A simple electron gun

[ChristofferB]

Hi all!

Now that I have a decent high vacuum system, I'd like to put it to use.

I want to build a simple ion gun, able to operate from my 0-3 kV 10 mA PMT PSU.

See attached schematic.

The vacuum feedthroughs are all ready.

My plan is to soft solder everything, cups and tubes soldered from 20 um copper foil, elements supported by heavy enameled copper wire.

The basic test is to see if I can get any current to shop up on my electrometer (will be a vacuum tube ion chamber sans chamber).

Later, with a viewport maybe drawing a dot on a piece of CRT screen.


I am a complete noob at ion optics, however, so I'd like the opinion of you all before I power it up.

Since the filament supply will be at high voltage (very inconvenient - can polarity be swapped?), I will make a plastic enclosure right at the feedthrough to house it, and operate the potentiometers by 10 cm glass rod/tube elongations.


Have I missed anything?

Thanks in advance!

[Gyro]

I suspect that you mean electron optics rather than ion optics.

An electron lens normally consists of two cups (or discs) which are at the same potential, with a cylinder between them (on which the potential can be varied) so that you can get curved field gradients at each end of the cylinder.

Looking at your diagram, do you actually need the cup closest to the filament. In a CRT, this is normally G1, which allows the beam to be modulated or cut off. With just the second cup as the anode, electrons ought to be accelerated towards it, with the ones escaping through the centre hole fanning out gently.

Regarding potentials, I can't see any reason why you shouldn't ground the filament and put a positive voltage on the Anode cup. This would probably work better anyway as you are building it in a metal fitting - If you make the filament negative, then the fitting effectively become all anode, attracting electrons to the sides rather than repelling them. Having the enclosure at cathode potential might actually help to collimate the beam.

The anode could maybe be a disc rather than a cup too.

[ChristofferB]

Looking at your diagram, do you actually need the cup closest to the filament. In a CRT, this is normally G1, which allows the beam to be modulated or cut off. With just the second cup as the anode, electrons ought to be accelerated towards it, with the ones escaping through the centre hole fanning out gently.

I'm unsure, actually, almost every diagram of an electron gun features a "wenhelt" cylinder, I believe for setting the focal lenght of the beam.

Regarding potentials, I can't see any reason why you shouldn't ground the filament and put a positive voltage on the Anode cup. This would probably work better anyway as you are building it in a metal fitting - If you make the filament negative, then the fitting effectively become all anode, attracting electrons to the sides rather than repelling them. Having the enclosure at cathode potential might actually help to collimate the beam.

That's a good point, a grounded anode wouldn't work at all in my case. I guess it's how it is done on a glass crt electron gun. Positive voltage it is!

The anode could maybe be a disc rather than a cup too.

The anode is a flat disc, likely 1mm copper.


Thanks for the input! I'll flip the design voltage.

[Gyro]

Peter Millett has an excellent archive of tube era reference books, including several (later ones) which cover electron lenses in great theoretical detail. I suggest  'Vacuum Tubes, Karl Spangenberg, 1948' but there are others that may be easier going...

http://www.tubebooks.org/technical_books_online.htm


P.S. You might want to consider the idea of magnetic focussing too (assuming that your enclosure is non-magnetic stainless)!