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 08, 3:38 pm
Poster: d.preston

On Nov 08, 3:38 pm, d.preston wrote:

>>>why not insulate the inner electrode.
>>>ie. cvd coat the inner electrode with silicon dioxide or aluminum oxide.
>>>the inner electrode will capacitively couple and the insulating layer will lower the electrodes emmission.
>>No one has tried that here yet. Someone should. Remember the same number of electrons will hit the inner grid as before and the same heat will be there. Some oxides work better as emitters than the bare metals. Some investigation would be needed.
>Probably some sort of tungsten carbide is what you want... you want very high thermal emissivity (so you can radiate the heat away at relatively low temps).
>Actually, the best strategy would be something like is used in power grid tubes (which the fusor, basically, is) and put a suppressor grid in front of the inner grid to deflect the electrons away. The real problem is the mechanical tolerances necessary.
>Study that Eimac handbook...

using our micro machining technique we can hold tollerences of +- .00001 center to center distances. +- .0001 on edge or hole diameter distances as standard.

I designed a cad model and ploted/assembled in paper a "strip" inner electrode, outer grid, and sphere. If anyone is interested I can e-mail them a picture.

for this prototype study I chose to build the inner electrode and grid from mobius triangles. basically i took a dodecahedron and did a central projection into a sphere. The electrodes are perfectly spherical with .005 strip radialy on edge

This forms 12 pentagonal openings.

The outer sphere I designed as a rhombicosidodecahedron.

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

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.

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.


dan preston