Low Power Cusped Field Thruster Developed for the Space-Borne Gravitational Wave Detection Mission in China
Round 1
Reviewer 1 Report
Appreciation of the article “Low power cusped field thruster development in Harbin Institute of Technology”, submitted by Hui Liu, Ming Zeng, Xiang Niu, Hong-Yan Huang and Da-Ren Yu to a special issue - "Plasmas for Space Propulsion" – of “Applied Sciences” (ISSN 2076-3417).
TITLE: “Low power cusped field thruster development in Harbin Institute of
Technology”
AUTHORS: Hui Liu, Ming Zeng, Xiang Niu, Hong-Yan Huang, Da-Ren Yu
DATE: June, 2021
SUMMARY ASSESSMENT
In this article, the authors present a micro thruster, the cusped field thruster, better known by the initials HEMPT (High Efficiency Multistage Plasma Thruster), which is a potential candidate device of the essential attitude control system for the mission of detecting gravitational waves in space, but eventually not only. The history, general physics (without any equations - in the entire text there is only one equation for the angular frequency of the plasma) and optimization developments are presented, including prototype design, experimental research and simulation studies, in particular, but not limited to, those carried out at the Harbin Institute of Technology in China. Progress in downscaling this type of low power thruster is indicated, and is introduced a new concept of a cusped field thruster utilizing a microwave discharge.
The article is very interesting, is well structured and may be of interest for low power applications in free space due to its low complexity and potential long life over a wide range of thrust and large thrust throttling ability.
The Abstract, Introduction and Conclusions are as expected, with no major comments to make. A long list of references is provided, with about 80 entries.
The text is written comprehensively and specifically with regard to English, generally of a very reasonable level, I have only made, here and there, small suggestions for changes.
I suggest the Editors that the article be published in the “Applied Sciences” special issue journal after being revised according to the small recommendations and suggestions set out below.
GENERAL RECOMMENDATIONS:
1.- I do not like - nor do I think it is advisable - with the title of the article, since it appears as an article of propaganda for a given institution, which devalues, from the outset, its content. And as for me, this article is much more than that. Therefore, authors should look for a more appropriate title where no reference is made to the institution to which they are linked. I can suggest a title ("Low power cusped field thruster developed for the space-borne gravitational wave detection mission") but the authors are sure to find a more appropriate one.
2.- In the many places in the text where the value of a certain quantity is indicated, the unit must not be glued to the numerical value, a space must be given between the numerical value and the unit. For example, but it's just one of many, many possible examples, on line 47 it says "... affected by 2-30N level...", when it should read "... affected by 2-30 N level.. ." Please correct throughout the text of the article.
3.- In the many places in the text where a call to a particular reference(s) is made, this call must not be pasted to the previous word, a space must be given between that word and the reference number. For example, but this is just one of many, many possible examples, on line 32 it reads "... in the satellites[1].", when it should read "... in the satellites [1]." Correcting, please, throughout the entire text of the article.
4.- Some figures, particularly 5, 10, 13 and 16 are very small and almost illegible.
Other figures such as 15 (a, b, c, d, e and f) and 18 (a, b and c) are on different pages and must appear on the same page. It's a matter of formatting.
SOME OTHERS SPECIFIC CONSIDERATIONS AND SUGGESTIONS:
- In the Abstract or, in line 70, in the worst case, it must be indicated that HEMPT stands for High Efficiency Multistage Plasma Thruster.
- In the article, the authors only refer to Kr as the propellant used (either experimentally or in computer simulations). Is there any reason for this? Why not Argon or the more traditional Xenon? Can you please make a reference to the preference for Kr?
- lines 50 and 51: “Also, the requirements of the laser instruments demand thruster noise no more than 0.1μN.” => Also, the requirements of the laser instruments demand a thruster noise of no more than 0.1 μN.
- lines 180 and 181: “… the reduction of propellent utilization ultimately results in the total performance down..” => … the reduction in propellant usage ultimately results in a decrease in overall performance.
- lines 186 and 187: In Figure 9, there's a problem with the caption: it's confusing. Perhaps: Experimental results of the thrust (a) and results of the simulation of the electron number density (in m-3? cm-3? arbitrary?...) for different values (or: as an effect) of the magnetic field strength in a cusped field thruster. Correct the caption appropriately, please. Also in this Figure 9, what is the meaning of F54, F58, F62 and F66?
- lines 188 and 189: “Studies on magnetic topology show that the shape of the last magnetic separatrix influents the plume divergence significantly[58].” => Studies on magnetic topology show that the shape of the last magnetic separatrix influences the plume divergence significantly [58].
- line 233: the title of subsection 3.2 should move to the next page - formatting problem only.
- lines 264 and 265: “Experimental results show that a long channel leads to significant plume divergence and total efficiency down,…” => “Experimental results show that a long channel leads to significant plume divergence and a reduction in overall efficiency,…”
- lines 273 and 274: The caption to Figure 16 can, and should, be improved: Differences among thrusters with different channel lengths: (a) propellant consumption and (b) ion acceleration efficiency. In figure (c), are presented the results of the simulation of ion loss on channel walls for different channel lengths.
- lines 276 and 277: “Similar with the traditional annular hall thruster,…” => Similar to the traditional annular hall thruster,…
- line 293: In Figure 17, what is the meaning of CFTK0, CFYK4 and CFTK8? Please make a reference to its meaning in the figure caption.
- line 369: where is "Figure 23" => Figure 22.
june 29th, 2021
Comments for author File: 
Comments.pdf
Author Response
Response to Reviewer 1’s comments:
We appreciate your review and thoughtful decision on our manuscript. Your suggestions contribute to improving our work, and they are earnestly considered. The manuscript is revised as follows.
- Comment: I do not like - nor do I think it is advisable - with the title of the article, since it appears as an article of propaganda for a given institution, which devalues, from the outset, its content. And as for me, this article is much more than that. Therefore, authors should look for a more appropriate title where no reference is made to the institution to which they are linked. I can suggest a title ("Low power cusped field thruster developed for the space-borne gravitational wave detection mission") but the authors are sure to find a more appropriate one.
Re: As your suggestion, we revised the title to a more appropriate one that contains no linked institution name. However, low power cusped field thruster was significantly developed in Europe, only referred to in outline in this review. On the other hand, this manuscript is majorly about the development in China. So finally, we decided to entitle this manuscript “Low power cusped field thruster developed for the space-borne gravitational wave detection mission in China."
- Comment: In the many places in the text where the value of a certain quantity is indicated, the unit must not be glued to the numerical value, a space must be given between the numerical value and the unit. For example, but it's just one of many, many possible examples, on line 47 it says "... affected by 2-30?N level...", when it should read "... affected by 2-30 ?N level.. ." Please correct throughout the text of the article.
Re: These common-sense clerical errors are revised separately following your suggestion, highlighted with red font.
- Comment: In the many places in the text where a call to a particular reference(s) is made, this call must not be pasted to the previous word, a space must be given between that word and the reference number. For example, but this is just one of many, many possible examples, on line 32 it reads "... in the satellites[1].", when it should read "... in the satellites [1]." Correcting, please, throughout the entire text of the article.
Re: These errors are revised as your suggestion and highlighted in red.
- Comment: Some figures, particularly 5, 10, 13 and 16 are very small and almost illegible. Other figures such as 15 (a, b, c, d, e and f) and 18 (a, b and c) are on different pages and must appear on the same page. It's a matter of formatting.
Re: Following your suggestion, Figure 5, 10 are blown up to an appropriate size, and Figure 13, 16 as well as 17 are redrawn with larger fonts. Figure 15 and 18 are put in the right position to make pics in the same page.
- SOME OTHERS SPECIFIC CONSIDERATIONS AND SUGGESTIONS
Re: Specific revisions are made as your suggestion. Captions and figures that short of information are completed and the overall format is adjusted.
Particularly, as for the reviewer's consideration of the propellant, it's one of the lapses that we didn’t claim the fact that the cusped field thruster commonly uses Xe propellant. It’s now indicated in line 115.
Author Response File: 
Author Response.docx
Reviewer 2 Report
Reviewer enjoyed exploring authors' findings on cusped field thruster. Some minor revisions are required as follow.
- In line 32, there is typo as 'In order o meet' -> 'In order to meet'
- Please add a space between number and unit. For example, '2-30uN' should be '2-30 uN'. Authors should check it out for entire manuscript.
- The resolution of Figs. 16 and 17 should be enhanced.
Author Response
Response to Reviewer 2’s comments:
Thank you very much for your positive opinion on our work and also for the constructive suggestions on this manuscript. The manuscript is revised as follows.
- Comment: In line 32, there is typo as 'In order o meet' -> 'In order to meet'
Re: This clerical error is now corrected in line 32.
- Comment: Please add a space between number and unit. For example, '2-30uN' should be '2-30 uN'. Authors should check it out for entire manuscript.
Re: Following your suggestion, these format errors are revised one by one throughout this manuscript.
- Comment: The resolution of Figs. 16 and 17 should be enhanced.
Re: Figure 16 and 17 are redrawn with bigger fonts and larger in picture size.
Author Response File: Author Response.docx
