Why is my W filament burning in my DIY electron gun project?

[emtae55]

TL;DR

I made electron gun by myself with W filament, but the filament was burned :(

I'm in a project that make electron gun myself. I made acryl cylinder for the body and used W(tungsten) filament that using in real SEM. I made the body vaccum(about 11kPa), and applied voltage to the filament(about 6V). Then, the filament started to be heated. But problem was appeared. I didn't change the voltage and current, but the intensity of light of filament went increased by itself, and then it was burned and the electron gun was broken down.
How can i solve this problem??

 

[256bits]

The tungsten is burning with the oxygen still in the vacuum chamber.

 

[emtae55]

The tungsten is burning with the oxygen still in the vacuum chamber.

But the air pressure of the chamber was 11kPa. Isn't it insufficient?

 

[nsaspook]

But the air pressure of the chamber was 11kPa. Isn't it insufficient?

No. That's about 83 Torr, barely rough vacuum. You need at least low milliTorr medium vacuum to high vacuum for a hot filament not to quickly burn.
https://www.engineeringtoolbox.com/vacuum-d_837.html

 

[256bits]

But the air pressure of the chamber was 11kPa. Isn't it insufficient?

No. Not for the small mass of the tungsten filament, which is somewhere of the order of micrograms in mass.

A gross calculation.
Knowing that:
1 g of tungsten has 5.44 x 10_-3moles of atoms,
Atmospheric air has 2 x 10(exp -3 ) moles of atoms oxygen in one liter.
Oxidation of tungsten combines 1 to 1 ratio of oxygen with tungsten,

You have to reduce the oxygen content in your chamber to below that of the level of tungsten atoms in your filament.

With a filament of microgram mass, that is 1 millionth of atmospheric pressure ( density ), or milliTorr.
Which is in line with what is mentioned above.

 

[Keith_McClary]

IIRC, in the manufacture of vacuum tubes they use a sacrificial piece of some metal (magnesium, sodium?) that consumes the oxygen. I don't remember if it needs to be heated by an electric current.

 

[Tom.G]

IIRC, in the manufacture of vacuum tubes they use a sacrificial piece of some metal (magnesium, sodium?) that consumes the oxygen. I don't remember if it needs to be heated by an electric current.

In vacuum tubes, the 'Getter' as it is called, is heated as the final stage of evacuation. An overview can be found here:
https://vacaero.com/information-resources/vac-aero-training/1166-getter-materials.html
above found with:
https://www.google.com/search&q=getter+material+in+vacuum

Cheers,
Tom