Bubble Formation in ITER-Grade Tungsten after Exposure to Stationary D/He Plasma and ELM-like Thermal Shocks

Round 1

Reviewer 1 Report

The manuscript studies the formation of bubbles in ITER-grade tungsten with simultaneous ELM-like transient heat loads and D/He plasma in the linear plasma device PSI-2. The paper is clearly written and the results are interesting. Here are the main comments and suggestions:

1) It would be helpful to add a picture of the sample before exposure.

2) The experimental setup should be instructed in more detail.

3) Table 1 should be cited in the paper and the main parameters should be explained.

4) What is the plasma parameters on Te and ne? What is the stationary heat load on the sample?

5) Was the surface temperature measured during the exposure? How is the surface temperature evolved during the exposure?

6) Many references in the paper are shown with "Error! Reference source not found".

7) The conclusions include so many points. It is better to modify it into one paragraph with only key points.

Author Response

Dear reviewer,

thank you for your useful feedback. I address the different points in the attached Word document.

Best regards,

Mauricio Gago and co-authors

Author Response File: Author Response.docx

Reviewer 2 Report

This paper present an investigation of the bubbles formed when ITER-grade W was exposed in PSI-2 to simultaneous ELM-like thermal loading using laser pulses and a high fluence, low energy D/He plasma.

The study is very relevant for the understanding of the behavior of future ITER PFMs and also for studies aiming at developing new materials or those studying post-mortem materials in current machines. It addresses a complex issue of concomitant interaction of particles and heat flux  with tungsten. It focuses on the characterization of micrometric bubbles and shows clearly that they are produced only in the presence of helium and are larger for absorbed powers above 0.6 GW.m^-2.

The article is clearly presented with appropriate citations, and the technical content is good, the figures showing the results are very useful. What I feel is missing in the discussion/conclusions is the values achieved by the surface temperature (based on measurements or, failing that, based on an estimates). Indeed, temperature is probably the most important parameter for bubble growth, diffusion and coalescence. For example, the micrometer size of the bubbles could indicate that the temperature reached is above 1600 K as proposed by [26]. To our knowledge, the increase in fluence by a factor smaller than 2 showing a significant increase in bubble density and size is not previously reported in the literature.

Please find my specific comments below including some minor corrections.

Line 61 : 5 mm samples

It is not clear or a word is missing. You mean “5 millimeter size samples were used” or “5 mm thick samples were used” ?

Line 64 : using one different sample per treatment.

What does this mean? Were the samples polished separatly  and/or did samples come from a different sample provided by PLANSEE AG before being cut and polished.

Could this last point explain the variation of microstructure mentioned on line 91? 

Line 79 : Exposing samples is of importance,

Is the word “hot” or “temperature” missing ?

Table 1 :

F_HF is not introduced nor used for the discussion. Could you correct or remove it please.

For Fluence(m^-2) the order of magnitude is missing -> Fluence (10²⁵ m^-2)

Please fill each cell with value or clearly show the merged cells.

Lines 84, 85,107, 130, 142, 153 and 156 : Error! Reference source not found.

please correct this error message.

Line 87 :

The SEM of B sample ( 0.8 GWm^-2) does not reflect trends expected and previously published by the authors for the surface changes i.e. higher is the power, larger are the cracks and droplets [19]. The areas remained free of cracks and the very small droplets which are protruding upwards for sample B seem also to be in contradiction with the high roughness value given in table 1. 

Could you please comment?

Line 90 :

The authors refer to initial conditions of the samples that would explain different behavior. These arguments are acceptable, but they would be more powerful if they were supported by comparative characterizations of the roughness and microstructure of the initial samples. 

Simultaneous exposures to a D/He plasma and high heat flux are difficult experiments which are performed using complex instrumentation. Can't we also consider a slight variation of experimental conditions during the exposure in PSI-2 leading to lower power on B sample to explain different behavior?

Line 93: lower surface temperature during the thermal shock

Is the surface temperature is measured in-situ during the experiment?

Line 94 : At higher Pabs the energy deposited on the surface might then be so high that cracking occurs despite a higher ductility.

This argument does not seem appropriate for the B sample which has fewer cracks. Can you please precise or clarify ?

Figure 2 : Please indicate where the inset images were taken. It is surprising to observe bubbles at the surface of each sample while FIB cross sections show bubbles only in depth for the higher powers.

Line 132 :  helium bubble

Without further analysis, the term helium bubble should be used with caution as it assumes that the bubbles are filled with helium only. Although created by the presence of helium, the bubbles could subsequently be empty or filled with deuterium and/or impurities initially present in the W-grade.

Lines 130 to 146 :  Generally, FIB sections allow for accurate and relevant observations of the subsurface, however they are local and unique measurements. Therefore, the images should be analyzed for trends rather than values, unless the authors have performed multiple FIBs on the same sample. In my opinion the values of 0.9% and 2% are useless to convince that the observed bubble density is higher on the FIB of sample D.

I agree that the depth of bubble formation is temperature dependent. But this depth is high compared to usual observations after similar plasma exposure conditions. So it seems that this depth depends more on the mechanism of W protuberance formation than on the mechanism of bubble diffusion.

Line 141: This explains why no bubbles can be seen in the first few micrometers in sample B and C

On the other hand, annealing cycles up to 1350K induced 5 nm subsurface annealing resulting in helium nanobubbles removal and surface smoothing as published in [DOI:10.1088/1741-4326/ac94e3]. I wonder if a similar phenomenon at higher temperatures and on a larger scale could not be involved during the temperature cycling of droplets which are protruding. This would induce surface annealing and removal of the bubbles closest to the surface and would explain why no bubbles can be seen in the first few micrometers.

Author Response

Dear reviewer,

thank you for your useful feedback. I address the different points in the attached Word document.

Best regards,

Mauricio Gago and co-authors

Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

The authors have addressed all the questions and suggestions. The paper is now appropriate for publication.

Reviewer 2 Report

I thank the authors for their detailed response and modifications to their paper.