Spatial Distribution Analyses of Axially Long Plasmas under a Multi-Cusp Magnetic Field Using a Kinetic Particle Simulation Code KEIO-MARC

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The authors present numerical simulations of hydrogen plasma generated by electron impact ionization in a long multi-cusp magnetic trap.  They employ a code that was developed by another group and is applied to the geometry of interest here to study the uniformity of the plasma produced as a function of the strength of the permanent magnets used for form the cusp field, the number of such magnets, and the number of electron sources (filaments).  The work appears to have been performed and analyzed well.  I have a number of relatively minor suggestions that the authors should consider before I would be prepared to recommend this paper for publication.

1. Treatment of the ends of the 3 m long system is not described very completely.  The authors indicate that the ends are maintained at “floating potential.”  By this, I assume they mean that no net electric current goes to the end caps, and the potential is adjusted to make sure this is the case.  This means that particles are continually lost to the end caps.  Presumably that is the primary loss channel.  The authors should explain more fully how the ends are treated in the calculation.
2. The authors should provide some indication of how the code has been benchmarked against experiments to give the reader some confidence that it accurately reflects the important physics in the kinds of systems the authors describe here.
3. The title should indicate that the work described here is numerical simulation, not experiment.
4. The acronym AIT-PID should be defined on page 1.
5. In the first line on page 4, the authors use the work “neutrons” when they certainly mean “neutrals.”
6. No discussion of the Debye length is provided.  It appears to be very short compared to the dimensions of the system and justifies the used of sheath potential as an initial energy for the emitted electrons.  The authors might add reference to this quantity.

Comments on the Quality of English Language

The English is mostly good, with a few minor errors and/or awkward phrasing.

Author Response

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Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

In this manuscript, the authors present results of simulations in which they varied the strenght of the magnetic field created by the magnets as well as the numer of magnets as design parameters for an axially 3m-long plasma. The field intensity was varied from 0.25 to 1T and the number of magnets from 6 to 12 by step of 2. The aim of the work was to find out the optimal design parameters to obtain an axially uniform plasma or at leat to mitigate the non-uniformity of the plasma electron density over a distance of 3 m. I have some comments summarized below:

• The effects on the electron density uniformity by varying the B-field intensity and the number of magnets were studied independently. Unless I missed some thing, the authors did not varied simultaneously the number of magnets and the intensity of the field.
• The authors did not explain why the number of magnets are even (variation from 6 to 8, to 10, then to 12).
• Figure 3 is mentioned in the text before figure 2.
• In many places, the authors mention the FFR size without explaining the real meaning of size. Is it the radius or the diameter of the circular cross section?
• In figure 6, it is better the give the y-axis labels near the y-axis instead of caption. For a) indicate if it is the radius or diameter of the FFR.
• In line 231 it is written that the B-field intensity is 0.5T for figures 7 and 8 while in the caption of figure 7 the field value is 0.75T.
• Figure 8. Same remark about the y-axis label as for figure 6.

Author Response

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Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

This manuscript is about the  numerical modeling of a plasma source, attempting to recover optimized plasma profiles, with specific reference to the electron density which should be as spatially uniform as possible. 

The conclusions of the work are not unexpected: the stronger is the confining magnetic field, the smaller are the losses at the boundaries, and the more uniform the profile. Similar conclusions are reached by varying the number of magnets, and therefore the spatial uniformity of the magnetic field. 

The work is very well written and organized. Premises, analysis and conclusions are all reported very clearly and the results are convincing.

The only--minor--flaw that I could spot is that perhaps the authors might have provided some more details about the numerical code. For example, if I understand well, it is a particle code: how many particles employed the authors in the simulations, and therefore which uncertainty of statistical origin could be expected in the results? 

I acknowledge that the code was described elsewhere, though, and maybe the authors did not think it was important to provide again its details. 

In conclusion, I think that the manuscript may be published in the present form.        

Author Response

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Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

The authors have responded in a satisfactory way to my minor criticisms of the first submission.  I now recommend this paper for publication.

Reviewer 2 Report

Comments and Suggestions for Authors

The answers of the authors to my comments are sufficient to accept the manuscript for publication without any further amendments.