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)?
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.
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.
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.
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.
