The Focus Fusion Society › Forums › Lawrenceville Plasma Physics Experiment (LPPX) › How's the new tungsten cathode working out?
Eagerly checking the site for updates. I hope the hurricane didn’t hold you up too much. Could you already fire shots with the bath cleaned equipment?
“The problem, we concluded, was that during each shot the inner layer of the steel, facing the plasma, was heated by the plasma to about 1000° C, enough to break up the chromium oxide in the stainless steel. Chromium oxide protects steel from further oxidation (rusting) at room temperature, but can’t withstand high temperatures, even for the few ms until the heat dissipated through the bulk of the steel.”
I assume an alloy like Hastelloy N has been rejected as not being as heat tolerant? Still with only 6-8% Chromium it should reduce the problem. Perhaps would accept a titanium coating better? I don’t know much about the properties of all the possible different alloys, but hastelloy N is a preferred choice for its resistance to oxidation at high temperatures.
Perhaps a relatively cheap solution would be a SiC coating? Higher melting point than SS. Doubt it could be done on site however. Would probably have to send in the chamber to be coated by a company like Electro-Coatings. They have a specialty coating they call Nye-Carb.
Then there is our friend CNT – perhaps it could be installed as a layer of conducting insulation to conduct heat away from problem areas. Don’t know how expensive that would be or if it would work better than a titanium coating – of course you need a place to conduct heat to.
P.S. If one of my suggestions is adopted, do I get a consultation fee? 🙂
Instead of “replace[ing] the vacuum chambers inner surface with titanium,” could you try lining the inner surface with titanium foil?
This would let you get some data on the new tungsten electrodes while you research the coating at your leisure. You could also try both titanium and titanium nitride. Any idea how many joules are transferred to the surface each shot?
well if the cause is chrome steel that should be easy to fix. I’d replace it with titanium.
edit 1 : They’ve had > 6 weeks to fix it, so I’m guessing its already fixed and they’re now onto something else.
edit 2 : wow, i’ve just seen their Oct 6th report, my titanium recommendation was the right one!
Another way could be to use local laser cleaning:
http://i.imgur.com/K47fhr6.gifv
you might however need to do it while the chamber is sealer and filled with inertial gas, which would be quite complicated (need a robotic hand or something).
Over a month on from the October 6th report where the problem of oxidizing chrome steel was diagnosed, FF1 parts have been prepared for titanium coating and have now been sent off to be coated.
Currently waiting for them to come back.
Cooperation Aids Oxygen Elimination; Vacuum Chamber Getting Titanium Nitride Coating
I like the idea that a 150degC bake might resolve the oxygen problem, but I am concerned that this will lead to problems with the mylar. PET recrystallizes as 80degC and melts at between 120 and 150degC.
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!)
Fingers crossed here for the Lawrenceville lads and lasses!
>…lads and lasses!
Are you a yorkshireman?
Leicestrian with Cheshire parents 🙂
I followed the links on the post of 13 December 2015
It would be nice to read articles that had dates posted on them. We have seen many articles that discuss arcing before and other issues. A date posted would ensure that the article is the most recent.
Thank you.
“Replacement of the damaged Mylar and other plastic insulators is going ahead rapidly and we expect to attempt another bake-out in late January or early February, leading to oxygen elimination and renewed firing of FF-1.”
This from a post Jan 25th. Are you firing again yet?
No , not firing, but soon. everything takes longer than you expect. Steady progress!Too busy for reports right now.