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

  • Electronics
  • Thread starter emtae55
  • Start date
  • Tags
    Filament
In summary, the person is attempting to create an electron gun using an acrylic cylinder and a tungsten filament. They have applied voltage to the filament and heated it, but the filament unexpectedly increased in intensity and burned, causing the electron gun to fail. The issue is likely due to insufficient vacuum pressure in the chamber, as the tungsten is burning with oxygen still present. The recommended pressure for a hot filament is at least low milliTorr to high vacuum. In vacuum tube manufacturing, a sacrificial piece of metal is often used to absorb oxygen during the final stage of evacuation.
  • #1
emtae55
24
0
TL;DR Summary
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??
 
Physics news on Phys.org
  • #2
The tungsten is burning with the oxygen still in the vacuum chamber.
 
  • #3
256bits said:
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?
 
  • #4
emtae55 said:
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
 
  • Like
  • Informative
Likes 256bits and anorlunda
  • #5
emtae55 said:
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.
 
  • Like
Likes nsaspook
  • #6
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.
 
  • Like
Likes Nik_2213
  • #7
Keith_McClary said:
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
 
  • Like
  • Informative
Likes Nik_2213, dlgoff, Asymptotic and 1 other person

1. Why is my W filament burning out quickly?

There could be several reasons for this. One possibility is that the voltage or current being applied to the filament is too high, causing it to overheat and burn out. Another possibility is that the filament is not properly secured in the electron gun, causing it to vibrate and wear out quickly. It is also possible that the filament itself is defective or of poor quality.

2. How can I prevent my W filament from burning out?

To prevent your W filament from burning out, make sure to use the appropriate voltage and current for your specific filament. You can also try using a voltage regulator to ensure that the voltage stays within a safe range. Additionally, make sure the filament is securely attached to the electron gun and check for any defects in the filament itself.

3. Can I use a different type of filament in my DIY electron gun project?

Yes, you can use a different type of filament in your electron gun project. However, make sure to do your research and use a filament that is compatible with your specific electron gun design. Different types of filaments may require different voltages and currents, so be sure to adjust accordingly.

4. How do I know if my W filament is burned out?

If your W filament is burned out, it will no longer produce electrons and your electron gun will not function properly. You may also notice a change in the color or appearance of the filament, such as a blackened or melted spot. If you suspect your filament is burned out, you can test it with a multimeter to check for continuity.

5. Can I replace the W filament in my electron gun?

Yes, you can replace the W filament in your electron gun. However, make sure to use a filament that is compatible with your electron gun design and follow proper safety precautions when handling the filament. It is also important to properly secure the new filament in the electron gun to prevent it from burning out quickly.

Similar threads

  • Electrical Engineering
Replies
12
Views
956
Replies
25
Views
6K
  • DIY Projects
Replies
5
Views
2K
Replies
11
Views
2K
Replies
15
Views
8K
  • DIY Projects
Replies
5
Views
2K
  • Introductory Physics Homework Help
Replies
6
Views
2K
  • STEM Academic Advising
Replies
8
Views
1K
  • STEM Educators and Teaching
Replies
3
Views
2K
  • Introductory Physics Homework Help
Replies
2
Views
5K
Back
Top