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: Size counts???
Date: Nov 09, 8:47 am
Poster: Richard Hull

On Nov 09, 8:47 am, Richard Hull wrote:

>When building a Farnsworth Hirsch fusor does the volume of the plasma have any effect on successful fusion??. For example in a fission bomb, geometry and size (critical mass) are important for function as well as neutron reflectors to bounce and slow down neutrons so they can interact. I guess what I'm asking is whether the neutrons have any effect on fusion or are just a by-product of the fusion.
> They are by definition neutral but can they help knock loose (strip) other atoms??. I'd appreciate any thoughts about general size, such as whether lower voltages would then be usable as the electric field would be denser, but on the other hand there would not be much space for ion acceleration.

The above are all good questions and naturally occur to the aspiring fusorite. Some have been touched on here before.

First, if you are looking at a dense gas machine of the sort all of us in the neutron club have produced and used, the size by the laws of physics must be 8" or smaller in diameter. (preferably 6" or less). This assumes voltages in the 20-40kv range are applied.

The reason is strictly one of mean free path. You are not going to have a significant fraction of you fusion energy deuterons survive the acceleratory run at the inner grid in a machine any larger than 8" at the micron pressure level (10e-3 torr).

If you lower your pressure and raise your voltage, then you can make as large a machine as you wish. Tradeoff are common here. At reduced pressures you are in a whole new, more expensive and difficult area of vacuum technology and the table top fusor made on the cheap will be just a distant memory.

At 10e-5 torr a 1 meter diameter fusor is a real possibility. Of course, so is the $50,000 investment level and this assumes you are one hell of scrounger. You would need equipment not even dreamed of by the fusorite with a 6" system. I have fusor V on the drawing board and all the parts on hand, and it will be a 10" diameter version. It is part of my slow, plodding progression upward.

Remember the fusion neutrons in a fusor are fast neutrons and will not interact much with the gas, grid, or even the shell of the system very much. Most will wind up outside the fusor at full energy. (mean free path again and low cross section of the materials). Yes, there can be some stripping possible here but it would approach zero on the statistical scale compared to the fusion neut count.

Size may count for some folks in some situations, but for us it is just a way to spend more dollars and enter a world where we would know less and less about the technology, forcing a very steep and expensive learning curve.

The advantages are uncertain at best for a research machine.

Richard Hull