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The other critical mass

Anonymous Wed 10 Mar 14:56:31 2021 No.12815396 View Reply Original Report

Quoted By: >>12815428 >>12816061

So theres the critical mass for fission, but what is the critical mass for fusion?
By that i mean, what is the minimal mass of hydrogen you would need to heat up to a critical temperature in such a way to stimulate enough fusion reactions to overcome the natural cooling process?

Anonymous

Anonymous Wed 10 Mar 2021 15:12:49 No.12815428 Report

Quoted By: >>12815516 >>12815989

>>12815396
There is. It's actually what determines whether or not a star ends up on the main sequence or slowly fades as a brown dwarf. That limit is about 0.08 Solar masses. The above is an approximation because fusion, stellar dynamics and structure are wack.

Anonymous

Anonymous Wed 10 Mar 2021 15:47:51 No.12815516 Report

Quoted By: >>12815598

>>12815428
But that happens because the cooling rate in big stars is so small they can get away with a very small rate of fusion. Can the same happen with a smaller mass, implying a higher rate of cooling, if the rate of fusion was also smaller?

Anonymous

Anonymous Wed 10 Mar 2021 16:11:35 No.12815598 Report

Quoted By: >>12815691

>>12815516
No. I don't believe that's what happens. In fact, large stars undergo more fusion than low mass stars, and are also hotter, and therefore radiate more energy than low mass stars.

You have to understand, prior to fusion ignition, all stars are driven by gravitational collapse as their energy source. Lower mass means less energy, which means lower temperature, which, at a certain point, means no fusion ignition.

It is also worth noting, cooling losses in the protostar phase, which is what the bit before fusion ignition is called, are negligible compared to the rate at which energy is gained from gravitational collapse.

Anonymous

Anonymous Wed 10 Mar 2021 16:42:12 No.12815691 Report

Quoted By: >>12815869

>>12815598
What if the critical mass had an external source of heat? I dont mean a star but sonething doable in a lab. Because theres no gravitational collapse as a heating device, it could be some external device (doesnt matter what, its a conceptual discussion). Once hot enough, fusion would start, at some rate, but im wondering if the cooling would still be always higher than the natural heating because small masses have a bigger surface to volume ratio.
Of course this would not be a stable system without gravitational confinement, but i wondered if it would still produce more energy than cooling rate before the gas expanded and the reactions stopped, if even for a milisecond.

Anonymous

Anonymous Wed 10 Mar 2021 17:48:28 No.12815869 Report

Quoted By: >>12816021

>>12815691
Critical mass is the mass of material required to initiate a self-sustaining fusion reaction.

Fusion is inherently not self-sustaining unless confined, because of the high Coulomb potential it is necessary to overcome to initiate fusion events. Even if you could just spontaneously pump enough energy to initiate fusion, fusion would still be unsustainable, as the expansion of gas IS the cooling, which will always be higher than the rate of generation of energy unless the gas is confined.

And the only way to confine a gas with just mass alone is through gravity.

The answer to your question is yes, of course you can get fusion reactions that briefly produce more energy than they lose. Any exothermic fusion reactions can do that, even on the smallest scale.

But that's not what critical mass is.

Anonymous

Anonymous Wed 10 Mar 2021 18:24:28 No.12815989 Report

Quoted By:

>>12815428
Would this change with different isotope fractions? Like if you had a big ball of strictly deuterium or a D/T mix?

Anonymous

Anonymous Wed 10 Mar 2021 18:35:25 No.12816021 Report

Quoted By:

>>12815869
>Any exotermic fusion reaction can do that
I mean they produce energy, my question is if the rate of energy production can be higher than the cooling before the gas expansion reduces the rate of reaction. And if so, for how long? How fast are these reactions compared to the time it takes for the gas to expand?

Anonymous

Anonymous Wed 10 Mar 2021 18:52:23 No.12816061 Report

Quoted By: >>12816201

>>12815396
>minimal mass of hydrogen to cause fusion
'bout 2 AMU, I reckon.

Anonymous

Anonymous Wed 10 Mar 2021 19:45:18 No.12816201 Report

Quoted By:

>>12816061
That wasnt the question

OP Wed 10 Mar 2021 20:04:19 No.12816276 Report

Quoted By:

Let me rephrase the question:
Imagine you could confine a small mass of hydrogen with an artificial gravitational field or inside a hollow sphere of a magical transparent material that cannot melt. So pretend confinement of the mass is not a problem.
If you were to heat up this mass of hydrogen with an external energy source, to the point where fusion reactions started, and then stop the external heating, the sphere would produce some heat with fusion reactions, but also lose heat as it cools down.
Is there any critical mass at which the self-heating overcomes the cooling?
If yes, i would expect that gas expansion (basically a loss of confinement) would quench the reaction. But assuming there are no confinement problems i wondered if heat production could still overcome the cooling.
Intuitively this seems unlikely for very small masses because the area increases with the square and the volume with the cube, so small masses will cool faster, relative to their heat production.
Imagine if this was done in a place like Neptune, if neptune were to be ignited, and somehow restrained from expanding, would it have a self sustaining reaction?
If IRL it cant be restrained from expanding, could the self sustained reaction still occur for a brief moment since physical expansion takes time?

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