New developments?

[Lerner]

This is what we’ve decide to do, near-term and medium-term. Since we have gone at times for hundreds of shots without breaking insulators, we don’t think we need a thicker one right now, with 1 MA or so currents. It seems to be a question now of avoiding any lateral stresses that accumulate when we are centering the electrodes and insulator. So first, we are separating the centering of the electrodes from final assembly, so that when we assemble, everything will already be vertical. Second, we’ve enlarged a cushioning gasket to eliminate a small ring of the insulator that was unsupported. Third, we are eliminating a second, ‘centering’ o-ring, so there will be no lateral pressure conveyed from the anode to the insulator. I estimate that together these steps should take any stresses on the insulator down by at least a factor of 5. And, the plastic shims we were using to measure the gap between the insulator and the electrodes could have left behind residue that explosively vaporized when the plasma hit it. So we are changing the probes to eliminate this problem.
In the medium term, stresses will increases as we go to higher currents. So, thanks in part to the ideas from the forum, and searching the literature, we’ve got three “finalists” to look at for a new insulator material—silicon nitride, zirconia, and cubic boron nitride. We’re seeing what suppliers can come up with and quotes. Anybody use any of these?

[asymmetric_implosion]

Be wary of Boron nitride. I’ve used it in plasma facing conditions before and it turns to powder pretty easily. I have no experience with silicon nitride or zirconia. Best of luck.

[Lerner]

Interesting–I have seen reports of it in plasma condtions. Might depend on how it is made, but I’ll make sure to check if the supplier has any plasma applications.

[asymmetric_implosion]

In my limited experience with it, boron nitride works well in quasi steady state plasma conditions like glow discharges. When I used it in pulse discharges (~100 A, a few microseconds) it visibly eroded in a short period of time and nitrogen showed in interesting places.

[Lerner]

hmmm…Does not sound promising. I wonder how we can safely test any of these three possibilities. If we just leave a smaple at the bottom of the chamber, it won’t get exposed to anything near the energy density at the insulator. Unless we put it in the way of the beam, which is hardly a fair test.

[asymmetric_implosion]

You could try putting samples parallel to the plasma flow between two cathode rods near the top of the cathode rods. You could mount 4 samples and try the three proposed insulators and alumina. It might not be the fairest test but it would be apples-to-apples. If you test it with the ion beam or electron beam you need to use samples at least 0.25″ thick. I put ceramics in the base of the anode for some experiments and thin sheets will crack easily under the stress of either beam. Alumina fairs very well in the tests I’ve completed.

[vansig]

Does the stress manifest the same way as heat-stress on glass?
this is due to relatively high coefficient of expansion on heating, versus the low overall heat conductivity.

borosilicate glass is therefore used in laboratory glassware, since it has a much lower coefficient of expansion.

when i look at the chemistry of this, it seems that to improve heat stress,
stuff is added to the glass that has negative coefficient of expansion, and this relieves
the overall expansion stresses.

[asymmetric_implosion]

In the case of plasma shock behind the insulator, it is more like taking a hammer to the insulator than a heat stress. The impact happens on a time scale much faster than the time to transfer heat into the material. We saw evidence of plasma chemistry on the inside surface of the insulator. The alumina discolors slightly from pristine white to a stained yellow. It remains a hypothesis but the data at the 0.25 MA scale seems to support it.

[vansig]

found this interesting composite material, containing barium titanate nanoparticles, coated in a plastic.
http://apl.aip.org/resource/1/applab/v95/i13/p132904_s1

[andrewmdodson]

I see a lot of science getting tossed around, cool :]

If someone could draw up a diagram of a geometry I can throw it in CAD real quick and run a finite element simulation to determine flashover. I can try with a few of the different materials proposed as a basis for model properties…

[prokaryote]

So, any updates for LPPX? It’s been over two months now. Just wondering.

Thanks in advance.

[AaronB]

We’re putting the finishing touches on the new report now.

[prokaryote]

Just saw the update, very cool! Hope the change to iridium wire works well and eliminates the metallic contamination.

[prokaryote]

It’s that time again! Any updates? Has the scheduled monthly updates been changed to every so often? Just asking. Thanks!

[DerekShannon]

Thanks for checking in — The highlight of the next progress report is our visit to Google last month, but we are waiting on the big G to post the video of Eric’s talk(s) (there were two, one general and one technical) on their “Solve For X” website. We did hear by the end of this week, and it’s Friday, so stay tuned……

[Author]

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