DIY Scanning Electron Microscope - Electron Gun Detail

[bigbirdcrap]

GREAT video thanks! i'm a high school student trying to build a transmission electron microscope (low quality - fairly low magnification) and this info helped a lot! electron gun assembly is exactly what i need

[artemonstrick]

As i've seen your HV-PSU as able to produce 0-10kV. How did you manage to make negative voltage? You can't just switch the ends and call it negative.

And why it was so crucial for you making 0V anode and wehnelt negative?

[bkraz333]

@artemonstrick Two of my power supplies can be configured to output positive or negative HV. Another two have isolated outputs, so the wires can be switched to reverse polarity. Having the anode at 0V is useful because all of the structural metal parts in the chamber can also be 0V and not affect the beam with electrostatic attraction, and this config also avoids the hassle of insulating nearly every part in the chamber. Good luck, post your results!

[butterfingers8008]

how dangerous is a fully functioning electron gun where the electrons leave the apparatus? ]

[bkraz333]

@butterfingers8008 Not very dangerous. Remember that all CRT TVs used electron guns, so if there were a major hazard or if the device could be repurposed, its use would have been restricted. Electron beams do pass through atmospheric air at all well, so the main danger from an electron beam is caused by bremsstrahlung. For accel voltages over 10KV, hard x-rays will be generated, which requires lead shielding.

[Films4You]

What stops any high voltage arcing? Is it the Vacuum?

[bkraz333]

@Films4You Yes, the only reason that high voltage arcs across terminals in air is because the molecules become ionized and form a low-resistance path. In a perfect vacuum, there are no molecules to support an ionization path between the terminals. It is still possible to have field emission or thermionic emission of electrons in a vacuum, but no arcing.

[Films4You]

@bkraz333 Thnk you very much for this excellent and detailed explanation :)

Well done with all your hard, clever, work :)

[mondolmk]

Great description of a self biased electron gun. Keep up the good videos.

[Dibblah1900]

Very nice video. Thanks! If you're not a lecturer for your day job, you should be.

[morto360]

What defines the resolution of the SEM?

Or whats the difference between a 200.000.000 USD SEM and yours?

One would think that resolution is controlled by how fine you can control the beam of electrons.. but that can't be 200mill worth... or?

[bkraz333]

@morto360 I think the most expensive SEMs are still under one million USD, but perhaps with add-on equipment it could could more. The more expensive SEMs have better vacuum systems which require the use of more exotic materials and machining practices in the chamber. They have more carefully wound electromagnetic lenses with extremely stable high-precision current supplies. In principle, operation is the same as my SEM, but everything must be done more carefully with tighter tolerances.

[morto360]

@bkraz333 I saw a video about the best SEM atm, and they said it cost about 220millions, thats why I was wondering... but I guess all this facts that you mentioned could add up a lot to the price.

[mmavipc]

lol that first frame, your face, jesus

[Arnthorg]

Could you have used an electron gun from an old CRT instead of making it yourself from scratch ?

[bkraz333]

@Arnthorg Electron guns in commercial CRTs use metal-oxide cathodes that can never be exposed to air, even when cold, or else the cathode will be damaged. Additionally, the CRT guns are built to provide a fairly large-area source of electrons, while the SEM requires a tiny point-source of electrons.

[xXdenhartXx]

Thanks.

[Sine1040]

Isn't the Wehnelt cylinder the focussing grid (G3) ? and the first grid after the cathode the intensity control grid (G1) ?

[bkraz333]

@Sine1040 CRT guns are built differently because achieving a very fine point-source of electrons is not the goal in building a picture tube.

[BHR477]

the explanation is great ,, just keep these videos coming :)

[AntiProtonBoy]

Quality.