Erosion Rate Measurements for DART Spacecraft Ion Propulsion System^†

Round 1

Reviewer 1 Report

Dear Authors,

I have the following suggestions/comments for the article:

1. The title needs to made more specific w.r.t the content, somehow it look a bit general (optional).

2. The statistical outputs or the glance needs to be addressed in abstract. Pl revise.

3. Introduction part is well written, the only improvement i may suggest is additional recent references. I have gone through some research recently. You may check if it comes in the domain you worked on. (optional)

4. Section 2 indeed is well explained but adding tabular/flowchart like information would make it better to understand. Somehow the information would be easy to grab for the readers.

5. Captioning of Figure 2 needs a revision. Things are confusing, mark them with arrows and use appropriate text size.

6. Similarly Fig. 3 is clear for the authors but from the readers point of view some information seems to be missing or merged, the coordinate system, the abbreviations, the references etc. Pl revise.

7. Going through the assumptions, do state the effects and authenticity for them, like if neutralized flow is not included why and how it effects the study?

8. The data set/size population is good enough to carry the results? particularly for Fig. 6/7.

9. Cite the Equation numbers in the text.

10. Don't you think the numerical model with all the parameters needs to be discussed in a single section?

11. Technical writing needs a revision for figure specifically.

12. Figure 11 needs to be presented clearly as per previous comments.

13. Table 3 is complete for all the data set?

14. some basic redundant equations needs to omitted.

15. Comparison/validation part needs to be discussed separately.

16. For conclusion part repetition is not a good idea instead summarize.

Best wishes

Author Response

• title needs to be made more specific…(optional); No change
• statistical outputs or the glance in abstract; Added a sentence to abstract
• add references to intro (optional); Added some material but no references
• section 2 well explained but add tabular/flowchart info; Couldn’t come up with suitable material
• revise Fig 2 caption, increase text size in Fig…; Text size mod, added arrows, revised caption
• Fig 3 unclear; Revised the figure
• Assumptions – effects and authenticity for them; Added sentence about neutralizer flow
• Data set/size good enough (see Fig 6/7)?; At times more data would be helpful
• Cite the equation numbers Equation numbers have been updated
• Numerical model in single section?; Main model is in 3.3, other model in 3.2
• Figure technical writing; Some figure captions updated
• Figure 11; Updates in text
• Table 3 is complete?; Yes – there are just 2 parameters to hilite
• Eliminate basic redundant equations (optional); No change
• Discuss comparison/validation separately; Updated discussion section
• Avoid repetition in conclusion; Updated conclusion section

Reviewer 2 Report

This paper shows novel research results which is of interest for the space and electric propulsion community. The paper is well written and the approach can be understood.

Let me make a few detailed comments:

Background pressure effects should be show in more detail, e.g. a table with experiments and chamber pressures. 

Table 1 is not described in the text.

Subscripts in the text are inconsistently formatted, e.g. Ia and Ia. Please check all of these.

Line 22:  .. current sources were examined.

Line 81: the authors only mention that QCM measurements are challenging. A few more words about the issues should be inserted.

Section starting at line 119: did the authors investigate the impact of somewhat bulky equipment passing in front, at 1m, of a large thruster? A change in plasma parameters will result.

Line 136: it is stated that rotational symmetry was assumed. This is not a priori given for a thruster with an off-axis neutralizer at distances < 1m. The authors must show arguments for the symmetry assumption.

Line 175: ... cylindrical vacuum chamber with 2.4m...

Figure 6: please show an argument for the exponential fit of the data points.

Figure 7: please show an argument for the fitted function and the extrapolation for <100V.

Figure 8: text and diagram should be larger for better readability, zooming in does not solve this.

Figure 9: same as Fig. 8

Line 481:  ... to be Gaussian...

Line 507: see above, details on the symmetry assumption are needed.

Line 645: it should be Figure 15 and not 13.

Line 657: it should be Figure 16 and not 14.

Line 664-666: a statement about the thruster grids age should appear much earlier in the text. For me, such a question came up at the very beginning.

Line 695: please put a bit more weight on this finding.

Figure 16: again, an argument for the linear fit (and not a curved function) is needed.

Author Response

• Background pressure effects – more detail; Limited data here, fully described in text
• Table 1 not described; Added 2 references to Table 1
• Subscripts inconsistent; Updated accel current subscripts
• Line 22 – current sources; Revised text
• Line 81 – QCM measurements; Revised text, issue is minimized at high deposition angles
• Line 119 - bulky equipment in plume; LIF did not involve anything bulky in front of the thruster. QCM work was done later and QCM is never in front; 50 degrees off axis at 1-m was closest point. Even if plasma properties change near the equipment the effect on molybdenum atoms is expected to be minimal.
• Line 136 – rotational symmetry; Added text - neutralizer has very little influence on neutral density at the grids. Ion current is minimized by neutralizer position but may not be negligible. Ions will tend to be distributed with reasonable rotational symmetry. Ion beam current azimuthal asymmetry at the grids has been measured for one configuration but the data are not publicly available.
• Line 175 – cylindrical vacuum chamber; Revised text
• Figure 6 – show argument for exponential fit; Revised text – fit is phenomenological.
• Figure 7 – argument for fitted fcn and extrapolation; Revised figure and text; much less extrapolation and the fit is phenomenological
• Figure 8 – text & diagram should be larger; Revised figure for readability
• Figure 9 – text & diagram should be larger; Revised figure for readability
• Line 481 – gaussian; Capitalized
• Line 507 – symmetry; Revised text
• Line 645 – Figure #; Corrected figure number
• Line 657 – Figure #; Corrected figure number
• Lines 664-666 – grid age; First mention of grid age in the intro
• Line 695 – give more weight; Added reference to Table 3 and strengthened mention of neutralization length in the conclusion

Figure 16 – argument for linear fit; We have no theoretical argument for linear vs nonlinear – added statement in the text to this effect