Fusion Message Board

In this space, visitors are invited to post any comments, questions, or skeptical observations about Philo T. Farnsworth's contributions to the field of Nuclear Fusion research.

Subject: Re: thermionic cathode
Date: Nov 09, 9:22 am
Poster: Richard Hull

On Nov 09, 9:22 am, Richard Hull wrote:

>I would like some opinions on recommended the cathode, anode and grid diameters.


Again, extensively covered on older postings, here. Tom Ligon also comments on this exact question in my first fusor video tape.

The ratio is not highly organized or set in stone. Gunned systems can work on a 1.5:1 ratio. Simple fusors are best to work in the 4:1 or 5:1 range. Chose your outer size and use the above to determine the inner grid.

The ratio is un-explored, for the most part, and probably has a tremendous latitude in range. RH

>btw tungsten carbide: W2C is conductive about 6% of what pure W is. also given conditions this may be converted to another carbide of W ie. WC which is even more conductive (about 40% of pure W). Aditionally if you heat W2C in a non carburizing atmosphere to about 2400k (white hot) you will burn off the carbon and transform back into straight W. In a D2 atmosphere I would guess that the carbon would react with the D and the W would get embrittled and go to shit...? just a guess.

All, that is 100% of all inner grid metals suffer total and complete hydrogen embrittlement. This is normal, and totally irrelevant and unimportant in the extreme.

If the grids are silver soldered this is an advantage as the curved metal can have its joints weakened and erroded away by deposition and still maintain shape due to this wonderful embrittling mechanism. I have commented on this in my videos and know from experience the results.
We should welcome the embrittlement. RH

>I think I might go with a Iron-crome-Aluminum-yitrium alloy to start. such an alloy can be pre treated to form a yetrium oxide under-coating with a aluminum oxide over coating. which is very thermally stable as the expansion coe. match. additionally the coating is self healing at elevated temp. provide there is some O2. Working temp will be slightly lower than W but at temp the strength and errosion is much lower.
>second choice i think would be silicon dioxide coated W.

Again, an important criterion is the outgassing and behavior in deep vacuums at temperature. structural strength is of very little concern in our application.

Richard Hull