Reply To: Site availability is poor

[tlh24]

1. Oxygen & Alumina
I read the latest developments with some interest, as I’ve fiddled with vacuum brazing tungsten for a while. While TiN seems like a decent enough solution to the Cr2O3 problem, one may worry that, since this ceramic is still attacked by acids, it will release some NH3 under a hydrogen plasma. (I like @bcreighton7 suggestion of SiC a bit better; have some experience sputtering amorphous SiC. Pure metals like Au,Pt,Ir,Mo might work too; would reflect some of the IR and stay cooler.)

My larger worry, though, is the alumina insulator. I’m not sure how close it is to the hydrogen plasma, but even though the ceramic is stable, it can definitely be reduced by hot hydrogen. For example, sintering WC-Co in alumina will cause aluminum to diffuse onto the surface of the cutting parts, leading to difficulty in brazing due to resulting surface Al2O3:
https://app.aws.org/wj/supplement/WJ_1984_10_s308.pdf
Secondly most alumina is a Al2O3-SiO2 ceramic, as this is much stronger than pure Al2O3 alone. The SiO2 is readily reduced in hydrogen, releasing water. But I see they are moving to a high-purity insulator.

A look at the Ellingham diagrams also suggests that for a high enough H2/H2O ratio, reduction of alumina is favorable; particularly more so since the vapor pressure of Al is high. Of course, the same Ellingham diagram reveals Cr2O3 reduction is well over 100x faster for a given temperature! Still, for these brief events, I’d bet that temperature is mostly a function of radiation hence distance.

Perhaps somebody can show a better picture of the insulator setup?
Or perhaps the insulator can be coated (e.g. ebeam) in BeO?

2. Tungsten
I’m not sure what pressure the experiments run at, but the vapor pressure of tungsten is finite. Using data from plansee’s website, looks like the evaporation rate of W @ 2500C (underestimate) is 0.1 mg/cm^2/hr. Assuming maybe 100 cm^2 of electrode (overestimate) for 1ms, this is ~3e-12 g, or 1e10 W atoms. Still seems like a lot — I will have to read the “runaway electrons” paper …

Was there any thought of using thoriated W to get the work function & surface temp down? Thorium has a much higher vap pressure, of course.

3. Mylar & bake-out

Where is the polyimide?! (I’m sorry for dumb questions, particularly on electrode materials. I have not read many post in this forum; am new here!)

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