Ion Gun Construction / Proposition

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

Post by Richard Hull »

In the long and the short of it, anyone here successfully manufactures an ion gun producing 10 ma of pure deuteron beam at the extractor will be the "gun king". Very few professional guns can approach this level without spending thousands per gun.

Two such guns of this type in a 6" fusor, firing dead on, opposite one another, would far exceed anything yet seen in the entire effort to date.

Those working on guns would do well to test them in a linear test bed to verify their actual output before presuming to be able to measure this quantity in a spherical collider like the fusor. My guess is that most early amateur guns made here would be in the 1-2ma range or less.

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 »

Yes; shipped out to all the people that emailed.
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Re: Ion Gun Construction / Proposition

Post by LehighUBoy »

Phil Fostini wrote:
> Yes; shipped out to all the people that emailed.
Do you have any remaining?
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Re: Ion Gun Construction / Proposition

Post by pfostini »

Yes, thats why I chose that particular tube.
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Re: Ion Gun Construction / Proposition

Post by pfostini »

I have a BC-720 scintillator placed 6" from the poiser. I am using an HP digital scope to look at the wave form. I have NIM modules ( ebay) SCA and timer counter etc. I am also using a ludlum BF3 tube/ moderator. Did my background checks on the BC-720 ( .8cpm). I made some BC-720's but this is the store bought model which will be my standard.

The ungunned fusor runs at slightly higher pressure. When the gun is on I am able to lower the pressure and have good control over the high voltage. I am sure other things are going on but this was the first try. I have not varied the guns parameters much but I was doing a lot of with and withouts. My fusor is an 8" sphere and when it gets cloths iron hot (ttsss) it looses some CPM. Or it could be my switcher getting too hot. I just got in a 125 KV power supply - 275 pounds of iron.

I think that once we get the hang of IG's they will be a big boost.
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Re: Ion Gun Construction / Proposition

Post by pfostini »

That sound pretty close to what I am seeing. Less than a MA but the improvement on the fusor was striking. I was thinking if the guns could be tested in some type of linear chamber. Gun at one end and a target at the other and if we used lenses the distance from the gun to the target should be the same distance as the poiser is to the shell.

I have modeled my detector after yours mentioned in neutron counting FAQ ( 2" BC-720 ). Placed at 6" from the poiser.I spent the last few weeks doing daily background checks and gamma rejection tests to make sure I am only reading neutrons.

I was also looking at a multipactor design for non-filament guns. I'll keep you posted.
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Re: Ion Gun Construction / Proposition

Post by hellblazer »

<heh>

Very cool.

My thought has always been that the current fusors are really poor ion guns. Take Richard's energy loss budget for any ion gun, and then apply it to the fusor, and even the worst ion gun will likely win (assuming the best for the fusor glow discharge).

I think the fusor is a decent accellerator, but from the australian experiments, it seems that the ions only have 25% of the cathode energy. Also, 60% of the ions were deduced to be H2+, decreasing the effective cross section. If the ion generator rips apart most of the moleculesthe H2 gas in addition to ionizing them (say in a duoplamsmatron), this alone could really increase neutron yield while decreasing power draw overall.

Not even bringing beam collision effects into play.

There might be a large percentage gain in just getting the neutrals out of the chamber so they don't collide with the ions doing the work. But this would require differential pumping in the guns to get rid of the neutrals. (at least that is what I believe it is for).

Just some random thoughts.

But again... Very cool results.
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Re: Ion Gun Construction / Proposition

Post by hellblazer »

Attached are the dwf drawings from the multipactor ion gun patent (http://www.hellblazer.com/personal/mr_f ... erator.pdf ). It's a straight clone of the patent. Note it isn't differentially pumped, so that may be a good sign for us amateurs.

You can view and print the dwf files with VoloView, available as a free download (for registration, which isn't onerous) from AutoDesk at http://usa.autodesk.com/adsk/index/0,,8 ... 12,00.html .

Don't know if this is of any use to anyone, but at least it's a drawing with the parts worked out.
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3l
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Re: Ion Gun Construction / Proposition

Post by 3l »

More than one way to skin a chicken. :)

I approached ions the same way I've done fusors.
Do all of it.
I had loads of microwave ovens sitting arround....
Yes... I started fiddling with them while I was waiting on Ebay UPS deliveries. I tried the Bell Jar's plasma stuff. It really works. Don't loose couple this stuff please use the conductive epoxy for God's Sake!
Test first then believe!

I've been adapting the microwave plasma generator in the Bell Jar plasma experiments into a pretty safe ion source. I used a 600 watt magnatron as the source. It is coupled to a 2" copper pipe and cemented to the exit emitter of the magnetron. (I'm a chicken when it comes to high power microwave) You simply arrange Helmhotz coils on the test tube and make a feed entrance for deuterium gas at the front of the test tube I simply glued a plastic tube with model cement and used a fine tube for D2 gas input into a hole I drilled into the test tube. To extract the beam glue an extraction electrode in with ceramic cement.
With a simple drilled baffle plate made out of steel the ions exit the holes while the microwave stay in....I simply copied the hole spacing on the front of the microwave oven. ( I hate scrapping off paint ) If you were really cheap just cut it out of a microwave oven door screen. The really lazy approach is to just drill a test tube size hole in the door the cover it with a microwave shield. The vacuum inside the ion generator is pumped externally without too much danger or problems of interfacing simply goes into the vacuum chamber of the fusor. I'm still playing with it..... I will release details as I have them. Microwave power without the design hassle of turning it loose in your fusor. I was gonna announce it a month from now...but Phil has broken the ice on the gun issue.
I will post pictures as soon as I get my new camera out of layaway.

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

Post by r_c_edgar »

Hi everyone,

Please direct the discussion to the new "Ion Gun Design and Construction " forum. Finish off any running dialogs for the sake of continuity, but anything that's even sort of a new topic should go in the new forum.

Thanks!

--Ryan Edgar
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Re: Ion Gun Construction / Proposition

Post by Starfire »

Thank you Ryan -- again -- Your work is appreciated.
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