Opportunities for Bright Beam Generation at the Argonne Wakefield Accelerator (AWA)

Round 1

Reviewer 1 Report

The manuscript “Opportunities for Bright-Beam Generation at the Argonne Wakefield Accelerator (AWA)”, by E. Frame et al., reports about a simulation campaign, performed using different numerical tools, aimed at assessing the possibility to produce very low emittance beams (high 5D brightness) in the upcoming improved AWA configuration.

An optimization for three different photocathods is presented, showing the possibility to achieve O(100 nm) normalized emittances once the Pareto condition on emittance/bunch length is reached. Both ideal (1D) and realistic (3D simulated) external electromagnetic fields are employed and compared, in order to quantify the effect of asymmetric/symmetric resonant cavities coupling on the final emittance. Finally, the effects of magnetic bunch compression are investigated.

Overall, the manuscript is well written and the claims supported by numerical data; some educated guesses about beam dynamics are present which also seem to be fitting. The manuscript can be improved with few minor changes:

1) In the manuscript, the Mean Transverse Energy is employed as parameter for measuring the performances of different cathods. Most researchers are more familiar with the “thermal emittance” which seems to be a strongly related parameter. Can Authors please explicitly state how the two quantities compare? A reference would also do.

2) Page 2, line 39, the laser length is quoted. Is that value rms of FWHM?

3) Section 3.1: the first lines refer to simulations with Q >= 1 nC, which are not reported in the manuscript. I would suggest to remove the statement in order to avoid confusion in the reader. Moreover, I suggest to move from Figure 2 caption to the main text the information about the actual bunch charge taken into consideration.

4) Section 3.2. The optimized emittances for the three cathods under investigation are reported. Authors, please also add the actual laser spot size on cathode allowing to produce the optimized emittance, since Table 1 only provides a range. Also, it is necessary to quote the final bunch current and/or the 5D brightness value. 

5) From Figure 3, it appears that emittance compensation is employed to reach the reported low emittances. However, From Figure 4, top row, it seems that the bunch envelope is not matched during velocity bunching. This is possibly due to the absence of solenoids around the first two R.F. cavities. Authors, please include a comment about that.

With the suggested improvements, the manuscript can be granted publication in Instruments.

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 2 Report

The paper presents the results of simulations of electron beam propagation in the AWA beamline. The paper is interesting, but will benefit from streamlining. It is often hard to keep track of what was simulated and what results correspond to what beam parameters and field conditions. For example,

1. Please clarify if beam brightness optimizations in Section 3 were performed for 1 nC beam or 100 pC beam?

2. What is the "RF gun amplitude" and how is it related to the field on the cathode?

3. What were the magnetic fields of the solenoids for the optimized configuration? The very broad ranges of magnetic fields are provided in Table 1.

4. What was the linac gradient for the optimal conditions? What was the final beam energy?

5. The conclusion states that 100 nm beam emittance was obtained for the optimized configuration. I do not believe that the text of the paper supports this conclusion.

6. Finally, please describe plans for the experiments that will be performed to validate simulations.

Overall, the paper is interesting and of a good quality. It can be published after minor revisions.

The English quality is good, but the paper may benefit from proofreading to correct misprints.

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

I thank Authors for replying to all of the point I raised. In my opinion now the manuscript can be granted publication in its present form.

Reviewer 2 Report

The paper is improved and all my comments are addressed. I especially like the addition of Table 2 that summarizes the optimized parameters.

The paper may still benefit from another round of proofreading to correct the language flow here and there.