Electron gun - salvage or build - Page 1
Forum section: electronics / projects

author: ChristofferB
author_rank: Frequent Contributor
timestamp: May 21, 2020, 04:01:38 pm
content: 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!

author: CatalinaWOW
author_rank: Super Contributor
timestamp: May 21, 2020, 04:47:52 pm
content: 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.

author: Gyro
author_rank: Super Contributor
timestamp: May 21, 2020, 06:07:52 pm
content: 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.

author: LaserSteve
author_rank: Super Contributor
timestamp: May 21, 2020, 06:56:34 pm
content: 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

author: ChristofferB
author_rank: Frequent Contributor
timestamp: May 21, 2020, 07:11:33 pm
content: Quote from: CatalinaWOW on May 21, 2020, 04:47:52 pmA 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 pmThat 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!

author: LaserSteve
author_rank: Super Contributor
timestamp: May 21, 2020, 09:59:14 pm
content: Oh, there are designs out there for demountable accelerator assemblies, that use larger size stainless steel tubing and tile, ceramic rods, or alumina blocks to make the gun holder with threaded rods. Have you watched the older of the two Tektronix "Crt Plant" videos? I might even drop you some sealing glass beads in the mail. I just bought 1500 of them for the laser project. I'm pretty sure C.L. Stong has a good design in one of his books. If you have that little spot welder, that you see in the Mullard or Tek videos, it would go quickly.Just don't listen to a certain "Make your own tubes" sage out there on YouTube that says Hard Glass is impossible for the hobbyist. He pushes people to leaded glass, ie code 0010, or code 80, and that is a recipe for tubes that crack, unless your very, very, good. If you know where to look, for 7052 or 8052 Glass, and thin Kovar rods, its not. I cheated once and used the tin plated Kovar base plate out of a Mini-Circuits mixer, the little blue and green beads around the leads are a compression seal made of glass frit. They are Hermetic. You know, SRA-7 or similar with the six or eight pins... The tin plating lets you solder.Weldable /silver solder able header feedthroughs are still around if you know what your looking for and search Ebay hard enough. Usually 8 pin OCTAl tube base styles show up, still made for Military and Aerospace relays world wide.If you don't mind pumping in real time, Hysol 1C White is a ceramic loaded epoxy that is chemically Almost identical to Torr Seal... That would help you make cheap glass to metal seals and I've used it to 10^-6 Torr.Steve

author: LaserSteve
author_rank: Super Contributor
timestamp: May 21, 2020, 10:29:37 pm
content: The green feeds like this picture are actually glass insulated and hermetic.. Usually the glass is a grey or green, sometimes dark blue, and the tip here is they are solderable as shown. Picture shamelessly swiped off Ebay for educational purposes. The glass is melted as a preformed paste into the seal frame in a hydrogen furnace. Once it cools the glass is under considerable compression due to dissimilar expansion or it would crack. I've seen them on Ebay out of Russia, Moldova, Poland and Ukraine by the bag for say 25$.Steve

author: ChristofferB
author_rank: Frequent Contributor
timestamp: May 22, 2020, 12:15:09 am
content: Damn, you're right, they're litterally a dime a dozen! Can they be trusted for high vacuum applications? The ideal solution would then be to get a KF-style flange turned in brass, so these could be soft soldered directly in!Alternately, an aluminum blind flange with a large hole, onto which a brass plate is vacuum epoxied., through which feedthroughs can be soldered.I'm not completely sure wich tektronix video you mean, but I should definitely scour the old C.L. Stong articles!thanks for the tips!

author: jmelson
author_rank: Super Contributor
timestamp: May 22, 2020, 01:24:10 am
content: Yikes, turbomolecular pump! Don't run it high vacuum side up, unless the manufacturer requires that. Too easy to drop debris into it.best to run it high vac side down, with the pump above the chamber.Otherwise, you can get this picture.Jon

author: LaserSteve
author_rank: Super Contributor
timestamp: May 22, 2020, 03:06:31 am
content: WE try to avoid soft solder in vacuum, but for you, the green or grey GOST standard parts with the hermetic glass are a winner. I'm assuming you do not want to pay 250-350 USD for a commercial feed in KF or other flange. Silver solder is OK. If you want to permanently seal your device, other means must be taken, but for a beginning high vacuum person, they are probably far better then anything you can make, short of Kovar to Aluminosilicate glass or Tungsten to 3320 to Pyrex. . Avoid thermal shock to the seals. They will limit your anode voltage, but for the grids, cathode, etc, not too shabby.If this is an older turbo, you might want to ensure it has the proper lubricant. A manual for the Turbo is a must, as is the proper cooling. J. Melson is right, a screen over the turbo is a very good idea, because otherwise your setup has to be perfectly clean with no broken parts ever. One thing I insist on is a TC or other gauge right at the roughing pump feed on the turbo. I watch mine in real time and have an alarm set up. You'll hear the turbo "load up" if there is a leak, and if the controller is any good it shuts the Turbo down if it senses drag. Suddenly loading the Rotor is not a sound I like to hear, so be careful.There are a couple of types of electron gun, some need much tighter tolerances, some need amazing curved shapes, and others sacrifice spot size for ease of building.Enjoy,...Video gets better at the middle:https://youtu.be/jHGAnJjnNY0Video CRThttps://youtu.be/MsMsZaSz3FkFor an experimental unit there are other ways without the specialized materials. PS, Please skip the Hf , RF, and PbO steps in the above videos if you try this...You don't need low melting glass rods to hold your gun together, either.One other thing for a beginner, never ever trust your glass, always have a tough screen around it to protect you during an implosion. I've seen a few glass things do a pretty good impression of a grenade if mishandled. Always do some stress calculations on that glass, the amount of pressure per unit area when under vacuum is amazing.Applied Science has some videos you may be interested in. I imagine you want to do something like Auger analysis? Or you'd just like to see a beam?Steve

author: LaserSteve
author_rank: Super Contributor
timestamp: May 22, 2020, 04:05:27 am
content: Zinc Orthosilicate Phosphor, Zn2SiO3:MnTwo Moles Zinc OxideOne Mole Silicaadd .5 to 1.25% Manganese Oxide or Manganese Chloride by weight as the activator. (No activator, no Glow)A slight excess of silica is desired.Potassium chloride or Sodium Chloride (15% by weight, added to basic mixture) may be used as a flux.Fire at 1000'C or higher for green.Add an excess of silica (2 MOLS or more, instead of one) and fire at 850'C to make the amorphous excess silica form that is medium persistence yellow.Grind or Ball Mill the ingredients together as fine as possible, without the KCL, or NaCl, the reaction can take a full 15 hours, with the flux, firing time is 40-60 minutes.You may find the need to fire, regrind, and fire again for the green form.Fire in Air, which is unusual for a Phosphor.Test under 256 or 365 nm illumination.From Dr. Gorton Fonda's recipe..Otherwise just buy ZNS:CUThere are plenty of facile synthesis or sol-gel synthesis of Phosphors on line, especially for ZnS:Cu.Yen and Weber, The Phosphor Handbook, is interesting for Chemists.YMMV as purity and particle size of the ingredients REALLY matter, ie start with Cabosil if you can.

author: Circlotron
author_rank: Super Contributor
timestamp: May 22, 2020, 04:29:04 am
content: Quote from: ChristofferB on May 21, 2020, 04:01:38 pmTeam up with someone who makes vacuum tubes at home for the audio crowd but hasn't got a pump. You would also need an induction heater for de-gassing during evacuation.

author: BreakingOhmsLaw
author_rank: Frequent Contributor
timestamp: May 22, 2020, 10:52:22 am
content: Hi,Used to work in a Philips picture tube factory.Evacuating CRT was done like this:Step 1: Piston Pump to about 50mbar.Step 2: Turbomolecular pump like the one you have in your setup.Step 3: Diffusion Pump while heating the CRT to 200C. After that, CRT was sealed.Step 4: Barium getter pill evaporated with an external RF Field.Heating the CRT was essential to achieve full vacuum, otherwise many molecules stick to the internal parts.

author: ChristofferB
author_rank: Frequent Contributor
timestamp: May 22, 2020, 04:02:08 pm
content: Quote from: LaserSteve on May 22, 2020, 03:06:31 amIt's a pretty new pump, an EXT255H from 2012, I think it's "as-new". And i have a pirani gauge on the foreline with a set point output to trigger an alarm / emergency pump shutdown etc.I plan on doing as little glass blowing as possible, I've spent a lot of time on it so far, and I have never produced anything I'd risk my turbo onI really like the idea of using the solderable soviet surplus feedtroughs, maybe using 2mm stiff copper wire, and let that be the entire support. That, or some blind holes tapped in a blind flange.All the tips on phosphor mixing are great! Do you know how to bind the phosphors to a surface?I've also seen people having success with using automotive spark plugs as high voltage vacuum feedthroughs, by the way. Might be good for anode voltages.

author: 1sciguy
author_rank: Contributor
timestamp: May 22, 2020, 04:50:08 pm
content: 450kV power supply for my electron gun at work. I designed the pressure vessel so it can be filled with SF6 insulating gas. We use GaAs photocathodes that are laser driven. I designed the laser system. 50ps pulses at 500 MHz by gain-switching laser diodes and then amplifying with fiber lasers. I actually combined three of these lasers with 120 degree phase shift for a 1.5Ghz pulse train. This was all at a National Lab and I have retired. I would not attempt such a project on my own without serious funds. Photocathodes are cool because all you have to do is bias your cathode at HV and hit it with a laser to get the electrons off. Of course there is chemistry involved to process the cathode in vacuum to lower the work function so it will spit off electrons based on light energy striking it. Thermionic guns are pretty simple, so yes, you could easily take these items from a CRT if you have a means to float your drive electronics at high voltage.

author: LaserSteve
author_rank: Super Contributor
timestamp: May 22, 2020, 05:00:53 pm
content: Binding is an artform. There are some videos of tube manufacturing on YouTube that cover the process, search for "Phosphor Settling"Silicate Solutions, Acetate Solutions, often with a buffer solution floating on top to mitigate turbulence. , then usually some means for decanting the buffer and un-used phosphor from the CRT bottle. usually by mechanically tilting very slowly to avoid any turbulence. In your case you probably can do it separate from inside your final structure.The US Patent server is your friend on that one..You then have to decide if you want your phosphor to be able to store charge, or add a conductive layer on top of it. (Aluminizing) Charge storage can be bad as it deflects the beam from a charged spot. Simple tubes generally do not have the bleed layer.BTW, the black fluid stuff in the videos for conduction is called "Aquadag".Steve

author: wizard69
author_rank: Super Contributor
timestamp: May 22, 2020, 11:30:24 pm
content: I'm laughing now because we run a vacuum process at about 250 mtorr. To think I thought that was high vacuumQuote from: BreakingOhmsLaw on May 22, 2020, 10:52:22 am

author: ChristofferB
author_rank: Frequent Contributor
timestamp: May 23, 2020, 12:16:24 am
content: Quote from: 1sciguy on May 22, 2020, 04:50:08 pmWhoa that looks impressive!I did read about laser electron sources, but since I haven't worked with lasers, I wanted to avoid nesting projects too deep.

author: ChristofferB
author_rank: Frequent Contributor
timestamp: May 23, 2020, 12:18:03 am
content: Quote from: LaserSteve on May 22, 2020, 05:00:53 pmIf I were to coat an aluminium hemisphere with a phosphor (as used in LEED systems) wouldn't that issue solve itself?