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: Grid loss not losses (cooling)
Date: Jul 13, 7:42 pm
Poster: Richard Hull

On Jul 13, 7:42 pm, Richard Hull wrote:

>I suppose that if you are cooling it actively, then there isn't the need for a refractory metal like tantalum to build the grid. What about using something like silver, which has high heat conductivity (or suitably doped conductive Diamond ) to connect to your cold finger. Or, just pass a suitable coolant through the actual grid elements, which could be something like hypodermic tubing.


All of the above were suggested here in Richmond last year by one of our groups of amateur scientists. (I have a meeting every month of science types who putter around with high energy physics.) She suggested hypodermic tubing also with flowing LN2!!! Boy that is really out there for the amateur budget!!

I think the cold finger idea of another guy in our group is about the best I have heard and jibs with what most have noted here. I would use 9999 silver if I were to do it. Silver is a superb conductor with low outgassing and very cheap. Casting a rod from maple leaf or liberty bullion coins would be dirt cheap. I would prefer the maple leaf as those Canucks give you an extra 9 for the same price. (sorry "uncle Sam", you are an order of magnitude shy).

Tygon tubing for insulation would allow a pumped alcohol fluid cooled by dry ice or such to ciculate well from a junk aquarium diaphram pump.

>A suitable coolant might be high pressure hydrogen gas.. low viscosity, high conductivity, low density, etc... It is used in many HV cooling applications like high speed turbogenerators. He might also be a good choice?


The hydrogen gas is a bit scary especially at high pressures. That pressure won't go thru the insulating tygon and a leak would be disasterous.
>Any idea how much heat is actually being dissipated on the grid? Since it is in a vacuum, it doesn't take much to get it pretty hot

The actual power in the inner grid is minimal compared to the disappation of the outer wall. In theory, it would seem that they would both deal with the same power. They do of course carry the same energy, being a simple series circuit, but the electrons accelerate to large kinetic energies and slam into the outer wall casing heating it to very high levels. The heating of the inner grid is by ion bombardment and at the point of current runaway by field emission.

My fusor III runs away at various pressures and different current an voltage points. But a rough guess would be about 5 microns with 21-24kv and near 10-15ma of current. That means about 220-300 watts of continuous DC input. The bulk of those watts heat the chamber casing (outer positive electrode). The inner grid heating is probably no more than 10-20% of that total. This being the summation of ion or deuteron current due to bombardment, IR loses in the tantalum wire as an electric conductor for total current in the system (very low loss here), and field emissive heating (electrons being electrostatically shorn from the wire atoms).

I have mulled over how to separate the three for quantitative values, and hope to post something on this soon. Of all of these, the IR losses might be computed from simple tables and surface area considerations. For precision, and based on grid construction, the thing would be a night mare of resisitive wye-delta matrices. Still, that's better than the reactive wye-deltas which drove me nuts in college years ago (during the age of the huge log-log EE slide rules)

Richard Hull