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

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Discussion Overview

The discussion revolves around the feasibility of using a bell jar for building a Farnsworth-Hirsch-Fusor, specifically addressing the pressure requirements needed for operation, which range from 0.1 to 10 Pa. Participants explore the capabilities of bell jars in maintaining vacuum levels and the necessary equipment for the project.

Discussion Character

  • Exploratory
  • Technical explanation
  • Homework-related

Main Points Raised

  • One participant expresses uncertainty about whether an ordinary bell jar can maintain a vacuum at the required pressure levels of 0.1-10 Pa.
  • Another participant suggests that while some bell jars may reach pressures around 10-100 Pa, achieving below 1 Pa may depend on factors such as the seal quality and the vacuum pump used.
  • It is proposed that replacing the gasket with a new material like viton or silicone and ensuring cleanliness could improve the vacuum performance of the bell jar.
  • A participant mentions having access to a vacuum pump that can reach down to 4 x 10^-4 mbar, which is relevant for the project.
  • There is a suggestion to test the bell jar's vacuum capability and a query about the availability of a pressure gauge to measure the vacuum level.
  • One participant notes the need for additional materials, specifically Deuterium and two spherical electrodes, for the project.

Areas of Agreement / Disagreement

Participants generally agree that a bell jar may not be suitable for achieving very low pressures without modifications, but there is no consensus on the exact capabilities of the bell jar in question or the specific pressure it can maintain.

Contextual Notes

Limitations include uncertainty regarding the specific pressure limits of the bell jar, the potential impact of outgassing, and the need for a pressure gauge to accurately measure vacuum levels.

Who May Find This Useful

Individuals interested in vacuum technology, high school students working on physics projects, and hobbyists exploring fusion-related experiments may find this discussion relevant.

Farnsworth
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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?
 
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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).
 
Last edited:
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).
 
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?
 
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.
 

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