Advanced Laser–Plasma Diagnostics for a Modular High-Repetition-Rate Plasma Electron Accelerator

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

see file Report.pdf

Comments for author File: Comments.pdf

Comments on the Quality of English Language

Two minor issues:

page 8, line 288: were -> where

page 8, line 288: in enclosed -> is enclosed

 

Author Response

Dear Reviwer,

Thnak you for your thorough review and valuable feedback on our manuscript "Advanced Laser Plasma Diagnostics for a Modular High-Repetition Rate Plasma Electron Accelerator". We apprechiate your insights and have addressed comments in the following.

 

Comment1:

Page 1, lines 4-6: “The core methods involve comprehensive metrological assessments,
including rigorous temporal laser pulse characterization and contrast measurements,
supplemented by detailed spatio-temporal distribution analyses of the laser system.”
Comment: It is not clear to me, if the measurement systems for the characterization of the
laser pulse are part of the presented e-KAIO module, as they are not included in the GUI
described in the manuscript. Please explain.

Response 1:

We agree that it was not clearly stated whether the additional diagnostics performed using TIPTOE and INSIGHT are part of the acceleration module itself or separate measurements. To address this, we modified Figure 2 (page 5), which provides a clearer overview of the setup used. The laser beam is sampled at the exit of the laser system and analyzed outside the chamber (including a similar vacuum window). Once the measurements are completed, the beam is coupled into the vacuum system, neglecting dispersion in air (valid for the system’s bandwidth without pulse compression).

 

Comment 2:

Page 1, lines 8-10: “We demonstrate the full functionality of the laser-plasma accelerator
module and perform target density and laser-plasma characterizations.”
Comment: For the demonstration of the full functionality of the laser-plasma accelerator
module, one would expect that an accelerated electron beam was obtained. The authors
should soften this claim. Also, which laser-plasma parameter measurements do the authors
refer to with the term laser-plasma characterization?

Response 2:

We agree that the claims made in the abstract were too bold, so we softened our statements by adjusting the abstract accordingly (i.e., on page 1, the abstract now reads: ‘functionality of the laser-plasma accelerator modules diagnostics’). Regarding the laser plasma diagnostics, we primarily refer to pump-probe shadowgraphy and density measurements. Although we only measured the atomic density of the gas jet, the PHASICS measurement in the SideView arm can also be used to measure plasma density. Additionally, a spectrometer could be added to the top or side view to characterize the light emitted from the plasma.

Comment 3:

Page 2-3, Section 2: Design Criteria for LPA at High Repetition Rates
Comment: Although the design criteria are outlined in detail in this section, the actual
chosen design parameters, both the laser (focal spot size, pulse duration, laser intensity) as
well as the target parameters (gas type, density) envisaged in the design are not all given and
could be presented in a table for better readability of the manuscript.

Response 3:

We fully agree with the assessment and added Table 1 (page 3) in the introduction section for potential results using a similar laser system, when including post-compression as was originally intended for the measurement campaign. Additionally, we summarize the achieved results in Table 2 (page 12) and provide justification for why the potential results were not reached, along with possible improvements leading to hopefully better results in the future.

Comment 4:

Page 4-5, Section 3.1: Interaction Chamber
Comment: The diagnostics of the e-KAIO chamber presented in this section should be
described in more detail and quantitative information should be given. The parameters of the
imaging systems (magnification/resolution) coupled to the CCD cameras should be given. Is
the top view imaging envisaged to be used to collect scattered light during the interaction or
is it intended only for the positioning of the gas-jet nozzle?
Also, it is not clear from the text, if the active beam stabilization mentioned in line 185 is an
automatized loop or an adjustment to be made “by hand”?
Is a diagnostic for the measurement of the beam charge included in the system?

Response 4:

We agree that there was a lack of information regarding the diagnostic performance and have improved on those points. More detailed information is now provided in the respective subchapters (see lines 305-317 for the target density and the caption of Figure 7, and page 10 for the focus diagnostic). The TopView is primarily used to determine the gas jet’s position relative to the laser axis. Additionally, one could add a spectrometer to analyze the light emitted from the plasma (see the additional caption in Figure 2 on page 5).

The beam stabilization is fully automated, and an additional remark has been added to the manuscript to clarify this point (see lines 187-191). However, so far, no diagnostic has been implemented to determine the beam charge. This could be done downstream of the system in a potential electron beam-line (see lines 213-215) or within the acceleration module, as free ports are available for additional diagnostics.

Comment 5:

Page 6-7, Section 3.3: Laser system and diagnostics
Comment: Same as point 1: Are the diagnostics used for the characterization of the laser
system as described in this section part of the e-KAIO chamber? If yes, why are they not
included in the user interface?

Response 5:

We appreciate your feedback, and we acknowledge that the exact setup was not described clearly enough. Thank you for bringing this to our attention. For further details, please refer to Response 1.

Comment 6:

Page 9-10, Section 4.2: Nozzle and Gas Density Characterization
Comment: Same as point 5: Are these diagnostics included in the e-KAIO system?

Response 6:

Please see Responses 1 and 5. Additional remark in line 307.

Reviewer 2 Report

Comments and Suggestions for Authors

This manuscript describes a design for a user-friendly target chamber module for a laser wakefield accelerator. This chamber can be used together with an industrial kHz, few mJ, fs laser system to produce and accelerate electrons to the MeV energies. The described work is interesting and -if successful- would allow to make laser wakefield accelerators available to a broad area of users, a game-changer for LWFA technology.

While I applaud the idea and effort behind the work, the manuscript and its content is, in my opinion, too vague for a scientific article and needs to be improved before the manuscript can be considered for publication, for example:

- The abstract is misleading and does not summarize the measurements/discussion presented in the manuscript:

-- Which results in the manuscript show the 'good stability and reproducibility of the laser system' (p.1, l.7)? In my opinion, two single-point measurements pulse intensity as a function of time are insufficient to make that claim. Additionally, the term 'good stability' must be defined in the context of the LWFA especially with the claim 'confirming its suitability for advances scientific and industrial applications'.

-- The sentence (p.1, l.8-10) 'We demonstrate the full functionality...' could mean that electron production and acceleration is shown (which is not) or that parameters required to achieve that were measured (unspecified in the text; the gas density and laser peak power seem to be too low). If you mean only the functionality of the diagnostics, this must be clarified to avoid confusion.

- Please read every sentence carefully and ensure that words (especially adjectives) and sentences are scientifically accurate. To illustrate the problem a few examples from the manuscript (though many more need to be corrected):

--p.1, l.5: why is it a comprehensive metrological assessment and not just a metrological assessment? p.1, l.5: what makes it a rigorous temporal laser pulse characterization instead of a temporal laser pulse characterization? if these (and other) adjectives are indeed accurate, their meaning must be defined.

--p.3, l.110: what is considered a 'very high target pressure'?

--p.10, l.317: 'to minimize the focal spot size', did you mean to write: to achieve a sufficiently small focal spot size? There would be ways to make the focal spot size even smaller.

 

Additionally, the manuscript should be clearer or more explicit on the following:

- To be able to understand how close the design is to achieve electron injection and acceleration, parameters the manuscript should provide a table of required laser peak intensities (or a0) for a given plasma electron density (to achieve injection and acceleration to a defined electron energy and maybe acceleration regimes). Please state clearly what parameter ranges have been achieved so far using this system and what is the strategy to improve.

-- Though never mentioned before, in the summary (p.11, l.347) it is mentioned that a post-compression stage is foreseen. I agree that a pulse length of ~40fs seems too long to achieve electron injection at 3x10**19/cm3 and the given focal spot size. Current limitations and planned improvement strategies to achieve goal parameters and electron injection and acceleration should be discussed earlier.

- There is a difference between gas and plasma density measurements (profiles). Since only gas density is measured, but plasma density (profile) is what matters for the interaction, further discussion on the relationship between the two should be provided.

- A discussion about the expected laser energy transmission would be insightful (especially since the low laser intensity seems to be a limitation for now). On p.10, l.327 it is stated that the assumption is 5.5mJ on target (from the on p.6, l.229 stated 9mJ, so 61%). What is this assumption based on and why was it not measured? What losses are expected from what components? How would post-compression affect transmission and what approach would be used? What is the Strehl ratio and a0 of the pulse measurement shown on Fig.6.

--Is coating of the mirrors an issue (especially those in proximity of the gas jet). How does chamber pressure relate to the time the system can be used before changing or cleaning mirrors? Coating of optics challenges many similar experiments.

Comments on the Quality of English Language

The manuscript is clearly understandable.

Author Response

Dear Reviwer,

Thnak you for your thorough review and valuable feedback on our manuscript "Advanced Laser Plasma Diagnostics for a Modular High-Repetition Rate Plasma Electron Accelerator". We apprechiate your insights and have addressed your comments in the following.

Comment 1:

The abstract is misleading and does not summarize the measurements/discussion presented in the manuscript:

-- Which results in the manuscript show the 'good stability and reproducibility of the laser system' (p.1, l.7)? In my opinion, two single-point measurements pulse intensity as a function of time are insufficient to make that claim. Additionally, the term 'good stability' must be defined in the context of the LWFA especially with the claim 'confirming its suitability for advances scientific and industrial applications'.

-- The sentence (p.1, l.8-10) 'We demonstrate the full functionality...' could mean that electron production and acceleration is shown (which is not) or that parameters required to achieve that were measured (unspecified in the text; the gas density and laser peak power seem to be too low). If you mean only the functionality of the diagnostics, this must be clarified to avoid confusion.

Response 1:

We agree that the claims made in the abstract were too bold, so we softened our statements by adjusting the abstract accordingly (i.e., on page 1, the abstract now reads: ‘functionality of the laser-plasma accelerator modules diagnostics’). Regarding the laser plasma diagnostics, we primarily refer to pump-probe shadowgraphy and density measurements. Although we only measured the atomic density of the gas jet, the PHASICS measurement in the SideView arm can also be used to measure plasma density, which we assume to be a fully ionized argon plasma (Z=8), so the plasma denisty is n_e = 8*n_a.

Regarding the laser stability we agree, that the temporal characterization is ot sufficient to claim good stability of the laser system.  The avergae power stability is only hinted to be measured in the GUI (upper right in Figure3 page 6 and see text line 193-195). Also see addition in line 236-240.

Comment 2:

Please read every sentence carefully and ensure that words (especially adjectives) and sentences are scientifically accurate. To illustrate the problem a few examples from the manuscript (though many more need to be corrected):

--p.1, l.5: why is it a comprehensive metrological assessment and not just a metrological assessment? p.1, l.5: what makes it a rigorous temporal laser pulse characterization instead of a temporal laser pulse characterization? if these (and other) adjectives are indeed accurate, their meaning must be defined.

--p.3, l.110: what is considered a 'very high target pressure'?

--p.10, l.317: 'to minimize the focal spot size', did you mean to write: to achieve a sufficiently small focal spot size? There would be ways to make the focal spot size even smaller.

Response 2:

We fully agree that when making strong claims, additional justification must be provided to further support those claims. We adapted the abstract and clearly state that we are referring to laser metrology. These measurements typically assess the laser average power and focal intensity, assuming a Gaussian profile. However, the temporal characterization is then assumed to be valid for the full spatial beam profile, which is not strictly true (due to spatio-temporal couplings, or STCs). To address this, we use the INSIGHT method to ensure there are no STCs, validating this assumption for our case. This assumes that there are no STCs induced by the OAP mirror.

Regarding the target pressure, we assume a high pressure for ne/nc > 0.25 (see text addition at line 112).

Thank you for the comment on the focal spot size. We adjusted the text accordingly and stated the limiting factors to minimize it (See page 10, line 327-329).

 

Comment 3:

- To be able to understand how close the design is to achieve electron injection and acceleration, parameters the manuscript should provide a table of required laser peak intensities (or a0) for a given plasma electron density (to achieve injection and acceleration to a defined electron energy and maybe acceleration regimes). Please state clearly what parameter ranges have been achieved so far using this system and what is the strategy to improve.

Response 3:

We fully agree with the assessment and added Table 1 (page 3) in the introduction section for potential results using a similar laser system, when including post-compression as was originally intended for the measurement campaign. Additionally, we summarize the achieved results in Table 2 (page 12) and provide justification for why the potential results were not reached, along with possible improvements leading to hopefully better results in the future.

Comment 4: 

Though never mentioned before, in the summary (p.11, l.347) it is mentioned that a post-compression stage is foreseen. I agree that a pulse length of ~40fs seems too long to achieve electron injection at 3x10**19/cm3 and the given focal spot size. Current limitations and planned improvement strategies to achieve goal parameters and electron injection and acceleration should be discussed earlier.

Response 4:

We fully agree with your statment and took the measures discussed in Response 3 above.

Comment 5: 

There is a difference between gas and plasma density measurements (profiles). Since only gas density is measured, but plasma density (profile) is what matters for the interaction, further discussion on the relationship between the two should be provided.

Response 5:

We fully agree with your comment. For LWFA mostly the plasma density profile matters, which is not measured here. Although we only measured the atomic density of the gas jet, the PHASICS measurement in the SideView arm can also be used to measure plasma density. Unfortunaltey those measurements were not perfromed du to a lack of time, but are possbile with the given setup. See addition in text page 10, lines 309-320.

Comment 6: 

 A discussion about the expected laser energy transmission would be insightful (especially since the low laser intensity seems to be a limitation for now). On p.10, l.327 it is stated that the assumption is 5.5mJ on target (from the on p.6, l.229 stated 9mJ, so 61%). What is this assumption based on and why was it not measured? What losses are expected from what components? How would post-compression affect transmission and what approach would be used? What is the Strehl ratio and a0 of the pulse measurement shown on Fig.6.

Response 6:

We fully agree, that there was a lack of the experimental overview in order to explain the losses to the target. To address this, we modified Figure 2 (page 5), which provides a clearer overview of the setup used. In addition we discuss the loss due to broad band optics and multiple relfections used (See lines 327-340). This is because initially it was planned to used the post-compressed beams, which habe a broad spectrum. The given laser energy on target was measured and not "assumed" as stated previously. Thank you for this remark!

Regarding the Strehl ration we found a value of S=0.8 due to the OAP abberations (line 335).

For the post-compression scheme future experiments are planned using a multi-pass-cell scheme, where a total trmission of 80% can be reached, when compressing the beams down to 6fs. See addition in lines 360-373.

Comment 7: 

Is coating of the mirrors an issue (especially those in proximity of the gas jet). How does chamber pressure relate to the time the system can be used before changing or cleaning mirrors? Coating of optics challenges many similar experiments.

Response 7:

As we are using gas jet targets, we don’t assume degradation to the optics close to the interaction region. Obviously, this is different for solid-state targets, where extra measures need to be taken, such as debris shielding. Additionally, extra care is taken by limiting the distance to the gas jet to 150 µm and ensuring that the laser doesn’t hit the differential pumping cube on top of the gas jet.

In case there are any concerns we are not aware of, we’d be happy to discuss this point further and benefit from any insights we may be missing.

 

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

The authors have answered all questions and addressed all issues raised to my satisfaction. The manuscript was modified accordingly, thus I recommend to accept the manuscript in its present form.

Author Response

We appreciate your positive feedback and are glad that all questions and issues have been satisfactorily addressed. We are continuously working on improving our experiments and are determined to demonstrate electron acceleration with our device in future studies. Thank you for recommending the acceptance of our manuscript in its present form.

Reviewer 2 Report

Comments and Suggestions for Authors

The authors have addressed my comments. I suggest publication of this manuscript.

Author Response

Thank you for your constructive comments. We are pleased to hear that our revisions have met your expectations and that you support the publication of our manuscript.