Electronics > Projects, Designs, and Technical Stuff
Electron gun - salvage or build
ChristofferB:
Hey guys!
I've almost assembled my ultrahigh vacuum setup (see attached). Once it's done, and I can reach 10^-9 mbar, I need something to use it for...
LEED (Low Energy Electron Diffraction) would be an interesting topic to approach, and no matter what, being able to create a beam of electrons would be cool.
So here's the question: Should I construct an electron gun, or should I crack open a oscilloscope CRT and just salvage one, where all the distances and geometries and materials are known to work? I have a vidicon tube as well, which has a very compact electron gun.
What do you think?
Thanks in advance!
CatalinaWOW:
What beam characteristics do you need for your proposed experiments? Current. Velocity. Velocity distribution. Beam diameter and shape. I think much of the answer to your question lies there. While salvaging an electron gun will save much work and avoid the need for tools and processes you may or may not have, if it isn't suitable for purpose it doesn't matter.
Gyro:
I have a vague memory that the final cathode coating is formed from its constituent chemicals when it is heated for the first time under vacuum. If so, that might render a recycled electron gun un-useable without a cathode replacement.
LaserSteve:
Your memory is correct. The CRT cathode will decompose and spit metal oxide crap and Co2 into your vacuum chamber for hours, and then the metal shell becomes your emitter, which is barely emissive. There were a few scandium cathodes out at the end of the CRT dynasty in big, expensive, long life tubes, but which got which was a trade secret. So you have to assume all tubes have carbonate cathodes. The rest of the gun is designed for an inductive bake out, so it is reusable.
For cathodes that see air or leaks on a repeat basis I'd suggest Lanthanum Hexaboride. SEM cathodes might be a place to start, because the time you spend learning to wind and coat tungsten could be better spent experimenting. Tungsten gets a insulating coating of Al2O3 applied using electrolysis, then a mixture of barium, strontium, and calcium carbonates in a binder which has to be decomposed during tube processing to form an emissive metal/metal oxide monolayer. Bare tungsten emits, it just does not perform well, and you end up jacking up the current to white heat to get emission. At which point the life can be measured in hours.
Outgassing and conditioning cathodes can come with a learning curve. You end up needing a Variac on the cathode transformer. One of pure Wolfram's lovely hidden secrets is that it sags around 700-750'C then gets stronger as it heats up. Don't leave it at 700 for any length of time when conditioning and outgassing. Etch it before use, too. Been there, done that, got the Tee Shirt. There is a reason for those platinum and iridium additives.
My experience with hot cathodes: SEM, Regenerating old scope tubes, Ion Lasers, X-ray tube installation, and electron beam evap.
Steve
ChristofferB:
--- Quote from: CatalinaWOW on May 21, 2020, 04:47:52 pm ---What beam characteristics do you need for your proposed experiments? Current. Velocity. Velocity distribution. Beam diameter and shape. I think much of the answer to your question lies there. While salvaging an electron gun will save much work and avoid the need for tools and processes you may or may not have, if it isn't suitable for purpose it doesn't matter.
--- End quote ---
A point beam 0.1-0.5 mm in diameter would be ideal. Energy can be set by the accelerating anode, so that really just depends on what PSU I can get.
Right now the application isn't very fixed, so having many parameters to play with would be interesting.
--- Quote from: LaserSteve on May 21, 2020, 06:56:34 pm ---Your memory is correct. The CRT cathode will decompose and spit metal oxide crap and Co2 into your vacuum chamber for hours, and then the metal shell becomes your emitter, which is barely emissive. There were a few scandium cathodes out at the end of the CRT dynasty in big, expensive, long life tubes, but which got which was a trade secret. So you have to assume all tubes have carbonate cathodes. The rest of the gun is designed for an inductive bake out, so it is reusable.
For cathodes that see air or leaks on a repeat basis I'd suggest Lanthanum Hexaboride. SEM cathodes might be a place to start, because the time you spend learning to wind and coat tungsten could be better spent experimenting. Tungsten gets a insulating coating of Al2O3 applied using electrolysis, then a mixture of barium, strontium, and calcium carbonates in a binder which has to be decomposed during tube processing to form an emissive metal/metal oxide oxide monolayer. Bare tungsten emits, it just does not perform well, and you end up jacking up the current to white heat to get emission. At which point the life can be measured in hours.
Outgassing and conditioning cathodes can come with a learning curve. You end up needing a Variac on the cathode transformer. One of pure Wolfram's lovely hidden secrets is that it sags around 700-750'C then gets stronger as it heats up. Don't leave it at 700 for any length of time when conditioning and outgassing. Etch it before use, too. Been there, done that, got the Tee Shirt. There is a reason for those platinum and iridium additives.
Steve
--- End quote ---
That certainly is valuable information. I've seen 3-terminal SEM electron gun capsules with just the filament and cathode, they aren't that pricy.
This leaves pretty much only the anodes of a CRT gun interesting for experimentation, the way I see it. Maybe the deflection plates are useful as well, if we assume my allignment of everything isn't spot-on.
It also sounds like building them yourself from scratch is such a big project in itself that it's not something one 'does'.
Thanks!
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