experimental ion gun

For the design and construction details of ion guns, necessary for more advanced designs and lower vacuums.
DavidHansen
Posts: 44
Joined: Thu Dec 26, 2002 8:15 pm
Real name:

experimental ion gun

Post by DavidHansen »

I have a 3.25" diameter by 5" long block of steel, and I am planning to attempt construction of an ion gun with this. I picked it up for next to nothing, and I have full access to both a metal lathe with an 8" max. chuck, and a Bridgeport milling machine. I know this could likely be a waste of steel, but I have nothing to lose. Here's my plan for the gun design:

At the cathode end, we have a 3.25" diameter OD by 2.75" ID by 2" long chamber. Within this chamber is the thermionic filament, and an anode plate (tungsten) with a 1 mm hole. Immediately after the anode plate is a two-element Einzel lens. Connected to the end of the chamber closest the Einzel lens is a 1.5" long tube with an 0.75" OD and 0.25" ID. This tube is surrounded by a magnetic solenoid, as is the cathode chamber. The opposite end of this tube opens into a 1.5" long 2.5" OD by 2" ID chamber containing the extractor cone. This chamber will be directly connected to a 2.75" conflat. So, the ions will be formed by the cathode, pass through the anode plate, pass through and be focused by the Einzel lens, be further focussed by the solenoidal field as they travel down the tube, and finally exit through the extractor cone.

I'm open to suggestions. Remember, I have nothing to lose, although it would be nice if this worked even a little bit.

David Hansen
hellblazer
Posts: 157
Joined: Wed Sep 18, 2002 12:16 pm
Real name:

Re: experimental ion gun

Post by hellblazer »

The only thing I can suggest is that you have to be very careful with the einzel lense. If you have two tubes facing each other within another tube, the fields will be pretty severely distorted. The way to do this is to turn the tubes such that one slides partly inside the other. This way, the field between the tubes - i.e. the lense - isn't distorted by the containing tube. I'll see if I can whip up an example which shows the effect and what the concern is.

But in any event, this is pretty darn cool.
DavidHansen
Posts: 44
Joined: Thu Dec 26, 2002 8:15 pm
Real name:

Re: experimental ion gun

Post by DavidHansen »

Correct me if I'm wrong, but for positive ion focussing, the second Einzel element is at a higher negative potential than the first, right? The chamber itself will be at ground potential. If I bias the cathode filament to -300V or so, is that sufficient enough for me to ground the anode, or must it have a +HV potential on it?

I'll probably run the extractor cone off an NST with a full-wave rectifier and smoothing cap. HV feedthrus will be thick-walled glass tubing. My budget right now is about $80 - I spent all my current science money on getting the parts for my demo fusor - oeprational soon.

David Hansen
hellblazer
Posts: 157
Joined: Wed Sep 18, 2002 12:16 pm
Real name:

Re: experimental ion gun

Post by hellblazer »

This is a good question. Kept me perplexed a while until I asked about it on the sci.accellerators forum.

If I understand things correctly, having the anode at ground is fine. You don't need a positive potential relative to ground. The issue is, can you accelerate the ions out from the plasma behind the anode? I would think you need an initial accellerator in front of the anode to pull the ions out. Then they'd be focused by the lense after they had been accellerated. It could be that you have enough of a field from the negative inner cylinder of the lense to pull them out and accellerate them just fine... Better minds than me will have to comment. I'm just learning this stuff, too.

As to the lens potentials, I think you can work it both ways - it should be symmetric. For example, if the ions are already accellerated, you can use 0, +110, 0. I believe what is important is the potential shape and it doesn't matter if it's a hill or a valley. The positive is a decell-accell lense. The negative case is an accell-decell lense. I haven't played enough with einzel lenses to know the advantages/properties of each type.

Obviously, if you are using the negative potential of the middle cylinder, then you have to use the negative configuration.

The issue I was concerned about is the field distortion from the gaps of the cylinders to the outside container's wall. So if the lens is surrounded by a conducting tube at ground potential, I thought it would have different field than one where the gaps are shielded.

Turns out this is not the case at all. The outer conducting tube serves as the faraday shield which screens out any other field distortions. It's exactly the same potential configuration, so there's no diference. I've attached the jpeg of the potential field if you're interested. In your case, the hill becomes a valley.
Attachments
distorted.JPG
hellblazer
Posts: 157
Joined: Wed Sep 18, 2002 12:16 pm
Real name:

Re: experimental ion gun

Post by hellblazer »

I should add that if the only accelleration field is the center cylinder of the einzel lense, what will happen is that the ions will be decelerated as they pass the field - thus the term acell-decell. Attached is a simion run of the lens under question.

The center cylinder has a potential of -300 V, the other two cylinders are at ground potential. The initial potential of the ions is at 2 eV of energy, with a downward spread.

As you can see for the simulation, once past the central cylinder, the ions are re-attracted to the negative field and actually reverse course.

Something to keep in mind. A way around this is to have another accellerator past the lense itself, but if this will also become a decellerator once the ions are past it. The only real way around this issue is to either accellerate the ions with enough field so they reach the target with the proper energy, or to reverse the situation such that the ions are repulsed by a positive field, accellerated towards the ground potential. That way, nothing is attracting them back and they just keep flying.

Anyways, hope this helps out. Good luck on the project. I'm eager to see how it turns out.
Attachments
acell-decel.jpg
DavidHansen
Posts: 44
Joined: Thu Dec 26, 2002 8:15 pm
Real name:

Re: experimental ion gun

Post by DavidHansen »

I was hoping to use a two-element lens, similar to the one in the following post: viewtopic.php?f=12&t=4932#p32446

Remember also, the -5 kV potential on my extractor cone will be acting on the ions, as well as the inner grid potential (when used in a fusor).

David Hansen
hellblazer
Posts: 157
Joined: Wed Sep 18, 2002 12:16 pm
Real name:

Re: experimental ion gun

Post by hellblazer »

Ah. I didn't realize you were using an extractor. My bad.

If that's the case, then this will work out just dandy.
DaveC
Posts: 2346
Joined: Sat Jun 30, 2001 1:13 am
Real name:

Re: experimental ion gun

Post by DaveC »

David -

From my experience with the ion gun beam profiles, ( your link is to a trajectory I had posted earlier, that Hal extracted for us, the theoretical beam plots are beautifully clean and sweet. I find that a grounded enclosure, almost always improves things. Lensing really is quite simple.

However... you might wish to think about using a larger source aperture...for your ions, so as to get more output. Remember, the ionizing efficiency is usually pretty low...<1%, A thermionic filament/cathode with an open mesh cage around it at +50 to 100 Volts will give a reasonably good amount of ions. If your extracting electrode is at a potential of ~ - 50 to - 100 v, most of the ions produced will be collected. Directing these into the lens system then is easy. You should most likely see and increase of 10X or more ion current, by eliminating or increasing the aperture diameter.

Dave Cooper
raneyt
Posts: 66
Joined: Fri Jun 29, 2001 8:40 am
Real name:

Re: experimental ion gun

Post by raneyt »

David,

It sounds like you'll be making the ion gun's cathode section from the "block of steel?" Do you have a source for stainless steel tubing? It sounds like a lot of work, even with a Bridgeport, to bore out the 2.75" ID hole. Did you make the 1-mm hole in the tungsten plate? If so, I'd like to learn about your technique. I haven't tried drilling tungsten. I had thought about using an abrasive technique similar to drilling glass.

Good luck with your efforts, sounds like you're on the right track.

Regards,

TIM
User avatar
Richard Hull
Moderator
Posts: 14992
Joined: Fri Jun 15, 2001 9:44 am
Real name: Richard Hull

Re: experimental ion gun

Post by Richard Hull »

About the only mechanical way to drill tungsten is with a diamond burr. These are commonly avaialble to the jewelry and rock hound business in a variety of sizes. The other way to skin this cat is to get a tungsten rod chucked up and make a small wax dam in circle form where the hole is to be and use a fine diamond powder/water slurry to punch through.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
DavidHansen
Posts: 44
Joined: Thu Dec 26, 2002 8:15 pm
Real name:

Re: experimental ion gun

Post by 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
Posts: 66
Joined: Fri Jun 29, 2001 8:40 am
Real name:

Re: experimental ion gun

Post by 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
Posts: 44
Joined: Thu Dec 26, 2002 8:15 pm
Real name:

Re: experimental ion gun

Post by 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
Posts: 44
Joined: Thu Dec 26, 2002 8:15 pm
Real name:

Re: experimental ion gun

Post by 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
Attachments
ion gun 2.GIF
hellblazer
Posts: 157
Joined: Wed Sep 18, 2002 12:16 pm
Real name:

Re: experimental ion gun

Post by hellblazer »

Man, that helps a lot for me - someone who thinks more in pictures. Thanks.
DaveC
Posts: 2346
Joined: Sat Jun 30, 2001 1:13 am
Real name:

Re: experimental ion gun

Post by 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
Posts: 66
Joined: Fri Jun 29, 2001 8:40 am
Real name:

Re: experimental ion gun

Post by 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
Posts: 44
Joined: Thu Dec 26, 2002 8:15 pm
Real name:

Re: experimental ion gun

Post by 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
Posts: 66
Joined: Fri Jun 29, 2001 8:40 am
Real name:

Re: experimental ion gun

Post by 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
Posts: 44
Joined: Thu Dec 26, 2002 8:15 pm
Real name:

Re: experimental ion gun

Post by 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
Posts: 86
Joined: Sat Feb 22, 2003 1:25 pm
Real name:

Re: experimental ion gun

Post by Brett »

Tungsten isn't electroplated from a water solution, it's electroplated using a molten salt electrolyte.
JohnCuthbert
Posts: 339
Joined: Mon Aug 19, 2002 4:30 pm
Real name:

Re: experimental ion gun

Post by 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
Posts: 44
Joined: Thu Dec 26, 2002 8:15 pm
Real name:

Re: experimental ion gun

Post by DavidHansen »

Maybe I won't plate it... :-P

David Hansen
DavidHansen
Posts: 44
Joined: Thu Dec 26, 2002 8:15 pm
Real name:

Re: experimental ion gun

Post by 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
Posts: 2346
Joined: Sat Jun 30, 2001 1:13 am
Real name:

Re: experimental ion gun

Post by 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
Post Reply

Return to “Ion Gun Design and Construction (& FAQs)”