Disappointment

13 12 2011

All photos.

Today I did another test of the CRT based electron gun in the vacuum chamber.

Spoiler alert: I didn’t see an electron beam.

This time I put a grounded piece of phosphor right in front of the beam path:

 

Here is a closeup of the grounding wire:

I printed the phosphor holder with 1% infill so it’s basically hollow, then I drilled it with holes to allow gas to escape:

 

The vacuum reached 1.66 millitorr which is not amazing, but good enough for this run.

I tested the wiring with the unbroken CRT, and it looked good:

 

Then I wired the power supply to the vacuum chamber feedthrough:

The I fired it up, you can see the hot cathode working:

But no beam.

:(

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5 responses

19 12 2011
Gima

Hello
Perhaps the electrons don’t have enough energy or are too scattered in order to create a bright spot. You could try to replace the phosphorus plate with a metal plate, and then you should test if there is current in the ground wire.

ps sorry for my bad english.

29 12 2011
Bob Reite

I assume that the cathode and other electrodes are negative with respect to your grounded piece of phosphor screen. Assuming that you are using the same electrode voltages as used in the picture of the operating CRT, my guess is that the geometry of your vacuum chamber is different enough from the original aquadag coating of the glass CRT that you can’t get a good focus. The other possibility is that by opening up the CRT and exposing the activated cathode to oxygen in the atmosphere, the cathode has become “poisoned” and won’t have good emission. I’m not sure where you can get new electron gun assemblies these days, the last CRT rebuilder closed his business July 2011 and I don’t know where he got his parts from.

Other thoughts. As Gima said, check for current. In a CRT gun of that size, you should expect between 10 and 20 microamps of beam current. Check for current in the ground return from the metal vacuum chamber as well as the phosphor screen. Try changing the focus voltage. If you can’t find a new oxide cathode, change the design to a “hot” filament. A filament from a car tail light bulb should have close to the correct geometry if you position it lengthwise in place of the original heater/cathode assembly and it won’t be bothered by exposure to the atmosphere as long as it’s not powered up again until it’s under vacuum.

26 01 2012
olivier

Hi,
Your pressure is too high, you need to be at the very least in the 1e-2 millitorr and really 1e-4 would be better. Your electrons are scattered and absorbed before the reach the screen (assuming they are emitted straight enough…).
Your vacuum vessel looks decent and you are using copper gaskets, good. what sort of pump do you have (turbopump would be my choice)? I m not too kean on the wiring of your gun: regular plastics degas like mad (your fluo screen support might also be a problem…), plus you get a large amount of gas trapped inside the wire which will degas forever. Try using bare copper wire, keep them appart with ceramic or glassfiber insulation. Look up instrumentation for UHV surface science, like e-beam heater and ion guns, you should get some ideas. But basically if you want to go UHV (which you need for e-beam) you want to use only metal, ceramics or glass, possibly kapton.
Good luck with your project ! I hope you succeed!
O.

11 02 2012
Vacuum Check « Prometheus Fusion Perfection

[…] have Swiss TV journalist Yves Gerber with me in the lab today. I wanted to try the electron gun again while he was […]

2 06 2012
nikita

how to make the electron gun

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