Ion Gun Construction / Proposition

For the design and construction details of ion guns, necessary for more advanced designs and lower vacuums.
hellblazer
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Re: Ion Gun Construction / Proposition

Post by hellblazer »

This is such a great idea!

Here's some links to ion gun designs:

This is an excellent overview with internals to the duoplasmatron you wanted:
http://linac2.home.cern.ch/linac2/seminar/seminar.htm

This is another fairly good overview
http://www.casetechnology.com/source.html

For more indepth design, there's the online PDF books by Stanley Humphries (these books cost $300 in material form)

Charged Particle Beams http://www.fieldp.com/cpb/cpb.html

Principles of Charged Particle Acceleration
http://www.fieldp.com/cpa/cpa.html
hellblazer
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Re: Ion Gun Construction / Proposition

Post by hellblazer »

The issue, as I understand things, is that once the ions get past the negative electrode, the electrode will become an ion decellerator unless there is something more to keep accellerating it. The inner grid of the fusor would be at a negative potential, so there would be accelleration due to that field. Hmmm... I'll have to see if I can model this and see if the ions have enough momentum to keep going down the tube long enough to be accellerated by the fusor cathode field.

(normal caveats - I've never done constructed an ion gun, so I'm likely making silly assumptions and mistakes)
pfostini
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Re: Ion Gun Construction / Proposition

Post by pfostini »

Some more results from our efforts. Nothing is permanent ( at least nothing a grinder can't handle). I will hold off on the conflat because I had to pay for that! The titanium electrode was from CO2 laser fabrication.
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Starfire
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Re: Ion Gun Construction / Proposition

Post by Starfire »

An amateur radio rig on 10 meters will certainly provide the 28Mhz @ 500 watts ( and KW's depending on the rig and linear amp ) it would only be a matter of matching the out-put. If that is required.
3l
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Re: Ion Gun Construction / Proposition

Post by 3l »

Very nice Phil!

The ion gun can get pretty hairy on cost due to the material issues of refractory metals and the shapes ect. You have done a great job of adapting the existing tech to a useful form for fusor work. Cudos. The device you built will work nicely without rf excitation. We should run it without excitation first to work out any unsuspected bugs. As the designs progress then raise the power. I would say 15 amps would be a good starting point. Beam focus and optics might be
an area that will chew up some time but things are moving along nicely. Low power will allow you to use a sheet of quartz to see how the focus of your optics is going. I would say build the first generation without Rf
to see how it goes then Use RF in the updated version.

Fusion is fun!
Larry Leins
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LehighUBoy
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Re: Ion Gun Construction / Proposition

Post by LehighUBoy »

Phil Fostini wrote:
> Some more results from our efforts. Nothing is permanent ( at least nothing a grinder can't handle). I will hold off on the conflat because I had to pay for that! The titanium electrode was from CO2 laser fabrication.

My only suggestion is optimize the accelerating cylinder lengths, specifically maximize the distance between the filament and first negative potential. If possible surround the vacuum side lead with ceramic type insulator in order to minimize field distortions. Electrical comments are ground both metal shells and provide a positive potential to the filament (as you mentioned before). In general, your initial modifications look good, specifically designing one end to allow the solenoid to slide over the device!

-garrett
pfostini
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Re: Ion Gun Construction / Proposition

Post by pfostini »

The cathode side electrode at the moment has its far end up against the BeO bore. Would it be OK to maximize the distance by making it shorter? Cut it in half and push it as far away from the cathode as I can?
I will look around and see what I have for ceramic insulation, I have also quartze tubing that may work.
pfostini
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Re: Ion Gun Construction / Proposition

Post by pfostini »

Quartz we have plenty of also. That sounds like a stratagy. Is 15 amps the cathode current or accelerator current DC. I could start scrounging for power supply materials.
LehighUBoy
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Re: Ion Gun Construction / Proposition

Post by LehighUBoy »

Phil Fostini wrote:
> The cathode side electrode at the moment has its far end up against the BeO bore. Would it be OK to maximize the distance by making it shorter? Cut it in half and push it as far away from the cathode as I can?

Attempt to segment the cathode side electrode into fourths and place one section at a maximum distance from the filament. The insulation arrangement is diagramed in the attached image (I apologize for the crude nature of the drawing).

-garrett
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pfostini
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Re: Ion Gun Construction / Proposition

Post by pfostini »

> Attempt to segment the cathode side electrode into fourths and place one section at a maximum distance from the filament.

Segment the 1" Ti electrode into .250 and take `one of the .250" pieces and place it where you show it , correct? I will need to find a way to cut the Ti, I also have stainless if that can withstand the temperatures. But if Ti is better overall I would prefer we make units with a good lifetime. The cathode end when completed would be welded back onto the bore and we do not want to go in there too much. I get away with cutting the weld two more times after this one. The cathode have a rated life of several thousand hours.

It would not be any problem to do the insulator as shown. If I have enough ceramic that we can all share then I will use that but if not we will use quartz tubing.

Hey your drawings are better than my scanned in hand sketches.
pfostini
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Re: Ion Gun Construction / Proposition

Post by pfostini »

I do have plenty of Ham gear around (N1RDT) and a Lepel 1.5KW. My wife and I loaded an Xray transformer into a plastic tube and I am getting ready to "float" it in oil. They are not light.
LehighUBoy
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Re: Ion Gun Construction / Proposition

Post by LehighUBoy »

Phil Fostini wrote:
> > Attempt to segment the cathode side electrode into fourths and place one section at a maximum distance from the filament.
>
> Segment the 1" Ti electrode into .250 and take one of the .250" pieces and place it where you show it , correct?

correct
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Richard Hull
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Re: Ion Gun Construction / Proposition

Post by Richard Hull »

You guys seem to be going for the jugular!! 15-30 amp filaments! Refractory materials!?

I would have thought a superb, yet simple, ion gun might be constructed first based on the basics outlined in " making scientific instruments". A simple 1 or 2 amp coated filament dispenser cathode operating at very low temps, light duty 200 volt acceleration with coaxial magnetic field should do the trick for test purposes and let mistakes be made cheaply.

I do know for a fact that the ion guns on the Farnsworth device had to be differentially pumped (read vacuum nightmare to the un-annointed).

The ion guns being discussed here sound rather overkill for a trial. Mistakes can bore a hole in a fusor shell in seconds. The Farnsworth team did it all the time with much more puney guns.

Were it me doing this, I would go for two ultra simple guns ONLY and coaxially align and operate them to "fire for effect" before moving forward. Along the way I would modify the guns to suit the effort and better my gun design techniques before making up a phalnax of them in a most super machine. Once I obtained all I could from a low powered, six gunned simple machine, only then would I go for a monster gunned system.

Still, I applaud the effort and hope all works out well.

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
guest

Re: Ion Gun Construction / Proposition

Post by guest »

Sound advice Richard, but there is much to be learnt with Ion guns. If a stable device can be produced will this not facillate a more controlled fusion and allow a better measurement inviroment? The comment of "fire for effect " is taken onboard - I seek a device which is prolific in production of Ions and yet can be responsive to fine adjustment and control.
pfostini
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Re: Ion Gun Construction / Proposition

Post by pfostini »

If we can make these tubes produce the ions then the cost is only in the mating flange to the fusor.
pfostini
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Re: Ion Gun Construction / Proposition

Post by pfostini »

> > Segment the 1" Ti electrode into .250 and take one of the .250" pieces and place it where you show it ,

One modified electrode below. I did the same with the anode.
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DaveC
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Re: Ion Gun Construction / Proposition

Post by DaveC »

I was going to respond yesterday in almost the tenor that Richard Hull expressed here. I think an ion gun putting out a few mA or perhaps a bit more.. should be more than adequate to hone your techiques.

If you think about this in terms of ions, a few mA 's of ions is a large supply, some 10E16/sec. This is many orders of magnitude above the neutron count rate. So improvements to fusion efficiency could reduce the need for large currents.

With some time spent bringing ions to a fine focus... and then aiming a couple beams to the same place in the center, an enormous increase in ion density will be achieved over the standard demo fusor.

I have been modelling electrostatic lenses lately. you can quite easily get electron beams focussed inside of 1 micron diameter, using very simple lenses. Will try to get a few posted soon. may be helpful for everyone's designs.

Most RGAs and Mass Spectrometers use a variable voltage electron source firing across the gas flow to create ions by impact ionization. The ionization potentials of nearly everything is below 30 volts. By 70 to 100 volts, their typical ionization potentials, all molecules are in pieces.

So you don't need to get too exotic to get lots of ions.

Dave Cooper
pfostini
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Re: Ion Gun Construction / Proposition

Post by pfostini »

Dave Cooper wrote:
. I think an ion gun putting out a few mA or perhaps a bit more.. should be more than adequate to hone your techiques.
>
>I was curious about the way ions guns are measured. Is the current draw above on the cathode or the current of the DC voltages from cathode to anode?
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Richard Hull
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Re: Ion Gun Construction / Proposition

Post by Richard Hull »

Positive ion currents are measured at a cathode in an ideal situation. (Ion gun in an evacuated tube firing positive ions at a simple negative cathode plate) A simple milliamp meter placed in the cathode return line will do.

Please note that the energy required to get a full 5ma of ions is pretty hefty and a large percentage of the ionizing energy is pretty much lost within the ionizer chamber of the gun system. I would rate ion guns as pretty inefficient, overall, in their absolute conversion of input energy to actual realized ion current energy. I am sure there are professional guns in the 50% range but as an amateur, expect far less. Maybe 10%...

The bulk of the losses are in the filament and initial ionizer grid currents. Only a tiny fraction of the actual ions produced in the ionizer cavity get spit out down range to a cathode. Again, on to every parade, a little rain must fall. As Don Lancaster says, " You can't win!....Its the only game in town and it is rigged from the start" You can only hope to control your net loses.

Cheating is allowed on your part, of course. Some of the more efficient guns use permenant coaxial, cylindrical, alnico magnets to do a lot of the heavy ionization. (effectively, for free) Such magnets are custom designed and costly though. The Farnsworth team used them in their custom designed guns.

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
pfostini
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Re: Ion Gun Construction / Proposition

Post by pfostini »

So if I were to understand correctly. The filament current is not part of the measurment of the gun. The current from the filament to first grid in the ionization chamber is not the ionization current you refer to. The current that travels to the cathode plate (cylinder) is what is measured, yes?

With this design the filament is designed to run at 15-20 amps AC for heating it. The ionization grid which would be the grid closest to the filament has a high current potential applied between itself and the filament. This would establish a discharge? From that discharge the cathode plate pulls away positve ions from that ionization? What current would be advisable for this ionization between filament and the first grid?

I would like to understand this so we can use what we have to make this work. If the materials are exotic, it is only because it is what I have available. The cathode being what it is , is what is in the unit. I do have smaller ones if required but I am trying to make this so all interested can participate. If this is an ion gun of mega porportions then reducing the input to control the output should work.

Let me know if I view how an ion gun works is correct so I can put the proper instrumentation on it. Looking at the fusor counts so far we are soon going to have to come up with some type of ion gun.
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Richard Hull
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Re: Ion Gun Construction / Proposition

Post by Richard Hull »

The ion gun can be broken into a number of component parts.

The electron gun is a filament producing electrons. Only a few will ionize the gas within the chamber through collisions. Many of the ions in the ionizer area will recombine before they can be extracted. A large number will crash into the extractor or acceleration components within the gun and or neutrals. Thus, the blued extractor holes found on used ion guns. Only those few positive ions lucky enough to make it through the microscopic extractor cone will be useful ions. If extracted into only a moderate vacuum, their mean free path is limited and the current of ions out of the gun head's extractor will be reduced by the time the stream hits its target. Also coloumbic scattering and broadening of the beam will reduce the on target beam energy. Refocusing is possible to a limited degree.

It is loss #1 plus loss #2, plus loss #3, etc., etc. There are NO gains. The effort, again, is one of controlling and limiting losses ONLY. Decent ion gun design is an art within the science.

The filament current relates to the maximum number of electrons AVAILABLE. The current filament-to-ionizer grid or accelerator grid is the relative number of ions PRODUCED in the ionizer chamber. Any additional focusing or extractor currents relate often to the number of lost ions due to collisions with components.

Only the current gunbody-to-cathode(target) in the tube is related to the ion gun's actual ion current.

Even this can be a fiction of sorts as some ions will recombine to become fast neutrals and re-ionize to positive ions of whatever gas is in the gun to cathode area. If these are not the ions out of the gun that you wanted at the cathode they will be part of the cathode current anyway.

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
pfostini
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Re: Ion Gun Construction / Proposition

Post by pfostini »

I got up the courage to put the gun in the previous post into action. Here is what I found.

Fusor at room temp no gun in use. Voltage at 40KV & 11 ma. Pressure at 8 microns. best count was 2000 cpm. With that said;

With the gun on and wired as follows : filament 15 amps AC, A potential of -400 volts 50 ma applied to the accelerating electrode and a negative 2000 volt at the far end with a current draw of 200 microamp at best. The neutron count increased to 2500 cpm and the control of the fusor was great. By controling the gun I could raise and lower the fusor voltage very well. I did not have the proper power supply setup. The 400 volts was not enough and I think 700 volts would allow more current to flow. I repeated the test several times. When my fusor runs hot it normally dropped to 1700 cpm hot. With the IG on the count still remained above 2000CPM.
DaveC
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Re: Ion Gun Construction / Proposition

Post by DaveC »

There are some ionizer designs that are more efficient than others...but the thing to remember is that the gas density is low, of necessity. That means you have to flood the area with a lot of electrons to hit just a few gas molecules to ionize them.

AS Richard has explained, the filament current is relatively high amperage, but low voltage and only serves to heat the filament to allow electrons to be extracted.

These electrons only need to be accelerated by no more than 100 volts to do their job. So the power into the ionizer is milli Amp times 100 volts , so a few watts or so.

The ion power, though is ion current times accelerating voltage . in this case 10 kV or more..., micro-amps at kV become tens of watts.

Generally, because the ionizer is not very efficient in terms of ions produced per electron fired, it will be hard to get ion currents above milli-Amps for the average installation.

But thats a lot of ions, still.

The role of the magnetic field, is to spiral the electrons around and around so they have increased probability of hitting a gas molecule. Don't know figures to quote here, but it can greatly increase the ion current in some cases.

The standard Ionization Gage Bayard Alpert type uses about 8 to 10 mA electron current to ionize the gases in the tube... which can give upwards of a few tens of microamps ion current. the current is proportional to pressure, hence Richard's comment in an earlier post about needing differential pumping to get high ion currents, from high pressures. Not really sure how much of all the ions you produce that way, even get to the point where they would be accelerated.

But I am quite certain you can get good results with ion currents in the nano to microamps levels since the beam current density at focus will be much higher than in the std Fusor..

Dave Cooper
3l
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Re: Ion Gun Construction / Proposition

Post by 3l »

How are you counting your neutrons?
You are making good strides... Wish I knew why the count falls in an ungunned fusor .
Wow if we get more posts ,I think a gunned fusor construction forum is in order....What do you think?

Tally Ho!
Larry Leins
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LehighUBoy
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Re: Ion Gun Construction / Proposition

Post by LehighUBoy »

Phil Fostini wrote:
> The neutron count increased to 2500 cpm and the control of the fusor was great.

Excellent! Are you planning to distribute any as you previously mentioned?

-garrett
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