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 potential problems
Date: Sep 04, 12:54 pm
Poster: Richard Hull

On Sep 04, 12:54 pm, Richard Hull wrote:

>Dear Fusion Researchers,
>I am attempting to build a fusor to experiment with and am running into
>some trouble. I hope you can help steer me in the right direction:
>I have constructed a crude vacuum chamber and electrode for my fusor
>project. I have copies of Farnsworth's patents and am trying to model my
>project after his description. The problem I am running into is
>obtaining a high voltage across my electrodes.
>Farnsworth describes a 15 amp current flow across his at voltages as
>high as 140 Kv. This is a very high power and such a device would be
>actively cooled as he describes.

That should be 15ma not 15 amp. If you are reading it it is a misprint. R Hull

>My power supply is capable of 30 kV and up to 30 ma. I am trying to
>limit the power input to 10 ma and 25 Kv to give me a total power in of
>250 watts. I have been able to pump the vacuum down to 10-5 Torr but
>with no resistors the direct current only gives me a kV at best???
>The only way I can get my grid potential up to 20 Kv is to place a 200 M
>ohm resistor after my cathode to ground. Needless to say I've blown
>quite a few resistors that way.
>Am I doing this right???

Have you metered the vacuum to 10e-5 torr? At that low a pressure, you should be able to put 40kv across it with little or no current drain!
I suspect you are closer to a 10e-1 torr. At that pressure it is impossible to place more than about 800-1200 volts across a fusor. (it will try to draw nearly infinite current.) At 10e-3 torr you can easily place 20kv across a fusor and still be in the latter stages of glow discharge.

The art is to hover at or about 10 to 0 microns, (10e-2 - 10e-3 torr), and jockey the voltage. As you glow clean the system, the voltage can be raised and the current, thereby, controlled. It is a delicate process. R Hull


>Did Farnsworth use a resistor to keep his Cathode potential so high?


No, not to keep his voltage high, but to limit his current due to mishaps. If you use a huge resistor while working at too high a pressure, (above 50 microns), all of your voltage will develop across it and not your fusor! (throwing away power.) Just because your power supply says its puting out 25kv never means that this is the voltage across the fusor, especially with a series limiter resistor. Always monitor the voltage across the fusor, shell to inner grid, and don't worry about the power supply voltage. R Hull


He doesn't show this in his patent? He would need an internal resistance of
>his device of at least 10 K ohms since even with a high vacuum in the
>10-7 Torr area there would still be little internal resistance at that
>voltage, then once a plasma is generated the resistance would drop.


There will be no plasma, at least glow mode plasma, at 10e-7 torr. It is a constant race of voltage against pressure. Place 70,000 volts across 10e-7 torr in a normal, simple fusor device and you should get almost zero current flow. (some field emission currents will occur) R. Hull


>If I understand the theory correctly the grid is at a high potential
>with respect to ground and the electron space charge building up inside
>the cathode, and this is the region where the ions oscillate?
>Even using the Child-Langmuir equation with a perfect vacuum a 30 Kv
>potential and a 10 ma current with an inter electrode separation of .01
>meters my electrode surface area has to be just 100 microns x 100
>microns. I'm sure Farnsworth's electrodes had a much higher surface


Yes and most simple fusors have about 10 times the separation you mention (normally .1 meter is the smallest electrode separation used). More data about the physical setup you are using is needed.

Richard Hull