Plasma Spraying of a Microwave Absorber Coating for an RF Dummy Load
, Stefan Illy 2, Tobias Ruess 2, Jörg Weggen 2 and Venancio Martinez-Garcia 1,*Round 1
Reviewer 1 Report
This is a very interesting manuscript that can be recommended for publication. Some minor comments that can be addressed:
- Table 2: particles size -25+5 : how particle size can be negative?
- Table 3: Better “Plasma gun current”, “Plasma gun voltage” and “Plasma gun power”, because the terms “Plasma current”, “Plasma voltage” and “Plasma power” look confusing.
- Table 3: Why Vargon in one line has a value of 44, in the another line of 8?
- P6, line170: “CuCr1Zr” – why 1 is there?
- p8, line 224: “Bond strength values of 10 10 ± 0,7 MPa” – not clear the exact number befor +- sign.
Author Response
Comment 1: Table 2: particles size -25+5 : how particle size can be negative?
Answer to reviewer: we modified the text to explain the terminology. “All the spray powders had the same particle size distribution of -20 +5 µm (prevalent mesh size notation based on ASTM B214 sieve analysis, referring to a particle size distribution of D10 = 5 and D90 = 20 respectively).”
Comment 2: Table 3: Better “Plasma gun current”, “Plasma gun voltage” and “Plasma gun power”, because the terms “Plasma current”, “Plasma voltage” and “Plasma power” look confusing.
Answer to reviewer: we added the modifications.
Comment 3: Table 3: Why Vargon in one line has a value of 44, in the another line of 8?
Answer to reviewer: the Vargon of 44 slpm is for the “Plasma gas mixture” and Vargon of 8 slpm is for the “Powder feed gas”.
Comment 4: P6, line170: “CuCr1Zr” – why 1 is there?
Answer to reviewer: CuCr1Zr copper alloy is the formerly CuCrZr. The information is added in the text. “Test samples of CuCr1Zr copper alloy (formerly CuCrZr),”
Comment 5: p8, line 224: “Bond strength values of 10 10 ± 0,7 MPa” – not clear the exact number befor +- sign.
Answer to reviewer: we corrected the bond strength values and added the fracture mode. “The bond strength analysis shows values of 9.67 ± 0.83 MPa with adhesive fracture mode, which are consistent with the results obtained in other works.”
Reviewer 2 Report
Dear Authors,
Thank you for your interesting work.
The project is important and the results are complementary. However, in order to strengthen the scientific soundness, some improvements are needed.
I would strongly suggest to add some discussion and also link your results with existing literature (9 references and probably 5+ are from your team....). But please, follow all comments attached in .pdf file.
Comments for author File: 
Comments.pdf
Author Response
Comment 1: I would strongly suggest to add some discussion and also link your results with existing literature (9 references and probably 5+ are from your team....).
Answer to reviewer: we added more references. In the field of thermally sprayed ceramic coatings for MW absorb applications in fusion reaction there are only very view publications and they are related with our previous works.
Comment 2: Line 16
Answer to reviewer: we added the powder composition. “alumina-titania 60/40 mixed oxide ceramic”
Comment 3: Figure 2
Answer to reviewer: unfortunately a change of the color scheme is a relatively big effort. We modified the figure caption, indicating that the injected beam starts at a very shallow angle - thereby the reader should easily recognize where the beamlet is injected. The figure description is modified "Trace of a single ray starting at the input position (on-axis) with a very shallow angle and propagating through the proposed load geometry. The loss of power is qualitatively indicated by the color (starting from red to magenta)."
Comment 4: Figure 3
Answer to reviewer: we intentionally did not "over-optimize" the shape of the absorbing layer due to the following reasons:
a) As mentioned, the simulation model is based on relatively simple assumptions and has its limitations.
b) it is not easy to "hit" the requested absorption properties of the layer perfectly.
c) the shape of the Gaussian input beam may differ slightly for different experimental setups d) we also plan to operate the load at other frequencies, where the absorption properties are significantly different.
Comment 5: Line 141
Answer to reviewer: the particle size distribution of -20 +5 µm is from Floristan et al. reference.
Comment 6: Table 2
Answer to reviewer: the particle size distribution is corrected to -35 +5 µm.
Comment 7: Line 150
Answer to reviewer: the coating is used as-sprayed without post-treatment. The coating is developed to work as-sprayed with homogeneous radiation absorption. We added “Microwave absorption measurements have been performed on flat sample coatings as-sprayed with different thicknesses from 30 μm to 150 μm at 140 GHz and 170 GHz.” at the end of section 3.3. “Coating deposition and characterization”.
Comment 8: Line 160
Answer to reviewer: we added cleaning after bead blasting. “Surface activation of the inner tube surface has been done by bead blasting with glass beads (300 µm) and degreased using acetone before and after the bead blasting process in order to enhance the mechanical adhesion of the coating to the substrate.”
Comment 9: Line 204
Answer to reviewer: we include more discussion to explain the no presence of agglomerates. “The particle size distribution represents a monomodal distribution. There was no significant fine fraction nor agglomerates in the delivered powder. The analysis of the SEM images is in accordance with the results of the laser granulometry, showing irregular shaped particles, as can be seen from Figure 6b”.
Comment 10: Line 210 and 217
Answer to reviewer: the desired coating thickness is obtained by adjustment of the amount of injected powder. “The coating thickness per past is adjusted by modifying the amount of powder injected during the process adjusting the rotation speed of the feed disc.”
Comment 11: Line 224
Answer to reviewer: we corrected the bond strength values and added the fracture mode. “The bond strength analysis shows values of 9.67 ± 0.83 MPa with adhesive fracture mode, which are consistent with the results obtained in other works.”
Comment 12: Line 231
Answer to reviewer: it is difficult to obtain quantitative data from the XRD analysis without. The original powder change with the plasma during the dwell time obtaining different phases of alumina, titania and aluminium titanate.
Comment 13: Figure 10b)
Answer to reviewer: we change the explanation “The relative permittivity of Al2O3/TiO2 60/40 is calculated to be ϵ_r≈17.5, in contrast to ϵ_r≈14.8 of the 87/13 mixture. Changing the relative permittivity of the mixture affects the resonance condition, as shown in Fig. 10b), which directly changes the absorption of the coating thicknesses. In addition, the absorption of the coating at 170 GHz is higher compared to 140 GHz up to a coating thickness of ≈ 110 μm.”
Comment 14: Figure 11
Answer to reviewer: we added in the text “The maximum coating thickness tolerances for the application are specified in ± 10 µm. To represent the good stability of the coating process, the tolerance limits shown are specified in ± 5 µm.”
Reviewer 3 Report
The article is about plasma spraying of a microwave absorber coating for a RF dummy load and provide some news in the flied of materials engineering. However, some improvement are required:
- In abstract and introduction you must to precise what is RF, even is known.
- Introduction must to be rewrite due to the fact that the authors have disconsidered state-of-the-art in the field: references are too old and absolutely insufficient. Very poor references list!
- Section „ Technical Requirements and Design Principles” is too long and contains too much information that are not related to the title of the section. Must to be rewrite in order to ensure a comprehensive knowledge about technics and design.
- Table 1 and 2 must to be removed and data converted to text.
- If the powder is from Oerlikon Metco AG (according characteristics from table 2), why was necessary to characterize by laser granulametry and SEM: remove figure 6.
- Figure 8 and figure 9 must to be discussed together. Please improve!
Major revision!
Author Response
Comment 1: In abstract and introduction you must to precise what is RF, even is known.
Answer to reviewer: we modified the text. "Gyrotrons are capable to operate in the radio frequency (RF) range of 100 – 200 GHz with an output power of 1-2 MW. In case of ITER, they are operated at a frequency of 170 GHz running at a continuous power of 1 MW."
Comment 2: Introduction must to be rewrite due to the fact that the authors have disconsidered state-of-the-art in the field: references are too old and absolutely insufficient. Very poor references list!
Answer to reviewer: in the field of high power RF loads and MW absorb coatings there are only very view publications. We includes more references and rewrote the introduction
Comment 3: Section „ Technical Requirements and Design Principles” is too long and contains too much information that are not related to the title of the section. Must to be rewrite in order to ensure a comprehensive knowledge about technics and design.
Answer to reviewer: we prefer to keep this section. This section justifies the ceramic MW absorber coating in the copper tube and is basic theory to calculate the ideal thickness in each section of the tube.
Comment 4: Table 1 and 2 must to be removed and data converted to text.
Answer to reviewer: we prefer to keep the tables 1 and 2 for a better understanding by the reader.
Comment 5: If the powder is from Oerlikon Metco AG (according characteristics from table 2), why was necessary to characterize by laser granulametry and SEM: remove figure 6.
Answer to reviewer: we justify the necessity to characterize by laser granulometry and SEM the commercial powder to ensure the powder specifications and quality control. “The particle size distribution of the Al2O3/TiO2 60/40 mixture powder was determined by means of laser diffraction particle size analysis in a Mastersizer S from Malvern Panalytical Ltd (United Kingdom) in order to ensure the powder specifications”. “The particle size distribution analysis of the APS powder for quality control is summarized in Figure 6a.”
Comment 6: Figure 8 and figure 9 must to be discussed together. Please improve!
Answer to reviewer: We have put XRD of powder and coating into one graph because it is of importance to see changes in phase composition. In the text, we have added some more explanation to phase composition in powder section (referring to Fig 9) and in coating characterization section.
Round 2
Reviewer 3 Report
I still remain to my oppinions not accepted by the authors.
