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: Pulsed fusor
Date: Oct 13, 09:00 am
Poster: Richard Hull

On Oct 13, 09:00 am, Richard Hull wrote:

>Is the fusor a constant voltage device (i.e. small increase in voltage gives big increase in current) or does it have a more resistive characteristic? It looks like a glow discharge, which is notoriously negative resistance, but, the geometry is fairly unusual, so there may be other effects.

Jim shows his experience here with glow discharge devices. He is correct in the assumption about the fusor being just a big neon lamp or voltage regulator.

With any device, with most any gas, at most any reduced pressure, the device will have several "special points" related to ionization, zener or negative resistance region and arc modes. If the pressure changes even slightly, this whole business slides and shifts about wildly. Electrode geometry enters in but for a given device all the points of electrical interest only move about under different pressures.

Negative resistance is common in all glow discharge systems. The fusor is a great voltage regulator! The secret is to jockey the pressure to get the system to do what you want it to do. There is a limit! Glow discharge, especially stable glow discharge, gets hairy in the under 3 micron region. Most simple supplies wind up as relaxation oscillator component acessories hung on the fusor and stability is a problem. in the 10e-4 torr decade, dispenser cathodes are wise items to install.

The lower pressures are for advanced folks only and while the mean free path starts looking real good, (bigger diameter systems) the gas density for collision slides off. I am sure some data exists here, but a lot is also held close to the chest by some researchers.

As I have noted before, a lot of ionization characteristics change in strange manners at this 5 micron to 5X10e-4 torr range. I call it the "strange decade". No one alive can appreciate this until they have dinked around electrically in this "nether world" of vacuum. It is intriguing to say the least.

Vacuum gear is not really set up to pump this region. Too low for forepumps which don't do well in the molecular flow regime and too high for turbo pumps and diff pumps designed to regularly hit 10-9 and 10-7 torr, respectively. So we must leak in our gas and pump at the same time or differentially pump to play in this region with any stability. Good valves, seals and some feedback loops under computer control are needed for ultra-stable operation.

Richard Hull