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Comment by djyaz1200 | Hacker News Reader

I wonder if many of the stars in the sky are from groups that almost nailed containment and stability on their Tokamak.

The Sun consumes a mass equivalent of a mount Everest worth of hydrogen via fusion to shine for just an hour (or thereabouts, if I did my math right :)). For perspective, this amount of energy is more than enough to power the Earth's current electrical usage for over a billion years.

That's all before getting into how a containment failure doesn't imply "and then everything nearby just started a self sustaining fusion reaction". The confinement itself is a key part of what enables the conditions for the fusion to continue.

I am a plasma researcher, though not in the fusion field. Containment and stability are required on tokamaks to keep a plasma burning. Losing either of these will quench the reaction. The best way to control a plasma - magnetic fields, also causes significant instabilities, which is why fusion is so difficult.

Could you elaborate on how magnetic fields cause instabilities? As a layman, it's not immediately obvious to me why that would be the case.

Because the plasma itself is charged and moves within the field, generating eddy currents which self-interact in complex and unpredictable ways. At a much much larger scale, the twisting of magnetic fields from convection within the sun causes sunspots and other phenomena around the solar surface.

Have you tried implementing bacteria with them?

No. It just does not make sense from physical rules. Fusion only happens in a very high vacuum, at ridiculous temperatures, with very specific fuel, in the confined space. Just the cooling effect of having oxygen atoms there(in the plasma) stops the reaction, let alone touching anything so cold(millions of times colder and denser) as the walls or the outside gas.

Also stops immediately if no fuel is given.

Naive question here : how do we expect to collect thermal energy from it if we can't allow it to cool even a little ?

You can extract heat, in fact you'd have to extract it or let it leak out somewhere. The whole point of it is that it generates more energy than you put in, so energy has to come out somewhere for it to maintain stability.

Gp is just saying that if you cracked it open like an egg (or just had a minor leak even) all that would happen is it would stop fusing. The room this happened in would be a bad place to be, but it's just going to start a fire or something, not destroy the world.

Neutron radiation doesn't get contained, and leaves the reactor easily carrying heat with it. That heat has to go somewhere, and so that's what we take energy from.

Seems implausible. The fusion presumably wouldn’t keep going if it breached the walls.

Also, to be bright enough that we would see it from here as a star, I imagine it would require enough material that one might as well just let gravity do the job rather that use a Tokamak?

Maybe there are efficiency gains that are large enough that it wouldn’t actually require as much material as a star? I wouldn’t guess so though.

> wonder if many of the stars in the sky are from groups that almost nailed containment and stability on their Tokamak

Different fusion systems. Stars fuse, in general, by statistically overloading the weak force. (The Sun is volumetrically about an order of magnitude less powerful than a human being. Like 200 to 1,110 W/m^3.)

In smaller volumes, e.g. on Earth, we have to break the strong force. This releases more energy, I think. But it also requires temperatures and energy densities far higher than that which stars produce.

Not sure if that strengthens or weakens your hypothesis...

Strong and weak force don't come into it in either case. Fusion requires overcoming electrostatic repulsion, that's about it. The problem is the Sun is gigantic but it's fusion process is actually very inefficient. To make it practical on Earth we need more particle interactions, and thus higher temperatures, to make it Q>1

> Strong and weak force don't come into it in either case. Fusion requires overcoming electrostatic repulsion, that's about it

You're wrong and right. Electrostatic repulsion is the barrier, and at its limit, defines electron degeneracy pressure. But the strong force is the ultimate source of energy of the reaction, and the weak force is important in stellar reactions.

The weak force initiates proton-proton fusion [1]. (We still struggle to empirically measure its cross section because it's so low. Weak force be weak.) DT fusion, on other hand, has to crack open the energy in those delicious gluons with raw temperature. This is why PP fusion occurs around 4 MK while DT fusion needs over 1,000 MK.

[1] Anthony Phillips' The Physics of Stars

I think that it is inaccurate to say that weak force "initiates" fusion.

Fusion is initiated by bringing nuclei very close one of another, overcoming the electrostatic repulsion.

When fusion is successful, the output energy is a consequence of the strong forces, i.e. it is the difference between the binding energies caused by strong forces in the output and input reactants.

The role of the weak force is that it can determine the probability of success of the fusion.

When the input nuclei have enough neutrons, the nuclei that have collided may remain fused. Otherwise, even after being fused for an extremely short time, the compound nucleus will break again, regenerating the input nuclei which are repulsed, so fusion fails.

In cases when fusion would fail due to a bad proton/neutron ratio in the fused nucleus, e.g. for the case of proton-proton fusion, during the very short time when the input nuclei are fused, weak forces may transform a proton into a neutron, preventing the separation of the fused nuclei and allowing fusion to succeed.

So overcoming the electromagnetic forces initiates fusion, strong forces determine the amount of energy obtained per fusion event and weak forces can determine the probability for fusion to succeed when nuclei collide.

> Fusion is initiated by bringing nuclei very close one of another, overcoming the electrostatic repulsion

Protons overcoming their electrostatic repulsion doesn't mean fusion--formation of a deuteron does [1]. Protons overcoming their repulsion creates the initial conditions for fusion, but in most cases no fusion occurs. The weak force "chooses" whether fusion occurs or two protons come unusually close and fly apart.

This is a bit of a pedantic line. But nuclear physisist say the weak force initiates fusion because if we take something with as low a cross section as proton-proton interaction to be the starting point of fusion, we might as well extend it to protons being in a star at all. (A greater fraction of protons in a star will fuse than proton-proton interactions graduate to fusion.)

Without the weak force, we have no stellar fusion. Without the weak force, artificial fusion is still possible. That's both a blessing and a curse, since the weak force permits lower-temperature fusion.

> When the input nuclei have enough neutrons

Irrelevant for proton-proton fusion.

[1] https://physics.stackexchange.com/questions/526471/why-is-th...

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