From my eternally pessimistic view, I see the biggest challenge with fusion being the race for resources. As climate change escalates, more and more money will go toward infrastructure repair, response to the migration of climate refugees, and of course, yet more pandemics. Countries will become de-stabilized by civil unrest and financial upheavals. Under those conditions, how long will governments continue to fund research that doesn't have an immediate promise of ROI?
Actually we are quite scarce on ressources to do fusion itself. Nuclear fusion reactors do have, amongst others, two big challenges to overcame, namely:
1. Consumption of tritium required to keep the reaction running
2. Neutron radiation during operation.
Tritium already is a big problem; it is non existant on earth. So far there is a global supply of ~20kg, which were mostly extracted from
CANDU nuclear power plants during runtime over decades. Since less CANDU reactors are running now than in the past, supply wents down. And this makes also tritium very, very expensive, around 30.000 US$/g at the moment. ITER alone will need for all experiments roughly 20 kg.
So in order to run a fusion reactor for years we do need cheep sources of tritium. Since the moon has tritium, some people thought about mining it there, it is estimated to have roughly 1.1 million tons of it.
The other idea is that the first fusion reactors are also going to do tritium breeding. This is so far highly theoretical, since nobody tried it out yet. And many physicists are quite skeptical about that.
The other is neutron radiation: this is quite powerful. And since the reactor will be exposed to it for years, it is expected to weaken the structure and change its properties. Another unsolved problem.
