experimental ion gun

[DavidHansen]

Tim,
The ENTIRE GUN will be created out of this 3.25" dia by 5" block of steel. The 2-3/4" hole will be created using the 2" end mill we have for the Bridgeport, and then chucking the thing in the lathe and boring it out to the proper diameter.
The tungsten anode will actually be a cup, not a plate. The end of the cup will have the ion extraction hole. I am pretty sure that tungsten carbide drill bits can handle tungsten metal, but if they can't, I do have some diamond-coated nickel bits.
I'll try to draw a diagram of my system and post it in the next few days.

David Hansen

[raneyt]

Dave-

I'll be interested to hear of your efforts. I'd recommend you pre-drill the hole, but not with an end mill. If you have a 2-flute center-cutting end mil, you can plunge cut and make a hole (to a limited extent), but it wasn't designed for hole drilling, per se. If you don't have a choice, that's the way it goes. It should be interesting, though.

Richard-

Yes, I remember talking about drilling tungsten. The method you mentioned works well for glass and I'd imagine it'll work for tungsten, but boy, we're talking about a tiny hole. I'll stick with molybdenum for the time being.

Regards,

TIM

[DavidHansen]

Tim,

I will predrill with a 1" drill bit before I use the 2" end mill (which, by the way, has 4 flutes, as I recall).

Right now I'm machining a crucible for someone, so I'll probably start on the ion gun next week.

David Hansen

[DavidHansen]

In the meantime, here's a basic plan for my gun system. I'm not sure as to what the exact parameters should be, so I'd like all the comments and advice I can get.

The anode, cathode, and accelerating electrode will be tungsten-plated molybdenum. The gun shell and the extractor cone will be steel. The lens elements will be copper. HV feedthrus are steel.

David Hansen

[hellblazer]

Man, that helps a lot for me - someone who thinks more in pictures. Thanks.

[DaveC]

David - I think you will have some focusing problems with your einzel lens located after the anode. In order to focus, the einzel lens will need to be above the anode potential. If you relocate the lens to be at the left of the anode then it will focus at low voltages and the anode will then accelerate the focused beam.

For a good ionizer, I would expect a broad beam to be more effective than a narrow or thin beam. Also, it really isn't necessary to have very high voltages to get good ionzation. Just about 100 V or even less, since all the gases of interest ionize below 20 eV. Electron current is a bit more important, since it measures the number of electrons produced per second.

I presume your extractor cone then is at a negative potential to draw out the positive ions.

Years ago we built an ionizer similar in general design.. but no lens.. I think the coil was operated so as to give about 500 to 1000 Gauss. You need to check that the electron radius of gyration is less than the radius of your enclosure.

Radius, R = MV/qB = (1/B) *Sqrt( 2MU/q)

where V is velocity in m/s, B is Magnetic Field in Tesla, q is the electronic charge, M is mass of the electron in kg, and U is potential of the electron beam , in volts.

If the electron beam is not moving at too high a speed (aka low potential) you may not need to spiral it to get good ionizing efficiency, making your gun simpler.

Dave Cooper

[raneyt]

Sounds good, David.

How about some details on the tungsten plated molybdenum? And when you say "steel," do you mean stainless steel? All this is neat stuff. Thanks for posting the diagram.

Regards,

TIM

[DavidHansen]

The gun is on hold until I get my demo fusor system working, which should only be two weeks at the most.

The molybdenum anode cup will be machined on a lathe, and the molybdenum will then be electroplated with a 3mm layer of tungsten - some fancy calculations determine how long current must be applied to form a 3mm layer. The tungsten stock will be in the form of TIG electrodes, from McMaster-Carr. Right now, I need to find a place to buy molybdenum in the small quantity I need (1.5" diameter x 1" long)

The ion exit aperture in the anode will be drilled with a diamond grit bit and plenty of cutting fluid.

David Hansen

[raneyt]

David,

Thanks for the details. I know at one time Advanced Alloys sold metals in small quantities of the more uncommon metals(www.advancedalloys.com); they may be worth a try. I've had a lot of luck doing "google searches" on the web when I need an uncommon item. On the electroplating, you didn't mention the electrolyte. Machining the molybdenum won't be too difficult. I was pleasantly surprised that it machines much like cast iron. Best of luck with your efforts. Later.

Regards,

TIM

[DavidHansen]

Electrolyte... hmm... lemme consult the CRC Handbook o' Chem. This is a tough question... not many tungsten salts appear soluble in water. Tungsten nitrate? Got any suggestions?

David Hansen

[Brett]

Tungsten isn't electroplated from a water solution, it's electroplated using a molten salt electrolyte.

[JohnCuthbert]

There is a thesis on the web (sorry, I don't know how to copy the address) on the subject of electrodeposition of NiW alloys from solutions containing tungstates. This might be adapted but I don't know.
(search on Google for tungstates and electroplating and you should find the reference)

[DavidHansen]

Maybe I won't plate it...

David Hansen

[DavidHansen]

Dave,
I'm not entirely sure why you want me to put the lens between the anode and the cathode. I thought the principle was to create ions between the cathode and anode, pull them out through the anode with the accelerating potential, focus them with the Einzel lens, and pull them out the end of the gun with the extractor cone. Why would one put the lens between the anode and cathode?

David Hansen

[DaveC]

David...
The thought I was suggesting.. is to make the ions with a modest ionizing voltage.. 30 - 100 volts. Then using a two cylinder einzel lens along with the anode's attraction to focus them at the fusor's center. Somewhere in this thread... though, is the trajectory I ran with the anode as a simple ring...in a grounded enclosure. It did a fair job of focusing all by itself.

But the key for lenses is have them at the lowest possible potential so it doesnt get too difficult providing the voltage for them.

Hope that's clear enough to follow.

Dave Cooper

[raneyt]

David,

The only thing I found was a brief summary about a "gas plating" process sometimes used with tungsten. As other folks have pointed out, tungsten is not plated in a conventional aqueous process, but uses molten salt electrolytes or vapor deposition. It sounds like I'd be easier to wear a hole through a tungsten disc than to do any kind of relatively exotic plating processes.

Regards,

TIM

[DavidHansen]

Is there more than one anode? This is quite confusing. You wish to place the einzel AFTER the anode, but make the anode a ring?

A diagram would be much appreciated.

David Hansen

[Brett]

Thinking about this, you might be able to "plate" tungsten at home using a variation of the way quartz-halogen bulbs work; At high temperatures, tungsten-halogen compounds break down, depositing metalic tungsten, at lower temperatures, halogens attack tungsten.

So you could put the electrode in a bulb with a chunk of tungsten, heat the whole thing up, making sure that the electrode was the hottest thing in there, and eventually that's where the tungsten would collect.