Getting Rid of Water Introduced by Leaky Valve

[Henning]

It looks like the water in the leaky valve (which isn’t leaky anymore as I understand) hinders improvement in yields.

Wouldn’t it be a good idea to reheat the valve and vacuum chamber over 100°C whilst running the turbo pump? Like the heating that was done to bake out the oxygen?

The high temperatures might damage the valve, so you need to decide if it’s possible at all.

[Francisl]

Heating the chamber and valve would get rid of the loose water. New water is formed continuously with every shot as the oxygen from the tungsten oxide is released and combines with the deuterium fuel. Tungsten oxide is a surprisingly complex material. The color and temperature of the tungsten oxide reveals its oxygen content. Temperatures above 550 degrees C in a hydrogen atmosphere can remove the surface oxide layer. See the references here and here.
The reinforcing structure can’t take those temperatures. So coating the electrode surfaces with silver seems to be a reasonable solution.

[Anonymous]

Hmm, so the tungsten oxide layer on the electrode came from some previous initial reaction with the water that leaked in? Is there some way to manually strip the oxide layer from the surface of the electrode to bring the electrode back to its original pristine state?

They’ve said they plan to silver plate the tungsten electrode, so that the silver will minimize reactivity with the oxygen. Are there any tradeoffs likely to happen from the silver plating?

I know they want to get to the engineering phase later, but maybe there are some improvements they could think about even now, to make the system more robust and less susceptible to being derailed by small things like water. After all, if a tiny water leak is enough to mess up your apparatus, then how resilient and reliable will it be once it’s brought into production?

[Francisl]

I don’t know anything about the leaky valve. At room temperatures tungsten doesn’t react with water so that shouldn’t be a problem. The tungsten blank was formed by sintering and then machining. It had to be frequently heat treated to relieve stresses caused by machining. Sintering causes materials to be slightly porous and the high temperature makes tungsten reactive with oxygen in the air which caused the tungsten oxide.
I don’t know how thick the oxide layer is or how mobile the oxide ions are in tungsten.
According to this report a thin oxide layer should burn off with repeated use. The problem is not going away as quickly as hoped.
Coating the electrodes with a thin layer of silver should reduce or stop the migration of oxide ions to the surface. The only way to know is to try it. If the layer of silver is only a few atoms thick, it should quickly conduct the heat to the tungsten and far less silver should evaporate than if a thick layer is used.
Another advantage of using a silver coating is that it can be used as a base for adding other metals on top of it. Each arm of the cathode could have a different metal coating to see which one or which combination works best.
We have to remember that this is an experiment and problems or mistakes are just another opportunity to learn something new.

[Henning]

I’m worried that coating the expensive tungsten electrodes with silver will make them worthless.

Wouldn’t it be better to mill cheaper copper electrodes and coat them? Copper is easier to handle anyway. Or any other easily millable metal.

As I understand, the surface evaporates, so the underlying material is unimportant. Or am I wrong?

Yes it will delay the experiments further, and will cost several thousand dollars.

[Francisl]

Are the copper electrodes still available that were used before the tungsten? Maybe they could be coated with different metals and compared to the results with the bare metal.
Nanomaterials react differently than bulk materials. I’m guessing the same holds true for very thin coatings versus thick coatings.

It would be nice if a lab could perform these experiments with small coupons under dpf type conditions.

[Anonymous]

Why can’t there be some kind of ventilation/purification system that maintains the purity level of the plasma? The Russian GIT-21 device mentioned from the Prague conference report was said to have puffed in fresh plasma between its electrodes before firing – so why can’t FF-1 do that too? Fresh plasma is obviously going to be of higher purity than plasma that’s sitting around absorbing more evaporated electrode material.

[Henning]

Henning wrote: I’m worried that coating the expensive tungsten electrodes with silver will make them worthless.

Wouldn’t it be better to mill cheaper copper electrodes and coat them? Copper is easier to handle anyway. Or any other easily millable metal.

As I understand, the surface evaporates, so the underlying material is unimportant. Or am I wrong?

Yes it will delay the experiments further, and will cost several thousand dollars.

Erm, now I remember why a non tungsten-cathode is a bad idea. Firstly the sawtooth at the inner rim next to the insulator needs to be tungsten, otherwise they will evaporate too quickly. Secondly the cathode should be monolithic. So these two factors rule out a copper cathode. With beryllium it’s another issue, as it’s melting point is higher.

[Francisl]

Why can’t there be some kind of ventilation/purification system that maintains the purity level of the plasma? The Russian GIT-21 device mentioned from the Prague conference report was said to have puffed in fresh plasma between its electrodes before firing – so why can’t FF-1 do that too? Fresh plasma is obviously going to be of higher purity than plasma that’s sitting around absorbing more evaporated electrode material.

I like your idea. Can we expand on it? The vaporized electrode ions are much heavier than the deuterium ions. If the fresh plasma is entering the chamber at the same speed as the plasma sheath in the axial phase, then maybe the heavy ions can be left behind because they can’t accelerate as fast as the deuterium ions. Another benefit is the insulator may be exposed to less stress for a shorter time.

[Francisl]

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