Why do you need a diffusion pump?

[brennanriddell]

I am new to the whole fusion scene, but have gathered some information. From what I've read, it appears that a vacuum of 75 microns or lower is required. If I had a mechanical pump, such as this FJC pump ( http://www.amazon.com/FJC-6912-Vacuum-P ... GX8N2SF9XM ), then why would I need the diffusion pump?

Also, being a high school student, I don't have that much money to fuse with. When it comes to vacuum chambers, what do I really need to handle that pressure? I have access to machine shops that could/will TIG weld whatever flanges and feedthroughs I need onto the chamber, I just need to know what the cheapest chamber I could use that is safe.

Best regards,

-Brennan

[Richard Hull]

A vacuum of 75 microns or lower.......50 microns is a better minimum...........this is demanded for even starting a diffusion pump. The diffusion pump or a far more expensive turbo-molecular pump is demanded to do fusion!

A general rule..........Your mechanical (forepump) should pull a vacuum below 30 microns below 20 is better. The diffusion or turbo pump should take you to about .01 microns (10e-5 torr). We don't talk fractional microns, but go to exponential torr pressures under 1 micron.

I can't understand where you got the idea that 75 microns is a good fusor vacuum 10e-4 to 10e-5 torr is a good fusor base line fusion vacuum prior to introducing deuterium.

Richard Hull

[brennanriddell]

Thank you for the response!

I don't know where I got 75 microns either. If I mechanically pumped down to 22-25 microns, would it be possible at all to fuse dueterium, or does it have to be really low in the 1 micron range?

-Brennan

[Richard Hull]

You need to be down 10 to 100 times lower than 1 micron before you admit deuterium to do reliable, detectable, easy fusion. 1 micorn doesn't cut it!

Note I have done fusion at just under 1 micron using a micromaze, but it was abyssmal and I had really good neutron detection gear, (early fusor III work). Others are never this fortunate as most are instrumentally impoverished to measure neutrons to a convincing degree at very low levels.

Richard Hull

[Dennis P Brown]

As to your other questions - welding a high vacuum capable joint is not trivial and most people have professionals do that type of work. That said, learning how to weld steel to create high vacuum grade interface is well worth learning if you have the equipment and time. That all said, your best bet for a fusion chamber is to buy a useful stainless steel chamber of some type that has a few existing ports to start your project. Many types of systems work - a standard vacuum "Tee" that uses (roughly) 3 inch diameter steel tubing and 0-ring seals would be a useful first chamber. Small or even larger vacuum hardware is available rather cheap on ebay when you look carefully and can adapt fittings.

To join the plasma club a simple mechanical (two stage) vacuum pump and a neon sign transformer will work fine. However, to do real fusion a far more powerful high voltage/current transformer(30+ KV and over 10 ma) is required (which is lethal) as well as deuterium gas - both these items tend to be rather expensive. The diffusion pump is also needed but really the cheapest one of these big three items you would need to do detectable fusion.

[RealBorg]

Fusor operation requires that
1) mean free path > grid size
2) dielectric strength > grid distance / voltage
3) deuterium gas

has anyone ever calculated the smallest size / highest possible pressure for a fusor?
dielectric breakdown would most likely prevent a 59nm sized fusor operating at ambient pressure.

typical fusor operation begins in the range below 10 micron or a mean free path larger than 5cm.
this means air is no longer acting like a fluid but rather like a number ping pong balls continously bouncing the walls of a big room - you can wait indefinitely until all of them can be removed through a small opening (=mechanical pump)...

to answer your question: you don't absolutely need a diffusion or turbomolecular pump. if you have a sufficiently thight chamber you could also use molecular adsorption or ion sputtering to get into that range. i have also proposed some novel ideas for molecular pumping in another thread which are pending review and practical evaluation.

Tom

[Richard Hull]

In my early days of actually doing fusion I had no diffusion pump, but the micromaze, no longer sold, did allow near or just below 1 micron pumping. I used the blast and bury technique and continuous gas flow to rid the atmosphere of waste products in the chamber and up the D2 content. It is highly wasteful of D2 and takes a long time to get what little fusion you can squeeze out of the system. Such an effort demands the best of neutron detection gear as you will never do much fusion and to detect it, you need good instrumentation, long runs and a good bit of statistics to try and prove you are really getting neutrons above the noise floor.

Few of even the best fusioneers with the best gear ever fuse at over 20 microns of D2. I checked my log and about 18 microns is the most D2 I ever worked in Fusor IV. 15 microns is a normal high here. Higher pressures are where the mega neutron per second fusion occurs, Generally around 8 microns+. You need a plus ultra system to do this. A really good 6-8 spherical fusor can hit the mega mark around 10 microns of D2.

Richard Hull