Probably a dumb question, but what are the potential environmental impacts of manufacturing lots of helium? Is it a greenhouse gas? Even if we pivoted to 100% fusion in the next 100 years, is there a chance of releasing enough helium that it would be a problem in practice? I assume we won't need to worry about running out of hydrogen considering all of the water in the world (provided we can efficiently get hydrogen from the more abundant ocean water rather than the limited fresh water)?

> Is it a greenhouse gas?

Certainly not in the conventional sense (although perhaps there's another way that helium might act similarly to a GHG that I'm not aware of).

Greenhouse gasses are molecules, i.e. multiple atoms bonded together. Those molecules can absorb photons of infrared light, which cause them to vibrate (as if the atoms were held together by springs). After some time, the vibration stops and an infrared photon is emitted.

The problem is: those photons are emitted in a random direction, unrelated to the photon that was absorbed. Half the time they will go roughly upwards, half the time they'll go downwards.

A photon of visible light (from the Sun) can travel down through the atmosphere without interacting much with the greenhouse gasses, since it has too much energy to be absorbed. This visible photon can be absorbed by other materials at ground level, e.g. by a plant, and its energy will eventually result in around 20 lower-energy infrared photons being emitted back up (on average).

These 20 infrared photons are readily absorbed by the greenhouse gasses, and each time they're absorbed, they get re-emitted in a random direction: half the time heading upwards again, but half the time heading back to the ground. This is how energy gets "trapped" by greenhouse gasses.

Helium is almost completely unreactive: it doesn't form molecules in the atmosphere, it just bounces around as individual atoms. Without bonds to vibrate (or asymmetries to spin), the only way it can absorb energy is by speeding up, and even this isn't very effective since its mass is so low. Fast-moving helium is also more likely to escape the Earth's gravity completely.

Hmm. I'd imagine all matter than interacts with photons to be GHGs in some format.

The frequency band for absorption and emission might be different, but the helium could absorb a shorter wavelength, then release a longer wavelength (not infrared) that can be absorbed by something else, then emitted yet again to be infrared

In fact, my understanding is that, not counting nuclear fusion, helium is a non-renewable resource critical for some kinds of critical uses like medicine; and that our current habit of putting it in balloons is considered rather reckless by people taking a longer view of things. If we could capture that helium, it would make things a lot better.

Exactly, helium is likely bound to exhaustion along with natural gases deposits: the only source of helium, currently. It can't be replaced with another gas, at least not in supercoducting magnets, so it's a problem. In the future, fusion power plants will be optimised to reduce helium losses, recycle helium produced by the reactions and may also have to extract helium from the atmosphere. The latter is a rather expensive process given that the concentration is just a few ppm.

Capturing and using gases doesn't typically prevent them from ending up in the atmosphere, unfortunately.

Like others have said though - helium escapes our atmosphere naturally and quite literally leaves Earth. That's how we're loosing all of our supply - it's just being vented into space constantly.

Helium doesn't persist in the atmosphere. The mean free velocity of helium gas is above escape velocity, so all of it leaves Earth for outer space.

Whoa, I didn't realize that was possible. Fascinating.

Don't worry, even if all energy humans used was from D+T fusion today it would only create 60% of the helium that we use. Also, helium is so light that it leaves the atmosphere so we really don't have access to much anyway.

What do we use it for? Isn’t it non reactive?

It's most valuable use is as a cryogenic. If you want sub 10K there are very few materials to choose from.

The lower the temperature, the higher your critical current is for a given superconductor. The reason HTS are a big deal isn't that you can have a superconductor at liquid nitrogen temperatures, but that you can have a very high field at liquid helium temperatures.

Rough estimate, meeting humanity's energy demand (132 PWh per year - that's all energy, not just electricity), with a reasonably efficient fusion station, we'd make 3,000-5,000 tonnes of "waste" helium per year. But as others have alluded to, it's quite a useful element and the general concern is we don't have enough.

Not much - it's lighter than the atmosphere so rises to the top and floats off into space.

UK accents will acquire a slightly higher pitch over time

Yet another way Monty Python was ahead of its time.

We have a shortage of helium on Earth, we are running out fast, all hydrogen produced can be put to good use.

That does not seem right, we have literal oceans of hydrogen?

sorry I meant helium, that was a brainfart

Yes but no at the same time.

We still have plenty of helium. It's a byproduct of oil extraction and is still often vented because it would not be profitable of capturing it.

Helium balloons will stop working and everyone will have high- pitched voices.

I had assumed both were chain reactions due to stars. That’s interesting

Well, technically, that's right. Fusion is a chain reaction. (albeit, "chain" might be a stretch here. Let's say it is a continuous process in stars.) But it's driven by gravity, not neutrons. You just need to bring about 70 times the mass of Jupiter into one place and that's it. Child's play, essentially ;).

The enormous pressure inside stars is what sustains the reaction. Neutrons coming off a fusion reaction, even if moderated, can’t really “squeeze together” other particles in any sense.

Also, per unit volume, the sun produces about as much power as a compost pile.

This lead me to the following - what if you make a compost pile the size (mass) of Sun? Meaning, it won't be made of hydrogen, but rather some carbon-based molecules. I'm not sure about other atoms in these molecules, but I think carbon is stable enough not to initiate nuclear reactions. So probably fusion won't start. What then?

Gravity would compress it. Depending on the exact composition fusion could start, ie it would become a star. According to Wikipedia carbon fusion requires a mass of 8 suns to start but there's a bunch of hydrogen in compost so maybe that would fuse.

Carbon is capable of fusion, in a sufficiently compressed stellar mass. Everything up to Iron-56 can theoretically sustain fusion.

Maybe it could work via the CNO cycle: https://en.wikipedia.org/wiki/CNO_cycle ?

Does the volume of the sun include its Corona in that last statement?

No, iirc that statement is for the part of the sun where fusion actually happens. Per volume, not a lot fusions happen in the plasma on the sun. The crux is that the volume of the compost heap grows order cubed to the radius, while the area through which the heat escapes grows quadratic. And the sun is like, a _really_ big compost heap.

In fusion on earth, we want to be considerably more efficient than the fusion process in the sun, as we don't have as much space to work with. ITER is already a pretty big machine.

That’s just the core. Go out to the photosphere, let alone the corona, and it’s about about a thousand times less, 0.27 W/m^3: https://www.wolframalpha.com/input/?i=%28solar+luminosity+%2...

Fusion can be a chain ration. In stars its caused by intense uninterpreted gravitational pressure. In the H-Bomb its triggered by a smaller fission explosion. Fusion chain reactions just require so much energy its hard (if not impossible) to harness on a commercial level. One small break in the the chain reaction containment and it falls apart.

Which is infinitely better then rapidly overheating and vaporizing and melting the reactor housing and spreading radiation which will stay dangerous for 200 years.