Fusor Mark 2 Construction

[Andrew Seltzman]

I have started construction on my Mark 2 fusor:

http://www.rtftechnologies.org/physics/fusor-mark2.htm

Andrew Seltzman
www.rtftechnologies.org

[zuowei]

you also need calculate it, especially basic parameters.

[3l]

Hi Andrew:

Four guns will demand ion pumping for sure if you don't do turbo!
I use a diff pump with out a cryonic trap on top so I use a small pump to take it's place.
My one gun unit needs small ion pump to maintain the proper vacuum in the beam line.
My 2L /sec mini pump uses 7kv at a gang of amps.
My first homemade ion supply made of mots was woefully inadequite so I got a Varian Starcell Pump on Ebay.
The shipping was killer it weights 75 lbs.
Mine runs in the 10^-4 th to 10^-5 Torr region.
Differentail pumping takes practice to get uniform results.
Good luck on the multigun quest.

Happy Fusoring!
Larry Leins
Fusor Tech

[TheSkepticalRealist]

Reality check: Larry can you please post pictures the setup you describe? It sounds fascinating!

[Adam Szendrey]

Have you looked at the Images Du Jour forum? Loads of images of various systems, some (if not all, since every fusor is a great achievement i think) are really amazing.

Adam

[3l]

Hi Tsr:

If you have an open wallet on Ebay ,this equipment is there to buy. All the stuff in Andrew's four gun fusor were on Ebay last month.
Yes I'm posting images of the whole system as soon as I get done welding up a new manifold connection. My old one got cracked when we tryed to mount the Baratron in a new spot.
A big goof but it won't be long.
Here's a picture of the ion pump , I'm using for a beam line pump.
The ion power supply was a real buy on Ebay just $9.99 for the unit and 70 dollars to ship it. The ion pump was about 100 dollars on Ebay ,so all totaled I've have a little over two hundred dollars in ion pumping. There are many size units availible as most universities and government labs convert to turbo pumps.......ion ,sublimation pumps and diff pumps are going for a song. Not free but fairly cheap if you look around.

Happy Fusoring!
Larry Leins
Fusor Tech

[Brian McDermott]

If anybody is trying a gunned fusor, one thing you may want to try instead of
an wire inner gird woud be to replicate one of the ITT cathodes like th one on
the Mk2'. They were designed to reduce electron losses by not allowing
secondary electrons to "see" the anode. The design is described briefly in
this post (the full link dosen't want to appear, you may have to copy and
paste):

view.php?
site=fusor&bn=fusor_historynews&key=996515058

[DaveC]

Pressures of 10^-4 to 10 ^-5 torr are rather high for using ion pumps. Though they will operate, their lifetimes will be rather short, even for the starr cell designs. Check the varian site for their Torr-liter seconds liftetime. It is inversely proportional to absolute pressure. At 10 ^-8 torr, it is a number of years, but at 10^-4 torr it is days.

You can sometimes regenerate the pump electrodes with an AC high current, high temperature bakeout (Neon sign transformer or equivalent limited current output transformer works well). But all told, high pressure operation is not what they were designed to do.

Either diffusion or Turbo pumps are the preferred item for this pressure regime. And diffusion pumps are among the most robust, forgiving and generally least cost. In order to make the pressure regulation work well, all of these pumps need to be operated with a restricted inlet... throttled. Best way is to use a length of narrow bore tubing..capillary ....between the pump and the regulating valve. Even a needle valve without the capillary line will be dicey to hold pressures steady.

Also, the injection gas valve should have a capillary sized feed line ahead of the regulating valve. And use of a single valve for multiple ion guns seems like an important control simplification.

Dave Cooper

[Richard Hull]

Lots of folks that are not vacuum savvy operate a lot of items in the lines at pressures that are outside of optimum.

Dave is correct on the ion pumps. Most vacuum wise folks never even turn them on until they are in the low 10e6 torr range. The ion pump can carry a well sealed system down to 10e8-10e9, maybe. Then it is time for a Ti sub pump if you need to go farther or get rid of a complex ion.

RGA's just are not made for vacuums much more crud filled than 10e-6 torr but there are goof balls who operate them in the micron range (for a little while). For a used e-bay RGA this is fine, but if you pay the long buck for a new system this would have been a big mistake. The best RGA for use at 10e3-10e4 torr is your own intuition and the knowledge that your system has a gang of mass 20 stuff still in it and the only way you will get rid of this peak is to use a cryo-trap coupled with a massive bakeout.

The fusor works just fine with water in it, though it is its own operational bakeout system and ion pump. You can and will crud it up with all manner of deposition from atmospheric gunk to varying amounts of the inner grid. This is normal, and unavoidable. Far more of the fusor's input energy goes into depositing part of the inner grid on the outer shell than goes into fusion.

Richard Hull

[3l]

Hi Richard & Dave:

The targeted gun I'm running works best at 10^-5 Torr.
It is a tough job providing enough gas for the generation of ions and the mean free path.
Yes the ion pump fills up.
Bake and bombard on a nst is the order of the day.
The diff pump really needs a cryotrap to really work well at 10^5 but I use the ion pump as a stop gap.
until I can get or build a cryotrap.
The RGA is not for use in the gunned system.
It would be useless there.
I have something else going in the high vacuum area.
Eventually I will turbo pump then the RGA will be useful.

Happy Fusoring!
Larry Leins
Fusor Tech

[DaveC]

Larry -

If you throttle your DIff pump with a small diameter line, you can use it up to about 100 microns, without any significant backstreaming.

But as you say, you cant use it wide open at the higher pressures.

Dave Cooper

[Andrew Seltzman]

Thanks for the input.

To be more specific, my fusor will resemble Gene Meeks Mark II Prime with respect to it's ion gun operation. The fusor will be run at somewhere between 1-10 mTorr, and the guns will be kept at the same pressure or higher. Based on the operation of my Mark 1 prototype, I have observed the mean free paths of ion beams emanation from the center grid to be greater then 1.5” at 40 mTorr (In this case I am considering mean free of an ion beam to be the distance an ion beam can travel before it dissipates).

In this scenario I believe that the beams from the gun assemblies can successfully travel the required 2.5” from the extractor cone to the geometric center of the fusor.

The central grid will probably consist of 316ss wire at first, then move up to a 316ss sphere machined to focus the ion beams at the center like in the farnsworth design (see attached pic).


Andrew Seltzman
www.rtftechnologies.org

[Andrew Seltzman]

Where is the best place to buy inexpensive 2.75 CF hardware.
I am looking for these parts:

10 of 2-3/4" SS Conflat Half Nipples, non rotating, 2.5" total length or greater

8 of 2-3/4" SS Conflat flange, non rotating, 1.5" ID

I see that OE Technologies sells the half nipples for $17.50, this seems like the best place to get them.
Do they sell pre-bored flanges? I can't find them on the site.

Is OE Technologies the best place?

Andrew Seltzman
www.rtftechnologies.org

[DaveC]

Andrew - at those prices I would say OE has the the edge. For new stuff, we use either NorCal or Duniway Stockroom... but Kurt Lesker is probably right there as well in price... All of these places will custom bore for you at very low price adders.

Dave Cooper

[Richard Hester]

There's also LDS Vacuum.

[Andrew Seltzman]

Progress update:
Final design has been completed and most of the parts have been acquired. The vacuum hub has been completed.

Andrew Seltzman
www.rtftechnologies.org

[Richard Hull]

Looking pretty sharp there Andrew! Are you really going to do the guns?!

Richard Hull

[Andrew Seltzman]

Thanks,

As best as I can predict, the fusor will have the guns as planned. Acording to my calculations, the gun ports will fit in their proper positions on the hemisphere, and will not interfere with each other.

Andrew Seltzman
www.rtftechnologies.org