Grid to anode distance

[Steven Sesselmann]

Hi Guys,

During my last experiments, I discovered that the anode cathode distance was critical in regards to electron leakage from the cathode, and I wonder if this is also a factor in standard Farnsworth fusors.

Do fusors with large anode cathode distances produce more x-rays and draw more current?

My instinct tells me that most of your fusors may have the gap too large, ions don't need a long runway to get their speed up to 50-60 Kev!

I think the anode cathode interspace could be as small as 10-20 mm.

Could some of you guys with working fusors post your anode cathode gaps below, so we can get some consensus of what the average gap is, and what works better.

Steven

[tligon]

I think your reasoning is sound. Dr. Bussard seemed to think the electron cloud in a Polywell would not be very far inside the magrid, and the ions would pop up to speed quickly.

If you want a uniform field between the outer and inner grids, then the spacing between the grids should be relatively larger than the openings of the inner grid. I suspect that's more of a mathematical nicety to keep Monsieur Poission tame than an actual operational requirement. In fact, some non-uniformity may encourage a good "star mode".

[Frank Sanns]

Funny you should mention that. I was about to install my next grid configuration and I was reviewing my results. The photo shows my normal 10" lattice outer grid with a 5" and a 3" grid dual purpose outer and inner latice grids. My smaller wire grids are not shown but I have used as small as 0.75 inches. The ruler in the photo is 12 inches long (30.5 cm).

My conclusions with my normal 10" lattice outer grid is that larger wire inner grids produce more neutrons than the smaller inner grids BUT the smaller inner grids are much easier to control. The 0.75" grid just about ran itself within minutes of being put into service. Smaller grids also give the best stars.

If I had to design for maximum neutrons, I would have to say the 10 inch outer grid with a 5 inch inner wire grid does best. Rebuilding my fusor this time, I am using the 5" lattice grid as the outer grid and a 2 or 3 inch inner wire grid because I would like to observe what is going on beyond the outer grid amongst other things.

Frank S.

[Steven Sesselmann]

Tom, you are quite correct, ions are near sighted, they only see things in focus when they are quite near, therefore if your cathode is close to your anode you need more mesh in the cathode, which in turn gives you less transparency. I hope to show that transparency may not be so important.

Frank, your thoughts are 2.5 inches or about 63 mm between grids, that sounds reasonable, in which case I would not have more than 2.5" between the wires in the grid.
Do you install the grids so the wires align?

Many Farnsworth fusors are suffering from heating of the shell, I put this down to electrons hitting the shell, not ions, in fact, if you generate ions below ground potential non of them will ever hit the shell.

Someone should try making an inner grid from two thin stainless steel "Ikea" bowls and drill opposing holes all over, the grid being about an inch smaller than the anode shell, it should work, but it need a bit of help to generate ions.

Steven

[Dustinit]

Steven,
Interesting you should mention that.
Here is a picture of an experimental grid by Hirsch.
It has 2 fine mesh screens with a voltage applied in order to
retain electrons within the grid to help neutralise space charge and
reduce electron streaming loss to the anode. Apparently only about 1kv between
the (meshs? meshes? mesh's?) screens was enough to stop the streaming loss.
He also had ion guns aligned with the holes.
On the cutaway the spacing looks on the order of 20 to 40 mm.
Interestingly, after fusion research, he went on to work in the oil industry.
Read in that what you will.
Dustin.

[tligon]

Mine produces a narrow e-beam which can produce a LOT of heat locally, but the general heating may be from another source.

The simple glow discharge machines ought to produce a lot of collisions between ions and neutrals ... the cross section is huge for this. The result of a fast ion hitting a cold neutral is a fast neutral and a cold ion. The fast neutral will go straight to the walls and make heat. I would expect this would not form beams, and would be distributed.

[Frank Sanns]

Steven,

Many of my grids are not symetrical by design so it is not really possible to align them.
I know my stars are not as "purdy" but I learn more that way.

One thing for sure, the beams do not start at the exit openings on the inner grid. The beams start on the openings on the opposite side of the inner grid.

If my grids were symetrical, I would have never learned that important piece of information.

Frank S.