What is the smallest amount of hydrogen needed for fusion

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Discussion Overview

The discussion revolves around the minimum amount of hydrogen required for fusion to occur under its own gravity, exploring the conditions necessary for different fusion processes and the longevity of fusion reactions. It includes theoretical considerations and potential applications in fusion energy.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • Some participants suggest that for proton-proton chain fusion to occur, a mass around 0.08 solar masses is necessary, while deuterium fusion can occur in brown dwarfs down to approximately 0.013 solar masses.
  • There is a concern raised about the concentration of hydrogen in the sea, with one participant noting that fusion reactors would likely need to utilize deuterium due to its low concentration in natural sources.
  • One participant proposes that the proton-boron fusion reaction may lead to the first commercially successful fusion reactor, citing the abundance and accessibility of both elements.
  • Another participant mentions the longevity of red dwarfs, which can live for hundreds of billions to trillions of years due to their slow burning rate, and questions the burn rate of brown dwarfs with deuterium.
  • There is uncertainty expressed regarding the availability of boron and deuterium, with one participant recalling a claim about the sufficiency of boron in seawater for long-term energy needs.
  • One participant expresses skepticism about achieving the necessary temperatures for certain fusion reactions, specifically referencing the challenges of reaching 600 keV in practical scenarios.

Areas of Agreement / Disagreement

Participants present multiple competing views regarding the types of fusion reactions that may be viable and the necessary conditions for those reactions, indicating that the discussion remains unresolved.

Contextual Notes

There are limitations regarding the assumptions about the availability of hydrogen and boron, as well as the specific conditions required for different fusion processes, which remain unresolved in the discussion.

Dragonfall
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What is the smallest amount of hydrogen needed so that fusion occurs automatically under its own gravity? How long would it last?

Not homework.
 
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For proton proton chain fusion to occur, you have to be in the ~.08 solar mass range. Below that, you can still fuse deuterium and be a brown dwarf down to the ~13 Jupiter mass range (or ~.013 solar mass range).
 
This something that the popular press never seem to consider. The 'endless supply' of fuel that's available for fusion, from the sea is actually in very low concentration (about 160ppm). Any fusion reactor that we could make would need to use deuterium rather than Hydrogen. We can't afford to wait for the p+p reactions to take place. It's another reason why the sea won't suddenly 'explode' as people fear.
 
I consider it more likely that the proton-boron reaction will produce the first commercially successful fusion reactor (see Polywell) and both elements are abundant and accessible.
 
Red dwarfs live for hundreds of billions to trillions of years because they burn very slowly. I'm not sure how fast brown dwarfs typically burn through their supply of deuterium.
 
dschlink said:
I consider it more likely that the proton-boron reaction will produce the first commercially successful fusion reactor (see Polywell) and both elements are abundant and accessible.
I don't know the statistics. Do we really have a lot of boring? I guess it could well be 'cheap' at any price.
 
sophiecentaur said:
I don't know the statistics. Do we really have a lot of boring? I guess it could well be 'cheap' at any price.

I can't find the reference, but I remember reading somewhere that the supplies of Boron in seawater was enough to fuel us for like a million + years or something. For Deuteium it was way more.
 
dschlink said:
I consider it more likely that the proton-boron reaction will produce the first commercially successful fusion reactor (see Polywell) and both elements are abundant and accessible.

http://en.wikipedia.org/wiki/Aneutronic_fusion

I hope you're kidding, I don't know of any way to produce temperatures of 600 keV (!) in any amount of material worthy of the word "temperature".
 

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