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:
Date: Sep 20, 08:12 am
Poster: Richard Hull

On Sep 20, 08:12 am, Richard Hull wrote:

>My first problem is in vacuum chamber size. My chamber is 5.5 inches in dia. and 11 inches long with a 5.5 inch dia. viewing window in the end so I am planning an outer grid of 5 inches dia. and an inner grid of 1.25 inches dia. My hope is there is enough leeway in size variations to allow these proportions. Also the chamber wall is steel and I'm not sure how this will affect the grids. I would like to see some results before going to the expense of building a new chamber.

The great beauty of the simple fusor is that there are no size restrictions and, in general little problem with scaling up or down in size. The smaller systems will require less vacuum expertise and cheaper equiptment can be used. A three inch outer grid and 1" inner grid are entirely practical solutions. You 5.5" system will be just fine. R Hull

>My power supply provides 8.5kv but I'm unsure of the ma rating and will have to measure it. What is the operable lower limit.


This is dependent on your final operating pressure. This is where experience with ionized gases comes in handy. A chamber working a 1000 microns all the way down to 100 microns will be a virtual dead short!! Your supply will not hit 500 to 1000 volts before it is hogging up hundreds of milliamperes if you got 'em, or burning out your supply if it does not!!

As the pressure in the chamber slips below 100 microns, you will notice a rise in voltage and reduction in current. (provided you have a current limited supply - see below). The upper voltage limit is virtually infinite provided you keep pumping out the chamber (rotary pump + diffusion pump). At 10 microns, (Near the limit of a simple rotary pump) a full 6-8 KV is possible with only a few tens of milliamps. With special techniques, (simple foreline trap worked by an expert), a rotary can be made to virtually hit 1-3 microns and here the voltge on the fusor really starts to rise and yet requires only a few milliamperes.

As to the Fusor supply, in general, it should be current limited. This can be as simple as a 5,000 ohm 100 watt wire wound resistor in series with the chamber's inner grid negative or minus connection to the power supply. Never put the resistor it in the ground or positive line of the supply!!!! You will float the chamber, outer grid, and base plate assemblies to lethal potentials!!! (this is were a hand made and not a store bought supply is best)

The supply itself should be a full wave filtered DC type of supply with some form of current limiting to avoid burning up the supply transformer or diodes. (this is especially important as the fusor neophyte struggles to get their "sea legs" for fusor operation) The supply should be fully varaible from zero volts to whatever KV level you think you will need. I use a variac on a simple linear type of full waave rectified system. My video tape on the fusor gives a diagram for a simple demo fusor supply.

For a demo system 0-6KV is fine. For a serious research system doing fusion, 20KV is a MINIMUM! Real neutron production won't become significant in deuterium gas until 20-40KV! Regadless, all supplies from the simple demo to the serious systems need about 60MA of available current!! This makes all of the supplies ABSOLUTELY AND INSTANTLY LETHAL! I can't over emphasize the need for electrical safety!

Also, with high voltages above 15KV, X-rays will start to emerge from the system and you will ultimately require lead shielding for the X-rays first and then thick parafin slabs to shield from the fast neutrons. Demo systems in air or deuterium operating under 10KV chamber voltages require no shielding whatsoever!

See all the general information on safety I have posted on the URL pointed to in Joshua Resnick's Web page near the end.

It is a German University web page which contains all of my original HVlist postings on construction and safety. Joshua has kindly supplied a hot key press to that site.

The 'perfesser' might want to snag it and post it somewhere on the Philo Fusion section as genral fusor construction data!

Richard Hull