Neutral Beams for Neutron Generation in Fusion Neutron Sources

Round 1

Reviewer 1 Report

The contribution of a beam to the fusion rate has been studied for years starting from pioneering work of Jasby and Kulsrud. TFTR showed some contributions to fusion by the beam. The issue is that the beam particles flowing through the plasma are subject to various instabilities, as well as plasma spin up and it is hard to predict what will happen in real plasma. See e.g., the recent review by Heidbrink  and White. There is no any V&V procedures shown, therefore it is hard to access utility of results.

Besides the proposed neutron source will have small life time due to issues with the first wall and will probably not be economically viable. Recent laser-based approaches can be better choice for this reason, see e.g. recent APL from MEC/ SLAC.

Author Response

The contribution of a beam to the fusion rate has been studied for years starting from pioneering work of Jasby and Kulsrud. TFTR showed some contributions to fusion by the beam.

• Several references to the pioneering works in the field are added

The issue is that the beam particles flowing through the plasma are subject to various instabilities, as well as plasma spin up and it is hard to predict what will happen in real plasma. See e.g., the recent review by Heidbrink  and White.

• This is correct, we added the comments in Conclusions, and the reference to Heidbrink’s paper 

There is no any V&V procedures shown, therefore it is hard to access utility of results.

• Indeed, the V&V procedures are not shown in the paper, but they were shown in our previous papers (ref. 18). The cross-checking procedures of NESTOR with other models helped us to discover the usage errors in beam-plasma calculations (ASTRA/NUBEAM) done in the past

Besides the proposed neutron source will have small life time due to issues with the first wall and will probably not be economically viable. 

• In FNS facility design, the FW issues are considered and evidently evaluated. The device viability analysis is brought in the refs 1-5.

Recent laser-based approaches can be better choice for this reason, see e.g. recent APL from MEC/ SLAC.

• Thank you for your kind advice. We’ll probably consider it in future.

Thank you very much for your evaluation of the work.

Reviewer 2 Report

The paper describes the results from a numerical code (NESTOR) to compare the neutron productions mainly between the beam-plasma interactions and the conventional thermonuclear fusion plasmas.  Authors’ message and intention seem to be clearly delivered through the code results including figures. Nevertheless, there are a couple of spots that need more specific explanations.  These include

1. Figure3. Authors may need to explain why the amplitude of peak increases toward the core of the plasma – Although Te increases inward, I would expect that the beam attenuates significantly as it propagates in and therefore, the fusion rate would decrease again.

2. Figure3. It would be better if Authors include on the plots a notation that indicates which psi is the core although we know psi = 0 is the core.

3. Figure4. Lines 151-154, P.4. The authors state the effect of the off-axis neutral beam injection associated with the results from Fig 4.  However, I suspect that the statement is not consistent with the results shown in Fig 3. Unless my interpretation is wrong, the height of the peak on Figure 3 should be such that those from psi = 0.5 – 0.8 have the highest values.  Could you clarify this?

4. Rt (first appears on p.5). Authors need to put a comment what Rt is when it first appears.

5. Figure 5. Although I can eventually figure it out from the text, it would be better if Authors include the notation that which lines correspond to which flux surfaces.  A simple arrow indicating core to edge would suffice.

6. Overall. In the calculations, do authors include density profiles?  It is mentioned that the Te profiles are included as parabolic forms but not for density, which I assume has non-negligible effects.  At least, the authors need to make a comment on this in the text (why they did not include density profile in the calculations etc).

7. General. Can the code (NESTOR) deal with the impurities?  At least, Authors can mention the role of impurities in the efficiency of generating neutrons (or why the model in the code does not include the impurity sources) probably in the conclusion section.   

Author Response

The authors are grateful to the Reviewer for careful reading and for the comments. We addressed them all.

1. The explanations of peak behaviour (Fig 3) are added
2. The core/edge labels are added (Fig 3)
3. The difference between Figs 3 and 4 is explained (by uniform and weighted fast ions distributions)  
4. Rt is defined
5. The arrow core-edge is added to Figs 5
6. Density and Te profiles are added - to Fig 3b. We definitely forgot to show them before, thank you! 
7. NESTOR is able to deal with impurities (although this is not shown). The impurities effects are briefly summarized in the conclusion. Again, thank you for this important suggestion!