
Comments 2: P2, Line 65, a full stop should be placed after Equation (1), but not before the equation.
Response 2: I modified following the suggestion.
Comments 3: P3, Line 72, in this Section 2, I only see one subsection, which is "2.1 MANET facility". I did not find the Subsection 2.2 . In this case, I think it is not necessary to divide Section 2 into subsections. Please re-arrange this section.
Response 3: I modified following the suggestion.
Comments 4: P4, the authors present their results in Section 3 in the form of a number of tables, i.e. Tables 1 -- 8. These results are obtained from numerical simulations. However, the authors did not describe how the simulations were performed. It is suggested that the authors should describe their methods in some detail. For example, did they use any particular softwares (tools) ? They could also consider to give some necessary equations used in the calculations.
Response 4: I added the name of the software used for the simulations in the article.
Comments 5: P8, Line 176, Kagra ---> KAGRA
Response 5: I modified following the suggestion.
Reviewer 2 Report
Comments and Suggestions for AuthorsGeneral appreciation:
The paper provides a nice, relevant, albeit simplified study of
passive shielding methods to protect future GW interferometers from induced magnetic noise.
It will become an important noise factor especially at low frequencies, as indicated in the introduction.
Further studies that provide more details on materials and geometries, would be recommended to
follow up on this work.
Nevertheless this is a good initial onset and should be published, provided that comments made below are taken into account.
Comments per section:
++++++++++++++++++++++
2. Magnetic noise investigation:
----------------------------------
It would be good to explain formula (1) in a bit more detail:
Where does the index, i , run over?
Why should the far field coupling function, CF_{BC} depend linearly on the
near-field coupling function CF_{i, SC}?
3. Magnetic noise mitigation:
---------------------------------
Important:
You seem to be employing some kind of modeling software for the materials and
geometry in order to compute the magnetic fields in or near critical areas.
Can you elaborate on this?
Details:
line 96: The former includes Helmholtz coil.
->Use either plural or the 'a' article if you remain singular
line 104: What do you mean by 'intersection area'? The area near the beam splitter?
line 109: 'a static external magnetic field direction' --> Is it also uniform? And is that justified?
Lines 111 to 114: 'increasing the coverage area of the arms with 111
high-permeability material beyond a certain size—determined by the geometry—does 112
not further reduce the magnetic field in the intersection region' --> Can you explain why this is so?
3.2. Eddy currents:
====================
Important:
This seems to shield only in case of AC magnetic fields. Please highlight this aspect.
In your simulation, you generate an external field of 1kHz. Why do you use this high frequency? You
correctly allude in your introduction that magnetic noise is particularly relevant in the low frequency range
between 1 and 100 Hz... So, why not generate external fields in this frequency range to be consistent?
Details:
Line 126: 'two pairs of cylinders are arranged in a Helmholtz configuration" --> What do you mean by cilinders?
The drawing seems to indicate that these are thin rings? What is their length?
Table 3 and 4: You seem to be able to determine the shielding factor in this case to 3 significant digits while in tables 1 and 2 you only
go up to 2 significant digits. Can you explain why this is so? Are there any uncertainties associated with these numbers?
References:
There exists extensive recent literature on measurements of magnetic coupling factors, and of the size and origin of Schumann induced
magnetic fields that can be correlated over large distances. Perhaps it would be a good idea to quote some of these relevant articles.
Author Response
Thank you very much for taking the time to review this manuscript. Please find the detailed responses below.
Comments 1: Magnetic noise investigation. It would be good to explain formula (1) in a bit more detail: Where does the index, i , run over? Why should the far field coupling function, CF_{BC} depend linearly on the near-field coupling function CF_{i, SC}?
Response 1: I have included more details in the article section also including a reference in which the formula is explored in depth. The indices run up to N, number of towers, location of couplings.
Comments 2: Magnetic noise mitigation. Important: You seem to be employing some kind of modeling software for the materials and geometry in order to compute the magnetic fields in or near critical areas. Can you elaborate on this? Details: line 96: The former includes Helmholtz coil.->Use either plural or the 'a' article if you remain singular. line 104: What do you mean by 'intersection area'? The area near the beam splitter?line 109: 'a static external magnetic field direction' --> Is it also uniform? And is that justified?Lines 111 to 114: 'increasing the coverage area of the arms with high-permeability material beyond a certain size—determined by the geometry—does not further reduce the magnetic field in the intersection region' --> Can you explain why this is so?
Response 2: I edited the article following your suggestions in the "Details". For the other comments, I have inserted the name of the software used in the text. Furthermore, the intersection area refers to the area between the arms and the mirror towers, as per CAD.
Comments 3: Eddy currents. Important: This seems to shield only in case of AC magnetic fields. Please highlight this aspect. In your simulation, you generate an external field of 1kHz. Why do you use this high frequency? You correctly allude in your introduction that magnetic noise is particularly relevant in the low frequency range between 1 and 100 Hz... So, why not generate external fields in this frequency range to be consistent? Details: Line 126: 'two pairs of cylinders are arranged in a Helmholtz configuration" --> What do you mean by cilinders?The drawing seems to indicate that these are thin rings? What is their length?Table 3 and 4: You seem to be able to determine the shielding factor in this case to 3 significant digits while in tables 1 and 2 you only go up to 2 significant digits. Can you explain why this is so? Are there any uncertainties associated with these numbers?
Response 3: For the AC fields in the text reference is made to Faraday's law. In the simulations we generated high frequency fields due to a limitation of the solver of the software used. Thanks for the comment, I've put an explanation in the "Discussion" section. As regards the cylinders, we mean the coils and their heights are indicated in the article (5 cm). For significant digits, we have included 3 in Table 4 to show the small variation.
Comments 4: References. There exists extensive recent literature on measurements of magnetic coupling factors, and of the size and origin of Schumann induced magnetic fields that can be correlated over large distances. Perhaps it would be a good idea to quote some of these relevant articles.
Response 4: I have inserted another more recent reference.
Thanks again for all the comments and suggestions.
