Electron gun- what's wrong here?

For the design and construction details of ion guns, necessary for more advanced designs and lower vacuums.

Electron gun- what's wrong here?

Postby David Kunkle » Tue Jul 12, 2016 1:55 pm

Trying to construct a simple electron gun. More like an electron shotgun- I don't need or want any collimation. Tried it last night for the first time and - nothing. Hope the pics are fairly obvious as to construction. That's the best my camera can do up close. The filament/cathode is run off a 12V DC source (12V, 6W filament). The copper wire ring/anode is run off a separate 12V DC, 1A source through a transformer to a 320V output. I have a few different ideas what's wrong, but hopefully it's obvious to the forum.
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Re: Electron gun- what's wrong here?

Postby Richard Hull » Tue Jul 12, 2016 5:19 pm

Draw your circuit! How were you detecting that it did not work? Was the high voltage AC or DC and involved in a closed circuit? Was there a milli or microammeter involved to see if current flowed? If it is a gun, where is the target. At best, with a suitable vacuum, you have a diode and not a gun.
You have supplied us with no real info.

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Re: Electron gun- what's wrong here?

Postby Dennis P Brown » Tue Jul 12, 2016 5:59 pm

If you are just "boiling" off electrons (12 volts) you need a focusing (nice but not critical) and accelerating field (essential) for the electrons or the gun to really work because the electrons can not just go anywhere while under vacuum! The idea for an "electron gun" is to have high enough voltage gradient so that the electrons are forced to travel through the massively insulating vacuum to the ground along a path you desire. Sorry but 12 volts can't possibly do that.
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Re: Electron gun- what's wrong here?

Postby John Futter » Wed Jul 13, 2016 8:46 am

David
your anode potential is not large
lots of things are going to be needed to know.
how hot is the filament in your case there will be no electron current until your filament is glowing near white hot ie like a torch bulb or brighter
anything less than this will not work.
I use standard 6 volt torch globes @ work as electron flood to eliminate charging on non conductive sputter targets.
I need the filament hotter than a normal torch globe and 35 volts anode to cathode to get 3mA of emitted electrons. And yes the filament only lasts a couple of hours
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Re: Electron gun- what's wrong here?

Postby Steven Sesselmann » Wed Jul 13, 2016 11:52 am

I agree, a schematic or a diagram would be most useful here.

Anyway, you can ground the copper ring, and float your low voltage supply at around -1000 volts, when the element is heated electrons will boil off and head for the copper ring, and in the process you will be making some low potential ions that head for the grid. This is a bit like having a second grid, check out Doug Coulters fusor.

Make sure you float the source safely and put a high ohm ballast on it just in case.

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Re: Electron gun- what's wrong here?

Postby David Kunkle » Wed Jul 13, 2016 5:09 pm

sch 001.JPG

Sorry, I can see now I should've provided more detail- thought maybe something simple like anode too close to filament. At any rate, this was supposed to be modeled after the e-gun used on the Prometheus site: https://prometheusfusionperfection.com/2012/06/

He used 500V, so I thought the 12V in/ 320V out supply I have would be in the ballpark. My one 12VDC PS is actually variable to 120V. I accidentally hooked up the 12V input/320V output PS to the variable PS when it was still set to 100V instead of 12V and blew up a capacitor rated at 16V. Didn't hurt anything else and still works fine after replacing the cap. So I figure worst case, I can boost the output on the 320V PS to close to 450-500V for the short times that I need it?

I have been using a 120V/40W filament as a passive emitter with an anode in the center of the chamber at 10-15kV, and 10-6 Torr range. Distance between the two is about 6". With that setup, the HV has no trouble pulling e-'s off the filament in the mA range. The HV PS goes to 110mA and easily gets pegged and shuts itself down.

Building the gun to make an active emitter to make a controlled flow to the chamber's center anode. The gun filament is instead 10" from the chamber's central anode and will not passively flow any e-'s to the central anode at that distance. Therefore, the above attempt at an e-gun. Detection was supposed to be reading in the mA range on the main HV PS.
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Re: Electron gun- what's wrong here?

Postby Steven Sesselmann » Thu Jul 14, 2016 2:40 am

David,

My suggestion for the simple ion source is to use a nipple and a feedtrough with two wires. You will also need a small AC isolation transformer and a source of DC high voltage. A cheap DC voltage source can be made from a simple inverter and two rectifier diodes. http://www.digikey.ca/product-detail/en ... -ND/652722

Connect the high voltage with positive to ground and negative to the filament loop through a 1M ohm load resistor and adjust the brightness of the filament with a variac on the isolation transformer.

For best results the gas feed should go through the flange so the gas flows over the filament.

Steven

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Re: Electron gun- what's wrong here?

Postby Jerry Biehler » Thu Jul 14, 2016 3:00 am

Use a good isolated filament transformer with a small variac to control the filament emission. Then tie negative to the filament and positive to the chamber ground. If you want to use that that ring as an extractor add a separate variable power supply for that.
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Re: Electron gun- what's wrong here?

Postby Dennis P Brown » Thu Jul 14, 2016 10:46 am

Without any type of focusing your electron beam will not be a beam at all but a very diffuse cloud and the majority of the "boiled off" electrons will be absorbed by your acceleration ring. What small amount of electrons that do accelerate past the ring will rapidly spread out. Installing an Einzel lens to focus the electrons would be simple to install and significantly increase the beam current delivered to your desired area in the vacuum chamber - especially if the target isn't practically on your emitter.
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Re: Electron gun- what's wrong here?

Postby Steven Sesselmann » Thu Jul 14, 2016 11:40 am

Dennis P Brown wrote:Without any type of focusing your electron beam will not be a beam at all but a very diffuse cloud and the majority of the "boiled off" electrons will be absorbed by your acceleration ring. What small amount of electrons that do accelerate past the ring will rapidly spread out. Installing an Einzel lens to focus the electrons would be simple to install and significantly increase the beam current delivered to your desired area in the vacuum chamber - especially if the target isn't practically on your emitter.


Dennis,
I think all David wants is more ions too keep his fusor alight, a diffuse cloud of electrons will ionise the gas just fine, no need for a focused beam here.

Steven
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