Building a Farnsworth-Hirsch-Fusor: Pressure Requirements for Bell Jar

[Farnsworth]

Hi,
I like to build a Farnsworth-Hirsch-Fusor for a school project. I want to use an ordinary bell jar but I am not quite sure if it will work at the pressure I need (0.1-10pa). Does anybody know how much pressure a bell jar can take?

 

[Gokul43201]

First off, I strongly suggest you enroll an adult supervisor who will make sure you don't fry yourself with the voltage.

I don't think any old bell jar will hold vacuum at the level of 1Pa. I've known bell jars to go down to about 10 - 100Pa, but I'm not sure if that pressure was limited by the leak-rate of the bell jar, outgassing from crud or the limit of the pump. The key is in the seal - if you replace the gasket with a new one (preferable viton or silicone), and clean up stuff nicely, you could get down to below 1Pa.

Also, you'll need access to a vacuum pump that'll go down to the mtorr range. These don't come cheap (a few hundred bucks used, if you're lucky).

 

[Farnsworth]

Thanks for your reply.
Of course I have an adult supervisor helping me.The bell jar is quite a new one. I have also got access to a good pump (according to the owners manual it goes down to 4*10^-4mbar).

 

[Gokul43201]

In that case, I'd try pumping it down and see how low it goes. There is no risk of the bell jar breaking under the pressure, if that's what you're asking about. Do you have a pressure gauge that you can use?

 

[Farnsworth]

unfortunately I don't have a pressure gauge, I guess that's one of the things I'll have to buy. I also need some Deuterium and two spherical electrodes.