> Helium is a by-product of nuclear fusion.

Fission too. Yet we are not collecting it. It's not clear if the same problems will apply to collecting it from fusion.

Helium is a by-product of fission, but the product of fusion. The formation from alpha-particles around fission reactions (and the amount being like 100g/year?) makes it not very worthwhile to collect.

What's the distinction between product and by-product then?

I called it a 'by-product' because it's not the product we're actually aiming for, not the point of the exercise - if oxygen were produced instead (by whatever science-ignoring magic) we wouldn't mind.

But in Fusion what we are doing is intentionally creating helium. It's the mechanism we want.

In fission it's a byproduct in that it's not our main intention.

Will fusion produced helium be cleaner than fission produced one?

The answer looks very simple until you remember it is produced inside a container, that may of may not contaminate it.