Laser Wakefield Photoneutron Generation with Few-Cycle High-Repetition-Rate Laser Systems

Round 1

Reviewer 1 Report

The manuscript describes a numeric simulation study of photoneutron generation using laser generated beams of electrons. The study aims to analyze the experimental capabilities of the e-SYLOS beamline at ELI-ALPS. After a brief description of LWFA the authors give the details of  a 3-dimensional particle in cell simulations for the expected electron beams in their facility. In the next chapter the authors use a particle transport simulation to calculate the neutron and photon yield in a metal converter. Descriptions of the geometry of the converter and different impinging electron beam energies are given. The final chapter gives an overview of different applications for the discussed neutron source.

The main idea of the article is to predict the neutron yields for upcoming experiments and their match with possible applications.

Major Issues:

·       In line 279, the authors stat that with already existing laser technology, a sufficient amount of Mo-99 may be generated, and give some details about the calculation in a footnote. A high power laser capable of accelerating electrons to MeV energies at a MHz rate is most certainly not in existence, and so this comment must be removed.

·       Section no. 3 relays on many past results in the literature, but except for its last sentence completely lack references to the relevant results (e.g. approximating the GDR in line 173).

·       in line 157: the claim that “with higher laser energies conversion efficiencies above 10% are expected.” Is not supported in the paper for the Wakefield scheme as the scheme is limited by beam loading.

·       Figure 8. is not explained in the text and is adapted from a different publication. It should be removed as the main point is already given in the text.

·       In section 4, I would recommend to the authors to give their estimate to conducting laser-based isotope- sensitive material analysis using their new beamline, such as been demonstrated at:

o   Zimmer, Marc, et al. "Demonstration of non-destructive and isotope-sensitive material analysis using a short-pulsed laser-driven epi-thermal neutron source." Nature communications 13.1 (2022): 1-11.

o   Kishon, I., et al. "Laser based neutron spectroscopy." Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment 932 (2019): 27-30.

This application would be easier to access than neutron radiography.

·        

Minor issues:

·       Line 20: MCNP is written only in initials.

Author Response

Thank you for your kind review.

In line 279, the authors stat that with already existing laser technology, a sufficient amount of Mo-99 may be generated, and give some details about the calculation in a footnote. A high power laser capable of accelerating electrons to MeV energies at a MHz rate is most certainly not in existence, and so this comment must be removed.

We have corrected to be more forward looking

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Section no. 3 relays on many past results in the literature, but except for its last sentence completely lack references to the relevant results (e.g. approximating the GDR in line 173).

Please note we added reference (line 185) for the GDR approximation but did not include the change in our answer.
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In line 157: the claim that “with higher laser energies conversion efficiencies above 10% are expected.” Is not supported in the paper for the Wakefield scheme as the scheme is limited by beam loading.

We have corrected and added reference.  Please note, in this paper we find 29% conversion efficiency. 

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Figure 8. is not explained in the text and is adapted from a different publication. It should be removed as the main point is already given in the text.

We respectfully would like to keep the figure as it shows the motivation for transmutation.  We have added more explanation in the text and caption.

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In section 4, I would recommend to the authors to give their estimate to conducting laser-based isotope- sensitive material analysis using their new beamline, such as been demonstrated at:

o   Zimmer, Marc, et al. "Demonstration of non-destructive and isotope-sensitive material analysis using a short-pulsed laser-driven epi-thermal neutron source." Nature communications 13.1 (2022): 1-11.

o   Kishon, I., et al. "Laser based neutron spectroscopy." Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment 932 (2019): 27-30.

This application would be easier to access than neutron radiography.

Thank you for the reference.  We added neutron spectroscopy as well as the rerefences.  

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Line 20: MCNP is written only in initials.

We expanded the initials: Monte Carlo N-particle

Reviewer 2 Report

The manuscript reports on simulation results for photoneutron generation with few-cycles new-generation laser systems available at large laser facilities currently being commissioned. The approach itself has already been investigated numerically and experimentally in the past, as reported in literature, however the neutron beam performances presented in this work anticipate enhanced features (mainly neutron fluxes) thanks to the state-of-the-art performances of the primary (laser) source. The topic is of high interest to the specialized laser-plasma community. Additionally, a broader interests is anticipated based on potential advantages of such a scheme with respect to conventional machines. I support the publication of this work, however there are a few important points to be clarified and changes implemented in the final version of the manuscript before its publication.

1. Fig.1c shows the electron energy distribution numerically estimated with a given range of laser/target parameters. Although this can be considered as "representative" based on the available laser technologies, the authors should extensively explain what is the room for optimization of such an initial electron energy distribution. In other words, is this distribution ideal for the given goal of optimizing the final photoneutron flux? If not, what shape/energy could be considered ideal? Also, what would be the key parameters to be optimized (laser pulse features, target, ...) with the goal to produce a neutron source with enhanced performances? Although there is some general sentence in the Conclusion, this is not sufficient to make this numerical work solid enough to support the author's statements. The Conclusions should be substantially extended by showing, at least qualitatively, the pathway towards a future optimization of the neutron source.

2. The authors write "The efficiency of the electron acceleration at these laser pulse parameters are very high, the accelerated electron bunch has a total energy of 32 mJ – as the total corresponding to a 29% laser-to-electron conversion efficiency. The cause of this high conversion efficiency is the strong coupling between the laser and the plasma and the high bunch charge (number of accelerated electrons)." This statement should be better supported by some physical explanation. Why there is a strong coupling and high charge?

3. The authors write (page 7): "The neutron spectrum from the LWFA electrons is the same as shown in Fig. 5 for the monoenergetic electron source." This is not clear to me. In which sense the neutron spectrum is the same? Furthermore, this spectrum is one of the most important of the overall work, and it seems to me that it is not shown in the manuscript. 

4. The last paragraph of the Conclusions, "Particle accelerators.... requirements.", contains too general statements/announcements that are not supported. The presented approach is definitively interesting and deserve future investigations, especially experimental ones. However, laser driven neutron source performances are presently far from those guaranteed by state-of-the-art conventional technologies.

Author Response

Dear Reviewer,

Thank you for your time to conduct a thorough review our our manuscript. 

Please see the attachment.  

Author Response File: Author Response.docx