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: HV Feed through
Date: Nov 20, 08:57 am
Poster: Richard Hull

On Nov 20, 08:57 am, Richard Hull wrote:

>I have started building a HV feed for my new chamber. One of my professors used to design HV power feed through devices and was a wealth of information. My design is based on the fact that the electric field between the conductors in a coaxial cable can be minimized if the ratio of outer diameter to inner diameter is e. So Do/Di = e. This formula is not trivial to derive but you don't have to be a rocket scientist either.


Thanks for sharing this!!!! This is sage advice from a real designer of real world stuff and counts in the grand scheme of things. As a grizzled old engineer myself, info like this is appreciated and cherished.

I gotta try this sometime soon.

The nearly .6" hot electrode will help hold the coronal stresses way down as opposed to say a .125 diameter rod, especially at voltages over 20kv.

The only problem I have with your design is the teflon. I have used it before and it does give some minor outgassing and with ion bombardment and the large surface area you have in chamber, it may refill the chamber with unwanted gases as fast as your pump tries to empty it. (unless you have a real monster pump like a Welch 1397.)

I agree that teflon is probably the best insulator on the planet right after sapphire and it will tolerate very high heat that melt other plastics, but the surface area of any absorbant or polymer should be kept to an absolute minimum, in vacuo! Let us know how this works out long term, especially at the higher energies. Teflon is truly good stuff! Vacuum technologists push for only fired and glazed alumina or porcelan for UHV work. We are not exactly at UHV levels though.

By the way, The original Farnsworth team used pure sapphire rods to suspend their inner grids! (I have photos of the original inner grid assemblies during various stages of manufacture) The Linde process for artificial sapphire was new back in the late fifties and early 60's and the special ground and grown 4" long 3/8" diameter rods cost thousands each! Today, a hundred bucks could get a rough boule rod of 1/4" diameter. Linde's effort involved direct contact and visits to the Fort Wayne ITT facility by their engineering reps!

The spark plug idea for fusor power input is still good, of course, for demo fusors or any neutron device which will not be pushed to really high voltages. It is also very cheap and offers a lot of simplification of design.

Richard Hull