High-Density Dynamics of Laser Wakefield Acceleration from Gas Plasmas to Nanotubes

Round 1

Reviewer 1 Report

The authors discuss LWFA in a high density plasmas by extremely short laser pulses. They provide 1D simulations and a few 2D simulations using the PIC code EPOCH.

The goal is to produce MeV energy electrons.

As the low density (close to the critical density) they propose using carbon nanotubes.

The paper contains no visible errors and can be published in MDPI Photonics as is.

Author Response

Dear reviewer,

Thank you very much for your positive evaluation.

Sincerely,

Bradley Nicks 

Reviewer 2 Report

This research article, entitled “High-Density Dynamics of Laser Wakefield Acceleration from Gas Plasmas to Nanotubes,” presents theoretical studies on the laser electron acceleration in different density regimes in consideration of the oncology application. The bulk electron acceleration in near-critical-density (NCD) plasma is an interesting topic. However, the contents of this article considerably overlap with the previous publication of reference [17], while the use of nanotubes for the high-density target is a new idea. This article is lengthy and difficult to understand because it contains several topics that were already introduced in the previous study. Thus, I would like to suggest rewriting this article, excluding the overlapped part with the previous study except the essential part to describe laser electron acceleration with nanotubes and focus on the dynamics and effects of using nanotubes for laser wakefield acceleration. Following are specific comments for rewriting this paper.

 

1. In the abstract, it should be mentioned that this study is theoretical works.
2. Laser proton therapy is not an issue of this paper, while it is described importantly in the second paragraph of the introduction.
3. If a₀=1, then the g(a₀)=√(1+a₀^2)-1 is not unity. The g(a₀) should be √2-1, and this value is below half of unity.
4. In figure 1, some descriptions in the figure on each component and structure would be better to be inserted to understand this figure.
5. At line 194, the phi is mainly known as the “Carrier Envelop Phase (CEP).” It is doubtful that there are no effects of the CEP on the electron acceleration, even for one or half-cycle pulse. This should be clarified.
6. What can happen if the CNT spacing is equal to or fraction of the plasma wavelength in the Grey regime (slightly under-dense plasma)? This situation can be formed by manipulating CNT density or ionizing laser properties. It would be interesting to investigate the resonant interaction between the modulated electron density and plasma wave.
7. I think the oncology analysis would be better to be done with the electron spectrum from the CNTs, not with the electrons from SM-LWFA.

Therefore, I would not recommend the publication of this article in photonics before rewriting the paper.

Author Response

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Author Response File: Author Response.pdf

Reviewer 3 Report

Review report:

Manuscript ID: photonics-1236297

Title: High-Density Dynamics of Laser Wakefield Acceleration from Gas Plasmas to Nanotubes

Authors: B. S. Nicks, E. Barraza-Valdez, S. Hakimi, K. D. Chesnut, G. H. DeGrandchamp, K. R. Gage, D. B. Housley, G. Lawler, D. J. Lin, P. Manwani, E. C. Nelson, G. M. Player, M. W. L. Seggebruch, J. Sweeney, J. E. Tanner, K. A. Thompson, and T. Tajima

 

The authors used 1D and 2D particle-in-cell (PIC) simulations to investigate electron acceleration by the interaction of relativistic intensity laser interacting with near critical density plasma. To generate such electron density the authors used complex array of carbon nanotubes. The scheme is driven by two femtosecond laser pulses. One pulse is used to extract electrons from the nanotube target and form a uniform electron density in the interaction region. Another delayed pulse is used to accelerate electrons to keV energy which the authors claim is via laser wakefield acceleration. The manuscript is well written, particularly the introduction and motivation. A brief road-map for the application of these accelerated electrons is also presented. Hence, I recommend that this article can be published in Photonics. However, I have some minor comments which require clarification before publication.

 

1. What is the advantage of near critical density plasma in wake filed acceleration?

2. Details of 1D and 2D simulation parameters such as box size, resolution, number of particles in each cell etc is missing.

3. Figure 3,4, it would be beneficial to show the colormap (heat-map) in the figure. Color axis represents different quantity.

4. The figure parameters are normalised in plasma frequency/wavelength/period. These units may be good for running a simulation. However, is very difficult to compare the figures with each other, specifically when the plasma density is varied several times.

5. Nanotube is a 3d phenomena. How 2d simulation is correctly implemented to simulate 3d properties?

6. In reference to the simulation with carbon nanotube target: Was the hexagonal structures shown in figure 12 used in nanotube target? What was the simulation box size, particle per cell in the simulation? Perhaps diverging colormap is better for the field plot in figure 14. Electron density plot instead of individual electrons in figure 14 would allow one to visualise wakefield structures, in case there is any!

 

Author Response

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Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

The manuscript has been improved a lot. Especially, the overlapped part with the previous study mostly dropped out, and the original part of this work has been prominent in the revised manuscript. Thus, I would like to recommend the publication of this article in Photonics after minor revision. The comments to improve this paper further are as follows.

 

1. My previous question, “What can happen if the CNT spacing is equal to or a fraction of the plasma wavelength in the Grey regime (slightly under-dense plasma)? This situation can be formed by manipulating CNT density or ionizing laser properties. It would be interesting to investigate the resonant interaction between the modulated electron density and plasma wave.”, was not answered. Therefore, I want to recommend considering this resonant case.
2. The scale parameter for electron density was n_c/n_e up to section 3, but the parameter has been changed to n_e/n_c in section 4. It can cause confusion.
   
   
3. There are many half-cut letters in figures; The labels of (a) and (b) in figure 2 and figure 3, and the captions in figure 5 (a) and figure 8 (a).
   
   
4. In figure 6, unnecessary lines around text or laser shape would be better to be removed.
   
   
5. In figure 8, the units of time and electron density in the upper part of the figure should be written properly. Also, the time unit in figure 9 should be corrected.
   
   
6. I think English expressions would be better to be carefully revised to improve readability.

Author Response

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Author Response File: Author Response.pdf